<dependency>

     <groupId>javassist</groupId>

      <artifactId>javassist</artifactId>

      <version>3.12..GA</version>

</dependency>

//创建字节码文件

public class CreateClass {

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

        ClassPool pool = ClassPool.getDefault();

        // 1.创建user类

        CtClass userClass = pool.makeClass("com.irish.entity.User");

        // 2.创建name 和age属性

        CtField nameField = CtField.make("private String name;", userClass);

        CtField ageField = CtField.make("private Integer age;", userClass);

        // 3.添加属性

        userClass.addField(nameField);

        userClass.addField(ageField);

        // 4.创建方法

        CtMethod nameMethod = CtMethod.make("public String getName() {return name;}", userClass);

        // 5.添加方法

        userClass.addMethod(nameMethod);

        // 6.添加构造函数

        CtConstructor ctConstructor = new CtConstructor(

                new CtClass[] { pool.get("java.lang.String"), pool.get("java.lang.Integer") }, userClass);

        ctConstructor.setBody("    { this.name = name; this.age = age; }");

        userClass.addConstructor(ctConstructor);

        // 生成class文件

        userClass.writeFile("F:/test");

        System.out.println("生成字节码，成功！");

    }

}

public class User {

    private String name;

    private Integer age;

}

//动态修改字节码文件

public class UpdateClass {

    @SuppressWarnings({ "rawtypes", "unchecked" })

    public static void main(String[] args) {

        try {

            ClassPool pool = ClassPool.getDefault();

            // 读取com.irish.User

            CtClass userClass = pool.get("com.irish.User");

            CtMethod method = new CtMethod(CtClass.voidType, "sum", new CtClass[] { CtClass.intType, CtClass.intType },

                    userClass);

            method.setBody("{System.out.println(\"sun:\" + ($1 + $2));}");

            // 添加方法

            userClass.addMethod(method);

            //将修改后的字节码写入到磁盘中，通过反编译工具，可以看到对类进行了修改

            userClass.writeFile("F:/test");

            // 动态执行方法

            Class clazz = userClass.toClass();

            Object newInstance = clazz.newInstance();

            Method sumMethod = clazz.getDeclaredMethod("sum", int.class, int.class);

            sumMethod.invoke(newInstance, , );

        } catch (Exception e) {

            e.printStackTrace();

        }

    }

}