Hazelcast IMDG Standard Support has expired. Extended support for version 4.1 ends in April 2024. Extended support for version 4.2 ends in September 2024.

We recommend that you try Hazelcast Platform.

In Hazelcast Platform, we’ve combined the in-memory storage of IMDG with the stream processing power of Jet. Find out more in our Platform documentation.

The following topics are a good place to start:

Implementing Java Serializable and Externalizable

A class often needs to implement the java.io.Serializable interface; native Java serialization is the easiest way to do serialization.

Let’s take a look at the example code below for Java Serializable.

public class Employee implements Serializable {
    private static final long serialVersionUID = 1L;
    private String surname;

    public Employee( String surname ) {
        this.surname = surname;
    }
}

Here, the fields that are non-static and non-transient are automatically serialized. To eliminate class compatibility issues, it is recommended that you add a serialVersionUID, as shown above. Also, when you are using methods that perform byte-content comparisons, such as IMap.replace(), and if byte-content of equal objects is different, you may face unexpected behaviors. For example, if the class relies on a hash map, the replace method may fail. The reason for this is the hash map is a serialized data structure with unreliable byte-content.

Implementing Java Externalizable

Hazelcast also supports java.io.Externalizable. This interface offers more control on the way fields are serialized or deserialized. Compared to native Java serialization, it also can have a positive effect on performance. With java.io.Externalizable, there is no need to add serialVersionUID.

Let’s take a look at the example code below.

public class Employee implements Externalizable {
    private String surname;
    public Employee(String surname) {
        this.surname = surname;
    }

    @Override
    public void readExternal( ObjectInput in )
      throws IOException, ClassNotFoundException {
        this.surname = in.readUTF();
    }

    @Override
    public void writeExternal( ObjectOutput out )
      throws IOException {
        out.writeUTF(surname);
    }
}

You explicitly perform writing and reading of fields. Perform reading in the same order as writing.