transient.

description:

　　transient is a Java keyword which marks a member variable not to be serialized when it is persisted to streams of bytes. When an object is transferred through the network, the object needs to be 'serialized'. Serialization converts the object state to serial bytes. Those bytes are sent over the network and the object is recreated from those bytes. Member variables marked by the java transientkeyword are not transferred, they are lost intentionally.

　　relevant link:　　Java Programming/Keywords/transient in wikipedia and transient Fields in the language specification.