详解Java中的字节码增强技术

<dependency>
    <groupId>org.ow2.asm</groupId>
    <artifactId>asm</artifactId>
    <version>9.4</version>
</dependency>

package com.xu.test;


/**
 * @author Administrator
 */
public class Main {

    public void print() {
        System.out.println("ASM");
    }

}

package com.xu.test;

import org.apache.commons.lang3.StringUtils;
import org.objectweb.asm.ClassVisitor;
import org.objectweb.asm.MethodVisitor;
import org.objectweb.asm.Opcodes;

/**
 * ASM 字节码增强技术
 *
 * @author Administrator
 */
public class CustomerClassVisitor extends ClassVisitor implements Opcodes {

    public CustomerClassVisitor(ClassVisitor api) {
        super(ASM9, api);
    }

    @Override
    public void visit(int version, int access, String name, String signature, String superName, String[] interfaces) {
        cv.visit(version, access, name, signature, superName, interfaces);
    }

    @Override
    public MethodVisitor visitMethod(int access, String name, String descriptor, String signature, String[] exceptions) {
        MethodVisitor mv = cv.visitMethod(access, name, descriptor, signature, exceptions);
        if (StringUtils.equals("print", name) && mv != null) {
            mv = new CustomerMethodVisitor(mv);
        }
        return mv;
    }

    class CustomerMethodVisitor extends MethodVisitor implements Opcodes {
        public CustomerMethodVisitor(MethodVisitor api) {
            super(ASM9, api);
        }

        @Override
        public void visitCode() {
            super.visitCode();
            mv.visitFieldInsn(GETSTATIC, "java/lang/System", "out", "Ljava/io/PrintStream;");
            mv.visitLdcInsn("start");
            mv.visitMethodInsn(INVOKEVIRTUAL, "java/io/PrintStream", "println", "(Ljava/lang/String;)V", false);
        }

        @Override
        public void visitInsn(int opcode) {
            if ((opcode >= Opcodes.IRETURN && opcode <= Opcodes.RETURN) || opcode == Opcodes.ATHROW) {
                mv.visitFieldInsn(GETSTATIC, "java/lang/System", "out", "Ljava/io/PrintStream;");
                mv.visitLdcInsn("end");
                mv.visitMethodInsn(INVOKEVIRTUAL, "java/io/PrintStream", "println", "(Ljava/lang/String;)V", false);
            }
            mv.visitInsn(opcode);
        }
    }

}

package com.xu.test;

import java.io.File;
import java.io.FileOutputStream;

import org.objectweb.asm.ClassReader;
import org.objectweb.asm.ClassVisitor;
import org.objectweb.asm.ClassWriter;

/**
 * @author Administrator
 */
public class Test {

    public static void main(String[] args) throws Exception {
        ClassReader reader = new ClassReader("com/xu/test/Main");
        ClassWriter writer = new ClassWriter(ClassWriter.COMPUTE_MAXS);
        // 处理
        ClassVisitor visitor = new CustomerClassVisitor(writer);
        reader.accept(visitor, ClassReader.SKIP_DEBUG);
        // 输出
        File file = new File("target\\classes\\com\\xu\\test\\Main.class");
        FileOutputStream stream = new FileOutputStream(file);
        stream.write(writer.toByteArray());
        stream.close();
        // 测试
        Class<?> cls = Class.forName("com.xu.test.Main");
        Main main = (Main) cls.getDeclaredConstructor().newInstance();
        main.print();
    }


}