Difference between Serializable and Externalizable in Java Serialization

1. In case of Serializable, default serialization process is used. while in case of Externalizable custom Serialization process is used which is implemented by application.

2. JVM gives call back to readExternel() and writeExternal() of java.io.Externalizalbe interface for restoring and writing objects into persistence.

3. Externalizable interface provides complete control of serialization process to application.

4. readExternal() and writeExternal() supersede any specific implementation of writeObject and readObject methods.

Though Externalizable provides complete control, it also presents challenges to serialize super type state and take care of default values in case of transient variable and static variables in Java. If used correctly Externalizable interface can improve performance of serialization process.

By implementating java.io.Serializable, you get "automatic" serialization capability for objects of your class. No need to implement any other logic, it'll just work. The Java runtime will use reflection to figure out how to marshal and unmarshal your objects.

In earlier version of Java, reflection was very slow, and so serializaing large object graphs (e.g. in client-server RMI applications) was a bit of a performance problem. To handle this situation, the java.io.Externalizable interface was provided, which is like java.io.Serializable but with custom-written mechanisms to perform the marshalling and unmarshalling functions (you need to implement readExternal and writeExternal methods on your class). This gives you the means to get around the reflection performance bottleneck.

In recent versions of Java (1.3 onwards, certainly) the performance of reflection is vastly better than it used to be, and so this is much less of a problem. I suspect you'd be hard-pressed to get a meaningful benefit from Externalizable with a modern JVM.

Also, the built-in Java serialization mechanism isn't the only one, you can get third-party replacements, such as JBoss Serialization, which is considerably quicker, and is a drop-in replacement for the default.

A big downside of Externalizable is that you have to maintain this logic yourself - if you add, remove or change a field in your class, you have to change your writeExternal/readExternal methods to account for it.

In summary, Externalizable is a relic of the Java 1.1 days. There's really no need for it any more

Read more: http://javarevisited.blogspot.com/2012/01/serializable-externalizable-in-java.html#ixzz2Z6AjeDuf