Java ASM 编辑已由 JVM 加载的类/方法

Question

我目前正在尝试在 JVM 加载 Java 类之后编辑它的字节码。

我使用 Java 8 和 ASM 5.0.3。 我无法更改命令行或 JVM 参数。

这是我正在尝试做的一个最小示例：

import org.objectweb.asm.*;

// Is in a library
class ExampleObject {
    public void exampleMethod(Object o) {
        System.out.println("Body " + o);
    }
}

public class Example {

    // Comes from the same library
    public static final ExampleObject exampleObject = new ExampleObject();

    public static void main(String[] args) {

        exampleObject.exampleMethod(1);
        // Output:
        // Body 1

        injectAtHead();

        exampleObject.exampleMethod(2);
        // Output:
        // Head 2
        // Body 2
    }

    public static void injectAtHead() {
        // Inject methodToInject at the head of exampleObject#exampleMethod
    }

    public static void methodToInject(Object o) {
        System.out.println("Head " + o);
    }
}

经过大量研究，我发现了很多关于使用 ASM 动态修改字节码的主题。 问题是他们都讨论在 JVM 加载类之前修改它的字节码。 所以，我不知道如何才能做到这一点，甚至不知道是否有可能。

Answer 1

这个评论几乎回答了问题。

经过几次适应，最初问题的解决方案是：

import net.bytebuddy.agent.ByteBuddyAgent;
import org.objectweb.asm.*;

import java.lang.instrument.ClassFileTransformer;
import java.lang.instrument.Instrumentation;
import java.security.ProtectionDomain;

import static net.bytebuddy.jar.asm.Opcodes.*;

public class Example {

    // Also comes from a library
    public static final ExampleObject exampleObject = new ExampleObject();

    public static void main(String[] args) {

        exampleObject.exampleMethod(1);
        // Output:
        // Body 1

        injectAtHead();

        exampleObject.exampleMethod(2);
        // Output:
        // Head 2
        // Body 2
    }

    public static void injectAtHead() {
        try {
            ByteBuddyAgent.install();
            Instrumentation instrumentation = ByteBuddyAgent.getInstrumentation();
            instrumentation.addTransformer(new SimpleClassFileTransformer(), true);

            instrumentation.retransformClasses(ExampleObject.class);
        } catch (Exception e) {
            e.printStackTrace();
        }
    }



    public static void methodToInject(Object o) {
        System.out.println("Head " + o);
    }

}


class ModifierMethodWriter extends MethodVisitor {

    private String methodName;

    public ModifierMethodWriter(int api, MethodVisitor mv, String methodName) {
        super(api, mv);
        this.methodName = methodName;
    }

    // This is the point we insert the code. Note that the instructions are
    // added right after
    // the visitCode method of the super class. This ordering is very
    // important.
    @Override
    public void visitCode() {
        // invoke methodToInject
        super.visitCode();
        super.visitVarInsn(ALOAD, 1);
        super.visitMethodInsn(INVOKESTATIC, "Example", "methodToInject", "(Ljava/lang/Object;)V", false);
    }

}

// Our class modifier class visitor. It delegate all calls to the super
// class
// Only makes sure that it returns our MethodVisitor for every method
class ModifierClassWriter extends ClassVisitor {
    private int api;

    public ModifierClassWriter(int api, ClassWriter cv) {
        super(api, cv);
        this.api = api;
    }

    @Override
    public MethodVisitor visitMethod(int access, String name, String desc, String signature, String[] exceptions) {

        if (!name.equals("exampleMethod")){
            return super.visitMethod(access, name, desc, signature, exceptions);
        } else {
            MethodVisitor mv = super.visitMethod(access, name, desc, signature, exceptions);
            ModifierMethodWriter mvw = new ModifierMethodWriter(api, mv, name);
            return mvw;
        }
    }

}

class SimpleClassFileTransformer implements ClassFileTransformer {

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

        if (!className.equals("ExampleObject")) {
            return classfileBuffer;
        }
        else {
            ClassReader classReader = new ClassReader(classfileBuffer);

            final ClassWriter cw = new ClassWriter(classReader, ClassWriter.COMPUTE_FRAMES | ClassWriter.COMPUTE_MAXS);

            // Wrap the ClassWriter with our custom ClassVisitor
            ModifierClassWriter mcw = new ModifierClassWriter(Opcodes.ASM5, cw);
            classReader.accept(mcw, 0);

            byte[] byteArray = cw.toByteArray();

            return byteArray;
        }
    }

}