title: Javassist, java
body: <p>I'm doing a task to create a java-application. The task is to change the method (calculation formula) without recompiling the application. After searching in Google, I realized that you can solve the problem with the help of javassist. So far I'm trying on a simple example. 
  So, the class containing the only method I want to change</p>
  
  <pre><code>package ru.testScandJavaCafee.service;
  public class Helper {
  public String createList()
  {
      System.out.println("++++");
      return "1000";
  }
  }
  </code></pre>
  
  <p>And the class from which I change the method</p>
  
  <pre><code>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;
  }
  </code></pre>
  
  <p>After starting I get the output</p>
  
  <pre><code>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
  </code></pre>
  
  <p>help to understand how to fix the error</p>