During deserialization, the private state of the object is restored. For example, a file descriptor contains a handle that provides access to an operating system resource. Being able to forge a file descriptor would allow some forms of illegal access, since restoring state is done from a stream. Therefore, the serializing runtime must take the conservative approach and not trust the stream to contain only valid representations of objects. To avoid compromising a class, the sensitive state of an object must not be restored from the stream, or it must be reverified by the class.
Several techniques are available to protect sensitive data in classes.
The easiest technique is to mark fields that contain sensitive data as private transient . Transient fields are not persistent and will not be saved by any persistence mechanism. Marking the field will prevent the state from appearing in the stream and from being restored during deserialization. Particularly sensitive classes should not be serialized at all. To accomplish this, the object should not implement either the Serializable or the Externalizable interface.