How to do bytecode manipulation with Javassist for solving linkage errors?

Question

I have posted several question related to javassit linkage error spring Exception when running application on WebSphere with java 8 . Now after some research work around this topics and error, i have got some useful information about bytecode manipulation with Javassist to solve linkage error .

For your reference ( Already posted this stack trace in another questions here )

  [10/13/17 ] 00000089 webapp        E com.ibm.ws.webcontainer.webapp.WebApp logServletError SRVE0293E: [Servlet                                                                       Error]-[weblge]: java.lang.ExceptionInInitializerError
        at java.lang.J9VMInternals.ensureError(J9VMInternals.java:134)
        at java.lang.J9VMInternals.recordInitializationFailure(J9VMInternals.java:123)
        at sun.reflect.NativeConstructorAccessorImpl.newInstance0(Native Method)
        at sun.reflect.NativeConstructorAccessorImpl.newInstance(NativeConstructorAccessorImpl.java:83)
        at sun.reflect.DelegatingConstructorAccessorImpl.newInstance(DelegatingConstructorAccessorImpl.java:57)
        at java.lang.reflect.Constructor.newInstance(Constructor.java:437)
        at org.springframework.beans.BeanUtils.instantiateClass(BeanUtils.java:163)
        at org.springframework.beans.BeanUtils.instantiateClass(BeanUtils.java:120)
        at org.springframework.web.context.ContextLoader.createWebApplicationContext(ContextLoader.java:345)
        at org.springframework.web.context.ContextLoader.initWebApplicationContext(ContextLoader.java:278)
        at org.springframework.web.context.ContextLoaderListener.contextInitialized(ContextLoaderListener.java:103)
        at com.ibm.ws.webcontainer.webapp.WebApp.notifyServletContextCreated(WebApp.java:1826)
        at com.ibm.ws.webcontainer.webapp.WebAppImpl.initialize(WebAppImpl.java:442)
        at com.ibm.ws.webcontainer.webapp.WebGroupImpl.addWebApplication(WebGroupImpl.java:88)
        at com.ibm.ws.webcontainer.VirtualHostImpl.addWebApplication(VirtualHostImpl.java:171)
        at com.ibm.ws.webcontainer.WSWebContainer.addWebApp(WSWebContainer.java:904)
        at com.ibm.ws.webcontainer.WSWebContainer.addWebApplication(WSWebContainer.java:789)
        at com.ibm.ws.webcontainer.component.WebContainerImpl.install(WebContainerImpl.java:427)
        at com.ibm.ws.webcontainer.component.WebContainerImpl.start(WebContainerImpl.java:719)
        at com.ibm.ws.runtime.component.ApplicationMgrImpl.start(ApplicationMgrImpl.java:1247)
        at com.ibm.ws.runtime.component.DeployedApplicationImpl.fireDeployedObjectStart(DeployedApplicationImpl.java:1514)
        at com.ibm.ws.runtime.component.DeployedModuleImpl.start(DeployedModuleImpl.java:704)
        at com.ibm.ws.runtime.component.DeployedApplicationImpl.start(DeployedApplicationImpl.java:1096)
        at com.ibm.ws.runtime.component.ApplicationMgrImpl.startApplication(ApplicationMgrImpl.java:799)
        at com.ibm.ws.runtime.component.ApplicationMgrImpl$5.run(ApplicationMgrImpl.java:2315)
        at com.ibm.ws.security.auth.ContextManagerImpl.runAs(ContextManagerImpl.java:5488)
        at com.ibm.ws.security.auth.ContextManagerImpl.runAsSystem(ContextManagerImpl.java:5614)
        at com.ibm.ws.security.core.SecurityContext.runAsSystem(SecurityContext.java:255)
        at com.ibm.ws.runtime.component.ApplicationMgrImpl.start(ApplicationMgrImpl.java:2320)
        at com.ibm.ws.runtime.component.CompositionUnitMgrImpl.start(CompositionUnitMgrImpl.java:436)
        at com.ibm.ws.runtime.component.CompositionUnitImpl.start(CompositionUnitImpl.java:123)
        at com.ibm.ws.runtime.component.CompositionUnitMgrImpl.start(CompositionUnitMgrImpl.java:379)
        at com.ibm.ws.runtime.component.CompositionUnitMgrImpl.access$500(CompositionUnitMgrImpl.java:127)
        at com.ibm.ws.runtime.component.CompositionUnitMgrImpl$CUInitializer.run(CompositionUnitMgrImpl.java:985)
        at com.ibm.wsspi.runtime.component.WsComponentImpl$_AsynchInitializer.run(WsComponentImpl.java:524)
        at com.ibm.ws.util.ThreadPool$Worker.run(ThreadPool.java:1909)
Caused by: java.lang.IllegalStateException: javassist.CannotCompileException: by java.lang.LinkageError: org.springframework.aop.framework.ProxyCreatorSupport
        at org.springframework.aop.framework.JavassistApplicationContext.<clinit>(JavassistApplicationContext.java:36)
        ... 34 more
Caused by: javassist.CannotCompileException: by java.lang.LinkageError: org.springframework.aop.framework.ProxyCreatorSupport
        at javassist.ClassPool.toClass(ClassPool.java:1170)
        at javassist.ClassPool.toClass(ClassPool.java:1113)
        at javassist.ClassPool.toClass(ClassPool.java:1071)
        at javassist.CtClass.toClass(CtClass.java:1275)
        at org.springframework.aop.framework.JavassistApplicationContext.<clinit>(JavassistApplicationContext.java:34)
        ... 34 more
Caused by: java.lang.LinkageError: org.springframework.aop.framework.ProxyCreatorSupport
        at java.lang.ClassLoader.defineClassImpl(Native Method)
        at java.lang.ClassLoader.defineClass(ClassLoader.java:357)
        at java.lang.ClassLoader.defineClass(ClassLoader.java:293)
        at sun.reflect.GeneratedMethodAccessor17.invoke(Unknown Source)
        at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:55)
        at java.lang.reflect.Method.invoke(Method.java:508)
        at javassist.ClassPool.toClass2(ClassPool.java:1183)
        at javassist.ClassPool.toClass(ClassPool.java:1164)
        ... 38 more

and i am getting this error on this java file , when i am trying do operation on org.springframework.aop.framework.ProxyCreatorSupport

package org.spring.aop.framework;

import javassist.ClassPool;
import javassist.CtClass;
import javassist.CtConstructor;
import javassist.LoaderClassPath;
import org.springframework.web.context.support.XmlWebApplicationContext;

public class JavassistApplicationContext extends XmlWebApplicationContext {

    static {
        ClassPool classPool = ClassPool.getDefault();
        try {
            ClassLoader classLoader = Thread.currentThread().getContextClassLoader();
            classPool.appendClassPath(new LoaderClassPath(classLoader));

            CtClass cc = classPool.get("org.spring.aop.framework.ProxyCreatorSupport");
            CtConstructor c = cc.getConstructors()[0];
            c.insertAfter("$0.aopProxyFactory = new org.spring.aop.framework.JavassistAopProxyFactory();");
            cc.toClass();
        } catch (Exception e) {
            throw new IllegalStateException(e);
        }
    }
}

on the line cc.toClass(); , i am getting this exception . I have tried different class loader methods on WebSphere to solve this exception and getting same error .

Now i understood that linkage problem is occur when tries to modify the class already loaded by referenced classloader . being able to solve this error , i should unload the class first from the reference classloader and i think it's difficult .

1. So i think the only method to solve this with the implementation of bytecode manipulation before the class loader by any class loader . isn't?
2. How to change my existing code to bytecode manipulation with java agent ? Any idea and suggestion ? please don't put as a duplicate question . This is only the reference work of the previous question .

Answer 1

You need to write a Java agent and attach it dynamically to transform a class at load time. You can find a cool tutorial here that explains how to do it.

Note that this method allows you to intercept the class while it's loaded by the JVM and modify it after this loading is actually done.