使用Javassist来动态创建，修改和代理类

<dependency>
    <groupId>javassist</groupId>
    <artifactId>javassist</artifactId>
    <version>3.3</version>
</dependency>

public class JavassistMain {
    public static void main(String[] args) throws Exception {
        //创建ClassPool
        ClassPool cp = ClassPool.getDefault();
        //生成的类的名称为com.guanjian.assist.JavassistTest
        CtClass clazz = cp.makeClass("com.guanjian.assist.JavassistTest");
        StringBuffer body = null;
        //创建字段，指定了字段类型、字段名称、字段所属的类
        CtField field = new CtField(cp.get("java.lang.String"), "prop", clazz);
        //指定该字段使用private修饰
        field.setModifiers(Modifier.PRIVATE);
        //设置prop字段的getter/setter方法
        clazz.addMethod(CtNewMethod.getter("getProp",field));
        clazz.addMethod(CtNewMethod.setter("setProp",field));
        //设置prop字段的初始化值，并将prop字段添加到clazz中
        clazz.addField(field,CtField.Initializer.constant("MyName"));
        //创建构造方法，指定了构造方法的参数类型和构造方法所属的类
        CtConstructor ctConstructor = new CtConstructor(new CtClass[]{}, clazz);
        //设置方法体
        body = new StringBuffer();
        body.append("{\n prop=\"MyName\";\n}");
        ctConstructor.setBody(body.toString());
        //将构造方法添加到clazz中
        clazz.addConstructor(ctConstructor);
        //创建execute()方法，指定了方法的返回值、方法名称、方法参数列表以及方法所属的类
        CtMethod ctMethod = new CtMethod(CtClass.voidType, "execute", new CtClass[]{}, clazz);
        //指定方法使用public修饰
        ctMethod.setModifiers(Modifier.PUBLIC);
        //设置方法体
        body = new StringBuffer();
        body.append("{\n System.out.println(\"execute():\" + this.prop);");
        body.append("\n}");
        ctMethod.setBody(body.toString());
        //将execute()方法添加到clazz中
        clazz.addMethod(ctMethod);
        //将上面定义的JavassistTest类保存到指定的目录
        clazz.writeFile("E:\\IOC\\target\\classes");
        //加载clazz类，并创建对象
        Class<?> c = clazz.toClass();
        Object o = c.newInstance();
        //调用execute()方法
        Method method = o.getClass().getMethod("execute", new Class[]{});
        method.invoke(o,new Object[]{});
    }
}

package com.guanjian.assist;

public class JavassistTest {
    private String prop = "MyName";

    public void getProp(String var1) {
        this.prop = var1;
    }

    public String setProp() {
        return this.prop;
    }

    public JavassistTest() {
        this.prop = "MyName";
    }

    public void execute() {
        System.out.println("execute():" + this.prop);
    }
}

public class JavassistUpdate {
    public static void main(String[] args) throws Exception {
        ClassPool cp = ClassPool.getDefault();
        CtClass clazz = cp.get("com.guanjian.assist.JavassistTest");
        //创建字段，指定了字段类型、字段名称、字段所属的类
        CtField field = new CtField(cp.get("java.lang.Integer"), "age", clazz);
        //指定该字段使用private修饰
        field.setModifiers(Modifier.PRIVATE);
        //设置age字段的getter/setter方法
        clazz.addMethod(CtNewMethod.setter("getAge",field));
        clazz.addMethod(CtNewMethod.getter("setAge",field));
        //将age字段添加到clazz中
        clazz.addField(field);
        clazz.writeFile("E:\\IOC\\target\\classes");
    }
}

package com.guanjian.assist;

public class JavassistTest {
    private String prop = "MyName";
    private Integer age;

    public void getProp(String var1) {
        this.prop = var1;
    }

    public String setProp() {
        return this.prop;
    }

    public JavassistTest() {
        this.prop = "MyName";
    }

    public void execute() {
        System.out.println("execute():" + this.prop);
    }

    public void getAge(Integer var1) {
        this.age = var1;
    }

    public Integer setAge() {
        return this.age;
    }
}

public class JavassistProxy {
    public static void main(String[] args) throws IllegalAccessException, InstantiationException {
        ProxyFactory factory = new ProxyFactory();
        //指定父类，ProxyFactory会动态生成继承该父类的子类
        //因为没有.java源文件，此处会飘红，但有.class文件可以执行
        factory.setSuperclass(JavassistTest.class);
        //设置过滤器，判断哪些方法调用需要被拦截
        factory.setFilter(new MethodFilter() {
            @Override
            public boolean isHandled(Method method) {
                if (method.getName().equals("execute")) {
                    return true;
                }
                return false;
            }
        });
        //设置拦截处理
        factory.setHandler(new MethodHandler() {
            @Override
            public Object invoke(Object self, Method thisMethod, Method proceed, Object[] args) throws Throwable {
                System.out.println("前置处理");
                Object result = proceed.invoke(self, args);
                System.out.println("执行结果:" + result);
                System.out.println("后置处理");
                return result;
            }
        });
        //创建JavassistTest代理类，并创建代理对象
        //因为没有.java源文件，以下多处会飘红，但有.class文件可以执行
        Class<?> c = factory.createClass();
        JavassistTest javassistTest = (JavassistTest)c.newInstance();
        //执行execute()方法，会被拦截
        javassistTest.execute();
        System.out.println(javassistTest.getProp());
    }
}