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.
