Javassist，java

Question

我正在做一个创建java应用程序的任务。任务是更改方法（计算公式）而不重新编译应用程序。在谷歌搜索后，我意识到你可以在javassist的帮助下解决问题。到目前为止，我正在尝试一个简单的例子。所以，该类包含我想要改变的唯一方法

package ru.testScandJavaCafee.service;
public class Helper {
public String createList()
{
    System.out.println("++++");
    return "1000";
}
}

以及我改变方法的类

package ru.testScandJavaCafee.controller;

import javassist.*;
import ru.testScandJavaCafee.service.Helper;

import java.io.IOException;
import java.lang.reflect.InvocationTargetException;
import java.lang.reflect.Method;


/**
 * Created by 15 on 06.05.2017.
 */

public class CoffeeTypeController {

public String createMethodHelper() throws NotFoundException, CannotCompileException, NoSuchMethodException, InvocationTargetException, IllegalAccessException, IOException, InstantiationException, ClassNotFoundException {

    ClassPool pool = ClassPool.getDefault();
    ClassPool.getDefault().insertClassPath(new ClassClassPath(ru.testScandJavaCafee.service.Helper2.class));

    CtClass cc = pool.get("ru.testScandJavaCafee.service.Helper");

    CtMethod cm = cc.getMethod("createList","()Ljava/lang/String;" );

    cm.setBody( "{  return \"200 \" ;}" );
    cc.toClass();

    Helper helper = new Helper();
    Method method1 = helper.getClass().getMethod("createList");
    String sum = (String)method1.invoke(helper);
    return sum;
}

开始后我得到了输出

javassist.CannotCompileException: by java.lang.LinkageError: loader (instance of  org/apache/catalina/loader/WebappClassLoader): attempted  duplicate class definition for name: "ru/testScandJavaCafee/service/Helper2"
    at javassist.ClassPool.toClass(ClassPool.java:1085)
    at javassist.ClassPool.toClass(ClassPool.java:1028)
    at javassist.ClassPool.toClass(ClassPool.java:986)
    at javassist.CtClass.toClass(CtClass.java:1110)
    at ru.testScandJavaCafee.controller.CoffeeTypeController.createMethodHelper(CoffeeTypeController.java:111)
    at ru.testScandJavaCafee.controller.CoffeeTypeController.doPost(CoffeeTypeController.java:56)
    at javax.servlet.http.HttpServlet.service(HttpServlet.java:650)
    at javax.servlet.http.HttpServlet.service(HttpServlet.java:731)
    at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:303)
    at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:208)
    at org.apache.tomcat.websocket.server.WsFilter.doFilter(WsFilter.java:52)
    at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:241)
    at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:208)
    at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:218)
    at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:110)
    at org.apache.catalina.authenticator.AuthenticatorBase.invoke(AuthenticatorBase.java:506)
    at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:169)
    at org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:103)
    at org.apache.catalina.valves.AccessLogValve.invoke(AccessLogValve.java:962)
    at org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:116)
    at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:452)
    at org.apache.coyote.http11.AbstractHttp11Processor.process(AbstractHttp11Processor.java:1087)
    at org.apache.coyote.AbstractProtocol$AbstractConnectionHandler.process(AbstractProtocol.java:637)
    at org.apache.tomcat.util.net.JIoEndpoint$SocketProcessor.run(JIoEndpoint.java:318)
    at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1142)
    at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:617)
    at org.apache.tomcat.util.threads.TaskThread$WrappingRunnable.run(TaskThread.java:61)
    at java.lang.Thread.run(Thread.java:745)
Caused by: java.lang.LinkageError: loader (instance of  org/apache/catalina/loader/WebappClassLoader): attempted  duplicate class definition for name: "ru/testScandJavaCafee/service/Helper2"
    at java.lang.ClassLoader.defineClass1(Native Method)
    at java.lang.ClassLoader.defineClass(ClassLoader.java:763)
    at java.lang.ClassLoader.defineClass(ClassLoader.java:642)
    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
    at java.lang.reflect.Method.invoke(Method.java:498)
    at javassist.ClassPool.toClass2(ClassPool.java:1098)
    at javassist.ClassPool.toClass(ClassPool.java:1079)
    ... 27 more

有助于了解如何修复错误

Answer 1

你错误就在这一行：

ClassPool.getDefault().insertClassPath(new ClassClassPath(ru.testScandJavaCafee.service.Helper2.class));

首先，您已经创建了池，因此应该将类路径插入该池。其次，类路径应该是代码所在类的类名。

pool.insertClassPath(new ClassClassPath(this.getClass()));

看看documentation。关于Class search path的部分

Answer 2

使用cc.getClass();

而不是：cc.toClass();

根据我的经验，下面的行看起来像是导致问题：

cm.setBody( "{  return \"200 \" ;}" );
cc.toClass(); <<<<<<<<<<<<< // duplicate class definition here.

.toClass本质上尝试转换为类但它已经存在，这就是为什么你得到重复的类定义。

CtClass寻找“.toClass（）”

此外，我不明白为什么你这样做，因为你不使用超过该线的cc。