package test;

public class Person {

    private String userName= "Tom";

    private void playGame() {

        System.out.println(userName+ "悄悄玩儿游戏");

    }

}

package test;

import java.lang.reflect.Field;

import java.lang.reflect.Method;

public class Private {

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

        Person person = new Person();

        Class c = person.getClass();

        //Class c2 = Class.forName("test.Person");

        //Person person2 = (Person)c2.newInstance();

        Method method = c.getDeclaredMethod("playGame");

        method.setAccessible(true);

        method.invoke(person);

        Field field = c.getDeclaredField("userName");

        field.setAccessible(true);

        field.set(person,"John");

        method.invoke(person);

    }

}

Tom悄悄玩儿游戏

John悄悄玩儿游戏