Class Loader

  • public class Loader
    extends Loader
    A class loader for reflection.

    To run a program, say MyApp, including a reflective class, you must write a start-up program as follows:

     public class Main {
       public static void main(String[] args) throws Throwable { cl
             = (;
         cl.makeReflective("Person", "MyMetaobject",
                           "");"MyApp", args);

    Then run this program as follows:

    % java Main arg1, ...

    This command runs Main.main() with arg1, ... and Main.main() runs MyApp.main() with arg1, ... The Person class is modified to be a reflective class. Method calls on a Person object are intercepted by an instance of MyMetaobject.

    Also, you can run MyApp in a slightly different way:

     public class Main2 {
       public static void main(String[] args) throws Throwable { cl = new;
         cl.makeReflective("Person", "MyMetaobject",
                           "");"MyApp", args);

    This program is run as follows:

    % java Main2 arg1, ...

    The difference from the former one is that the class Main is loaded by whereas the class Main2 is not. Thus, Main belongs to the same name space (security domain) as MyApp whereas Main2 does not; Main2 belongs to the same name space as For more details, see the notes in the manual page of javassist.Loader.

    The class Main2 is equivalent to this class:

     public class Main3 {
       public static void main(String[] args) throws Throwable {
         Reflection reflection = new Reflection();
         javassist.Loader cl
             = new javassist.Loader(ClassPool.getDefault(reflection));
         reflection.makeReflective("Person", "MyMetaobject",
                                   "");"MyApp", args);

    Note: does not make a class reflective if that class is in a java.* or javax.* pacakge because of the specifications on the class loading algorithm of Java. The JVM does not allow to load such a system class with a user class loader.

    To avoid this limitation, those classes should be statically modified with and the original class files should be replaced.

    See Also:
    Reflection, Compiler, Loader
    • Constructor Summary

      Constructor Description
      Constructs a new class loader.
    • Method Summary

      All Methods Static Methods Instance Methods Concrete Methods 
      Modifier and Type Method Description
      static void main​(java.lang.String[] args)
      Loads a class with an instance of Loader and calls main() in that class.
      boolean makeReflective​(java.lang.String clazz, java.lang.String metaobject, java.lang.String metaclass)
      Produces a reflective class.
      • Methods inherited from class java.lang.ClassLoader

        clearAssertionStatus, getDefinedPackage, getDefinedPackages, getName, getParent, getPlatformClassLoader, getResource, getResourceAsStream, getResources, getSystemClassLoader, getSystemResource, getSystemResourceAsStream, getSystemResources, getUnnamedModule, isRegisteredAsParallelCapable, loadClass, resources, setClassAssertionStatus, setDefaultAssertionStatus, setPackageAssertionStatus
      • Methods inherited from class java.lang.Object

        equals, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait
    • Method Detail

      • main

        public static void main​(java.lang.String[] args)
                         throws java.lang.Throwable
        Loads a class with an instance of Loader and calls main() in that class.
        args - command line parameters.
          args[0] is the class name to be loaded.
          args[1..n] are parameters passed to the target main().
      • makeReflective

        public boolean makeReflective​(java.lang.String clazz,
                                      java.lang.String metaobject,
                                      java.lang.String metaclass)
                               throws CannotCompileException,
        Produces a reflective class. If the super class is also made reflective, it must be done before the sub class.
        clazz - the reflective class.
        metaobject - the class of metaobjects. It must be a subclass of Metaobject.
        metaclass - the class of the class metaobject. It must be a subclass of ClassMetaobject.
        false if the class is already reflective.
        See Also:
        Metaobject, ClassMetaobject