转换类无效

问题描述 投票：5回答：1

基于本教程，我尝试使Java代理正常工作。https://www.baeldung.com/java-instrumentation#loading-a-java-agent

我得到了[Agent] Transforming class TestApplication我没有错误，但是看不到转换类的任何效果。

最终，我希望同时使静态负载和动态负载都能工作，但现在我只关注静态方式。


public class Static_Agent {

    public static void premain(String agentArgs, Instrumentation inst) {
        String[] tokens = agentArgs.split(";");
        String className = tokens[0];
        String methodName = tokens[1];

        System.out.println(">> "+className);
        System.out.println(">> "+methodName);
        transformClass(className, methodName, inst);
    }



    public static void transformClass(String className, String methodName, Instrumentation instrumentation) {
        Class<?> targetCls = null;
        ClassLoader targetClassLoader = null;
        // see if we can get the class using forName
        try {
            targetCls = Class.forName(className);
            targetClassLoader = targetCls.getClassLoader();
            transform(targetCls, methodName, targetClassLoader, instrumentation);
            return;
        } catch (Exception ex) {
            ex.printStackTrace();
        }
        // otherwise iterate all loaded classes and find what we want
        for(Class<?> clazz: instrumentation.getAllLoadedClasses()) {
            if(clazz.getName().equals(className)) {
                targetCls = clazz;
                targetClassLoader = targetCls.getClassLoader();
                transform(targetCls, methodName, targetClassLoader, instrumentation);
                return;
            }
        }
        throw new RuntimeException("Failed to find class [" + className + "]");
    }


    public static void transform(Class<?> clazz, String methodName, ClassLoader classLoader, Instrumentation instrumentation) {
        Transformer dt = new Transformer(clazz.getName(), methodName, classLoader);
        instrumentation.addTransformer(dt, true);
        try {
            instrumentation.retransformClasses(clazz);
        } catch (Exception ex) {
            throw new RuntimeException("Transform failed for class: [" + clazz.getName() + "]", ex);
        }
    }



}

public class Transformer implements ClassFileTransformer {


    /** The internal form class name of the class to transform */
    private String targetClassName;
    /** The class loader of the class we want to transform */
    private ClassLoader targetClassLoader;

    private String targetMethodName;

    public Transformer(String targetClassName, String targetMethodName, ClassLoader targetClassLoader) {
        this.targetClassName = targetClassName;
        this.targetClassLoader = targetClassLoader;
        this.targetMethodName = targetMethodName;
    }

    @Override
    public byte[] transform(ClassLoader loader, String className, Class<?> classBeingRedefined,
            ProtectionDomain protectionDomain, byte[] classfileBuffer) throws IllegalClassFormatException {
        byte[] byteCode = classfileBuffer;

        String finalTargetClassName = this.targetClassName.replaceAll("\\.", "/");
        if (!className.equals(finalTargetClassName)) {
            return byteCode;
        }

        if (className.equals(finalTargetClassName) && loader.equals(targetClassLoader)) {
            System.out.println("[Agent] Transforming class TestApplication");
            try {

                ClassPool cp = ClassPool.getDefault();
                CtClass cc = cp.get(targetClassName);
                CtMethod m = cc.getDeclaredMethod(targetMethodName);
                m.addLocalVariable("startTime", CtClass.longType);
                m.insertBefore("startTime = System.currentTimeMillis();");

                StringBuilder endBlock = new StringBuilder();

                m.addLocalVariable("endTime", CtClass.longType);
                m.addLocalVariable("opTime", CtClass.longType);
                endBlock.append("endTime = System.currentTimeMillis();");
                endBlock.append("opTime = (endTime-startTime)/1000;");

                endBlock.append("System.out.println(\"[Application] Withdrawal operation completed in:\" + opTime + \" seconds!\");");

                m.insertAfter(endBlock.toString());

                byteCode = cc.toBytecode();
                cc.detach();
            } catch (Exception e) {
                System.out.println("Exception"+e);
            }
        }
        return byteCode;
    }
}

public class TestApplication {

    public static void main(String[] args) {

        try {
            TestApplication.run();
        } catch (Exception e) {
            e.printStackTrace();
        }
    }


    public static void run() throws Exception {
        System.out.println("--- start ---");

        while (true) {
            test();
            Thread.sleep(4_000);
        }


    }


    static int count = 0;

    public static void test() {
        System.out.println(count++);
    }

}

我启动时带有：

java -javaagent:static_agent.jar="doeke.application.TestApplication;test" -jar application.jar

如果有帮助，项目在这里：https://github.com/clankill3r/java_agent

java agent javassist javaagents java-attach-api
1个回答
5
投票

感谢提出这个问题，让我有机会看一下Java Instrumentation。

[花了一些时间来交叉检查您的示例代码和提供的教程。问题不在于编程代码，而在于如何启动程序。

如果在Transformer.java中将某些记录器添加到transform（）方法中，则会在运行后发现代码路径已损坏：

ClassPool cp = ClassPool.getDefault();

并且，用相同的方法从以下位置替换异常捕获代码后：

} catch (Exception e) {

到：

} catch (NotFoundException | CannotCompileException | IOException e) {

它将给您更多提示，如下：

Exception in thread "main" java.lang.reflect.InvocationTargetException
        at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
        at sun.reflect.NativeMethodAccessorImpl.invoke(Unknown Source)
        at sun.reflect.DelegatingMethodAccessorImpl.invoke(Unknown Source)
        at java.lang.reflect.Method.invoke(Unknown Source)
        at sun.instrument.InstrumentationImpl.loadClassAndStartAgent(Unknown Source)
        at sun.instrument.InstrumentationImpl.loadClassAndCallPremain(Unknown Source)
Caused by: java.lang.NoClassDefFoundError: javassist/NotFoundException
        at doeke.static_agent.Static_Agent.transform(Static_Agent.java:60)
        at doeke.static_agent.Static_Agent.transformClass(Static_Agent.java:40)
        at doeke.static_agent.Static_Agent.premain(Static_Agent.java:28)
        ... 6 more
Caused by: java.lang.ClassNotFoundException: javassist.NotFoundException
        at java.net.URLClassLoader.findClass(Unknown Source)
        at java.lang.ClassLoader.loadClass(Unknown Source)
        at sun.misc.Launcher$AppClassLoader.loadClass(Unknown Source)
        at java.lang.ClassLoader.loadClass(Unknown Source)
        ... 9 more
FATAL ERROR in native method: processing of -javaagent failed

至此，根本原因更加明显。这是因为在启动程序时，那些与Javassist相关的类（例如ClassPool，CtClass，CtMethod等）在运行时无法引用其相应的库。

所以，解决方案是：

  1. 假设您已将static_agent.jar导出到与application.jar相同的“ build”文件夹中

  2. 所有其他文件夹结构与您提供的github中显示的相同

  3. 将命令的cd放入命令控制台中的build文件夹

  4. 如下修改原始程序启动脚本

Windows操作系统：

java -javaagent:static_agent.jar="doeke.application.TestApplication;test" -cp ../libs/javassist-3.12.1.GA.jar;application.jar doeke.application.TestApplication

Unix / Linux操作系统：

java -javaagent:static_agent.jar="doeke.application.TestApplication;test" -cp ../libs/javassist-3.12.1.GA.jar:application.jar doeke.application.TestApplication

您最终将获得预期的结果：

[Agent] In premain method.
>> doeke.application.TestApplication
>> test
[Agent] Transforming class
--- start ---
0
[Application] Withdrawal operation completed in:0 seconds!
1
[Application] Withdrawal operation completed in:0 seconds!
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