Javassistを使ってJavaのfinalクラスの拡張は・・・ ref: http://qiita.com/kurogelee/items/79ab9393c061153dd8ea

ExtendSample.java

package javassist.test;

import javassist.ClassPool;
import javassist.CtClass;

public class ExtendSample {
    public static void main(String[] args) throws Exception{
        ClassPool pool = ClassPool.getDefault();
        CtClass cc = pool.makeClass("dynamic.clazz.Sample");
        cc.setSuperclass(pool.get("java.lang.String"));
        Class<?> clazz = cc.toClass();
        Object obj = clazz.newInstance();
    }
}

file1.txt

Exception in thread "main" javassist.CannotCompileException: by java.lang.VerifyError: Cannot inherit from final class
    at javassist.ClassPool.toClass(ClassPool.java:1099)
    at javassist.ClassPool.toClass(ClassPool.java:1042)
    at javassist.ClassPool.toClass(ClassPool.java:1000)
    at javassist.CtClass.toClass(CtClass.java:1224)
    at javassist.test.ExtendSample.main(ExtendSample.java:11)
Caused by: java.lang.VerifyError: Cannot inherit from final class
    at java.lang.ClassLoader.defineClass1(Native Method)
    at java.lang.ClassLoader.defineClassCond(Unknown Source)
    at java.lang.ClassLoader.defineClass(Unknown Source)
    at java.lang.ClassLoader.defineClass(Unknown Source)
    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    at sun.reflect.NativeMethodAccessorImpl.invoke(Unknown Source)
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(Unknown Source)
    at java.lang.reflect.Method.invoke(Unknown Source)
    at javassist.ClassPool.toClass2(ClassPool.java:1112)
    at javassist.ClassPool.toClass(ClassPool.java:1093)
    ... 4 more