Java javassist 方法调用

Question

所以我将 java 代理与 javassist 一起使用，目的是将一些与监控相关的小代码注入不同类中的不同方法。

我的java代理代码：

public class ConverterAgent implements ClassFileTransformer {

public static void premain(String args, Instrumentation instrumentation){
    System.out.println(">>>>>>>>>> Intializing Java agent <<<<<<<<<<");
    ConverterAgent transformer = new ConverterAgent();
    instrumentation.addTransformer(transformer);

}

public static void agentmain(String args, Instrumentation instrumentation){
    System.out.println(">>>>>>>>>> Intializing Java agent <<<<<<<<<<");
    ConverterAgent transformer=new ConverterAgent();
    instrumentation.addTransformer(transformer); 
}


@Override
public byte[] transform(final ClassLoader loader, 
        String className, 
        Class<?> classBeingRedefined, 
        ProtectionDomain protectionDoman, 
        byte[] classFileBuffer)
                throws IllegalClassFormatException {



//javassist code goes here


return classFileBuffer;

}

}

我的 javassist 注射看起来像这样：

if ("className1".equals(className)){

//code

}


 if ("className2".equals(className)){

//same code as in first class

}


if ("className3".equals(className)){

//same code as in first and second class

}

所以我多次注入完全相同的代码，我想优化我的过程并为每次注入调用一个方法，这样我就不必一遍又一遍地复制相同的代码。但这就是我遇到问题的地方，我应该使用什么方法类型以及除了 Class 和方法名称之外它还需要什么参数。

Answer 1

将 class 字节码转换为您的转换方法，然后 return 修改“新”class 字节码。

这意味着您肯定return 是包含转换方法所需信息的字节[]。

所以你的方法应该是这样的：

public class DynamicTransformer implements ClassFileTransformer {

    public byte[] transform(ClassLoader loader, String className, Class classBeingRedefined,
    ProtectionDomain protectionDomain, byte[] classfileBuffer) throws IllegalClassFormatException {

        byte[] byteCode = classfileBuffer;

        // into the transformer will arrive every class loaded so you filter 
        // to match only what you need
        if (className.equals("com/full/path/to/be/instrumented/className1") ||
            className.equals("com/full/path/to/be/instrumented/className2") ||
            className.equals("com/full/path/to/be/instrumented/className3") ) {

            byteCode = myMethodThatTransform(className, byteCode);
        }

        return byteCode;
    }


    public byte[] myMethodThatTransform(String className, byte[] byteCode){\
        try {
            // retrive default Javassist class pool
            ClassPool cp = ClassPool.getDefault();
            // get from the class pool our class with this qualified name
            CtClass cc = cp.get(className);
            // get all the methods of the retrieved class
            CtMethod[] methods = cc.getDeclaredMethods()
            for(CtMethod meth : methods) {
                // The instrumentation code to be returned and injected
                final StringBuffer buffer = new StringBuffer();
                String name = meth.getName();
                // just print into the buffer a log for example
                buffer.append("System.out.println(\"Method " + name + " executed\" );");
                meth.insertBefore(buffer.toString())
            }
            // create the byteclode of the class
            byteCode = cc.toBytecode();
            // remove the CtClass from the ClassPool
            cc.detach();
        } catch (Exception ex) {
            ex.printStackTrace();
        }
        return byteCode;
    }
}

Java javassist 方法调用

Java javassist method calling

java

methods

code-injection

javaagents

javassist

将 class 字节码转换为您的转换方法，然后 return 修改“新”class 字节码。