如何使用Javassist进行字节码处理以解决链接错误？

Question

我已经发布了几个与javassit链接错误spring Exception有关的问题， 当在Java 8的WebSphere上运行应用程序时 。 现在，在围绕该主题和错误进行了一些研究之后，我获得了一些有关使用Javassist进行字节码操作以解决链接错误的有用信息。

供您参考（已经在另一个问题中将此堆栈跟踪信息发布在这里）

  [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

当我尝试在org.springframework.aop.framework.ProxyCreatorSupport上执行操作时，我在此java文件上收到此错误

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);
        }
    }
}

在cc.toClass(); ，我正在得到这个例外。 我已经尝试在WebSphere上使用不同的类加载器方法来解决此异常并得到相同的错误。

现在我知道， linkage problem is occur when tries to modify the class already loaded by referenced classloader . 为了能够解决此错误，我should unload the class first from the reference classloader ，我认为这很困难。

1. 因此，我认为通过任何类加载器在类加载器之前执行字节码操作来解决此问题的唯一方法。 是不是？
2. 如何使用Java代理将现有代码更改为字节码操作？ 有什么想法和建议吗？ 请不要重复提出问题。 这只是上一个问题的参考资料。

Answer 1

您需要编写Java代理并将其动态附加以在加载时转换类。 您可以在此处找到一个不错的教程，其中介绍了如何进行操作。

请注意，此方法允许您在JVM加载类时对其进行拦截，并在实际完成加载后对其进行修改。