This is a sample class loader using ClassPool. Unlike a regular class loader, this class loader obtains bytecode from a ClassPool.

Note that Javassist can be used without this class loader; programmers can define their own versions of class loader. They can run a program even without any user-defined class loader if that program is statically translated with Javassist. This class loader is just provided as a utility class.

Suppose that an instance of MyTranslator implementing the interface Translator is responsible for modifying class files. The startup program of an application using MyTranslator should be something like this:

import javassist.*;

public class Main {
  public static void main(String[] args) throws Throwable {
    MyTranslator myTrans = new MyTranslator();
    ClassPool cp = ClassPool.getDefault();
    Loader cl = new Loader(cp);
    cl.addTranslator(cp, myTrans);
    cl.run("MyApp", args);
  }
}

Class MyApp is the main program of the application.

This program should be executed as follows:

% java Main arg1 arg2...

It modifies the class MyApp with a MyTranslator object before the JVM loads it. Then it calls main() in MyApp with arguments arg1, arg2, ...

This program execution is equivalent to:

% java MyApp arg1 arg2...

except that classes are translated by MyTranslator at load time.

If only a particular class must be modified when it is loaded, the startup program can be simpler; MyTranslator is unnecessary. For example, if only a class test.Rectangle is modified, the main() method above will be the following:

ClassPool cp = ClassPool.getDefault();
Loader cl = new Loader(cp);
CtClass ct = cp.get("test.Rectangle");
ct.setSuperclass(cp.get("test.Point"));
cl.run("MyApp", args);

This program changes the super class of the test.Rectangle class.

Note 1:

This class loader does not allow the users to intercept the loading of java.* and javax.* classes (and sun.*, org.xml.*, ...) unless Loader.doDelegation is false. This is because the JVM prohibits a user class loader from loading a system class. Also see Note 2. If this behavior is not appropriate, a subclass of Loader must be defined and loadClassByDelegation() must be overridden.

Note 2:

If classes are loaded with different class loaders, they belong to separate name spaces. If class C is loaded by a class loader CL, all classes that the class C refers to are also loaded by CL. However, if CL delegates the loading of the class C to CL', then those classes that the class C refers to are loaded by a parent class loader CL' instead of CL.

If an object of class C is assigned to a variable of class C belonging to a different name space, then a ClassCastException is thrown.

Because of the fact above, this loader delegates only the loading of javassist.Loader and classes included in package java.* and javax.* to the parent class loader. Other classes are directly loaded by this loader.

For example, suppose that java.lang.String would be loaded by this loader while java.io.File is loaded by the parent class loader. If the constructor of java.io.File is called with an instance of java.lang.String, then it may throw an exception since it accepts an instance of only the java.lang.String loaded by the parent class loader.