CRUD - Java SDK

Write Operations

You can create objects in a realm, update objects in a realm, and eventually delete objects from a realm. Because these operations modify the state of the realm, we call them writes.

Realm handles writes in terms of transactions. A transaction is a list of read and write operations that Realm treats as a single indivisible operation. In other words, a transaction is all or nothing: either all of the operations in the transaction succeed or none of the operations in the transaction take effect.

Note: All writes must happen in a transaction.

A realm allows only one open write transaction at a time. Realm blocks other writes on other threads until the open transaction is complete. Consequently, there is no race condition when reading values from the realm within a transaction.

When you are done with your transaction, Realm either commits it or cancels it:

When Realm commits a transaction, Realm writes all changes to disk.
When Realm cancels a write transaction or an operation in the transaction causes an error, all changes are discarded (or "rolled back").

Tip: Whenever you create, update, or delete a Realm object, your changes update the representation of that object in Realm and emit notifications to any subscribed listeners. As a result, you should only write to Realm objects when necessary to persist data.

Important: By default, you can only read or write to a realm in your application's UI thread using asynchronous transactions. That is, you can only use Realm methods whose name ends with the word Async in the main thread of your Android application unless you explicitly allow the use of synchronous methods.

This restriction exists for the benefit of your application users: performing read and write operations on the UI thread can lead to unresponsive or slow UI interactions, so it's usually best to handle these operations either asynchronously or in a background thread.

Managed Objects

Managed objects are live Realm objects that update based on changes to underlying data in Realm. Managed objects can only come from an open realm, and receive updates as long as that realm remains open. Managed objects cannot be passed between threads.

Unmanaged objects

Unmanaged objects are instances of Realm objects that are not live. You can get an unmanaged object by manually constructing a Realm object yourself, or by calling [Realm.copyFromRealm(). Unmanaged objects can be passed between threads.

Run a Transaction

Realm represents each transaction as a callback function that contains zero or more read and write operations. To run a transaction, define a transaction callback and pass it to the realm's write method. Within this callback, you are free to create, read, update, and delete on the realm. If the code in the callback throws an exception when Realm runs it, Realm cancels the transaction. Otherwise, Realm commits the transaction immediately after the callback.

Example: The following code shows how to run a transaction with executeTransaction() or executeTransactionAsync(). If the code in the callback throws an exception, Realm cancels the transaction. Otherwise, Realm commits the transaction.

Java

java

realm.executeTransaction(r -> {
    // Create a turtle enthusiast named Ali.
    TurtleEnthusiast ali = r.createObject(TurtleEnthusiast.class, new ObjectId());
    ali.setName("Ali");
    // Find turtles younger than 2 years old
    RealmResults<Turtle> hatchlings = r.where(Turtle.class).lessThan("age", 2).findAll();
    // Give all hatchlings to Ali.
    hatchlings.setObject("owner", ali);
});

Kotlin

kotlin

realm.executeTransaction { r: Realm ->
    // Create a turtle enthusiast named Ali.
    val ali = r.createObject(TurtleEnthusiast::class.java, ObjectId())
    ali.name = "Ali"
    // Find turtles younger than 2 years old
    val hatchlings =
        r.where(Turtle::class.java).lessThan("age", 2).findAll()
    // Give all hatchlings to Ali.
    hatchlings.setObject("owner", ali)
}