Question

因此，使用下面的代码，我的输出是：

Starting application with the Agent
Visiting class: HelloWorld
Class Major Version: 51
Super class: java/lang/Object
Source: HelloWorld.java
Method: <init> desc = ()V cv = com.amir.agent.instrumentor.amirClassVisitor$1@79f1d448 and mv = null
Method: main desc = ([Ljava/lang/String;)V cv = com.amir.agent.instrumentor.amirClassVisitor$1@79f1d448 and mv = null
Method: foo desc = ()V cv = com.amir.agent.instrumentor.amirClassVisitor$1@79f1d448 and mv = null
Method ends here
Done instrumenting: HelloWorld

这让我很困惑。为什么我的methodVisitor会null？当null为methodVisitor时，ASM源代码似乎仅返回classVisitor的{{1}}，这在我的情况下是不正确的。

null

编辑：

我将此代码称为：

package com.amir.agent.instrumentor;

import org.objectweb.asm.AnnotationVisitor;
import org.objectweb.asm.Attribute;
import org.objectweb.asm.ClassReader;
import org.objectweb.asm.ClassVisitor;
import org.objectweb.asm.FieldVisitor;
import org.objectweb.asm.MethodVisitor;
import org.objectweb.asm.Opcodes;

public class amirClassVisitor {

    private byte[] outData = null;

    public amirClassVisitor() {
    }

    public void performInstrumentation(final String className,
                                       final byte[] classAsBytes) {
        final ClassVisitor cl = new ClassVisitor(Opcodes.ASM4) {
            @Override
            public void visit(final int version,
                              final int access,
                              final String name,
                              final String signature,
                              final String superName,
                              final String[] interfaces) {
                System.out.println("Visiting class: "+name);
                System.out.println("Class Major Version: "+version);
                System.out.println("Super class: " + superName);
                super.visit(version, access, name, signature, superName, interfaces);
            }

            @Override
            public void visitOuterClass(final String owner,
                                        final String name,
                                        final String desc) {
                System.out.println("Outer class: "+owner);
                super.visitOuterClass(owner, name, desc);
            }

            @Override
            public AnnotationVisitor visitAnnotation(final String desc,
                                                     final boolean visible) {
                System.out.println("Annotation: "+desc);
                return super.visitAnnotation(desc, visible);
            }

            @Override
            public void visitAttribute(final Attribute attr) {
                System.out.println("Class Attribute: " + attr.type);
                super.visitAttribute(attr);
            }

            @Override
            public void visitInnerClass(final String name,
                                        final String outerName,
                                        final String innerName,
                                        final int access) {
                System.out.println("Inner Class: " + innerName + " defined in " + outerName);
                super.visitInnerClass(name, outerName, innerName, access);
            }

            @Override
            public FieldVisitor visitField(final int access,
                                           final String name,
                                           final String desc,
                                           final String signature,
                                           final Object value) {
                System.out.println("Field: "+name+" "+desc+" value:"+value);
                return super.visitField(access, name, desc, signature, value);
            }

            @Override
            public void visitEnd() {
                System.out.println("Method ends here");
                super.visitEnd();
            }

            @Override
            public MethodVisitor visitMethod(final int access,
                                             final String name,
                                             final String desc,
                                             final String signature,
                                             final String[] exceptions) {
                final MethodVisitor mv = super.visitMethod(access, name, desc, signature, exceptions);
                System.out.println("Method: " +name+ " desc = " +desc+ " cv = " +this+ " and mv = " +mv);
                return mv;
            }

            @Override
            public void visitSource(final String source,
                                    final String debug) {
                System.out.println("Source: "+source);
                super.visitSource(source, debug);
            }

        };

        final ClassReader classReader = new ClassReader(classAsBytes);
        classReader.accept(cl, 0);
        System.out.println("Done instrumenting: " +className);

    }

    public byte[] result() {
        return outData;
    }

}

Answer 1

您打印的内容不是“我的methodVisitor”，而是super visitMethod调用返回的值，换句话说是ClassVisitor.visitMethod返回的默认值：

<强>返回：
访问方法字节代码的对象，如果此类访问者对访问此方法的代码不感兴趣，则为null。

由于抽象类ClassVisitor没有实现任何操作，因此它没有兴趣访问该方法，因此null是完美的返回值。

因此，您可以实例化实现所需行为的新MethodVisitor。或者，由于您似乎打算对课程进行检测，因此请让您的班级访问者委托给您传递给ClassWriter的super class constructor。这样，您将继承可以自定义的复制行为。然后，super.visitMethod将返回非null方法访问者，该访问者将复制该方法。

使用ASM的Java字节码检测，MethodVisitor为null

1 个答案: