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Migrate from the Java SDK to the Kotlin SDK

Atlas Device SDKs are deprecated. Refer to the deprecation page for details.

Note

What is the Kotlin SDK?

The Kotlin SDK is a new Realm client SDK built entirely with the Kotlin programming language. The Kotlin SDK uses an entirely different codebase from the Java SDK. It is designed specifically to take advantage of Kotlin language features such as coroutines and suspend functions. The Java SDK also supports some of these features, as well as Android applications written in Kotlin. But the Kotlin SDK is more Kotlin-idiomatic than the Java SDK.

Overview

The Java SDK and the Kotlin SDK differ in many ways. On this page, you'll find a high-level comparison of most of the ways the SDKs differ.

Kotlin SDK Architecture

The Java SDK provided live objects, queries, and realms that automatically update when underlying data changes. The Kotlin SDK still provides this live interface in write transactions, but otherwise relies on a new frozen architecture that makes Realm objects easier to work with. Here are some of the main differences between the Java SDK architecture and the Kotlin SDK architecture:

Frozen by default: All objects are now frozen. Unlike live objects, frozen objects do not automatically update after database writes. You can still access live objects within a write transaction, but passing a live object out of a write transaction freezes the object.
Thread-safety: All realm instances, objects, query results, and collections can now be transferred across threads.
Singleton: You now only need one instance of each realm. No need to open and close realms on individual threads.

Tip

Frozen Architecture in the Kotlin SDK

Opening Realms

The Java SDK automatically detects Realm Object Models defined in your application, and uses all of them in the schema of opened realms unless you specify otherwise. The Kotlin SDK requires you to manually specify the Realm Object Models to use in your realm schema. Additionally:

The Kotlin SDK does not provide the ability to set and access a default realm in your application. Since you can now share realms, objects, and results across threads, you can rely on a global singleton instead.
The Java SDK used RealmConfiguration.Builder().build() to generate instances of RealmConfiguration. With the Kotlin SDK, use the RealmConfiguration.create() companion method RealmConfiguration instead.
The Java SDK used the static Realm.getInstance() method to open a realm with a given config. With the Kotlin SDK, use the static Realm.open() method instead.

val config = RealmConfiguration.Builder()
 .build()
var realm: Realm
realm = Realm.getInstance(config)
Log.v(
 "EXAMPLE",
 "Successfully opened a realm: "
 + realm.path
)

RealmConfiguration config =
 new RealmConfiguration.Builder()
 .build();
Realm realm;
realm = Realm.getInstance(config);
Log.v("EXAMPLE",
"Successfully opened a realm: "
 + realm.getPath());

val config = RealmConfiguration.create(
 setOf(Frog::class, Sample::class))
val realm = Realm.open(config)
Log.v("Successfully opened realm:" +
 "${realm.configuration.name}")

Optionally, use RealmConfiguration.Builder to customize your configuration even more:

Kotlin SDK

val config = RealmConfiguration.Builder(
 setOf(Frog::class, Sample::class))
 .name(REALM_NAME)
 .deleteRealmIfMigrationNeeded()
 .directory(PATH)
 .encryptionKey(KEY)
 .build()
val realm = Realm.open(config)
Log.v("Successfully opened realm:" +
 realm.configuration.name
)

Tip

Open & Close a Realm

Realm Object Models

In the Java SDK, you declare Realm object models in one of two ways:

extend RealmObject
implement RealmModel

The Kotlin SDK uses default methods in the RealmObject interface instead. With the Kotlin SDK, inherit from RealmObject to declare a Realm object model. Annotations work the same way they did in java for fields with special properties, such as ignored fields, primary keys, and indexes.

open class Sample : RealmObject() {
 @PrimaryKey
 var stringField = "Realm"
 var byteField: Byte = 0xA
 // no support for chars: no charField
 var shortField: Short = 17
 var intField = 42
 @Index
 var longField = 256L
 var booleanField = true
 var floatField = 3.14f
 var doubleField = 1.19840122
 var timestampField = Date()
}

public class Sample extends RealmObject {
 @PrimaryKey
 public String stringField = "Realm";
 public Byte byteField = 0xA;
 // no support for chars: no charField
 public Short shortField = 17;
 public Integer intField = 42;
 @Index
 public Long longField = 256L;
 public Boolean booleanField = true;
 public Float floatField = 3.14f;
 public Double doubleField =
 1.19840122;
 public Date timestampField =
 new Date();
}

class Sample : RealmObject {
 @PrimaryKey
 var stringField: String = "Realm"
 var byteField: Byte = 0xA
 var charField: Char = 'a'
 var shortField: Short = 17
 var intField: Int = 42
 @Index
 var longField: Long = 256L
 var booleanField: Boolean = true
 var floatField: Float = 3.14f
 var doubleField: Double = 1.19840122
 var timestampField: RealmInstant =
 RealmInstant.from(
 100,
 1000)
 var objectIdField: ObjectId = ObjectId()
}

Tip

Supported Schemas Types

Relationships

Both the Java and Kotlin SDKs declare relationships through Realm object fields:

One-to-One

open class Child : RealmObject() {
 var frog: Frog? = null
}

public class Child
 extends RealmObject {
 public Frog frog = null;
}

class Child : RealmObject {
 var frog: Frog? = null
}

One-to-Many

With the Java SDK, you could define one-to-many relationships with fields of type RealmList. The Kotlin SDK still uses fields of type RealmList, but you should instantiate RealmList instances with the realmListOf() companion method.

open class Kid : RealmObject() {
 var frogs = RealmList<Frog>()
}

public class Kid
 extends RealmObject {
 public RealmList<Frog> frogs =
 new RealmList<Frog>();
}

class Kid : RealmObject {
 var frogs: RealmList<Frog> =
 realmListOf()
}

Schema Types

With the Java SDK, you needed to use the @Required annotation to make lists of primitives non-nullable in realm object models. The Kotlin SDK makes lists of primitives non-nullable by default. Use the ? operator to make a list of primitives nullable.

open class CollegeStudent : RealmObject() {
 @Required
 var notes = RealmList<String>()
 var nullableNotes = RealmList<String>()
}

public class CollegeStudent
 extends RealmObject {
 @Required
 public RealmList<String> notes =
 new RealmList<String>();
 public RealmList<String> nullableNotes =
 new RealmList<String>();
}

class Student : RealmObject {
 var notes: RealmList<String> =
 realmListOf()
 var nullableNotes: RealmList<String?> =
 realmListOf()
}

Writes

The Kotlin SDK introduces new names for the methods that write to realms.

Async

With the Java SDK, you could write asynchronously to a realm with realm.executeTransactionAsync(). The Kotlin SDK uses the suspend function realm.write() instead.

realm.executeTransactionAsync {
 transactionRealm: Realm ->
 val sample: Sample =
 Sample()
 sample.stringField = "Sven"
 transactionRealm.copyToRealm(
 sample
 )
}

realm.executeTransactionAsync(
 transactionRealm -> {
 Sample sample = new Sample();
 sample.stringField = "Sven";
 transactionRealm.copyToRealm(sample);
});

realm.write {
 // this: MutableRealm
 val sample = Sample()
 sample.stringField = "Sven"
 this.copyToRealm(sample)
}

Sync

With the Java SDK, you could write synchronously to a realm with realm.executeTransaction(). The Kotlin SDK uses realm.writeBlocking():

realm.executeTransaction {
 transactionRealm: Realm ->
 val sample: Sample =
 Sample()
 sample.stringField = "Sven"
 transactionRealm.copyToRealm(
 sample
 )
}

realm.executeTransaction(
 transactionRealm -> {
 Sample sample = new Sample();
 sample.stringField = "Sven";
 transactionRealm.copyToRealm(sample);
});

realm.writeBlocking {
 // this: MutableRealm
 val sample = Sample()
 sample.stringField = "Sven"
 this.copyToRealm(sample)
}

Tip

Queries

There are several differences between queries in the Java SDK and queries in the Kotlin SDK:

With the Java SDK, you can query objects in realms using a fluent interface or Realm Query Language (RQL). The Kotlin SDK only uses RQL.
The Java SDK uses realm.where() to query realms, whereas the Kotlin SDK uses realm.query().
With the Java SDK, you could query asynchronously with realmQuery.findAllAsync() and realmQuery.findFirstAsync(). In the Kotlin SDK, query asynchronously with realmQuery.asFlow(). Once you have a flow of results, you can collect the results.
With the Java SDK, you could query synchronously with realmQuery.findAll() and realmQuery.findFirst(). In the Kotlin SDK, query synchronously with realmQuery.find().

Filters

val samples =
 realm.where(
 Sample::class.java
 ).findAll()
val samplesThatBeginWithN =
 realm.where(
 Sample::class.java
 )
 .beginsWith(
 "stringField",
 "N"
 ).findAll()

RealmResults<Sample> samples =
 realm
 .where(Sample.class)
 .findAll();
RealmResults<Sample> samplesThatBeginWithN =
 realm
 .where(Sample.class)
 .beginsWith("stringField",
 "N")
 .findAll();

val samples: RealmResults<Sample> =
 realm.query<Sample>().find()
val samplesThatBeginWithN:
 RealmResults<Sample> =
 realm.query<Sample>(
 "stringField BEGINSWITH 'N'"
 ).find()

Sort, Distinct, Limit

val aggregates =
 realm.where(
 Sample::class.java
 )
 .distinct("stringField")
 .sort(
 "stringField",
 Sort.ASCENDING
 )
 .limit(2)
 .findAll()

RealmResults<Sample> aggregates =
 realm.where(Sample.class)
 .distinct("stringField")
 .sort("stringField",
 Sort.ASCENDING)
 .limit(2)
 .findAll();

val aggregates: RealmResults<Sample> =
 realm.query<Sample>()
 .distinct(Sample::stringField.name)
 .sort(Sample::stringField.name,
 Sort.ASCENDING)
 .limit(2)
 .find()

Tip

Deletes

In both SDKs, you can only delete live objects. The Kotlin SDK provides mutableRealm.findLatest() to access a live version of any frozen object. In a write transaction, you can directly query for live objects and delete them without using findLatest().

val sample =
 realm.where(
 Sample::class.java
 ).findFirst()
// delete one object synchronously
realm.executeTransaction {
 transactionRealm: Realm? ->
 sample!!.deleteFromRealm()
}
// delete a query result asynchronously
realm.executeTransactionAsync {
 backgroundRealm: Realm ->
 backgroundRealm.where(
 Sample::class.java
 ).findFirst()!!.deleteFromRealm()
}

Sample sample =
 realm.where(Sample.class)
 .findFirst();
// delete one object synchronously
realm.executeTransaction(
 transactionRealm ->
 sample.deleteFromRealm());
// delete a query result asynchronously
realm.executeTransactionAsync(
 backgroundRealm ->
 backgroundRealm
 .where(Sample.class)
 .findFirst()
 .deleteFromRealm());

val sample: Sample? =
 realm.query<Sample>()
 .first().find()
// delete one object synchronously
realm.writeBlocking {
 if (sample != null) {
 findLatest(sample)
 ?.also { delete(it) }
 }
}
// delete a query result asynchronously
GlobalScope.launch {
 realm.write {
 query<Sample>()
 .first()
 .find()
 ?.also { delete(it) }
 }
}

Tip

Notifications

In both SDKs, you can subscribe to change to collections of results. With the Java SDK, you could receive notifications whenever realm results changed with the following interfaces:

realmResults.addChangeListener()
RxJava through asFlowable()
Kotlin Extensions with toFlow()

The Kotlin SDK replaces all of these options with realmQuery.asFlow(). Once you have a flow of results, you can call collect to subscribe to changes. Any object of type UpdatedResults emitted by the flow represents a change to the results set.

realm.where(Sample::class.java)
 .findAllAsync()
 .addChangeListener {
 samples: RealmResults<Sample>?,
 changeSet: OrderedCollectionChangeSet ->
 // log change description
 Log.v(
 "EXAMPLE",
 ("Results changed. " +
 "change ranges: " +
 Arrays.toString(
 changeSet
 .changeRanges
 ) +
 ", insertion ranges: " +
 Arrays.toString(
 changeSet
 .insertionRanges
 ) +
 ", deletion ranges: " +
 Arrays.toString(
 changeSet
 .deletionRanges
 ))
 )
 }

realm.where(Sample.class).findAllAsync()
 .addChangeListener(
 (samples, changeSet) -> {
 // log change description
 Log.v("EXAMPLE",
 "Results changed. " +
 "change ranges: " +
 Arrays.toString(
 changeSet
 .getChangeRanges()) +
 ", insertion ranges: " +
 Arrays.toString(
 changeSet
 .getInsertionRanges()) +
 ", deletion ranges: " +
 Arrays.toString(
 changeSet
 .getDeletionRanges()));
 });

// in a coroutine or a suspend function
realm.query<Sample>().asFlow().collect {
 results: ResultsChange<Sample> ->
 when (results) {
 is InitialResults<Sample> -> {
 // do nothing with the
 // initial set of results
 }
 is UpdatedResults<Sample> -> {
 // log change description
 Log.v("Results changed. " +
 "change ranges: " +
 results.changeRanges +
 ", insertion ranges: " +
 results.insertionRanges +
 ", deletion ranges: " +
 results.deletionRanges
 )
 }
 }
}

Threading

With the Java SDK, realms, Realm objects, and results cannot be passed between threads. The Kotlin SDK freezes these objects by default, making them thread-safe. Unlike the live objects used by the Java SDK, the frozen objects found in the Kotlin SDK do not automatically update when underlying data changes. With the Kotlin SDK, you must use notifications to subscribe to updates instead.

realm = Realm.getInstance(config)
val sample =
 realm.where(
 Sample::class.java
 ).findFirst()
// save sample field in a
// separate variable
// for access on another thread
val sampleStringField =
 sample!!.stringField
val executorService =
 Executors.newFixedThreadPool(4)
executorService.execute {
 // cannot pass a realm
 // into another thread,
 // so get a new instance
 // for separate thread
 val threadRealm =
 Realm.getInstance(config)
 // cannot access original
 // sample on another
 // thread, use
 // sampleStringField instead
 val threadSample =
 threadRealm.where(
 Sample::class.java
 )
 .equalTo(
 "stringField",
 sampleStringField
 ).findFirst()
 Log.v(
 "EXAMPLE",
 "Separate thread sample: " +
 threadSample
 )
}

realm = Realm.getInstance(config);
Sample sample = realm
 .where(Sample.class).findFirst();
// save sample field in a variable
// for access on another thread
String sampleStringField =
 sample.stringField;
ExecutorService executorService =
 Executors.newFixedThreadPool(4);
executorService.execute(() -> {
 // cannot pass a realm
 // into another thread,
 // so get a new instance
 // for separate thread
 Realm threadRealm =
 Realm.getInstance(config);
 // cannot access original
 // sample on another
 // thread, use
 // sampleStringField instead
 Sample threadSample =
 threadRealm
 .where(Sample.class)
 .equalTo("stringField",
 sampleStringField)
 .findFirst();
 Log.v("EXAMPLE",
 "Separate thread sample: "
 + threadSample);
});

val realm = Realm.open(config)
val sample: Sample? =
 realm.query<Sample>()
 .first()
 .find()
launch(Dispatchers.Unconfined) {
 // can access the realm opened on
 // a different thread
 realm.query<Sample>().find()
 // can access realm object queried
 // on a different thread
 Log.v(sample!!.stringField)
}.join()

Tip

Frozen Architecture

Migrations

With the Java SDK, migrations were a manual process. The Kotlin SDK automates migrations, but also gives you access to a similar dynamic realm interface for custom tweaks to migration logic.

val config =
 RealmConfiguration.Builder()
 .migration { realm: DynamicRealm,
 oldVersion: Long,
 newVersion: Long ->
 val schema: RealmSchema =
 realm.schema
 if (oldVersion == 0L) {
 // perform schema migration
 schema.get("Sample")
 ?.addField(
 "new_field",
 String::class.java
 )
 }
 // migrate data
 schema.get("Sample")
 ?.transform {
 obj: DynamicRealmObject ->
 obj.set(
 "longField",
 42L
 )
 }
 }.build()
val realm: Realm =
 Realm.getInstance(config)
Log.v(
 "EXAMPLE",
 "Successfully opened a realm: "
 + realm.path
)

RealmConfiguration config =
 new RealmConfiguration.Builder()
 .migration((realm,
 oldVersion,
 newVersion) -> {
 RealmSchema schema =
 realm.getSchema();
 if (oldVersion == 0L) {
 // perform schema migration
 schema.get("Sample")
 .addField("new_field",
 String.class);
 }
 // migrate data
 schema.get("Sample")
 .transform(obj ->
 obj.set("longField",
 42L));
 }).build();
Realm realm;
realm = Realm.getInstance(config);
Log.v("EXAMPLE",
"Successfully opened a realm: "
 + realm.getPath());

// A Realm migration that performs
// automatic schema migration
// and allows additional custom
// migration of data.
RealmConfiguration.Builder(
 schema = setOf(Sample::class))
 .migration(AutomaticSchemaMigration {
 context:
 AutomaticSchemaMigration.MigrationContext ->
 val oldRealm:
 DynamicRealm =
 context.oldRealm
 val newRealm:
 DynamicMutableRealm =
 context.newRealm
 // dynamic realm gives access
 // to realm data
 // through a generic string
 // based API
 context.enumerate("Sample") {
 oldObject:
 DynamicRealmObject,
 newObject:
 DynamicMutableRealmObject? ->
 newObject?.set("longField",
 42L)
 }
 })
 .build()
val realm = Realm.open(config)

What Next?

Now that you understand the differences between the Java SDK and the Kotlin SDK, check out the rest of the Kotlin SDK documentation.

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