Java Byte Code

http://prismoskills.appspot.com/lessons/Super_Java/Insert_code_at_runtime.jsp
Javassist is a Java byte-code editor which allows to:

1. Define a new class at runtime
2. Modify a class file when the JVM loads it
3. Insert byte-code in the form of source text, which is compiled on the fly by Javassist
4. Add new methods to classes

public class DynamicCodeInsertion
{
    static String pkgName = "com.prismoskills.javassist.examples.annotation";
    public static void main(String[] args) throws Exception
    {
        ClassPool cp = ClassPool.getDefault();
        CtClass cc = cp.get(pkgName + ".FooCodeInsertion");
        // Without the call to "makePackage()", package information is lost
        cp.makePackage(cp.getClassLoader(), pkgName);
        CtMethod m = cc.getDeclaredMethod("helloWorld");
        m.insertBefore("{System.out.print(\"Oh, say no to \");}");
        m.insertAfter("{System.out.print(\"And say - Hi World\");}");
        // Changes are not persisted without a call to "toClass()"
        cc.toClass();

        (new FooCodeInsertion()).helloWorld();
    }

}
http://prismoskills.appspot.com/lessons/Super_Java/Dynamically_adding_annotations.jsp
public static void main(String[] args) throws Exception
{
    ClassPool cp = ClassPool.getDefault();
    CtClass cc = cp.get(pkgName + ".SomeClass");
    // Without the call to "makePackage()", package information is lost
    cp.makePackage(cp.getClassLoader(), pkgName);
    addAnnotation(cc, "someField", "java.lang.Deprecated");
    // Changes are not persisted without a call to "toClass()"
    Class<?> c = cc.toClass();

    Field fld = SomeClass.class.getDeclaredField("someField");
    System.out.println(c.getPackage());
    System.out.println(fld.getAnnotation(Deprecated.class));
}

public static void addAnnotation(
        CtClass clazz,
        String fieldName,
        String annotationName) throws Exception
{
    ClassFile cfile = clazz.getClassFile();
    ConstPool cpool = cfile.getConstPool();
    CtField cfield  = clazz.getField(fieldName);

    AnnotationsAttribute attr =
            new AnnotationsAttribute(cpool, AnnotationsAttribute.visibleTag);
    Annotation annot = new Annotation(annotationName, cpool);
    attr.addAnnotation(annot);
    cfield.getFieldInfo().addAttribute(attr);
}