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Migrate from the Java SDK to the Kotlin SDK

On this page

  • Overview
  • Kotlin SDK Architecture
  • Opening Realms
  • Realm Object Models
  • Relationships
  • One-to-One
  • One-to-Many
  • Schema Types
  • Writes
  • Async
  • Sync
  • Queries
  • Filters
  • Sort, Distinct, Limit
  • Deletes
  • Notifications
  • Threading
  • Migrations
  • What Next?


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.

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.

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.


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 method instead.

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

Kotlin SDK
val config = RealmConfiguration.Builder(
setOf(Frog::class, Sample::class))
val realm =
Log.v("Successfully opened realm:" +

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.

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

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.

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.

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

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

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

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().

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().

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.

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.


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.

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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