Getting started

Prerequisites

• Android Studio version 1.5.1 or higher
• JDK version 7.0 or higher
• A recent version of the Android SDK
• Android API Level 9 or higher (Android 2.3 and above)

Note: Realm does not support Java outside of Android. We no longer support Eclipse as an IDE; please migrate to Android Studio.

Installation

Install Realm as a Gradle plugin.

Step 1: Add the class path dependency to the project level build.gradle file.

buildscript {
    repositories {
        jcenter()
    }
    dependencies {
        classpath "io.realm:realm-gradle-plugin:5.15.2"
    }
}

Find the project level build.gradle file here:

Step 2: Apply the realm-android plugin to the top of the application level build.gradle file.

apply plugin: 'realm-android'

Find the application level build.gradle file here:

Once you make these two changes, simply refresh your gradle dependencies. If you’re upgrading from a version of Realm earlier than v0.88, you may also need to clean your gradle project (./gradlew clean).

Find a sample of the two modified build.gradle files here:

• Project level build.gradle
• Application level build.gradle

Other Build Systems

The Maven & Ant build systems are not supported. We’re tracking interest in supporting them on GitHub:

• Maven support
• Ant support

A ProGuard configuration is provided as part of the Realm library. This means that you don’t need to add any Realm specific rules to your ProGuard configuration.

Samples

Realm Java lets you efficiently write your app’s model layer in a safe, persistent, and fast way. Here’s what it looks like:

// Define your model class by extending RealmObject
public class Dog extends RealmObject {
    private String name;
    private int age;

    // ... Generated getters and setters ...
}

public class Person extends RealmObject {
    @PrimaryKey
    private long id;
    private String name;
    private RealmList<Dog> dogs; // Declare one-to-many relationships

    // ... Generated getters and setters ...
}

// Use them like regular java objects
Dog dog = new Dog();
dog.setName("Rex");
dog.setAge(1);

// Initialize Realm (just once per application)
Realm.init(context);

// Get a Realm instance for this thread
Realm realm = Realm.getDefaultInstance();

// Query Realm for all dogs younger than 2 years old
final RealmResults<Dog> puppies = realm.where(Dog.class).lessThan("age", 2).findAll();
puppies.size(); // => 0 because no dogs have been added to the Realm yet

// Persist your data in a transaction
realm.beginTransaction();
final Dog managedDog = realm.copyToRealm(dog); // Persist unmanaged objects
Person person = realm.createObject(Person.class); // Create managed objects directly
person.getDogs().add(managedDog);
realm.commitTransaction();

// Listeners will be notified when data changes
puppies.addChangeListener(new OrderedRealmCollectionChangeListener<RealmResults<Dog>>() {
    @Override
    public void onChange(RealmResults<Dog> results, OrderedCollectionChangeSet changeSet) {
        // Query results are updated in real time with fine grained notifications.
        changeSet.getInsertions(); // => [0] is added.
    }
});

// Asynchronously update objects on a background thread
realm.executeTransactionAsync(new Realm.Transaction() {
    @Override
    public void execute(Realm bgRealm) {
        Dog dog = bgRealm.where(Dog.class).equalTo("age", 1).findFirst();
        dog.setAge(3);
    }
}, new Realm.Transaction.OnSuccess() {
    @Override
    public void onSuccess() {
        // Original queries and Realm objects are automatically updated.
        puppies.size(); // => 0 because there are no more puppies younger than 2 years old
        managedDog.getAge();   // => 3 the dogs age is updated
    }
});

Browse the Realm database

If you need help finding your app’s Realm file, check this StackOverflow answer for detailed instructions.

Realm Studio

Realm Studio is our premiere developer tool, built so you can easily manage the Realm Database and Realm Platform. With Realm Studio, you can open and edit local and synced Realms, and administer any Realm Object Server instance. It supports Mac, Windows and Linux.

Stetho Realm

You can also use the Stetho-Realm plugin for Stetho, an Android debug bridge for the Chrome browser created by Facebook.

Stetho-Realm is not officially maintained by Realm.

Initializing Realm

Before you can use Realm in your app, you must initialize it. This only has to be done once.

Realm.init(context);

You must provide an Android context. A good place to initialize Realm is in onCreate on an application subclass:

public class MyApplication extends Application {
  @Override
  public void onCreate() {
    super.onCreate();
    Realm.init(this);
  }
}

If you create your own application subclass, you must add it to the app’s AndroidManifest.xml:

<application
  android:name=".MyApplication"
  ...
/>

Realms

A Realm is an instance of a Realm Mobile Database container. Realms can be local or synchronized. A synchronized Realm uses the Realm Object Server to transparently synchronize its contents with other devices. While your application continues working with a synchronized Realm as if it’s a local file, the data in that Realm might be updated by any device with write access to that Realm. In practice, your application works with any Realm, local or synchronized, the same way.

For a more detailed discussion about Realms, read The Realm Data Model.

Opening Realms

Open a Realm by instantiating a new Realm object. We’ve seen this used already in examples:

// Initialize Realm
Realm.init(context);

// Get a Realm instance for this thread
Realm realm = Realm.getDefaultInstance();

The getDefaultInstance method instantiates the Realm with a default RealmConfiguration.

Configuring a Realm

To control how Realms are created, use a RealmConfiguration object. The minimal configuration usable by Realm is:

RealmConfiguration config = new RealmConfiguration.Builder().build();

That configuration—with no options—uses the Realm file default.realm located in Context.getFilesDir. To use another configuration, you would create a new RealmConfiguration object:

// The RealmConfiguration is created using the builder pattern.
// The Realm file will be located in Context.getFilesDir() with name "myrealm.realm"
RealmConfiguration config = new RealmConfiguration.Builder()
  .name("myrealm.realm")
  .encryptionKey(getKey())
  .schemaVersion(42)
  .modules(new MySchemaModule())
  .migration(new MyMigration())
  .build();
// Use the config
Realm realm = Realm.getInstance(config);

You can have multiple RealmConfiguration objects, so you can control the version, schema and location of each Realm independently.

RealmConfiguration myConfig = new RealmConfiguration.Builder()
  .name("myrealm.realm")
  .schemaVersion(2)
  .modules(new MyCustomSchema())
  .build();

RealmConfiguration otherConfig = new RealmConfiguration.Builder()
  .name("otherrealm.realm")
  .schemaVersion(5)
  .modules(new MyOtherSchema())
  .build();

Realm myRealm = Realm.getInstance(myConfig);
Realm otherRealm = Realm.getInstance(otherConfig);

Get the absolute path of a Realm by using Realm.getPath.

It is important to note that Realm instances are thread singletons, meaning that the static constructor will return the same instance in response to all calls from a given thread.

The default Realm

The RealmConfiguration can be saved as a default configuration. Setting a default configuration in your custom Application class makes it available in the rest of your code.

public class MyApplication extends Application {
  @Override
  public void onCreate() {
    super.onCreate();
    // The default Realm file is "default.realm" in Context.getFilesDir();
    // we'll change it to "myrealm.realm"
    Realm.init(this);
    RealmConfiguration config = new RealmConfiguration.Builder().name("myrealm.realm").build();
    Realm.setDefaultConfiguration(config);
  }
}

public class MyActivity extends Activity {
  @Override
  protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    Realm realm = Realm.getDefaultInstance(); // opens "myrealm.realm"
    try {
      // ... Do something ...
    } finally {
      realm.close();
    }
  }
}

Opening a synchronized Realm

Read-only Realms

It’s sometimes useful to ship a prepared Realm file with your app—you may want to bundle some shared data with your application. In many cases you don’t want to accidentally modify that Realm, as the data is purely read-only. You can do this by bundling a Realm file in assets and using a readOnly configuration:

RealmConfiguration config = new RealmConfiguration.Builder()
    .assetFile("my.realm")
    .readOnly()
    // It is optional, but recommended to create a module that describes the classes
    // found in your bundled file. Otherwise if your app contains other classes
    // than those found in the file, it will crash when opening the Realm as the
    // schema cannot be updated in read-only mode.
    .modules(new BundledRealmModule())
    .build();

In-memory Realms

With an inMemory configuration, you can create a Realm that runs entirely in memory without being persisted to disk.

RealmConfiguration myConfig = new RealmConfiguration.Builder()
    .name("myrealm.realm")
    .inMemory()
    .build();

In-memory Realms might still use disk space if memory is running low, but all files created by an in-memory Realm will be deleted when the Realm is closed. Creating an in-memory Realm with the same name as a persisted Realm isn’t allowed—names still have to be unique.

When all in-memory Realm instances with a particular name go out of scope with no references, that frees all that Realm’s data. To keep an in-memory Realm “alive” throughout your app’s execution, hold onto a reference to it.

Dynamic Realms

When working with a conventional Realm, the model class is defined using RealmObject subclasses. This has a lot of benefits with regards to type safety. But sometimes, the types aren’t available until runtime, e.g., during migrations, or when working with string-based data like CSV files. Dynamic Realms to the rescue!

A DynamicRealm is a variant of the conventional Realm that makes it possible to work with Realm data without using RealmObject subclasses. Instead, all access is done using Strings instead of Classes.

Opening a Dynamic Realm uses the same configuration as a conventional Realm but the Dynamic Realm ignores any configured schema, migration, and schema version.

RealmConfiguration realmConfig = new RealmConfiguration.Builder().build();
DynamicRealm realm = DynamicRealm.getInstance(realmConfig);

// In a DynamicRealm all objects are DynamicRealmObjects
realm.beginTransaction();
DynamicRealmObject person = realm.createObject("Person");
realm.commitTransaction();

// All fields are accessed using strings
String name = person.getString("name");
int age = person.getInt("age");

// An underlying schema still exists, so accessing a field that does not exist
// will throw an exception
person.getString("I don't exist");

// Queries still work normally
RealmResults<DynamicRealmObject> persons = realm.where("Person")
    .equalTo("name", "John")
    .findAll();

A DynamicRealm gains flexibility at the expense of both type safety and performance; in general, you should be using normal Realms. Only use Dynamic Realms when you need that flexibility.

Closing Realms

Realm implements Closeable to take care of native memory deallocation and file descriptors, so always close your Realm instances when you’re done with them.

Realm instances are reference counted—if you call getInstance twice in a thread, you need to call close twice as well. This allows you to implement Runnable classes without having to worry about which thread will execute them: simply start it with getInstance and end it with close.

For the UI thread, the easiest way is to execute realm.close in the owning component’s onDestroy method. If you need to create a Looper thread other than UI, you can use this pattern:

public class MyThread extends Thread {

    private Realm realm;

    @Override
    public void run() {
        Looper.prepare();
        realm = Realm.getDefaultInstance();
        try {
            //... Setup the handlers using the Realm instance ...
            Looper.loop();
        } finally {
            realm.close();
        }
    }
}

For AsyncTask this is a good pattern:

protected Void doInBackground(Void... params) {
    Realm realm = Realm.getDefaultInstance();
    try {
        // ... Use the Realm instance ...
    } finally {
        realm.close();
    }

    return null;
}

If you are using Thread or Runnable for short-lived tasks:

// Run a non-Looper thread with a Realm instance.
Thread thread = new Thread(new Runnable() {
    @Override
    public void run() {
        Realm realm = Realm.getDefaultInstance();
        try {
            // ... Use the Realm instance ...
        } finally {
            realm.close();
        }
    }
});

thread.start();

If you’re working on an app with minSdkVersion >= 19 and Java >= 7, then you can use try-with-resources:

try (Realm realm = Realm.getDefaultInstance()) {
    // No need to close the Realm instance manually
}

Auto-Refresh

If you obtain a Realm instance from a thread associated with a Looper, the Realm instance comes with an auto-refresh feature. (Android’s UI thread is a Looper.) This means the Realm instance will be periodically updated to the latest version. This lets you keep your UI constantly updated with the latest content with almost no effort!

If you get a Realm instance from a thread that does not have a Looper attached, objects from that instance won’t be updated until you call the waitForChange method. Holding on to an old version of your data is expensive in terms of memory and disk space, and the cost increases with the number of versions between the one being retained and the latest. This is why it is important to close the Realm instance as soon as you are done with it in the thread.

If you want to check whether your Realm instance has auto-refresh activated or not, use the isAutoRefresh method.

Models

Create Realm models by extending the RealmObject base class:

public class User extends RealmObject {

    private String          name;
    private int             age;

    @Ignore
    private int             sessionId;

    // Standard getters & setters generated by your IDE…
    public String getName() { return name; }
    public void   setName(String name) { this.name = name; }
    public int    getAge() { return age; }
    public void   setAge(int age) { this.age = age; }
    public int    getSessionId() { return sessionId; }
    public void   setSessionId(int sessionId) { this.sessionId = sessionId; }
}

A Realm model class supports public, protected and private fields, as well as custom methods.

public class User extends RealmObject {

    public String name;

    public boolean hasLongName() {
      return name.length() > 7;
    }

    @Override
    public boolean equals(Object o) {
      // Custom equals comparison
    }
}

Field types

Realm supports boolean, byte, short, int, long, float, double, String, Date and byte[] field types. The integer types byte, short, int, and long are all mapped to long within Realm. In addition to those standard field types, Realm supports subclasses of RealmObject and RealmList<? extends RealmObject> to model relationships.

The boxed types Boolean, Byte, Short, Integer, Long, Float and Double can also be used in model classes. These types may have the value null.

Required fields

The @Required annotation can be used to tell Realm to disallow null values in a field, making it required rather than optional. Only Boolean, Byte, Short, Integer, Long, Float, Double, String, byte[] and Date can be annotated with @Required. If you add it to other field types, compilation will fail.

Fields with primitive types and the RealmList type are required implicitly. Fields with RealmObject type are always nullable.

Primary keys

To mark a field as a model’s primary key, use the annotation @PrimaryKey. The field type must be either a string (String) or an integer (byte, short, int, long, Byte, Short, Integer, and Long). Using a string field as a primary key automatically indexes the field: the annotation @PrimaryKey on a string implicitly sets the annotation @Index. Realm doesn’t support compound keys, i.e., using multiple fields as a single primary key.

Using primary keys makes it possible to use the copyToRealmOrUpdate or insertOrUpdate methods. These looks for an object with a given primary key, and either updates it (if an object with that key already exists) or creates it (if the key does not exist). If you call copyToRealmOrUpdate or insertOrUpdate on a class without a primary key, an exception will be thrown.

When you use primary keys, reads (queries) will be slightly faster, but writes (creating and updating objects) will be a little slower. The changes in performance will depend on the size of your Realm’s data set.

Note that Realm.createObject returns a new object with all fields set to their default values. If the object is a class with a primary key, this could create a conflict—there might be an object with that primary key set already. To avoid this, you can create an unmanaged object, set its field values, then add it to the Realm with copyToRealm or insert:

final MyObject obj = new MyObject();
obj.setId(42);
obj.setName("Fish");
realm.executeTransaction(new Realm.Transaction() {
    @Override
    public void execute(Realm realm) {
        // This will create a new object in Realm or throw an exception if the
        // object already exists (same primary key)
        // realm.copyToRealm(obj);

        // This will update an existing object with the same primary key
        // or create a new object if an object with no primary key = 42
        realm.copyToRealmOrUpdate(obj);
    }
});

Primary keys that are String types or boxed integers (Byte, Short, Integer, and Long) can have the value null unless the @PrimaryKey annotation is combined with @Required.

Indexing properties

To index a field, use the annotation @Index. Like primary keys, this makes writes slightly slower , but makes reads faster. (It also makes your Realm file slightly larger, to store the index.) It’s best to only add indexes when you’re optimizing the read performance for specific situations.

You can index String, byte, short, int, long, boolean and Date fields.

Ignoring properties

If you don’t want to save a field in your model to its Realm, use the annotation @Ignore. You might do this if, for example, your input contains more fields than your model, and you don’t wish to have many special cases for handling these unused data fields.

Fields marked static and transient are always ignored, and do not need the @Ignore annotation.

Counters

Realm offers MutableRealmInteger as a special integer type. MutableRealmInteger exposes an additional API that can more clearly express intent and generate better conflict resolution steps when using Synchronized Realms.

Traditionally, a counter would be implemented by reading a value, incrementing it and setting it (myObj.counter += 1). This will not work well in an asynchronous situation — for example when two clients are offline — because both parties will read a value, say 10, increment it, and store the value as 11. Eventually, when they regain connectivity and try to merge their changes, they’ll agree that the counter is at 11 rather than the expected 12.

MutableRealmIntegers are backed by traditional integer types, so no migration is required when changing a field from byte, short, int or long to MutableRealmInteger.

MutableRealmInteger is not an immutable type standard like primitive number types in Java. It is a live object like RealmObject, RealmResults and RealmList. This means the value contained inside the MutableRealmInteger can change when a Realm is written to. For this reason MutableRealmInteger fields must be marked final.

public class Party extends RealmObject {
{
    public final MutableRealmInteger guests = MutableRealmInteger.valueOf(0);
}

To change the counter value, simply call counter.increment() or counter.decrement().

Party party = realm.where(Party.class).findFirst();
realm.beginTransaction();
party.guests.get(); // 0
party.guests.increment(1); // 1
party.guests.decrement(1); // 0
party.guests.increment(5); // 5
party.guests.decrement(1); // 4
realm.commitTransaction();

To reset the counter, you can assign it a new value using counter.set().

Calling set() can potentially override increment() and decrement() operations coming from other devices pr. the normal last-write-wins merge rules, so mixin these operations should only be done if lossy counters are acceptable.

Party party = realm.where(Party.class).findFirst();
realm.beginTransaction();
party.guests.set(0);
realm.commitTransaction();

Overriding property names

The default behaviour is that Realm will use the name defined in the Java model class as the name to represent classes and fields internally in the Realm file. In some cases you might want to change this behaviour:

• To support two model classes with the same simple name but in different packages.
• To make it easier to work with cross platform schemas as naming conventions are different.
• To use a Java class name that is longer than the 57 character limit enforced by Realm.
• To change a field name in Java without forcing app users through a migration process.

In those cases you can override the name being used internally by defining a different name using the @RealmModule, @RealmClass or @RealmField annotations.

You can define a naming policy at the module level, which will affect all classes part of the module:

@RealmModule(
  allClasses = true, 
  classNamingPolicy = RealmNamingPolicy.LOWER_CASE_WITH_UNDERSCORES,
  fieldNamingPolicy = RealmNamingPolicy.LOWER_CASE_WITH_UNDERSCORES
)
public class MyModule {
  
}

You can define a custom name for the class or a field naming policy that will effect all fields in that class. This will override any module level settings:

@RealmClass(name = "__Person", fieldNamingPolicy = RealmNamingPolicy.PASCAL_CASE)
public class Person extends RealmObject {
  public String name;  
}

You can define a custom name for a field, this will override any Class and Module level settings:

public class extends RealmObject {
  @RealmField(name = "person_name") 
  public String name;  
}

Choosing an internal name that differs from the name used in the Java model classes has the following implications:

• Queries on a DynamicRealm must use the internal name. Queries on normal Realm instances must continue to use the name as it is defined in the Java class.
• Migrations must use the internal name when creating classes and fields.
• Schema errors reported will use the internal names.

Note that changing the internal name does NOT affect importing data from JSON. The JSON data must still follow the names as defined in the Realm Java class.

When it comes to parsing JSON using standard libraries like Moshi, GSON or Jackson. Then it is important to remember that these libraries define the transformation from JSON to Java while setting the internal Realm names define the transformation from Java to the Realm file. This means that if you want to import data into Realm from JSON using these libraries you still need to provide the annotations from both the JSON parser library and Realm.

Using Moshi, it would look something like this:

public class Person extends RealmObject {
    @Json(name = "first_name") // Name used in JSON input.
    @RealmField(name = "first_name") // Name used internally in the Realm file.
    public string firstName; // name used in Java
 }

See RealmNamingPolicy for more info.

Working with RealmObjects

Auto-updating objects

RealmObjects are live, auto-updating views into the underlying data; you never have to refresh objects. Changes to objects are instantly reflected in query results.

realm.executeTransaction(new Realm.Transaction() {
    @Override
    public void execute(Realm realm) {
        Dog myDog = realm.createObject(Dog.class);
        myDog.setName("Fido");
        myDog.setAge(1);
    }
});
Dog myDog = realm.where(Dog.class).equalTo("age", 1).findFirst();

realm.executeTransaction(new Realm.Transaction() {
    @Override
    public void execute(Realm realm) {
        Dog myPuppy = realm.where(Dog.class).equalTo("age", 1).findFirst();
        myPuppy.setAge(2);
    }
});

myDog.getAge(); // => 2

This not only keeps Realm fast and efficient, it allows your code to be simpler and more reactive. If your Activity or Fragment is dependent on a specific RealmObject or RealmResults instance, you don’t need worry about refreshing or re-fetching it before updating the UI.

You can subscribe to Realm notifications to know when Realm data is updated.

Customizing objects

It is possible to use a RealmObject almost like a POJO. Extend your class from RealmObject. You can let the fields be public, and can use simple assignments instead of setters and getters.

public class Dog extends RealmObject {
    public String name;
    public int age;
}

You can use Dog like any other class: you can add logic to your getter and setter methods (for example, for validation), and you can add any custom methods you wish.

To add your Dog object to a Realm, use the createObject or copyToRealm methods:

realm.executeTransaction(new Realm.Transaction() {
    @Overrride
    public void execute(Realm realm) {
        Dog dog = realm.createObject(Dog.class);
        dog.name = "Fido";
        dog.age  = 5;
    }
};

RealmModel interface

Instead of extending RealmObject, your classes can implement the RealmModel interface, adding the @RealmClass annotation:

@RealmClass
public class User implements RealmModel {

}

With this interface, all methods available on RealmObject become available through static methods. Note that a class that extends RealmObject does not need either the @RealmClass annotation or to implement RealmModel.

// With RealmObject
user.isValid();
user.addChangeListener(listener);

// With RealmModel
RealmObject.isValid(user);
RealmObject.addChangeListener(user, listener);

JSON

You can add a JSON object that maps to a RealmObject to Realm. The JSON object can be a String, a JSONObject or an InputStream. Realm will ignore any properties in the JSON not defined by the RealmObject. Add single objects through Realm.createObjectFromJson, and lists of objects through [Realm.createAllFromJson][api/io/realm/Realm.html#createAllFromJson-java.lang.Class-java.lang.String-).

// A RealmObject that represents a city
public class City extends RealmObject {
    private String city;
    private int id;
    // getters and setters left out ...
}

// Insert from a string
realm.executeTransaction(new Realm.Transaction() {
    @Override
    public void execute(Realm realm) {
        realm.createObjectFromJson(City.class, "{ city: \"Copenhagen\", id: 1 }");
    }
});

// Insert multiple items using an InputStream
realm.executeTransaction(new Realm.Transaction() {
    @Override
    public void execute(Realm realm) {
        try {
            InputStream is = new FileInputStream(new File("path_to_file"));
            realm.createAllFromJson(City.class, is);
        } catch (IOException e) {
            throw new RuntimeException(e);
        }
    }
});

If a field in the JSON object is null and the field is required by the Realm model, Realm will throw an exception. If the field is optional, its value will be set to the field default when creating an object, and to null when updating an object. If the Realm model has a field that isn’t present in the JSON object, the value will remain unchanged in the Realm model.

Adapters

Realm offers abstract utility classes to help bind data coming from OrderedRealmCollections (both RealmResults and RealmList implement this interface) to standard UI widgets.

• Use RealmBaseAdapter with ListView. See an example.
• Use RealmRecyclerViewAdapter with RecyclerView. See an example.

To use the adapters, add the dependencies to the application level build.gradle:

dependencies {
    compile 'io.realm:android-adapters:2.1.1'
}

Javadoc for the adapters can be found here and an example of their use can be found here.

Intents

Since RealmObjects are not Parcelable and cannot be passed directly, you must pass an identifier for the object you’re working with. For example, if an object has a primary key, pass the primary key value in the Intent extras bundle:

// Assuming we had a person class with a @PrimaryKey on the 'id' field ...
Intent intent = new Intent(getActivity(), ReceivingService.class);
intent.putExtra("person_id", person.getId());
getActivity().startService(intent);

Retrieve the primary key value from the bundle on the receiving end (Activity, Service, IntentService, BroadcastReceiver, etc.), and then open a Realm and query for the RealmObject:

// in onCreate(), onHandleIntent(), etc.
String personId = intent.getStringExtra("person_id");
Realm realm = Realm.getDefaultInstance();
try {
    Person person = realm.where(Person.class).equalTo("id", personId).findFirst();
    // do something with the person ...
} finally {
    realm.close();
}

The overhead for re-opening the Realm on a different thread is very small.

You can find working examples in the Object Passing portion of the threading example. The example shows you how to pass id’s and retrieve the RealmObject in common Android use cases.

Relationships

You can link any two RealmObjects together. Relationships are cheap in Realm: traversing links isn’t expensive in terms of speed or memory. Let’s explore the different types of relationships Realm lets you define between objects.

Many-to-one

To set up a many-to-one or one-to-one relationship, give a model a property whose type is one of your RealmObject subclasses:

public class Email extends RealmObject {
    private String address;
    private boolean active;
}

public class Contact extends RealmObject {
    private String name;
    private Email email;
}

Contact bob = realm.createObject(Contact.class);
bob.name = "Bob Newhart";

Email email1 = realm.createObject(Email.class);
email1.address = "bob@example.com";

bob.email = email1;

Each Contact has zero or one Email instances. Nothing would prevent you from using the same Email object with more than one Contact; the distinction between a many-to-one and a one-to-one relationship is up to your application.

Setting the relationship field to null will clear the reference:

bob.email = null;

This deletes the relationship between bob and email1, but email1 is still in the Realm.

Many-to-many

You can create a relationship to any number of objects from a single object via a RealmList<T> field declaration. Let’s rewrite our example to support multiple email addresses:

public class Contact extends RealmObject {
    public String name;
    public RealmList<Email> emails;
}

public class Email extends RealmObject {
    public String address;
    public boolean active;
}

RealmLists are containers of RealmObjects; a RealmList behaves like a regular Java List. You can use the same object in different RealmLists, and you can use this to model both one-to-many and many-to-many relationships.

realm.executeTransaction(new Realm.Transaction() {
    @Override
    public void execute(Realm realm) {
        Contact contact = realm.createObject(Contact.class);
        contact.name = "John Doe";

        Email email1 = realm.createObject(Email.class);
        email1.address = "john@example.com";
        email1.active = true;
        contact.emails.add(email1);

        Email email2 = realm.createObject(Email.class);
        email2.address = "jd@example.com";
        email2.active = false;
        contact.emails.add(email2);
    }
});

It is possible to declare recursive relationships which can be useful when modeling certain types of data.

public class Person extends RealmObject {
    public String name;
    public RealmList<Person> friends;
    // Other fields...
}

Setting the value to null for a RealmList field will clear the list. The list will be empty (length zero), but the objects that were in the list won’t be deleted from the Realm. The getter for a RealmList will never return null: the returned object is always a list. The length might be zero.

Inverse relationships

Relationships are unidirectional. Take the two classes Person and Dog as an example:

public class Dog extends RealmObject {
    private String name;
    private int age;
}

public class Person extends RealmObject {
    @PrimaryKey
    private long id;
    private String name;
    private RealmList<Dog> dogs;
}

You can follow the link from a Person to a Dog, but there’s no way to go from a Dog to its Person objects. You can resolve this by giving the Dog a @LinkingObjects annotation.

public class Person extends RealmObject {
  private String id;
  private String name;
  private RealmList<Dog> dogs;
  // getters and setters
}

public class Dog extends RealmObject {
  private String id;
  private String name;
  private String color;
  @LinkingObjects("dogs")
  private final RealmResults<Person> owners;
  // getters and setters
}

We’ve given Dog an owners field, and specified that it should contain all Person objects that have this Dog object in their dogs field.

The annotated field must be declared final, and must be of type RealmResults<T> where T is the type/class of opposite end of the relationship. Since relationships are either many-to-one or many-to-many, following inverse relationships could result in 0, 1 or more objects.

Like any other RealmResults set, you can query an inverse relationship.

Lists Of Primitives

Realm model classes can contains lists of primitive data types. This must be modeled using RealmList<T>, where T can be the following types: String, Integer, Boolean, Float, Double, Short, Long, Byte, byte[] and Date.

public class Person extends RealmObject {
  public String name;
  public RealmList<String> children = new RealmList<>();
}

Unlike lists of RealmModel’s, lists of primitives can contain null values. If null values shouldn’t be allowed, use the @Required annotation:

public class Person extends RealmObject {
  public String name;
  @Required
  public RealmList<String> children = new RealmList<>();
}

Before Realm Java 4.0.0, it was common to model lists of primitives using a special Realm<String/Int> class. You can migrate from this approach to lists of primitives using the following migration code:

// Model classes
public class RealmString extends RealmObject {
  public String value;
}

public class Person extends RealmObject {
  public String name;
  @Required
  public RealmList<String> children = new RealmList<>();
}

// Migration code
RealmObjectSchema objSchema = realmSchema.get("Person");
objSchema.addRealmListField("children_tmp", String.class)
        .setRequired("children_tmp", true)
        .transform(new RealmObjectSchema.Function() {
            @Override
            public void apply(DynamicRealmObject obj) {
                RealmList<DynamicRealmObject> children = obj.getList("children");
                RealmList<String> migratedChildren = obj.getList("children_tmp", String.class);
                for (DynamicRealmObject child : children) {
                    migratedChildren.add(child.getString("value"));
                }
            }
        })
        .removeField("children")
        .renameField("children_tmp", "children");

Schemas

The default schema for a Realm is simply all the Realm model classes in a project. However, you can change this behavior—for instance, you might want to restrict a Realm to only contain a subset of classes. To do this, create a custom RealmModule.

// Create the module
@RealmModule(classes = { Person.class, Dog.class })
public class MyModule {
}

// Set the module in the RealmConfiguration to allow only classes defined by the module.
RealmConfiguration config = new RealmConfiguration.Builder()
  .modules(new MyModule())
  .build();

// It is possible to combine multiple modules to one schema.
RealmConfiguration config = new RealmConfiguration.Builder()
  .modules(new MyModule(), new MyOtherModule())
  .build();

For library developers: Libraries that include Realm must expose and use their schema through a RealmModule. Doing so prevents the default RealmModule from being generated for the library project, which would conflict with the default RealmModule used by the app. The library’s RealmModule is also how the library exposes its Realm classes to the app.

// A library must create a module and set library = true. This will prevent the default
// module from being created.
// allClasses = true can be used instead of listing all classes in the library.
@RealmModule(library = true, allClasses = true)
public class MyLibraryModule {
}

// Library projects are therefore required to explicitly set their own module.
RealmConfiguration libraryConfig = new RealmConfiguration.Builder()
  .name("library.realm")
  .modules(new MyLibraryModule())
  .build();

// Apps can add the library RealmModule to their own schema.
RealmConfiguration config = new RealmConfiguration.Builder()
  .name("app.realm")
  .modules(Realm.getDefaultModule(), new MyLibraryModule())
  .build();

You can’t have multiple RealmModule declarations in a single file. If you have two or more RealmModules, you will have to split the declarations into several files with exactly one declaration per file.

See a complete example of how RealmModules work between library and app projects here.

Writes

Unlike read operations, write operations in Realm must be wrapped in transactions. At the end of a write operation, you can either commit the transaction or cancel it. Committing a transaction writes all changes to disk (and if the Realm is synced, queues it for synchronization with Realm Object Server). If you cancel a write transaction, all the changes are discarded. Transactions are “all or nothing”: either all the writes within a transaction succeed, or none of them take effect. This helps guarantee data consistency, as well as providing thread safety.

// Obtain a Realm instance
Realm realm = Realm.getDefaultInstance();

realm.beginTransaction();

//... add or update objects here ...

realm.commitTransaction();

Or to discard the changes by canceling the transaction:

realm.beginTransaction();
User user = realm.createObject(User.class);

//  ...

realm.cancelTransaction();

Write transactions block each other. This can cause ANR errors if you create write transactions on both the UI and background threads at the same time. To avoid this, use async transactions when creating write transactions on the UI thread.

If an exception happens inside a transaction, you’ll lose the changes in that transaction, but the Realm itself won’t be affected (or corrupted). If you catch the exception and the app continues, you’ll need to cancel the transaction. If you use executeTransaction, this happens automatically.

Thanks to Realm’s MVCC architecture, reads are not blocked while a write transaction is open. Unless you need to make simultaneous transactions from many threads at once, you can favor larger transactions that do more work over many fine-grained transactions. When you commit a write transaction to a Realm, all other instances of that Realm will be notified and be updated automatically.

Read & write access in Realm is ACID.

Creating objects

Wrap the createObject method in a write transaction.

realm.beginTransaction();
User user = realm.createObject(User.class); // Create a new object
user.setName("John");
user.setEmail("john@corporation.com");
realm.commitTransaction();

If you create an object instance first and use copyToRealm to add it to a Realm, you should wrap the copy action in a transaction, too. Realm supports as many custom constructors as you like as long as one of them is a public no-arguments constructor.

User user = new User("John");
user.setEmail("john@corporation.com");

// Copy the object to Realm. Any further changes must happen on realmUser
realm.beginTransaction();
User realmUser = realm.copyToRealm(user);
realm.commitTransaction();

Remember, Realm only manages the returned object (realmUser in this example), not the object originally copied (user). To make changes to the object in the database, make changes to the returned copy, not the original.

If you are only interestered in inserting the object and not using the managed copy right away, it is possible to use insert instead. This works in a similar way to copyToRealm but is much faster as not returning the object makes it possible to optimize it more.

If you are inserting many objects, the recommend approach is to use insert or insertOrUpdate.

List<User> users = Arrays.asList(new User("John"), new User("Jane"));

realm.beginTransaction();
realm.insert(users);
realm.commitTransaction();

Transaction blocks

Instead of manually keeping track of beginTransaction, commitTransaction, and cancelTransaction, you can use the executeTransaction method, which will automatically handle begin/commit, and cancel if an error happens.

realm.executeTransaction(new Realm.Transaction() {
    @Override
    public void execute(Realm realm) {
        User user = realm.createObject(User.class);
        user.setName("John");
        user.setEmail("john@corporation.com");
    }
});

Asynchronous transactions

Since transactions are blocked by other transactions, you might want to write on a background thread to avoid blocking the UI thread. By using an asynchronous transaction, Realm will run that transaction on a background thread.

realm.executeTransactionAsync(new Realm.Transaction() {
            @Override
            public void execute(Realm bgRealm) {
                User user = bgRealm.createObject(User.class);
                user.setName("John");
                user.setEmail("john@corporation.com");
            }
        }, new Realm.Transaction.OnSuccess() {
            @Override
            public void onSuccess() {
                // Transaction was a success.
            }
        }, new Realm.Transaction.OnError() {
            @Override
            public void onError(Throwable error) {
                // Transaction failed and was automatically canceled.
            }
        });

OnSuccess and OnError callbacks are both optional, but if provided, they will be called when the transaction succeeds or fails, respectively. Callbacks are controlled by the Looper, so they are only allowed on Looper threads.

RealmAsyncTask transaction = realm.executeTransactionAsync(new Realm.Transaction() {
            @Override
            public void execute(Realm bgRealm) {
                User user = bgRealm.createObject(User.class);
                user.setName("John");
                user.setEmail("john@corporation.com");
            }
        }, null);

The RealmAsyncTask object can cancel any pending transaction if you need to quit the Activity/Fragment before the transaction is completed. Forgetting to cancel a transaction can crash the app if the callback updates the UI!

public void onStop () {
    if (transaction != null && !transaction.isCancelled()) {
        transaction.cancel();
    }
}

Updating strings and byte arrays

Since Realm operates on fields as a whole, it’s not possible to directly update individual elements of strings or byte arrays. Instead, you’ll need to read the whole field, make a modification to the individual element, and write it back again in a transaction block.

realm.executeTransaction(new Realm.Transaction() {
    @Override
    public void execute(Realm realm) {
        bytes[] bytes = realmObject.binary;
        bytes[4] = 'a';
        realmObject.binary = bytes;
    }
});

Batch updates

If you need to update many objects at once, then the most efficient way is making a query that find the objects and use one of RealmResults.setX() methods, where X is the type of field you want to update.

// Updating a boolean field
realm.executeTransaction(new Realm.Transaction() {
    @Override
    public void execute(Realm realm) {
        RealmResults<Person> persons = realm.where(Person.class).equalTo("invited", false).findAll();
        persons.setBoolean("invited", true);
    }
});

It is also possible to use a generic set method called RealmResults.setValue(String fieldName, Object value), which automatically detects the type of input and converts it as appropriate. This is similar to how DynamicRealmObject.setX() and DynamicRealmObject.setValue() behave. Using RealmResults.setValue() is slower than using the specific typed method.

// Updating a boolean field using automatic input conversion as needed.
realm.executeTransaction(new Realm.Transaction() {
    @Override
    public void execute(Realm realm) {
        RealmResults<Person> persons = realm.where(Person.class).equalTo("invited", false).findAll();
        persons.setValue("invited", "true"); 
    }
});

Queries

Realm’s query engine uses a Fluent interface to construct multi-clause queries.

public class User extends RealmObject {

    @PrimaryKey
    private String          name;
    private int             age;

    @Ignore
    private int             sessionId;

    // Standard getters & setters generated by your IDE…
    public String getName() { return name; }
    public void   setName(String name) { this.name = name; }
    public int    getAge() { return age; }
    public void   setAge(int age) { this.age = age; }
    public int    getSessionId() { return sessionId; }
    public void   setSessionId(int sessionId) { this.sessionId = sessionId; }
}

To find all users named John or Peter, you would write:

// Build the query looking at all users:
RealmQuery<User> query = realm.where(User.class);

// Add query conditions:
query.equalTo("name", "John");
query.or().equalTo("name", "Peter");

// Execute the query:
RealmResults<User> result1 = query.findAll();

// Or alternatively do the same all at once (the "Fluent interface"):
RealmResults<User> result2 = realm.where(User.class)
                                  .equalTo("name", "John")
                                  .or()
                                  .equalTo("name", "Peter")
                                  .findAll();

This gives you a new instance of the class RealmResults, containing the users with the name John or Peter.

The method findAll executes the query; [RealmQuery][] includes a family of findAll methods:

• findAll finds all objects that meet the query conditions
• findAllAsync operates asynchronously on a background thread
• findFirst (and findFirstAsync) find the first object that meets the query conditions

For full details, dive into the RealmQuery API reference.

Queries return a list of references to the matching objects, so you work directly with the original objects that matches your query. RealmResults inherits from AbstractList and behaves in similar ways. For example, RealmResults are ordered, and you can access the individual objects through an index. If a query has no matches, the returned RealmResults object will be a list of size(0) (not null).

If you wish to modify or delete objects in a RealmResults set, you must do so in a write transaction.

Note that you can also query relationships: read about link queries.

Filtering

The where method starts a RealmQuery by specifying a model. The filter criteria is specified with predicate methods, most of which have self-explanatory names (e.g., equalTo). A predicate always takes a field name as its first argument.

Not all predicates can be used with all field types; consult the [RealmQuery][] API reference for details.

For all data types, you have the following predicates:

• equalTo
• notEqualTo
• in

To match a field against a list of values, use in. For example, to find the names “Jill,” “William,” or “Trillian”, you can use in("name", new String[]{"Jill", "William", "Trillian"}). The in predicate is applicable to strings, binary data, and numeric fields (including dates).

Numeric data types, including Date, allow these additional predicates:

• between (includes both end points, i.e., it is a bounded interval)
• greaterThan
• lessThan
• greaterThanOrEqualTo
• lessThanOrEqualTo

String fields allow these additional predicates:

• contains
• beginsWith
• endsWith
• like

All four string predicates have an optional third argument to control case sensitivity: Case.INSENSITIVE and Case.SENSITIVE. The default is Case.SENSITIVE.

The predicate like performs glob-style wildcard matching. The matching pattern consists of characters and one or more wildcards:

• * matches 0 or more Unicode characters
• ? matches a single Unicode character

For example, consider a Realm with four objects with a field called name which has the values William, Bill, Jill, and Trillian. The predicate like("name", "?ill*") will match the first three objects, and like("name", "*ia?") will match the first and the last object.

Binary data, strings, and lists of RealmObjects (RealmList) may be empty, i.e., have a length of zero. You can check for emptiness with:

• isEmpty
• isNotEmpty

If a field is not required, the value can have the value null (recall that fields which are RealmObjects can never be required, and the value can be null). You can check for null with:

• isNull
• isNotNull

Logical operators

Conditions are implicitly joined with logical and. Logical or joins must be applied explicitly using or.

public class User extends RealmObject {

    @PrimaryKey
    private String          name;
    private int             age;

    @Ignore
    private int             sessionId;

    // Standard getters & setters generated by your IDE…
    public String getName() { return name; }
    public void   setName(String name) { this.name = name; }
    public int    getAge() { return age; }
    public void   setAge(int age) { this.age = age; }
    public int    getSessionId() { return sessionId; }
    public void   setSessionId(int sessionId) { this.sessionId = sessionId; }
}

You can also group conditions using beginGroup and endGroup to specify order of evaluation:

RealmResults<User> r = realm.where(User.class)
                            .greaterThan("age", 10)  // implicit AND
                            .beginGroup()
                                .equalTo("name", "Peter")
                                .or()
                                .contains("name", "Jo")
                            .endGroup()
                            .findAll();

Negate conditions with not. You can use the not operator with beginGroup/endGroup to negate sub-conditions only. If you wanted to all find users who are not named “Peter” or “Jo,” the query could be:

RealmResults<User> r = realm.where(User.class)
                           .not()
                           .beginGroup()
                                .equalTo("name", "Peter")
                                .or()
                                .contains("name", "Jo")
                            .endGroup()
                            .findAll();

With this particular query, though, it’s easier to use in:

RealmResults<User> r = realm.where(User.class)
                           .not()
                           .in("name", new String[]{"Peter", "Jo"})
                           .findAll();

Sorting

You can define how the results should be sorted when doing the query with the sort method.

RealmResults<User> result = realm.where(User.class).sort("age").findAll();

Or you can sort any results already retrieved by Realm:

result = result.sort("age"); // Sort ascending
result = result.sort("age", Sort.DESCENDING);

Sorts are ascending by default; to change that, use Sort.DESCENDING as the second argument. It is possible to sort using multiple fields simultaneously.

Limiting Results

Most other database technologies provide the ability to ‘paginate’ results from queries (such as the ‘LIMIT’ keyword in SQLite). This is often done out of necessity to avoid reading too much from disk, or pulling too many results into memory at once.

Since queries in Realm are lazy, performing this sort of paginating is normally not needed when working with a local Realm file as Realm will only load objects from the results of the query once they are explicitly accessed.

However, there are a few cases where limiting results can be beneficial:

• When using Query-based Realms, data will be fetched from the server when queried for. In this case it can make sense to limit the number of results as it directly impact how much data is transfered from the server.

• When creating UI’s that depend on a limited query result like a “Top 10 List”. In this case, creating a limited query result will reduce the complexity of code needed to create such a screen.

In those cases, query results can be limited by applying the limit() keyword to the query.

RealmResults<Person> people = realm.where(Person.class)
  .sort("name")
  .limit(10)
  .findAllAsync();

Limited query results are auto-updated like any other query results. This means that if 10 elements are returned when the query first returns, these 10 objects can be replaced or removed as the underlying data set changes.

When using fine-grained notifications, objects that stops being part of the RealmResults will be reported as deleted. This does not necessarily mean that they are deleted from the underlying Realm, just that they are no longer part of the query result.

Unique values

To return only unique values, use the distinct predicate. For example, to find out how many different names you have in your Realm:

RealmResults<Person> unique = realm.where(Person.class).distinct("name").findAll();

You can only call distinct on integer and string fields; other field types will throw an exception. As with sorting, you can specify multiple fields.

Chaining queries

You can run additional queries on result sets:

RealmResults<Person> teenagers = realm.where(Person.class).between("age", 13, 20).findAll();
Person firstJohn = teenagers.where().equalTo("name", "John").findFirst();

You can also chain queries on child objects as well. Assume the above Person object has a list of Dog objects.

public class Dog extends RealmObject {
    private int age;
    // getters & setters ...
}

public class Person extends RealmObject {
    private int age;
    private RealmList<Dog> dogs;
    // getters & setters ...
}

You can query for all people between the age of 13 and 20 who have at least one dog which is one year old:

RealmResults<Person> teensWithPups = realm.where(Person.class).between("age", 13, 20).equalTo("dogs.age", 1).findAll();

Note that query chains are built on RealmResults, not RealmQuery. When you add more conditions to a RealmQuery object, you are modifying the query itself.

Link queries

It is possible to query links or relationships. Consider the model below:

public class Person extends RealmObject {
  private String id;
  private String name;
  private RealmList<Dog> dogs;
  // getters and setters
}

public class Dog extends RealmObject {
  private String id;
  private String name;
  private String color;
  // getters and setters
}

Each Person object has multiple dog relationships, as shown in this table diagram:

Now, we can find specific people with link queries:

// persons => [U1,U2]
RealmResults<Person> persons = realm.where(Person.class)
                                .equalTo("dogs.color", "Brown")
                                .findAll();

The field name in equalTo is a path through the relationships, using a period (.) as the separator. The query above reads “find all Persons who have Dogs whose color is Brown.” Note that the result will contain all Dog objects for Person objects that have at least one matching Dog:

persons.get(0).getDogs(); // => [A,B]
persons.get(1).getDogs(); // => [B,C,D]

Remember, we’re searching for people who have particular kinds of dogs, not the actual dogs themselves.

Let’s dig a little deeper:

// r1 => [U1,U2]
RealmResults<Person> r1 = realm.where(Person.class)
                             .equalTo("dogs.name", "Fluffy")
                             .equalTo("dogs.color", "Brown")
                             .findAll();

// r2 => [U2]
RealmResults<Person> r2 = realm.where(Person.class)
                             .equalTo("dogs.name", "Fluffy")
                             .findAll()
                             .where()
                             .equalTo("dogs.color", "Brown")
                             .findAll()
                             .where()
                             .equalTo("dogs.color", "Yellow")
                             .findAll();

The first query reads, “find all Persons who have dogs named ‘Fluffy’ and have dogs whose color is ‘Brown.’” The second query reads, “find all Persons who have dogs named ‘Fluffy.’ Within that result set, find all Persons who have dogs whose color is ‘Brown.’ Then, within that result set, find all Persons who have dogs whose color is ‘Yellow.’” So the first query finds two sets of Persons and returns the intersection of those sets; the second query operates differently, by taking the result set of each findAll and feeding it into the next where query to successively narrow down the results down. You could rewrite the second query by chaining:

RealmResults<Person> set1 = realm.where(Person.class).equalTo("dogs.name", "Fluffy").findAll();
RealmResults<Person> set2 = set1.where(Person.class).equalTo("dogs.color", "Brown").findAll();
RealmResults<Person> set3 = set2.where(Person.class).equalTo("dogs.color", "Yellow").findAll();

Using inverse relationships, you can expand your query possibilities. Let’s consider the same two model classes, Person and Dog. Instead of starting the query with Person, you can query first for the dog, and follow the inverse relationship to the persons.

RealmResults<Dog> brownFluffies = realm.where(Dog.class).equalTo("color", "Brown").equalTo("name", "Fluffy").findAll();
for (Dog brownFluffy : brownFluffies) {
    RealmResults<Person> owners = brownFluffy.getOwners();
    // ...
}

You can also use link queries with inverse relationships:

RealmResults<Dog> dogs = realm.where(Dog.class).equalTo("persons.name", "Jane").findAll();

Auto-updating results

RealmResults are live, auto-updating views into the underlying data. If another thread, process or even device modifies an object in a RealmResults set, the change is instantly reflected. Your code doesn’t need to re-run the query or manually refresh the data.

final RealmResults<Dog> puppies = realm.where(Dog.class).lessThan("age", 2).findAll();
puppies.size(); // => 0

realm.executeTransaction(new Realm.Transaction() {
    @Override
    void public execute(Realm realm) {
        Dog dog = realm.createObject(Dog.class);
        dog.setName("Fido");
        dog.setAge(1);
    }
});

puppies.addChangeListener(new RealmChangeListener() {
    @Override
    public void onChange(RealmResults<Dog> results) {
      // results and puppies point are both up to date
      results.size(); // => 1
      puppies.size(); // => 1
    }
});

This applies to all RealmResults: all objects, filtered and chained.

This property of RealmResults not only keeps Realm fast and efficient, but it allows your code to be simpler and more reactive. For example, if your Activity or Fragment relies on the results of a query, you can just store the Realm object or RealmResults in a field. Its data will always be up-to-date when accessed.

Even though you don’t have to refresh your RealmResults, your application may need to update its UI or run other tasks when data changes. You can subscribe to notifications when Realm data gets updated. Since results are auto-updating, it’s important to not rely on indices and counts staying constant.

Aggregation

RealmResults offers aggregation convenience methods for operations like sums and averages across result sets.

RealmResults<User> results = realm.where(User.class).findAll();
long   sum     = results.sum("age").longValue();
long   min     = results.min("age").longValue();
long   max     = results.max("age").longValue();
double average = results.average("age");

long   matches = results.size();

Iterations & snapshots

All Realm collections are live. This means that they always reflect the latest state. In most cases this is desirable, but what if you’re iterating over a collection with the purpose of modifying the elements? For example:

RealmResults<Person> guests = realm.where(Person.class).equalTo("invited", false).findAll();
realm.beginTransaction();
for (int i = 0; guests.size(); i++) {
    guests.get(i).setInvited(true);
}
realm.commitTransaction();

Typically you would expect this simple loop to invite all guests. Because the RealmResults is updated immediately, though, only half the guests end up being invited! An invited guest is removed immediately from the collection, which will shift all elements. When the i parameter gets incremented, it will miss an element.

To prevent this, you can take a snapshot of a collection’s data. A snapshot guarantees the order of elements will not change, even if an element is deleted or modified.

Iterators created from RealmResults will use a snapshot automatically, Iterators created from RealmList won’t. To delete elements while iterating a RealmList, Iterator.remove() should be used instead of RealmList.remove() or other APIs which would remove elements from the RealmList indirectly to avoid ConcurrentModificationException. RealmResults and RealmList have a createSnapshot method to create one manually.

RealmResults<Person> guests = realm.where(Person.class).equalTo("invited", false).findAll();

// Use an iterator to invite all guests
realm.beginTransaction();
for (Person guest : guests) {
    guest.setInvited(true);
}
realm.commitTransaction();

// Use a snapshot to invite all guests
realm.beginTransaction();
OrderedRealmCollectionSnapshot<Person> guestsSnapshot = guests.createSnapshot();
for (int i = 0; guestsSnapshot.size(); i++) {
    guestsSnapshot.get(i).setInvited(true);
}
realm.commitTransaction();

Deletion

You can delete the results of a query from the Realm:

// obtain the results of a query
final RealmResults<Dog> results = realm.where(Dog.class).findAll();

// All changes to data must happen in a transaction
realm.executeTransaction(new Realm.Transaction() {
    @Override
    public void execute(Realm realm) {
        // remove single match
        results.deleteFirstFromRealm();
        results.deleteLastFromRealm();

        // remove a single object
        Dog dog = results.get(5);
        dog.deleteFromRealm();

        // Delete all matches
        results.deleteAllFromRealm();
    }
});

Asynchronous queries

Most queries in Realm are fast enough to run synchronously, even on the UI thread. However for either complex queries or queries on large data sets, it can be an advantage to run the query on a background thread.

RealmResults<User> result = realm.where(User.class)
                              .equalTo("name", "John")
                              .or()
                              .equalTo("name", "Peter")
                              .findAllAsync();

Note that the query is not blocking—it immediately returns a RealmResults<User>. This is a promise similar to the concept of a Future in standard Java. The query will continue to run in a background thread, updating the returned instance of RealmResults once it’s finished.

If you want to be notified when the query completes and the RealmResults object is updated, you can register a RealmChangeListener. This listener will be called every time the RealmResults are updated to reflect the latest changes in the Realm (usually after a commit).

private OrderedRealmCollectionChangeListener<RealmResults<User>> callback = new OrderedRealmCollectionChangeListener<>() {
    @Override
    public void onChange(RealmResults<User> results, OrderedCollectionChangeSet changeSet) {
        if (changeSet == null) {
            // The first time async returns with an null changeSet.
        } else {
            // Called on every update.
        }
    }
};

private RealmResults<User> result;

public void onStart() {
    result = realm.where(User.class).findAllAsync();
    result.addChangeListener(callback);
}

Remember to unregister any listeners when exiting an Activity or Fragment to avoid memory leaks.

public void onStop () {
    result.removeChangeListener(callback); // remove a particular listener
    // or
    result.removeAllChangeListeners(); // remove all registered listeners
}

Use isLoaded to check if a query has completed:

RealmResults<User> result = realm.where(User.class).findAllAsync();
if (result.isLoaded()) {
  // Results are now available
}

Calling isLoaded on a RealmResults object obtained synchronously will always return true.

You can also wait until the query completes. This will block the thread, making the query synchronous again.

RealmResults<User> result = realm.where(User.class).findAllAsync();
result.load() // be careful, this will block the current thread until it returns

Note: You can only use asynchronous queries on a Looper thread. The asynchronous query needs to use the Realm’s Handler in order to deliver results consistently. Trying to call an asynchronous query using a Realm opened inside a thread without a Looper will throw an IllegalStateException.

Migrations

When working with any database, it is likely your model classes (i.e. your database schema) will change over time. Since model classes in Realm are defined as standard objects, changing the schema is as easy as changing the interface of the corresponding RealmObject subclass.

Local migrations

For Realms that are not synced to the Realm Object Server, performing a migration requires two changes to your RealmConfiguration: setting a new schema version, and writing code to perform the migration.

RealmConfiguration config = new RealmConfiguration.Builder()
    .schemaVersion(2) // Must be bumped when the schema changes
    .migration(new MyMigration()) // Migration to run instead of throwing an exception
    .build()

Using this, the migration code will be run automatically if needed. We provide built-in methods so you can upgrade your schema on disk, and the data you stored for previous versions of the schema.

// Example migration adding a new class
public class MyMigration implements RealmMigration {
  @Override
  public void migrate(DynamicRealm realm, long oldVersion, long newVersion) {

     // DynamicRealm exposes an editable schema
     RealmSchema schema = realm.getSchema();

     // Migrate to version 1: Add a new class.
     // Example:
     // public Person extends RealmObject {
     //     private String name;
     //     private int age;
     //     // getters and setters left out for brevity
     // }
     if (oldVersion == 0) {
        schema.create("Person")
            .addField("name", String.class)
            .addField("age", int.class);
        oldVersion++;
     }

     // Migrate to version 2: Add a primary key + object references
     // Example:
     // public Person extends RealmObject {
     //     private String name;
     //     @PrimaryKey
     //     private int age;
     //     private Dog favoriteDog;
     //     private RealmList<Dog> dogs;
     //     // getters and setters left out for brevity
     // }
     if (oldVersion == 1) {
        schema.get("Person")
            .addField("id", long.class, FieldAttribute.PRIMARY_KEY)
            .addRealmObjectField("favoriteDog", schema.get("Dog"))
            .addRealmListField("dogs", schema.get("Dog"));
        oldVersion++;
     }
  }
}

See our migration sample app for more details.

If there is no file on disk when Realm launches, no migration is needed, and Realm will just create a new .realm file & schema based on the latest models defined in your code. This means that if you are in the middle of development and changing your schema frequently—and it’s all right to lose all your data—you can delete your .realm file on disk instead of writing a migration. This can be helpful when tinkering with models early in the development cycle of your app.

RealmConfiguration config = new RealmConfiguration.Builder()
    .deleteRealmIfMigrationNeeded()
    .build()

Notifications

It is possible to register a listener to receive notifications for changes on a Realm or its entities. Realm notifications are sent when the Realm as a whole is changed; collection notifications are sent when individual objects are changed, added, or removed.

Stop notification delivery by calling the removeChangeListener or removeAllChangeListeners method. Notifications will also stop if the object on which the listener is registered gets garbage collected, or if its Realm instance is closed. You should keep a strong reference of the object you’re listening to as long as you need the notifications.

// Wrong way to register for notifications. Query result will 
// be GC'ed when the method exits causing the listener to stop 
// emitting notifications.
public void runQuery() {
  realm.where(Person.class)
    .findAllAsync()
    .addChangeListener(new RealmChangeListener() {
      public void onChange(RealmResults<Person> persons) {
        // Persons was updated
      }
    };
}

// Right way to register for notifications. The listener will 
// continue to emit notifications after the method exits.
RealmResults<Person> persons;
public void runQuery() {
  persons = realm.where(Person.class)
    .findAllAsync()
    .addChangeListener(new RealmChangeListener() {
      public void onChange(RealmResults<Person> persons) {
        // Persons was updated
      }
    };
}

Notifications are always delivered on the thread that they were originally registered on. That thread must have a running Looper. If the relevant write transaction happened on a different thread, the listener will be called asynchronously after the transaction is committed.

If the write transaction happened on the same thread, the listener will be called synchronously when the transaction is committed. However, in some cases the listener may be called when the transaction starts—if the Realm is advanced to the latest version, or Realm entities being observed were modified or deleted in a way that triggers notifications. In those cases, the listener runs within the context of the current write transaction, so an attempt to begin a new write transaction within the notification handler will throw an exception. You can use the Realm.isInTransaction method to determine if your code is executing within a write transaction.

Since asynchronous notifications are delivered through looper events, the delivery of notifications might be delayed by other events in the looper queue. When notifications can’t be delivered immediately, changes from multiple write transactions may be coalesced into a single notification.

Realm notifications

Your UI or other looper threads can get notified of changes in a Realm by adding a listener, which is executed whenever the Realm is changed:

public class MyActivity extends Activity {
    private Realm realm;
    private RealmChangeListener realmListener;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
      super.onCreate(savedInstanceState);
      realm = Realm.getDefaultInstance();
      realmListener = new RealmChangeListener<Realm>() {
          @Override
          public void onChange(Realm realm) {
            // ... do something with the updates (UI, etc.) ...
          }
        };
      realm.addChangeListener(realmListener);
    }

    @Override
    protected void onDestroy() {
        super.onDestroy();
        // Remove the listener.
        realm.removeChangeListener(realmListener);
        // Close the Realm instance.
        realm.close();
    }
}

A listener on a Realm receives the entire changed Realm.

Collection notifications

Collection notifications receive not the whole Realm, but instead fine-grained descriptions of changes. These consist of the indices of objects that have been added, removed, or modified since the last notification. Collection notifications are delivered asynchronously, first with the initial results and then again after each write transaction which changes any of the objects in the collection (or adds new ones).

These changes can be accessed via the OrderedCollectionChangeSet parameter that is passed to the change listener. This object holds information about the indices affected by deletions, insertions and changes.

The first two, deletions and insertions, record the indices of objects that have been added to or removed from the collection. This takes into account when you add objects to the Realm or delete them from the Realm. For RealmResults this also applies when you filter for specific values and the object was changed so that it is now matching the query or not matching anymore.

You’re notified about changes whenever a field of an object has changed, which was previously part of the collection and is still part of it. This happens as well when to-one and to-many relationships change.

public class Dog extends RealmObject {
  public String name;
  public int age;
}

public class Person exteds RealmObject {
  public String name;
  public RealmList<Dog> dogs;
}

Let’s assume you’re observing a list of dog owners as given by the model code above. You will be notified about modifications for a matched Person object for example when:

• You modify the Person’s name.
• You add or remove a Dog from the list of dogs belonging to a Person.
• You modify the age of a Dog belonging to that Person.

This makes it possible to discretely control animations and visual updates made to the content inside your UI, instead of arbitrarily reloading everything each time a notification occurs.

private final OrderedRealmCollectionChangeListener<RealmResults<Person>> changeListener = new OrderedRealmCollectionChangeListener<RealmResults<Person>>() {
    @Override
    public void onChange(RealmResults<Person> collection, OrderedCollectionChangeSet changeSet) {
        // `null`  means the async query returns the first time.
        if (changeSet == null) {
            notifyDataSetChanged();
            return;
        }
        // For deletions, the adapter has to be notified in reverse order.
        OrderedCollectionChangeSet.Range[] deletions = changeSet.getDeletionRanges();
        for (int i = deletions.length - 1; i >= 0; i--) {
            OrderedCollectionChangeSet.Range range = deletions[i];
            notifyItemRangeRemoved(range.startIndex, range.length);
        }

        OrderedCollectionChangeSet.Range[] insertions = changeSet.getInsertionRanges();
        for (OrderedCollectionChangeSet.Range range : insertions) {
            notifyItemRangeInserted(range.startIndex, range.length);
        }

        OrderedCollectionChangeSet.Range[] modifications = changeSet.getChangeRanges();
        for (OrderedCollectionChangeSet.Range range : modifications) {
            notifyItemRangeChanged(range.startIndex, range.length);
        }
    }
};

The RealmRecyclerViewAdapter provides this out of the box.

Object notifications

Realm supports object-level notifications. You may register a notification on a particular RealmObject in order to be notified if the object is deleted, or whenever any of the managed fields on the object have their values modified.

Only managed RealmObjects can have listeners registered on them.

These changes can be accessed via the ObjectChangeSet parameter that is passed to the change listener. The ObjectChangeSet holds information about which fields were changed and if the RealmObject was deleted.

The ObjectChangeSet.isDeleted will return true if the object was deleted. After that the listener won’t be called again.

The ObjectChangeSet.getChangedFields will return the names of changed fields if any of the object’s managed fields were changed. You can also use ObjectChangeSet.isFieldChanged to test if a given field was just changed.

private final RealmObjectChangeListener<Dog> listener = new RealmObjectChangeListener<Dog>() {
    @Override
    public void onChange(Dog dog, ObjectChangeSet changeSet) {
        if (changeSet.isDeleted()) {
            Log.i(TAG, "The dog was deleted");
            return;
        }

        for (String fieldName : changeSet.getChangedFields()) {
            Log.i(TAG, "Field " + fieldName + " was changed.");
        }
    }
};

Notifications for the same value

Realm will treat all changes as something that will raise a notification. However, in many cases it is not desirable to refresh the UI if the value hasn’t changed.

If you are updating single fields you can check the value in the Realm file before overridding it to avoid triggering notifications:

realm.executeTransaction(new Realm.Transaction() {
    @Override
    public void execute(Realm realm) {
        String newName = "Jane";
        Person managedPerson = getPerson();
        if (!newName.equals(managedPerson.getName())) {
            managedPerson.setName(newName);
        }
    }
});

If you are inserting objects using Realm.copyToRealm() or Realm.copyToRealmOrUpdate() you can use an ImportFlag to indicate that only fields that actually changed should be updated:

realm.executeTransaction(new Realm.Transaction() {
    @Override
    public void execute(Realm realm) {
        int id = 42;
        Person person = new Person(id, "Jane");
        realm.copyToRealmOrUpdate(person, ImportFlag.CHECK_SAME_VALUES_BEFORE_SET);
    }
});

This way notifications will only be raised for the fields that actually changed. When using this flag, null values in the input object are treated as a normal value like any other, so if the value in the Realm was non-null it will be overwritten with null and trigger a notification.

Encryption

The Realm file can be encrypted on disk by passing a 512-bit encryption key (64 bytes) to the configuration using RealmConfiguration.Builder.encryptionKey:

byte[] key = new byte[64];
new SecureRandom().nextBytes(key);
RealmConfiguration config = new RealmConfiguration.Builder()
  .encryptionKey(key)
  .build();

Realm realm = Realm.getInstance(config);

Realm transparently encrypts and decrypts data with standard AES-256 encryption using the first 256 bits of the given 512-bit encryption key. Realm uses the other 256 bits of the 512-bit encryption key for the HMAC. You must supply the same encryption key every time you open that Realm. See examples/encryptionExample for an example of how to securely store keys between runs in the Android KeyStore so that other applications cannot read them.

Working with synced Realms

Threading

Realm makes it effortless to work with data on multiple threads without having to worry about consistency or performance, because objects and queries are auto-updating at all times. You can operate on live objects in different threads, reading and writing to them, without worrying about what other threads are doing to those same objects. If you need to change data, you can use a transaction. The other objects in the other thread will be updated in near real time (the updates will be scheduled as an event on the Looper, so Looper threads will be updated as soon as the event is processed).

The only limitation is that you cannot randomly pass Realm objects between threads. If you need the same data on another thread, you need to query for that data on the other thread. Furthermore, you can observe the changes using Realms reactive architecture. Remember, all objects are kept up to date between threads—Realm will notify you when the data changes.

Threading example

Assume we have an app that displays a list of customers. In a background thread (an Android IntentService) we poll a remote endpoint for new customers and then save them to Realm. When the background thread adds new customers, the data in the UI thread will be automatically updated. The UI thread gets notified via a RealmChangeListener and at that point we tell the UI widget to update itself. No need to re-query because Realm keeps everything up to date.

// in a Fragment or Activity, etc

// Listeners will only be triggered as long as the query result is
// not garbage collected, so keep a strong class reference to it.
private RealmResults<Customer> customers;

@Override
public void onActivityCreated(Bundle savedInstanceState) {
    // ... boilerplate omitted for brevity
    realm = Realm.getDefaultInstance();
    // get all the customers
    customers = realm.where(Customer.class).findAllAsync();
    // ... build a list adapter and set it to the ListView/RecyclerView/etc

    // set up a Realm change listener
    changeListener = new RealmChangeListener() {
        @Override
        public void onChange(RealmResults<Customer> results) {
            // This is called anytime the Realm database changes on any thread.
            // Please note, change listeners only work on Looper threads.
            // For non-looper threads, you manually have to use Realm.waitForChange() instead.
            updateUi();
        }
    };
    // Tell Realm to notify our listener when the customers results
    // have changed (items added, removed, updated, anything of the sort).
    customers.addChangeListener(changeListener);
}

// In a background service, in another thread
public class PollingService extends IntentService {
    @Override
    public void onHandleIntent(Intent intent) {
        Realm realm = Realm.getDefaultInstance();
        try {
            // go do some network calls/etc and get some data and stuff it into a 'json' var
            String json = customerApi.getCustomers();
            realm.beginTransaction();
            realm.createObjectFromJson(Customer.class, json); // Save a bunch of new Customer objects
            realm.commitTransaction();
            // At this point, the data in the UI thread is already up to date.
            // ...
        } finally {
            realm.close();
        }
    }
    // ...
}

Once the background service adds new customers to the Realm, the customers list is automatically updated in the UI without any additional intervention on your part. The same goes for individual objects. Suppose that you’re only managing one object. Just change it on one thread and the UI thread automatically has the new data. If you need to respond to that change, just add a listener like we’re doing above.

Using a Realm across threads

The only rule to using Realm across threads is to remember that Realm, RealmObject, and RealmResults instances cannot be passed across threads. Instead, use an asynchronous query or asynchronous transaction, to offload the operation to a background thread and bring any results back to the original thread for you.

When you want to access the same data from a different thread, you can obtain a new Realm instance (i.e. Realm.getInstance(RealmConfiguration config) or its cousins) and get your objects through a query.

The objects will map to the same data on disk, and will be readable and writeable from any thread.

Android framework threads

Be careful when working with these classes:

• AsyncTask
• IntentService

The AsyncTask class contains the doInBackground method which executes a background thread. The IntentService class contains the onHandleIntent(Intent intent) method which executes in a worker thread.

If you need to use Realm in either of these methods you should open the Realm, perform your work and then close the Realm before exiting. Below are a couple of examples.

AsyncTask

Open and close the Realm in the doInBackground method, as shown below.

private class DownloadOrders extends AsyncTask<Void, Void, Long> {
    protected Long doInBackground(Void... voids) {
        // Now in a background thread.

        // Open the Realm
        Realm realm = Realm.getDefaultInstance();
        try {
            // Work with Realm
            realm.createAllFromJson(Order.class, api.getNewOrders());
            Order firstOrder = realm.where(Order.class).findFirst();
            long orderId = firstOrder.getId(); // Id of order
            return orderId;
        } finally {
            realm.close();
        }
    }

    protected void onPostExecute(Long orderId) {
        // Back on the Android mainThread
        // do something with orderId such as query Realm
        // for the order and perform some operation with it.
    }
}

IntentService

Open and close the Realm in the onHandleIntent method, as shown below.

public class OrdersIntentService extends IntentService {
    public OrdersIntentService(String name) {
        super("OrdersIntentService");
    }

    @Override
    protected void onHandleIntent(Intent intent) {
        // Now in a background thread.

        // Open the Realm
        Realm realm = Realm.getDefaultInstance();
        try {
            // Work with Realm
            realm.createAllFromJson(Order.class, api.getNewOrders());
            Order firstOrder = realm.where(Order.class).findFirst();
            long orderId = firstOrder.getId(); // Id of order
        } finally {
            realm.close();
        }
    }
}

Multi-process support

Realm can be accessed from multiple processes with some limitations. Everything including the notifications should just work when accessing the same Realm from different processes in the same APK.

Examples

Take a look at our examples to see Realm used in practice in an app. See here for more details on how to run the examples.

The introExample contains simple examples of how you use the Realm API.

The gridViewExample is a trivial app that shows how to use Realm as the backing store for a GridView. It also shows how you could populate the database with JSON using GSON plus how to use ABI splits to minimize the size of the final APK.

The threadExample is a simple app that shows how to use Realm in a multithreaded environment.

The adapterExample shows how to use the RealmBaseAdapter and RealmRecyclerViewAdapter to make Realm work with Android ListView and RecyclerView in an elegant way.

The jsonExample demonstrates Realm’s JSON facilities.

The encryptionExample shows you how to work with encrypted Realms.

The rxJavaExamples shows how Realm works together with RxJava.

The unitTestExample shows how you can write unit tests when working with Realm.

The multiProcessExample shows how to use Realm from different processes in the same APK.

Other libraries

This section describes how you can integrate Realm with other commonly used libraries for Android.

GSON

GSON is a library created by Google for deserializing and serializing JSON. GSON should work with Realm out of the box.

// Using the User class
public class User extends RealmObject {
    private String name;
    private String email;
    // getters and setters left out ...
}

Gson gson = new GsonBuilder().create();
String json = "{ name : 'John', email : 'john@corporation.com' }";
User user = gson.fromJson(json, User.class);

You can also see an example of how GSON can work with Realm in our GridViewExample.

Serialization

For full compatibility with libraries like Retrofit you will often want to be able to both deserialize and serialize an object. Serializing Realm objects to JSON does not work with GSON’s default behavior as GSON will use field values instead of getters and setters.

To make GSON serialization work with Realm you will need to write a custom JsonSerializer for each object that can be serialized and register it as a TypeAdapter.

This Gist shows how it can be done.

Primitive lists

Although Realm support import arrays in JSON to lists Of primitives natively, lack of query support of primitive list may still be a problem. You may want to import the JSON arrays of primitive types as a list of RealmObject. If it is not possible to change the JSON API, you can write a custom TypeAdapter for GSON that automatically maps between the primitive type from JSON and the wrapper object used by Realm.

In this Gist is an example of using a wrapper object for Integers, but the template can be used for all primitive arrays with datatypes supported by Realm.

Troubleshooting

Realm objects can contain fields that internally contain circular references. When this happens GSON can throw a StackOverflowError. We have seen this happen when a Realm object has a Drawable field:

public class Person extends RealmObject {
    @Ignore
    Drawable avatar;
    // other fields, etc
}

The Person class above contains an Android Drawable that has the @Ignore annotation applied. During GSON serialization the Drawable was being inspected and caused a StackOverflowError (GitHub Issue). To alleviate this, add the following code to your shouldSkipField method.

public boolean shouldSkipField(FieldAttributes f) {
  return f.getDeclaringClass().equals(RealmObject.class) || f.getDeclaringClass().equals(Drawable.class);
}

Please note the Drawable.class evaluation. This tells GSON to skip this field during serialization. Adding this will alleviate the StackOverflowError.

Jackson Databind

Jackson Databind is a library for binding JSON data to Java classes.

Jackson uses reflection to perform the data binding. This conflicts with Realm’s support for RxJava, as RxJava might not be available to the class loader. This can result in an exception that looks like this:

java.lang.NoClassDefFoundError: rx.Observable
at libcore.reflect.InternalNames.getClass(InternalNames.java:55)
...

This can be fixed by either adding RxJava to your project or create two empty dummy files that looks like the following.

// File 1
package io.reactivex;

public class Flowable {
}


// File 2
package io.reactivex;

public class Observable {
}

// File 3
package io.reactivex;

enum BackpressureStrategy {
    LATEST;
}

This issue has also been reported to the Jackson project here.

Kotlin

Realm is fully compatible with Kotlin programming language, but there are a few caveats to be aware of:

• The realm-android plugin has to be applied after kotlin-android and kotlin-kapt like:

apply plugin: 'kotlin-android'
apply plugin: 'kotlin-kapt'
apply plugin: 'realm-android'

• It is important that your model classes are open.

• A limitation of the Kotlin annotation processor indicates that adding the annotation @RealmClass is required in some cases.

• Storing enums in Realm Model classes in Kotlin should be done using the following pattern:

open class EnumTest: RealmObject() {

  // Beware that enums stored in Realm must not be obfuscated by ProGuard  
  class enum MyEnum {
    Value1, Value2
  }

  // Custom private backing field representing the enum
  private var strField: String = MyEnum.Value1.name

  // Public field exposing setting/getting the enum
  var enumField: MyEnum
      get() = MyEnum.values().first { it.name == strField }
      set(value) {
          strField = value.name
      }
}

// Queries
val results = realm.where<EnumTest>().equalTo("strField", MyEnum.Value1.name).findAll();

Note that queries needs to be done on the strField with String value, not the enumField which is ignored by Realm as it has no backing field.

• In Kotlin Long::class.java actually returns a Class reference to long not Long. The same is true for other primitive types like Integer, Float, Double and Boolean. Choosing the correct class has implications during a migration:

schema
  .addField("field", Long::class.java) // Non-nullable
  .addField("field", Long::class.javaObjectType) // Nullable
  .addField("field", Long::class.javaPrimitiveType) // Non-nullable

If Kotlin is used in a project, Realm automatically detects this and adds a number of extension methods that makes working with Kotlin easier. This includes among others:

• All methods that accept a class parameter in Java now has a reified variant in Kotlin, e.g. realm.where(Person.class).findAll() becomes realm.where<Person>().findAll()

• If a model class implements the RealmModel interface, default methods are now injected automatically, meaning that you can use the exact same call pattern regardless if you extends the base class RealmObject or implements the interface RealmModel.

• The query predicate in() now has a Kotlin alias named anyOf() as in is a keyword in Kotlin.

This extension library can be manually disabled by using the realm closure:

android {
  ...
}

realm {
  kotlinExtensionsEnabled = false // Disable extensions if needed
}

See this example for a working app combining Realm and Kotlin.

Parceler

Parceler is a library that automatically generates the boilerplate required to make an object respect the Parcelable interface. Due to Realm’s use of proxy classes, Parceler requires the following setup to work with Realm’s model classes.

Proxy classes in Realm uses the fully qualified name of the model class plus the RealmProxy suffix, so for example io.realm.model.Person becomes io_realm_model_PersonRealmProxy.class

// All classes that extend RealmObject will have a matching RealmProxy class created
// by the annotation processor. Parceler must be made aware of this class. Note that
// the class is not available until the project has been compiled at least once.
@Parcel(implementations = { some_package_PersonRealmProxy.class },
        value = Parcel.Serialization.BEAN,
        analyze = { Person.class })
public class Person extends RealmObject {
    // ...
}

If you are using Gradle for getting Parceler, please make sure the following lines are there (see here for more details):

compile "org.parceler:parceler-api:1.0.3"
apt "org.parceler:parceler:1.0.3"

There are some important restrictions to be aware of when using Parceler:

1. If your model contains a RealmList you need to register a special adapter.
2. Once an object has been parceled, it becomes detached from Realm and at this point behaves like an unmanaged object containing a snapshot of the data. Further changes to this object will not be persisted in Realm.

Retrofit

Retrofit is a library from Square that makes it easy to work with a REST API in a typesafe manner.

Realm will work with both Retrofit 1.* and 2.* out of the box, but note that Retrofit does not automatically add objects to Realm, instead you must manually add them using the realm.copyToRealm or realm.copyToRealmOrUpdate methods.

GitHubService service = restAdapter.create(GitHubService.class);
List<Repo> repos = service.listRepos("octocat");

// Copy elements from Retrofit to Realm to persist them.
realm.beginTransaction();
List<Repo> realmRepos = realm.copyToRealmOrUpdate(repos);
realm.commitTransaction();

Robolectric

Robolectric is a library that allows you to run JUnit tests directly in the JVM instead of in a phone or emulator. Currently, Robolectrics does not support native libraries like those that are bundled with Realm. This means that for now it is not possible to test Realm using Robolectric.

You can follow the feature request here: https://github.com/robolectric/robolectric/issues/1389

RxJava

RxJava is a Reactive Extensions library from Netflix that extends the Observer pattern. It makes it possible to observe changes to data as composable sequences.

Realm has first-class support for RxJava 2 Flowable and Observable

• Realm
• RealmResults
• RealmList
• RealmObject
• DynamicRealm
• DynamicRealmObject
• RealmResults changesets
• RealmList changesets
• RealmObject changesets
• DynamicRealmObject changesets

// Combining Realm, Retrofit and RxJava (Using Retrolambda syntax for brevity)
// Load all persons and merge them with their latest stats from GitHub (if they have any)
Realm realm = Realm.getDefaultInstance();
GitHubService api = retrofit.create(GitHubService.class);
realm.where(Person.class).isNotNull("username").findAllAsync().asFlowable()
    .filter(persons.isLoaded)
    .flatMap(persons -> Observable.from(persons))
    .flatMap(person -> api.user(person.getGithubUserName())
    .observeOn(AndroidSchedulers.mainThread())
    .subscribe(user -> showUser(user));

Asynchronous Queries are non-blocking—the code above will immediately return a RealmResults instance. If you want to ensure you’re only operating on a loaded list, filter the Flowable via the filter operator and check the list by calling the [RealmResults.isLoaded](api/io/realm/RealmResults.html#isLoaded--) method. By checking to see if the `RealmResults` are loaded you can then determine if your query has completed or not.

See the RxJava sample project for more examples.

RxJava is an optional dependency, which means that Realm doesn’t automatically include it. This has the benefit that you can choose which version of RxJava to use as well as avoid the method count bloat in projects that does not use RxJava. RxJava has to be manually added to the build.gradle file.

dependencies {
  compile 'io.reactivex:rxjava:2.1.4'
}

It is possible to configure how Realm create streams by creating a custom RxObservableFactory. This is configured using RealmConfiguration.

RealmConfiguration config = new RealmConfiguration.Builder()
  .rxFactory(new MyRxFactory())
  .build()

If no RxObservableFactory is defined, Realm defaults to RealmObservableFactory which is a class provided by Realm that supports RxJava <= 2.*.

If you are using RxJava 1 you can use this library by David Karnok to convert between RxJava 2 and RxJava 1 types.

Testing and debugging

See our unitTestExample for information on how Realm can be combined with JUnit3, JUnit4, Robolectric, Mockito and PowerMock.

Android Studio debugging

There’s a small “gotcha” to be aware of when working using Android Studio or IntelliJ: the debugger can provide misleading values depending on the debugging view you’re using.

For example, adding a watch in Android Studio on a RealmObject will display values of the fields. Unfortunately, these values are wrong because the field values are not used. Realm creates a proxy object behind the scenes and overrides the getters and setters in order to access the persisted data in the Realm. Adding a watch for any of the accessors will yield the correct values. See the image below:

In the image above, the debugger has stopped on line 113. There are three watch values, the person variable and the person.getName and person.getAge accessors. The code from lines 107 to 111 alters the person instance by changing the name and age. These values are then persisted in a transaction. On line 113, where the debugger is currently paused, the person watch instance is reporting on field values and they are incorrect. The watch values that use the accessor for person.getName and person.getAge report values that are correct.

Please note, the .toString method will output the correct values, but the watch panel will not (when watching a variable which is a RealmObject).

NDK debugging

Realm is a library that contains native code. We recommend that you use a crash reporting tool, such as Crashlytics, to track native errors so we are in a better position to help you if something goes wrong.

Debugging NDK crashes is usually cumbersome as the default stack trace provides minimal information that can be of use. Crashlytics will allow you to capture valuable NDK crash information. To enable NDK crash reporting in Crashlytics, please follow the steps outlined in this guide.

To enable NDK crash reporting for your project, add this to the root of your build.gradle file. Please note, the values androidNdkOut and androidNdkLibsOut are not needed.

crashlytics {
  enableNdk true
}

Current limitations

Realm generally tries to have as few constraints as possible, and we are continuously adding new features based on feedback from the community. However, Realm still has a few limitations. Please refer to our GitHub issues for a more comprehensive list of known issues.

Models

Realm models have no support for final and volatile fields. This is mainly to avoid discrepancies between how an object would behave as managed by Realm or unmanaged.

Realm model classes are not allowed to extend any other object than RealmObject. If declared, the default constructor (constructor with no parameters) must always be empty. The reason is that a default contructor will call methods which assume a Realm instance is present. But that instance isn’t create before the contructor returns. You can add other constructors for your convenience.

General

Realm aims to strike a balance between flexibility and performance. In order to accomplish this goal, realistic limits are imposed on various aspects of storing information in a Realm. For example:

1. The upper limit of class names is 57 characters. Realm Java prepends class_ to all names, and the browser will show it as part of the name.
2. The length of field names has a upper limit of 63 character.
3. It is not possible to have two model classes with the same name in different packages.
4. Nested transactions are not supported, and an exception is thrown if they are detected.
5. Strings and byte arrays (byte[]) cannot be larger than 16 MB.
6. Realm models have no support for final and volatile fields. This is mainly to avoid discrepancies between how an object would behave as managed by Realm or unmanaged.
7. If a custom constructor is provided, a public no-arg constructor must also be present.
8. Realm model classes are not allowed to extend any other class than RealmObject.

To avoid size limitations and a performance impact, it is best not to store large blobs (such as image and video files) directly in Realm. Instead, save the file to a file store (such as S3) and keep only the location of the file and any relevant metadata in Realm.

Sorting and querying on strings

Sorting and case-insensitive string matches in queries are only supported for character sets in Latin Basic, Latin Supplement, Latin Extended A, and Latin Extended B (UTF-8 range 0–591). Also, setting the case insensitive flag in queries when using equalTo, notEqualTo, contains, endsWith, beginsWith, or like will only work on characters from the English locale.

Realm uses non-standard sorting for upper and lowercase letters, sorting them together rather than sorting uppercase first. That means that '- !"#0&()*,./:;?_+<=>123aAbBcC...xXyYzZ is the actual sorting order in Realm. Read more about these limitations here.

Threads

Although Realm files can be accessed by multiple threads concurrently, you cannot hand over Realms, Realm objects, queries, and results between threads. The thread example shows how to use Realm in a multithreading environment. Read more about Realm’s threading.

Although Realm files can be accessed by multiple threads concurrently, they can only be accessed by a single process at a time. Different processes should either copy Realm files or create their own.

RealmObject’s hashCode

A RealmObject is a live object, and it might be updated by changes from other threads. Although two Realm objects returning true for RealmObject.equals must have the same value for RealmObject.hashCode, the value is not stable, and should neither be used as a key in HashMap nor saved in a HashSet.

Multi-process

• Accessing encrypted Realms from different processes simultaneously is not supported. There is a Realm Core issue (#1845) tracking this.
• Accessing the same Realms from different processes in different APKs is not supported. It is safe to do it but things like notifications won’t work as expected.
• Accessing synced Realms from different processes is not supported.

Incremental builds

Realms bytecode transformer supports incremental builds, but in a few cases a full build is required. The transformer is not able to detect these cases by itself.

• Adding or removing the @Ignore annotation from a field in a model class.
• Adding or removing the static keyword from a field in a model class.
• Adding or removing the transient keyword from a field in a model class.

Failing to perform a full build in these cases will either crash the app or cause fields to return the default value for the datatype (e.g. 0 or null) instead of returning the true value stored in Realm.

Best practices

Out of the box, Realm works seamlessly with Android. The main thing that you have to keep in mind is that RealmObjects are thread confined. The importance of understanding this comes into play more when you want to start passing Realm objects between activities, to background services, broadcast receivers and more.

Preventing “Application Not Responding” (ANR) errors

Typically Realm is fast enough to read and write data on Android’s main thread. However, write transactions are blocking across threads so in order to prevent accidental ANR’s we advise that you perform all Realm write operations on a background thread (not Android’s main thread). Learn how to perform operations on the background thread via Realms Asynchronous Transactions.

Controlling the lifecycle of Realm instances

Choosing the proper lifecycle for a Realm instance is a balancing act. Because RealmObjects and RealmResults are accessed through a lazy cache, keeping a Realm instance open for as long as possible not only avoids the overhead incurred in opening and closing it but is likely to allow queries against it to run more quickly. On the other hand, an open Realm instance holds significant resources, some of which are not controlled by the Java memory manager. Java cannot automatically administer these resources. It is essential that code that opens a Realm instance closes it when it is no longer needed.

Realm uses an internal reference counted cache so that, after getting the first Realm instance, getting subsequent instances on the same thread is free. The underlying resources are released, though, only when all of the instances on that thread are closed.

One reasonable choice is to make the lifecycle of the Realm instance congruent with the lifecycles of views that observe it. The examples below demonstrate this using a Fragment and an Activity, each with a RecyclerView that displays data retrieved from a Realm instance. In both examples the Realm instance and the RecyclerView Adapter are initialized in the create method and closed in the corresponding destroy method. Note that this is safe, even for the Activity: the database will be left in a consistent state even if the onDestroy and close methods are never called.

Clearly, if most of the Fragments associated with an Activity require access to the same dataset, it would make sense for the Activity, not the individual Fragments, to control the lifecycle of the instance.

// Setup Realm in your Application
public class MyApplication extends Application {
    @Override
    public void onCreate() {
        super.onCreate();
        Realm.init(this);
        RealmConfiguration realmConfiguration = new RealmConfiguration.Builder().build();
        Realm.setDefaultConfiguration(realmConfiguration);
    }
}

// onCreate()/onDestroy() overlap when switching between activities.
// Activity2.onCreate() will be called before Activity1.onDestroy()
// so the call to getDefaultInstance in Activity2 will be fast.
public class MyActivity extends Activity {
    private Realm realm;
    private RecyclerView recyclerView;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);

        realm = Realm.getDefaultInstance();

        setContentView(R.layout.activity_main);

        recyclerView = (RecyclerView) findViewById(R.id.recycler_view);
        recyclerView.setAdapter(
            new MyRecyclerViewAdapter(this, realm.where(MyModel.class).findAllAsync()));

        // ...
    }

    @Override
    protected void onDestroy() {
        super.onDestroy();
        realm.close();
    }
}

// Use onCreateView()/onDestroyView() for Fragments.
// Note that if the db is large, getting the Realm instance may, briefly, block rendering.
// In that case it may be preferable to manage the Realm instance and RecyclerView from
// onStart/onStop instead. Returning a view, immediately, from onCreateView allows the
// fragment frame to be rendered while the instance is initialized and the view loaded.
public class MyFragment extends Fragment {
    private Realm realm;
    private RecyclerView recyclerView;

    @Override
    public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) {
        realm = Realm.getDefaultInstance();

        View root = inflater.inflate(R.layout.fragment_view, container, false);

        recyclerView = (RecyclerView) root.findViewById(R.id.recycler_view);
        recyclerView.setAdapter(
            new MyRecyclerViewAdapter(getActivity(), realm.where(MyModel.class).findAllAsync()));

        // ...

        return root;
    }

    @Override
    public void onDestroyView() {
        super.onDestroyView();
        realm.close();
    }
}

Reuse RealmResults and RealmObjects

On the UI thread and all other Looper threads, all RealmObjects and RealmResults are automatically refreshed when changes are made to the Realm. This means that it isn’t necessary to fetch those objects again when reacting to a RealmChangedListener. The objects are already updated and ready to be redrawn on the screen.

public class MyActivity extends Activity {

    private Realm realm;
    private RealmResults<Person> allPersons;
    private RealmChangeListener realmListener = new RealmChangeListener() {
        @Override
        public void onChange(Realm realm) {
            // Just redraw the views. `allPersons` already contain the
            // latest data.
            invalidateView();
        }
    };

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        realm = Realm.getDefaultInstance();
        realm.addRealmChangeListener(listener);
        allPerson = realm.where(Person.class).findAll(); // Create the "live" query result
        setupViews(); // Initial setup of views
        invalidateView(); // Redraw views with data
    }

    // ...
}

Autoincrementing IDs

Autoincrementing IDs are not supported by Realm by design. This is primarily because it is impossible to generate such keys in a distributed environment, and compatibility between data stored in a local Realm and a synchronized Realm is a high priority. Note that Realm does not need primary keys in order to create relationships.

It is still possible to efficiently create primary keys with Realm that satisfy the use cases provided by autoincrementing IDs, but it is important to identify what the autoincrementing ID is used for:

1) To provide a unique identifier in order to identify the object. This can be replaced by a GUID, which guarantees uniqueness and can be created by a device even when it’s offline:

```java
public class Person extends RealmObject {
    @PrimaryKey
    private String id = UUID.randomUUID().toString();
    private String name;
}
```

2) To provide loose insertion order. An example is sorting tweets. This can be replaced by a createdAt field, which doesn’t need to be a primary key:

```java
public class Person extends RealmObject {
    @PrimaryKey
    private String id = UUID.randomUUID().toString();
    private Date createdAt = new Date();
    private String name;
}
```

3) To provide strict insertion order. An example is a list of tasks. This can be modelled using a RealmList which will guarantee insertion order, even if devices have been offline.

```java
public class SortedPeople extends RealmObject {
    @PrimaryKey
    private int id = 0
    private RealmList<Person> persons;
}

public class Person extends RealmObject {
    private String name;
}

// Create wrapper object when creating object
RealmConfiguration config = new RealmConfiguration.Builder()
.initialData(new Realm.Transaction() {
    @Override
    public void execute(Realm realm) {
        realm.insert(new SortedPeople());
    }
});

// Insert objects through the wrapper
realm.executeTransaction(new Realm.Transaction() {
    @Override
    public void execute(Realm realm) {
        SortedPeople sortedPeople = realm.where(SortedPeople.class).findFirst();
        sortedPeople.getPersons().add(new Person());
    }
});
```

If you have a use case where you still think autoincrementing IDs will be a better fit, you can use this helper class, but note that keys generated using this class are not usable if you:

1) Create Realm Objects in multiple processes. 2) Want to share the Realm between multiple devices at some point in the future.

For autoincrementing IDs that are safe to create across processes, you will need to query for the max value each time you begin a transaction: