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Problems with the tomcat classloader
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created by <a href="http://community.jboss.org/people/roli82001">Roland Käser</a> in <i>Javassist</i> - <a href="http://community.jboss.org/message/639418#639418">View the full discussion</a>
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<div class="jive-rendered-content"><p>Hello </p><p style="min-height: 8pt; height: 8pt; padding: 0px;"> </p><p>While developing a very small and lightweight j2ee linke server, I try to extend a abstract class at runtime using: </p><p style="min-height: 8pt; height: 8pt; padding: 0px;"> </p><p>ClassPool pool = ClassPool.getDefault();</p><p>                              pool.insertClassPath(new ClassClassPath(this.getClass()));</p><p>                              CtClass cc = pool.makeClass("HelloWorldClientBean"); </p><p>                              cc.setSuperclass(pool.get("ch.isn.dante.comserver.test.HelloWorldSessionBean"));</p><p style="min-height: 8pt; height: 8pt; padding: 0px;"> </p><p>Class beanClass = cc.toClass();</p><p>cc.defrost();</p><p style="min-height: 8pt; height: 8pt; padding: 0px;"> </p><p>Constructor constructor = beanClass.getConstructor(argsClass);</p><p>  </p><p>Object[] params = new Object[]{new ComServerSession(),null};   </p><p>HelloWorldSessionBean sessionBean = (HelloWorldSessionBean)constructor.newInstance(params); </p><p>System.out.println("Hello World Output: " + sessionBean.getHelloWorld());</p><p style="min-height: 8pt; height: 8pt; padding: 0px;"> </p><p>This works fine on the client side (regular java vm). </p><p style="min-height: 8pt; height: 8pt; padding: 0px;"> </p><p>On the server side I get first a very strange exception which was stangely related to the tomcats classloader. </p><p style="min-height: 8pt; height: 8pt; padding: 0px;"> </p><p>javassist.CannotCompileException: by java.lang.LinkageError: loader (instance of  org/apache/catalina/loader/WebappClassLoader): attempted  duplicate class definition for name: "ch/isn/dante/comserver/test/HelloWorldSessionBeanExecutable"</p><p>javassist.CannotCompileException: by java.lang.LinkageError: loader (instance of  org/apache/catalina/loader/WebappClassLoader): attempted  duplicate class definition for name: "ch/isn/dante/comserver/test/HelloWorldSessionBeanExecutable"</p><p>        at javassist.ClassPool.toClass(ClassPool.java:1089)</p><p>        at javassist.ClassPool.toClass(ClassPool.java:1032)</p><p>        at javassist.ClassPool.toClass(ClassPool.java:990)</p><p>        at javassist.CtClass.toClass(CtClass.java:1125)</p><p>...</p><p>Caused by: java.lang.LinkageError: loader (instance of  org/apache/catalina/loader/WebappClassLoader): attempted  duplicate class definition for name: "ch/isn/dante/comserver/test/HelloWorldSessionBeanExecutable"</p><p>        at java.lang.ClassLoader.defineClass1(Native Method)</p><p>        at java.lang.ClassLoader.defineClassCond(ClassLoader.java:631)</p><p>        at java.lang.ClassLoader.defineClass(ClassLoader.java:615)</p><p>        at java.lang.ClassLoader.defineClass(ClassLoader.java:465)</p><p>        at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)</p><p>        at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:39)</p><p>        at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:25)</p><p>        at java.lang.reflect.Method.invoke(Method.java:597)</p><p>        at javassist.ClassPool.toClass2(ClassPool.java:1102)</p><p>        at javassist.ClassPool.toClass(ClassPool.java:1083)</p><p style="min-height: 8pt; height: 8pt; padding: 0px;"> </p><p style="min-height: 8pt; height: 8pt; padding: 0px;"> </p><p style="min-height: 8pt; height: 8pt; padding: 0px;"> </p><p>To prevent this I wrote a very ugly hack to use the system class loader and add the classpath in which this superclass:</p><p style="min-height: 8pt; height: 8pt; padding: 0px;"> </p><table><tbody><tr><td style=";"><br/></td><td style=";"><br/></td><td style=";"><br/></td><td style=";">URL url = new File(serverServlet.getInstallDir() + "WEB-INF/classes/").toURL();</td></tr><tr><td style=";"><br/></td><td style=";"><br/></td><td style=";"><br/></td><td style=";">URLClassLoader sysloader = (URLClassLoader) ClassLoader.getSystemClassLoader();</td></tr><tr><td style=";"><br/></td><td style=";"><br/></td><td style=";"><br/></td><td style=";"> Class sysclass = URLClassLoader.class;</td></tr><tr><td style=";"><br/></td><td style=";"><br/></td><td style=";"><br/></td><td style=";"><br/></td></tr><tr><td style=";"><br/></td><td style=";"><br/></td><td style=";"><br/></td><td style=";"> Method method = sysclass.getDeclaredMethod("addURL", parameters);</td></tr><tr><td style=";"><br/></td><td style=";"><br/></td><td style=";"><br/></td><td style=";"> method.setAccessible(true);</td></tr><tr><td style=";"><br/></td><td style=";"><br/></td><td style=";"><br/></td><td style=";"> method.invoke(sysloader, new Object[]{url});</td></tr></tbody></table><p> and created the class using this modified classloader:</p><p style="min-height: 8pt; height: 8pt; padding: 0px;"> </p><table><tbody><tr><td style=";"><br/></td><td style=";"><br/></td><td style=";">Class beanClass = cc.toClass(sysloader);  </td><td style=";"><br/></td><td style=";"><br/></td><td style=";"><br/></td></tr><tr><td style=";"><br/></td><td style=";"><br/></td><td style=";">cc.defrost();</td></tr></tbody></table><p style="min-height: 8pt; height: 8pt; padding: 0px;"> </p><p style="min-height: 8pt; height: 8pt; padding: 0px;"> </p><p>Now I get a NoSuchMethodException:</p><p style="min-height: 8pt; height: 8pt; padding: 0px;"> </p><p>java.lang.NoSuchMethodException: HelloWorldServerBean.<init>(ch.isn.dante.comserver.server.ComServerSession, ch.isn.dante.comserver.server.ServerApplication)</p><p>        at java.lang.Class.getConstructor0(Class.java:2706)</p><p>        at java.lang.Class.getConstructor(Class.java:1657)</p><p>        at ch.isn.dante.comserver.server.ComServerSession.loadClass(ComServerSession.java:239)</p><p>        at ch.isn.dante.comserver.server.ComServer.doGet(ComServer.java:286)</p><p>        at ch.isn.dante.comserver.server.ComServer.doPost(ComServer.java:417)</p><p>        at javax.servlet.http.HttpServlet.service(HttpServlet.java:647)</p><p>        at javax.servlet.http.HttpServlet.service(HttpServlet.java:729)</p><p style="min-height: 8pt; height: 8pt; padding: 0px;"> </p><p>I can't see from which this error comes. When I  inspect the newly created class I get one constructor:</p><p style="min-height: 8pt; height: 8pt; padding: 0px;"> </p><p>public HelloWorldClientBean(ch.isn.dante.comserver.server.ComServerSession,ch.isn.dante.comserver.server.ServerApplication)</p></div>
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