What is Externalizable in JAVA?

Answer 1

Externalizable is an interface which contains two methods readExternal and writeExternal. These methods give you a control over the serialization mechanism. Thus if your class implements this interface, you can customize the serialization process by implementing these methods.

Answer 2

Source : http://www.byteslounge.com/tutorials/java-externalizable-example
The Externalizable interface provides the necessary means for implementing a custom serialization mechanism. Implementing the Externalizable interface means that we must override some of its methods, namely the writeExternal and readExternal methods. These methods will be called when you serialize (or deserialize) a given instance. When we implement the Serializable interface we don't need to implement any method: The serialization takes place automatically. Even if we want to implement our own serialization mechanism by implementing the Serializable interface (yes it is also possible by defining writeObject and readObject methods) we don't need to override or implement any method. The JVM calls the serialization methods from our class using reflection. In early JVM implementations reflection performance was kind of slow so the Externalizable interface was also used to overcome this disadvantage.