java javassist.CannotCompileException: by java.lang.LinkageError: loader
java javassist.CannotCompileException: by java.lang.LinkageError: loader
我需要在不重新编译应用程序的情况下更改方法（计算公式）。我知道这可以在 javassist 的帮助下完成。到目前为止，我正在尝试一个简单的例子。在post方法中，我调用了createMethodHelper()方法，必须要改Helper2方法。一切都好。但是经过多次调用（重新加载页面），报错 javassist.CannotCompileException: by java.lang.LinkageError: loader
因此，class 包含我想要更改的唯一方法
package ru.testScandJavaCafee.controller;
public class Helper2 {
public String createList()
{
System.out.println("++++");
return "1000";
}
}
以及我从中更改方法的class
public String createMethodHelper() throws NotFoundException, CannotCompileException, NoSuchMethodException, InvocationTargetException, IllegalAccessException, IOException, InstantiationException, ClassNotFoundException {
ClassPool pool = ClassPool.getDefault();
pool.insertClassPath(new ClassClassPath(this.getClass()));
CtClass cc = pool.get("ru.testScandJavaCafee.controller.Helper2");
cc.defrost();
CtMethod cm = cc.getMethod("createList","()Ljava/lang/String;" );
cc.defrost();
cm.setBody( "{ return \"300 \" ;}" );
cc.defrost();
Class c = cc.toClass();
cc.defrost();
ru.testScandJavaCafee.controller.Helper2 test = (ru.testScandJavaCafee.controller.Helper2) c.newInstance();
String sum = test.createList();
return sum;
}
第二次调用（重新加载页面）后，报错
javassist.CannotCompileException: by java.lang.LinkageError: loader (instance of org/apache/catalina/loader/WebappClassLoader): attempted duplicate class definition for name: "ru/testScandJavaCafee/controller/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:108)
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/controller/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
帮助了解如何修复错误
我从 documentation and this tutorial 对此进行了一些研究。而如果你想多次修改方法，你可以这样做：
public static void main(String[] args) {
try {
// get a new instance of version 1
Object newClassInstance1 = m2("com.tima.Helper2", "public String createList() { System.out.println(\"++++\"); return \"200\";}");
Method method1 = newClassInstance1.getClass().getMethod("createList");
// this executes the createList in the new class com.tima.Helper2
String sum1 = (String) method1.invoke(newClassInstance1);
System.out.println("com.tima.Helper2: " + sum1);
// this shows that the original Helper method was not modified
Helper h = new Helper();
System.out.println("com.tima.Helper: " + h.createList());
// this shows that the com.tima.Helper2 overrides Helper and can be used as Helper with a modified method
Helper h2 = (Helper) newClassInstance1;
System.out.println("com.tima.Helper2 as Helper: " + h2.createList());
// below does the same thing a second time
Object newClassInstance2 = m2("com.tima.Helper3", "public String createList() { System.out.println(\"++++\"); return \"300\";}");
Method method2 = newClassInstance2.getClass().getMethod("createList");
String sum2 = (String) method2.invoke(newClassInstance2);
System.out.println("com.tima.Helper3: " + sum2);
Helper h3 = new Helper();
System.out.println("com.tima.Helper: " + h3.createList());
Helper h4 = (Helper) newClassInstance2;
System.out.println("com.tima.Helper3 as Helper: " + h4.createList());
} catch (Exception e) {
e.printStackTrace();
}
}
public static Object m2(String className, String methodBody)
throws CannotCompileException, InstantiationException, IllegalAccessException, NotFoundException {
// get the pool
ClassPool classPool = ClassPool.getDefault();
// this seems optional, but if it isn't Main.class (my test class) should be replaced with this.getClass()
classPool.insertClassPath(new ClassClassPath(Main.class));
// get the helper class
CtClass helperClass = classPool.get("com.tima.Helper");
// create a new class
CtClass newCtClass = classPool.makeClass(className);
// make it child of Helper
newCtClass.setSuperclass(helperClass);
// this overrides the method in Helper
newCtClass.addMethod(CtNewMethod.make(methodBody, newCtClass));
// get a new instance
Class<?> newClass = newCtClass.toClass();
Object newClassInstance = newClass.newInstance();
return newClassInstance;
}
输出
++++
com.tima.Helper2: 200
++++
com.tima.Helper: 1000
++++
com.tima.Helper2 as Helper: 200
++++
com.tima.Helper3: 300
++++
com.tima.Helper: 1000
++++
com.tima.Helper3 as Helper: 300
基本上，每次调用该方法时，它都会创建一个新的 class，使 Helper class 成为它的超级 class 并覆盖 createList 方法.这里的明显问题取决于调用此方法的次数，最终会生成大量 classes。因此，您可能希望在创建之前添加检查这些 class 是否已加载。
你说这个方法 运行s 当你刷新/加载页面时所以如果你只希望它 运行 一次，我建议你把这个代码放在一个单例中 运行s 当您的应用程序启动时。我不确定您在 Web 应用程序中使用的是什么，但是 JSF 和 Spring 都有用于此类用途的单例 bean。
我需要在不重新编译应用程序的情况下更改方法（计算公式）。我知道这可以在 javassist 的帮助下完成。到目前为止，我正在尝试一个简单的例子。在post方法中，我调用了createMethodHelper()方法，必须要改Helper2方法。一切都好。但是经过多次调用（重新加载页面），报错 javassist.CannotCompileException: by java.lang.LinkageError: loader 因此，class 包含我想要更改的唯一方法
package ru.testScandJavaCafee.controller;
public class Helper2 {
public String createList()
{
System.out.println("++++");
return "1000";
}
}
以及我从中更改方法的class
public String createMethodHelper() throws NotFoundException, CannotCompileException, NoSuchMethodException, InvocationTargetException, IllegalAccessException, IOException, InstantiationException, ClassNotFoundException {
ClassPool pool = ClassPool.getDefault();
pool.insertClassPath(new ClassClassPath(this.getClass()));
CtClass cc = pool.get("ru.testScandJavaCafee.controller.Helper2");
cc.defrost();
CtMethod cm = cc.getMethod("createList","()Ljava/lang/String;" );
cc.defrost();
cm.setBody( "{ return \"300 \" ;}" );
cc.defrost();
Class c = cc.toClass();
cc.defrost();
ru.testScandJavaCafee.controller.Helper2 test = (ru.testScandJavaCafee.controller.Helper2) c.newInstance();
String sum = test.createList();
return sum;
}
第二次调用（重新加载页面）后，报错
javassist.CannotCompileException: by java.lang.LinkageError: loader (instance of org/apache/catalina/loader/WebappClassLoader): attempted duplicate class definition for name: "ru/testScandJavaCafee/controller/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:108)
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/controller/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
帮助了解如何修复错误
我从 documentation and this tutorial 对此进行了一些研究。而如果你想多次修改方法，你可以这样做：
public static void main(String[] args) {
try {
// get a new instance of version 1
Object newClassInstance1 = m2("com.tima.Helper2", "public String createList() { System.out.println(\"++++\"); return \"200\";}");
Method method1 = newClassInstance1.getClass().getMethod("createList");
// this executes the createList in the new class com.tima.Helper2
String sum1 = (String) method1.invoke(newClassInstance1);
System.out.println("com.tima.Helper2: " + sum1);
// this shows that the original Helper method was not modified
Helper h = new Helper();
System.out.println("com.tima.Helper: " + h.createList());
// this shows that the com.tima.Helper2 overrides Helper and can be used as Helper with a modified method
Helper h2 = (Helper) newClassInstance1;
System.out.println("com.tima.Helper2 as Helper: " + h2.createList());
// below does the same thing a second time
Object newClassInstance2 = m2("com.tima.Helper3", "public String createList() { System.out.println(\"++++\"); return \"300\";}");
Method method2 = newClassInstance2.getClass().getMethod("createList");
String sum2 = (String) method2.invoke(newClassInstance2);
System.out.println("com.tima.Helper3: " + sum2);
Helper h3 = new Helper();
System.out.println("com.tima.Helper: " + h3.createList());
Helper h4 = (Helper) newClassInstance2;
System.out.println("com.tima.Helper3 as Helper: " + h4.createList());
} catch (Exception e) {
e.printStackTrace();
}
}
public static Object m2(String className, String methodBody)
throws CannotCompileException, InstantiationException, IllegalAccessException, NotFoundException {
// get the pool
ClassPool classPool = ClassPool.getDefault();
// this seems optional, but if it isn't Main.class (my test class) should be replaced with this.getClass()
classPool.insertClassPath(new ClassClassPath(Main.class));
// get the helper class
CtClass helperClass = classPool.get("com.tima.Helper");
// create a new class
CtClass newCtClass = classPool.makeClass(className);
// make it child of Helper
newCtClass.setSuperclass(helperClass);
// this overrides the method in Helper
newCtClass.addMethod(CtNewMethod.make(methodBody, newCtClass));
// get a new instance
Class<?> newClass = newCtClass.toClass();
Object newClassInstance = newClass.newInstance();
return newClassInstance;
}
输出
++++
com.tima.Helper2: 200
++++
com.tima.Helper: 1000
++++
com.tima.Helper2 as Helper: 200
++++
com.tima.Helper3: 300
++++
com.tima.Helper: 1000
++++
com.tima.Helper3 as Helper: 300
基本上，每次调用该方法时，它都会创建一个新的 class，使 Helper class 成为它的超级 class 并覆盖 createList 方法.这里的明显问题取决于调用此方法的次数，最终会生成大量 classes。因此，您可能希望在创建之前添加检查这些 class 是否已加载。
你说这个方法 运行s 当你刷新/加载页面时所以如果你只希望它 运行 一次，我建议你把这个代码放在一个单例中 运行s 当您的应用程序启动时。我不确定您在 Web 应用程序中使用的是什么，但是 JSF 和 Spring 都有用于此类用途的单例 bean。