9.4 Serialized Applets

One of the uses of object serialization in Java 1.1 is for serialized applets. As discussed in Chapter 15, Java-Related HTML Tags, the <APPLET> tag has a new attribute, OBJECT, that can be used in place of the CODE attribute to specify a serialized object file instead of a class file. When such an <APPLET> tag is encountered, the applet viewer or Web browser creates the applet by deserializing it.

The reason that this is an interesting thing to do is that it allows an applet to be shipped in a pre-initialized state. The code for the applet need not even include the code that performed the initialization. For example, imagine a GUI builder tool that allows a programmer to build a GUI using a point-and-click interface. Such a tool could create a tree of AWT components within an Applet panel, and then serialize the applet, including all of the GUI components it contains. When deserialized, the applet would have a complete GUI, despite the fact that the applet's class file does not contain any code to create the GUI.

You can experiment with applet serialization with the JDK 1.1 appletviewer program. Start an applet running in appletviewer in the usual way. This loads the applet and runs its init() and start() methods. Next, select the Stop item from the menu to stop the applet. Now use the Save menu item to serialize the applet to a file. By convention, your serialized applet file should be given a .ser extension. If the applet refers to any non-serializable objects, you may not be able to serialize it. For example, you may encounter problems serializing applets that use threads or images.

Once you have serialized an applet, create an HTML file with an <APPLET> tag something like this:

<APPLET OBJECT="MyApplet.ser" WIDTH=400 HEIGHT=200></APPLET >

Finally, you can use appletviewer with this new HTML file. It should deserialize and display the applet. When created in this way, the applet's init() method is not called (since it was called before serialization), but its start() method is called (because the applet should have been stopped before serialization).