Darkstar: The Java Game Server

A. Serialization

Serialization is the process of taking the in-memory representation of an object and storing it (persisting) to a file so that it survives even after the program which created it exits. In comparison to other languages, Java provides straightforward support for serialization. However, there are several steps necessary for successful serialization.

First, your object must implement the java.io.Serializable interface. This interface simply marks your object as semantically able to be serialized. Not every type of Java object can be serialized. Its easy to imagine how a String can be written to and recovered from a file. After all, its just a list of characters, that's basically what a file is anyway. However, its much more difficult to imagine how an object like a Thread could be serialized. A Thread is much more than just data, its a running programming with its own context. As you might imagine, in Java Strings implement the Serializable interface, Threads do not.

Second, all of the instance variables in your object must be serializable. Fortunately, most of the data objects (Strings, Vectors, etc) in the main Java library are serializable. As I mentioned before, some, like Thread are not. When in doubt, look at the documentation. But what if your class needs to have some non-serializable object as an instance variable? This is unlikely in Darkstar, but perfectly possible. In that case, you need to mark that object as transient. For example:

protected transient Thread ...