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 {
        //生成一個類只需要ClassWriter組件即可
        ClassWriter cw = new ClassWriter(0);
        //通過visit方法確定類的頭部信息
        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"});
        //定義類的屬性
        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();
        //定義類的方法
        cw.visitMethod(Opcodes.ACC_PUBLIC+Opcodes.ACC_ABSTRACT, "compareTo",
                "(Ljava/lang/Object;)I", null, null).visitEnd();
        cw.visitEnd(); //使cw類已經完成
        //將cw轉換成字節(jié)數組寫到文件里面去
        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. 創(chuàng)建一個空類
        CtClass cc = pool.makeClass("com.test.javassist.Person");

        // 2. 新增一個字段 private String name;
        // 字段名為name
        CtField param = new CtField(pool.get("java.lang.String"), "name", cc);
        // 訪問級別是 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. 添加無參的構造函數
        CtConstructor cons = new CtConstructor(new CtClass[]{}, cc);
        cons.setBody("{name = \"xiaohong\";}");
        cc.addConstructor(cons);

        // 5. 添加有參的構造函數
        cons = new CtConstructor(new CtClass[]{pool.get("java.lang.String")}, cc);
        // $0=this / $1,$2,$3... 代表方法參數
        cons.setBody("{$0.name = $1;}");
        cc.addConstructor(cons);

        // 6. 創(chuàng)建一個名為printName方法，無參數，無返回值，輸出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);

        //這里會將這個創(chuàng)建的類對象編譯為.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!"));