如何实现Java底层技术之字节码操作与ASM框架

import org.objectweb.asm.ClassWriter;
import org.objectweb.asm.MethodVisitor;
import org.objectweb.asm.Opcodes;

public class HelloWorldGenerator {

    public static void main(String[] args) {
        // 1. 创建一个ClassWriter，用于生成字节码
        ClassWriter cw = new ClassWriter(0);

        // 2. 创建一个类
        String className = "HelloWorld";
        cw.visit(Opcodes.V1_8, Opcodes.ACC_PUBLIC, className, null, "java/lang/Object", null);

        // 3. 添加一个无参的构造方法
        MethodVisitor constructor = cw.visitMethod(Opcodes.ACC_PUBLIC, "<init>", "()V", null, null);
        constructor.visitCode();
        constructor.visitVarInsn(Opcodes.ALOAD, 0);
        constructor.visitMethodInsn(Opcodes.INVOKESPECIAL, "java/lang/Object", "<init>", "()V", false);
        constructor.visitInsn(Opcodes.RETURN);
        constructor.visitMaxs(1, 1);
        constructor.visitEnd();

        // 4. 添加一个名为"sayHello"的方法
        MethodVisitor method = cw.visitMethod(Opcodes.ACC_PUBLIC + Opcodes.ACC_STATIC, "sayHello", "()V", null, null);
        method.visitCode();
        method.visitFieldInsn(Opcodes.GETSTATIC, "java/lang/System", "out", "Ljava/io/PrintStream;");
        method.visitLdcInsn("Hello, World!");
        method.visitMethodInsn(Opcodes.INVOKEVIRTUAL, "java/io/PrintStream", "println", "(Ljava/lang/String;)V", false);
        method.visitInsn(Opcodes.RETURN);
        method.visitMaxs(2, 0);
        method.visitEnd();

        // 5. 字节码生成完毕，获取最终的字节数组
        byte[] byteCode = cw.toByteArray();

        // 6. 使用自定义的类加载器加载并执行生成的字节码
        ClassLoader cl = new MyClassLoader();
        Class<?> helloWorldClass = cl.defineClass(className, byteCode);
        try {
            helloWorldClass.getDeclaredMethod("sayHello").invoke(null);
        } catch (Exception e) {
            e.printStackTrace();
        }
    }

    static class MyClassLoader extends ClassLoader {
        public Class<?> defineClass(String name, byte[] b) {
            return defineClass(name, b, 0, b.length);
        }
    }
}