Learning Java, 4th Edition

Serialization

Using a DataOutputStream, you could write an application that saves the data content of your objects one at a time as simple types. However, Java provides an even more powerful mechanism called object serialization that does almost all the work for you. In its simplest form, object serialization is an automatic way to save and load the state of an object. However, object serialization has greater depths that we cannot plumb within the scope of this book, including complete control over the serialization process and interesting twists such as class versioning.

Basically, an instance of any class that implements the Serializable interface can be saved to and restored from a stream. The stream subclasses, ObjectInputStream and ObjectOutputStream, are used to serialize primitive types and objects. Subclasses of Serializable classes are also serializable. The default serialization mechanism saves the value of all of the object’s fields (public and private), except those that are static and those marked transient.

One of the most important (and tricky) things about serialization is that when an object is serialized, any object references it contains are also serialized. Serialization can capture entire “graphs” of interconnected objects and put them back together on the receiving end (we’ll demonstrate this in an upcoming example). The implication is that any object we serialize must contain only references to other Serializable objects. We can prune the tree and limit the ...