Javassist：如何将代理添加到类路径？

Question

我有一个代理，我正在动态地加载到一个正在运行的Java应用程序中，它在附加时打开一个简单的Swing JFrame。它还允许将新行添加到JFrame内部的文本区中。

我的目标是改变一些方法在加载代理的应用程序内部的工作方式。

public class MyAgent {
    public static void agentmain(String args, Instrumentation instrumentation) {
        UI.openWindow();
        UI.addMessage("Agent loaded: %s", args);
        instrumentation.addTransformer(new MyTransformer());
        instrumentation.redefineClasses(new ClassDefinition(Class.forName("app.TargetClass"), ...));
    }
}

UI窗口在可从代理访问的另一个类中管理。它成功打开窗口,并在加载代理时追加文本消息。

public class UI {
    private static SwingWindow swingWindow;
    
    public static void addMessage(String format, Object... args) {
        System.out.println("UI: " + String.format(format, args));
        swingWindow.appendToTextArea(format, args);
    }
    
    public static void openWindow() {
        try {
            SwingUtilities.invokeAndWait(() -> swingWindow = new SwingWindow());
        }
        catch (Exception e) {}
    }
}

public class MyTransformer implements ClassFileTransformer {
    @Override
    public byte[] transform(ClassLoader loader, String className, ..., byte[] classBuffer) {
        if (className.equals("app/TargetClass")) {
            UI.addMessage("Now transforming the class that I need!");

            try {
                ClassPool classPool = ClassPool.getDefault();
                CtClass targetClass = classPool.get("app.TargetClass");
                CtMethod targetMethod = targetClass.getDeclaredMethod("importantMethod");
                targetMethod.insertBefore("me.domain.agent.ui.UI.addMessage(\"Hello from Javassist!\")");
                byte[] byteCode = targetClass.toBytecode();
                targetClass.detach();
                return byteCode;
            }
            catch (Exception e) {
                UI.addMessage("Couldn't transform the class I needed.");
            }
        }

        return classBuffer;
    }
}

UI：转换类App.TargetClass:[源错误]没有此类：ME$Domain.Agent.UI.UI

但是,UI类在代理内部：

agent.jar
├── META-INF
└── me.domain.agent
    ├── ui
    │   └── UI.class
    ├── MyTransformer.class
    └── Agent.class

我尝试将代理的类加载器添加到Javassist的类池中：

classPool.insertClassPath(new LoaderClassPath(Agent.class.getClassLoader()));

但不管用。如何将对代理UI的调用添加到字节码中？

Answer 1

我决定使用ASM从字节码调用代理UI。找到类没有问题。

下面是基于ASM的类转换器的样子：

public class MyTransformer implements ClassFileTransformer {
    public void transformClass(ClassNode classNode) {
        MethodNode methodNode = findMethodNodeOfClass(classNode, "importantMethod", "()V");
        if (methodNode == null) {
            throw new TransformerException("app.TargetClass#importantMethod not found");
        }

        AbstractInsnNode firstInsn = findFirstInstruction(methodNode);
        if (firstInsn == null) {
            throw new TransformerException("No instructions in app.TargetClass#importantMethod");
        }

        InsnList insnList = new InsnList();
        insnList.add(new LdcInsnNode("Hello from ASM!"));
        insnList.add(new MethodInsnNode(INVOKESTATIC, Type.getInternalName(UI.class), "addMessage", "(Ljava/lang/String;)V"));
        methodNode.instructions.insertBefore(firstInsn, insnList);
    }

    @Override
    public byte[] transform(ClassLoader classLoader, String className, Class<?> classBeingRedefined, ProtectionDomain protectionDomain, byte[] byteCode) {
        if (className.equals("app/TargetClass")) {
            try {
                ClassNode classNode = new ClassNode();
                ClassReader classReader = new ClassReader(byteCode);
                classReader.accept(classNode, 0);
                this.transformClass(classNode);
                ClassWriter classWriter = new ClassWriter(ClassWriter.COMPUTE_MAXS | ClassWriter.COMPUTE_FRAMES);
                classNode.accept(classWriter);
                return classWriter.toByteArray();
            }
            catch (Exception e) {
                e.printStackTrace();
            }
        }

        return byteCode;
    }
}

这将在app.TargetClass#ImportMethod的第一个非标签指令之前插入静态调用ui.addMessage（“Hello from ASM！”）。