import java.io.File;

import java.io.FileNotFoundException;

import java.io.FileOutputStream;

import java.io.IOException;

import org.objectweb.asm.ClassWriter;

import org.objectweb.asm.Opcodes;

public class GeneratorClass {

    public static void main(String[] args) throws IOException {

        //生成一个类只需要ClassWriter组件即可

        ClassWriter cw = new ClassWriter(0);

        //通过visit方法确定类的头部信息

        cw.visit(Opcodes.V1_5, Opcodes.ACC_PUBLIC+Opcodes.ACC_ABSTRACT+Opcodes.ACC_INTERFACE,

                "com/asm3/Comparable", null, "java/lang/Object", new String[]{"com/asm3/Mesurable"});

        //定义类的属性

        cw.visitField(Opcodes.ACC_PUBLIC+Opcodes.ACC_FINAL+Opcodes.ACC_STATIC,

                "LESS", "I", null, new Integer(-1)).visitEnd();

        cw.visitField(Opcodes.ACC_PUBLIC+Opcodes.ACC_FINAL+Opcodes.ACC_STATIC,

                "EQUAL", "I", null, new Integer(0)).visitEnd();

        cw.visitField(Opcodes.ACC_PUBLIC+Opcodes.ACC_FINAL+Opcodes.ACC_STATIC,

                "GREATER", "I", null, new Integer(1)).visitEnd();

        //定义类的方法

        cw.visitMethod(Opcodes.ACC_PUBLIC+Opcodes.ACC_ABSTRACT, "compareTo",

                "(Ljava/lang/Object;)I", null, null).visitEnd();

        cw.visitEnd(); //使cw类已经完成

        //将cw转换成字节数组写到文件里面去

        byte[] data = cw.toByteArray();

        File file = new File("D://Comparable.class");

        FileOutputStream fout = new FileOutputStream(file);

        fout.write(data);

        fout.close();

    }

}

import javassist.*;

public class CreatePerson {

    public static void createPseson() throws Exception {

        ClassPool pool = ClassPool.getDefault();

        // 1. 创建一个空类

        CtClass cc = pool.makeClass("com.test.javassist.Person");

        // 2. 新增一个字段 private String name;

        // 字段名为name

        CtField param = new CtField(pool.get("java.lang.String"), "name", cc);

        // 访问级别是 private

        param.setModifiers(Modifier.PRIVATE);

        // 初始值是 "xiaoming"

        cc.addField(param, CtField.Initializer.constant("xiaoming"));

        // 3. 生成 getter、setter 方法

        cc.addMethod(CtNewMethod.setter("setName", param));

        cc.addMethod(CtNewMethod.getter("getName", param));

        // 4. 添加无参的构造函数

        CtConstructor cons = new CtConstructor(new CtClass[]{}, cc);

        cons.setBody("{name = \"xiaohong\";}");

        cc.addConstructor(cons);

        // 5. 添加有参的构造函数

        cons = new CtConstructor(new CtClass[]{pool.get("java.lang.String")}, cc);

        // $0=this / $1,$2,$3... 代表方法参数

        cons.setBody("{$0.name = $1;}");

        cc.addConstructor(cons);

        // 6. 创建一个名为printName方法，无参数，无返回值，输出name值

        CtMethod ctMethod = new CtMethod(CtClass.voidType, "printName", new CtClass[]{}, cc);

        ctMethod.setModifiers(Modifier.PUBLIC);

        ctMethod.setBody("{System.out.println(name);}");

        cc.addMethod(ctMethod);

        //这里会将这个创建的类对象编译为.class文件

        cc.writeFile("/Users/yangyue/workspace/springboot-learn/java-agent/src/main/java/");

    }

    public static void main(String[] args) {

        try {

            createPseson();

        } catch (Exception e) {

            e.printStackTrace();

        }

    }

}

Class<?> dynamicType = new ByteBuddy()

  .subclass(Object.class)

  .method(ElementMatchers.named("toString"))

  .intercept(FixedValue.value("Hello World!"))

  .make()

  .load(getClass().getClassLoader())

  .getLoaded();

assertThat(dynamicType.newInstance().toString(), is("Hello World!"));