19. What is serialization? How would you exclude a field of a class from serialization or what is a transient variable? What is the common use?

Serialization is a process of reading or writing an object. It is a process of saving an object’s state to a sequence of bytes, as well as a process of rebuilding those bytes back into a live object at some future time. An object is marked serializable by implementing the java.io.Serializable interface, which is only a marker interface -- it simply allows the serialization mechanism to verify that the class can be persisted, typically to a file.

Transient variables cannot be serialized. The fields marked transient in a serializable object will not be transmitted in the byte stream. An example would be a file handle or a database connection. Such objects are only meaningful locally. So they should be marked as transient in a serializable class.

Serialization can adversely affect performance since it:

• Depends on reflection.

• Has an incredibly verbose data format.

• Is very easy to send surplus data.

When to use serialization? Do not use serialization if you do not have to. A common use of serialization is to use it to send an object over the network or if the state of an object needs to be persisted to a flat file or a database. Deep cloning or copy can be achieved through serialization. This may be fast to code but will have performance implications. The objects stored in an HTTP session should be serializable to support in-memory replication of sessions to achieve scalability. Objects are passed in RMI (Remote Method Invocation) across network using serialization.

Also Refer: http://debugmaster.blogspot.com/2009/08/serialization-externalization.html