Serialization (Java)

Serialization (Java)

Akka has a built-in Extension for serialization, and it is both possible to use the built-in serializers and to write your own.

The serialization mechanism is both used by Akka internally to serialize messages, and available for ad-hoc serialization of whatever you might need it for.

Usage

Configuration

For Akka to know which Serializer to use for what, you need edit your Configuration, in the “akka.actor.serializers”-section you bind names to implementations of the akka.serialization.Serializer you wish to use, like this:

val config = ConfigFactory.parseString("""
  akka {
    actor {
      serializers {
        java = "akka.serialization.JavaSerializer"
        proto = "akka.serialization.ProtobufSerializer"
        myown = "akka.docs.serialization.MyOwnSerializer"
      }
    }
  }
""")

After you’ve bound names to different implementations of Serializer you need to wire which classes should be serialized using which Serializer, this is done in the “akka.actor.serialization-bindings”-section:

val config = ConfigFactory.parseString("""
  akka {
    actor {
      serializers {
        java = "akka.serialization.JavaSerializer"
        proto = "akka.serialization.ProtobufSerializer"
        myown = "akka.docs.serialization.MyOwnSerializer"
      }

      serialization-bindings {
        "java.lang.String" = java
        "akka.docs.serialization.Customer" = java
        "com.google.protobuf.Message" = proto
        "akka.docs.serialization.MyOwnSerializable" = myown
        "java.lang.Boolean" = myown
      }
    }
  }
""")

You only need to specify the name of an interface or abstract base class of the messages. In case of ambiguity, i.e. the message implements several of the configured classes, the most specific configured class will be used, i.e. the one of which all other candidates are superclasses. If this condition cannot be met, because e.g. java.io.Serializable and MyOwnSerializable both apply and neither is a subtype of the other, a warning will be issued.

Akka provides serializers for java.io.Serializable and protobuf com.google.protobuf.GeneratedMessage by default (the latter only if depending on the akka-remote module), so normally you don’t need to add configuration for that; since com.google.protobuf.GeneratedMessage implements java.io.Serializable, protobuf messages will always by serialized using the protobuf protocol unless specifically overridden. In order to disable a default serializer, map its marker type to “none”:

akka.actor.serialization-bindings {
  "java.io.Serializable" = none
}

Verification

If you want to verify that your messages are serializable you can enable the following config option:

val config = ConfigFactory.parseString("""
  akka {
    actor {
      serialize-messages = on
    }
  }
""")

Warning

We only recommend using the config option turned on when you’re running tests. It is completely pointless to have it turned on in other scenarios.

If you want to verify that your Props are serializable you can enable the following config option:

val config = ConfigFactory.parseString("""
  akka {
    actor {
      serialize-creators = on
    }
  }
""")

Warning

We only recommend using the config option turned on when you’re running tests. It is completely pointless to have it turned on in other scenarios.

Programmatic

If you want to programmatically serialize/deserialize using Akka Serialization, here’s some examples:

import akka.actor.*;
import akka.serialization.*;
import com.typesafe.config.*;

      ActorSystem system = ActorSystem.create("example");

      // Get the Serialization Extension
      Serialization serialization = SerializationExtension.get(system);

      // Have something to serialize
      String original = "woohoo";

      // Find the Serializer for it
      Serializer serializer = serialization.findSerializerFor(original);

      // Turn it into bytes
      byte[] bytes = serializer.toBinary(original);

      // Turn it back into an object,
      // the nulls are for the class manifest and for the classloader
      String back = (String)serializer.fromBinary(bytes);

      // Voilá!
      assertEquals(original, back);

For more information, have a look at the ScalaDoc for akka.serialization._

Customization

So, lets say that you want to create your own Serializer, you saw the akka.docs.serialization.MyOwnSerializer in the config example above?

Creating new Serializers

First you need to create a class definition of your Serializer, which is done by extending akka.serialization.JSerializer, like this:

import akka.actor.*;
import akka.serialization.*;
import com.typesafe.config.*;

    public static class MyOwnSerializer extends JSerializer {

      // This is whether "fromBinary" requires a "clazz" or not
      @Override public boolean includeManifest() {
          return false;
      }

      // Pick a unique identifier for your Serializer,
      // you've got a couple of billions to choose from,
      // 0 - 16 is reserved by Akka itself
      @Override public int identifier() {
          return 1234567;
      }

      // "toBinary" serializes the given object to an Array of Bytes
      @Override public byte[] toBinary(Object obj) {
        // Put the code that serializes the object here
        // ... ...
      }

      // "fromBinary" deserializes the given array,
      // using the type hint (if any, see "includeManifest" above)
      @Override public Object fromBinaryJava(byte[] bytes,
                     Class<?> clazz) {
        // Put your code that deserializes here
        // ... ...
      }
    }

Then you only need to fill in the blanks, bind it to a name in your Configuration and then list which classes that should be serialized using it.

Serializing ActorRefs

All ActorRefs are serializable using JavaSerializer, but in case you are writing your own serializer, you might want to know how to serialize and deserialize them properly, here’s the magic incantation:

import akka.actor.*;
import akka.serialization.*;
import com.typesafe.config.*;

        // Serialize
        // (beneath toBinary)
        final Address transportAddress =
                Serialization.currentTransportAddress().value();
        String identifier;

        // If there is no transportAddress,
        // it means that either this Serializer isn't called
        // within a piece of code that sets it,
        // so either you need to supply your own,
        // or simply use the local path.
        if (transportAddress == null) identifier = theActorRef.path().toString();
        else identifier = theActorRef.path().toStringWithAddress(transportAddress);
        // Then just serialize the identifier however you like


        // Deserialize
        // (beneath fromBinary)
        final ActorRef deserializedActorRef = theActorSystem.actorFor(identifier);
        // Then just use the ActorRef

A Word About Java Serialization

When using Java serialization without employing the JavaSerializer for the task, you must make sure to supply a valid ExtendedActorSystem in the dynamic variable JavaSerializer.currentSystem. This is used when reading in the representation of an ActorRef for turning the string representation into a real reference. DynamicVariable is a thread-local variable, so be sure to have it set while deserializing anything which might contain actor references.

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