Java Cookbook

Constructing a Class from Scratch

Problem

You need to load a class and run its methods.

Solution

Write and use your own ClassLoader.

Discussion

A ClassLoader , of course, is a program that loads classes. There is one class loader built into the Java Virtual Machine, but your application can create additional ones as needed. Learning to write and run a working class loader and use it to load a class and run its methods is a nontrivial exercise. In fact, you rarely need to write a class loader, but knowing how is helpful in understanding how the JVM finds classes, creates objects, and calls methods.

ClassLoader itself is abstract; you must subclass it to provide a loadClass( ) method that loads classes as you wish. It can load the bytes from a network connection, a local disk, RAM, a serial port, or anywhere else. Or you can construct the class file in memory yourself if you have access to a compiler.

You must call this class loader’s loadClass( ) method for any classes you wish to load from it. Note that it will be called to load all classes required for classes you load (parent classes that aren’t already loaded, for example). However, the JVM will still load classes that you instantiate with the new operator “normally” via CLASSPATH.

To write a class loader, you need to subclass ClassLoader and implement at least findClass( ) and loadClass( ). The loadClass( ) method needs to get the class file into a byte array (typically by reading it), convert the array into a Class object, ...