Java Enterprise Best Practices

Always Set the serialVersionUID

serialVersionUID is a class invariant that the RMI runtime uses to validate that the classes on both sides of the wire are the same. Here’s how it works: the first process marshals an object and sends it over the wire. As part of the marshalling process, the serialVersionUID of all relevant classes is also sent. The receiving process compares the serialVersionUIDs that were sent with the serialVersionUIDs of the local classes. If they aren’t equal, the RMI runtime will throw an instance of UnmarshalException (and the method call will never even reach your code on the server side).

If you don’t specify serialVersionUID, you will run into two problems. The first is that the system’s value for serialVersionUID is generated at runtime (not at compile time), and generating it can be expensive. The second, more serious problem is that the automatically generated values of serialVersionUID are created by hashing together all the fields and methods of the class and are therefore extraordinarily sensitive to minor changes. For these reasons, whenever you define a class that will be marshalled, you should always set the serialVersionUID, as in the following example:

public class ClassWhichWillBeMarshalled  implements Externalizable{
	public static final long serialVersionUID = 1L;
	//  . . . 
}