Custom Classloader And Java Agent

Question

I want to modifly the class in app which is running. So I use java agent to do this. My java code like this:

    /**
 * 
 * @param args
 * @param inst
 */
public static void agentmain(String args, Instrumentation inst) {
    try {
        inst.addTransformer(new GameClassFileTransformer(), true);
        parseFiles();
        List<Class<?>> retransformClasses = new ArrayList<>();
        for (JarClassInfo jarClassInfo : GameAgentMain.bugFiles.values()) {
            retransformClasses.add(Class.forName(jarClassInfo.getClassName()));
        }
        Class<?>[] classes = new Class[retransformClasses.size()];
        retransformClasses.toArray(classes);
        inst.retransformClasses(classes);
    } catch (Exception e) {
        LogUtil.error(e);
    }
}

The JarClassInfo is a class about the class I want to reload.There hava two fields in JarClassInfo ,first is className,second is bytecode(java type is byte[]),The classname is which class want to reload.But the class is loaded by

custom classLoad

.When the app run at :

retransformClasses.add(Class.forName(jarClassInfo.getClassName()));

That is a exception be thrown:

Exception in thread "Attach Listener" java.lang.reflect.InvocationTargetException
        at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
        at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:57)
        at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
        at java.lang.reflect.Method.invoke(Method.java:606)
        at sun.instrument.InstrumentationImpl.loadClassAndStartAgent(InstrumentationImpl.java:382)
        at sun.instrument.InstrumentationImpl.loadClassAndCallAgentmain(InstrumentationImpl.java:407)
Caused by: java.lang.ClassFormatError: Incompatible magic value 2329931675 in class file com/jingqi/game/module/chat/command/SendChatMsgCMD
        at java.lang.ClassLoader.defineClass1(Native Method)
        at java.lang.ClassLoader.defineClass(ClassLoader.java:800)
        at java.security.SecureClassLoader.defineClass(SecureClassLoader.java:142)
        at java.net.URLClassLoader.defineClass(URLClassLoader.java:449)
        at java.net.URLClassLoader.access$1(URLClassLoader.java:409)
        at java.net.URLClassLoader$2.run(URLClassLoader.java:361)
        at java.net.URLClassLoader$2.run(URLClassLoader.java:1)
        at java.security.AccessController.doPrivileged(Native Method)
        at java.net.URLClassLoader.findClass(URLClassLoader.java:354)
        at java.lang.ClassLoader.loadClass(ClassLoader.java:425)
        at sun.misc.Launcher$AppClassLoader.loadClass(Launcher.java:308)
        at java.lang.ClassLoader.loadClass(ClassLoader.java:358)
        at java.lang.Class.forName0(Native Method)
        at java.lang.Class.forName(Class.java:190)
        at com.jingqi.game.agent.GameAgentMain.agentmain(GameAgentMain.java:60)
        ... 6 more

My custom classloader(EncrypClassLoader) is used to decode my class. So the methd call stack indicate sun.misc.Launcher$AppClassLoader to load SendChatMsgCMD .The SendChatMsgCMD is not a normal class file(because I encrypt this class file).

So ,what method can be solved 。

Answer 1

Your custom classloader isn't doing what it's supposed to do: decrypt classes.
That's why you're​ getting​ the java.lang.ClassFormatError about the invalid magic value.

EDIT: you need to also realise that Class.forName() will load the class referred by the classname argument using the current class's classloader.