2.2.16锁对象的改变

Posted 2020-10-17 成功的路上总是离不开贵人的帮助，名师的指点和小人的刺激。

在将任何数据类型作为同步锁时，需要注意的是，是否有多个线程同时持有锁对象，如果同时持有相同的锁对象，则这些线程之间就是同步的，如果分别获得锁对象，这些线程之间是异步的。

package com.cky.thread;

/**
 * Created by edison on 2017/12/9.
 */
public class MyServIce {
    private String lock ="123";
    public void testMethodA (){
        try {
            synchronized (lock) {
                System.out.println(Thread.currentThread().getName() +
                "begin="+System.currentTimeMillis());
                lock="456";
                Thread.sleep(2000);
                System.out.println(Thread.currentThread().getName() +
                "end="+System.currentTimeMillis());
            }
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
    }
}

package com.cky.thread;

/**
 * Created by edison on 2017/12/8.
 */
public class ThreadA extends  Thread{
    private MyServIce service;

    public ThreadA(MyServIce service) {
        this.service = service;
    }

    @Override
    public void run() {
        super.run();
        service.testMethodA();
    }
}

package com.cky.thread;

/**
 * Created by edison on 2017/12/8.
 */
public class ThreadB extends  Thread{


    private MyServIce service;

    public ThreadB(MyServIce service) {
        this.service = service;
    }

    @Override
    public void run() {
        super.run();
        service.testMethodA();
    }
}

package com.cky.thread;

/**
 * Created by edison on 2017/12/9.
 */
public class Test2 {
    public static void main(String[] args) throws InterruptedException {
        MyServIce myServIce = new MyServIce();
        ThreadA a = new ThreadA(myServIce);
        a.setName("A");
        ThreadB b= new ThreadB(myServIce);
        b.setName("B");
        a.start();
        Thread.sleep(50);
        b.start();


    }
}

结果分析：

因为50毫秒过后线程B 获得的锁是456

继续测试：

去掉代码Thread.sleep(50)

package com.cky.thread;

/**
 * Created by edison on 2017/12/9.
 */
public class Test2 {
    public static void main(String[] args) throws InterruptedException {
        MyServIce myServIce = new MyServIce();
        ThreadA a = new ThreadA(myServIce);
        a.setName("A");
        ThreadB b= new ThreadB(myServIce);
        b.setName("B");
        a.start();
        //Thread.sleep(50);
        b.start();


    }
}

Bbegin=1512803944419
Bend=1512803946419
Abegin=1512803946419
Aend=1512803948419

　　

线程A和B持有的锁都是123，虽然将锁改成了456。但是结果还是同步的，因为线程A和B共同争抢的锁是123

注意：只要对象不变，即使对象的属性被改变，运行的结果还是同步的，测试如下

 

package com.cky.daemon;

/**
 * Created by edison on 2017/12/9.
 */
public class Service {
    public void methodA(UserInfo userInfo) {
        synchronized (userInfo) {
            try {
                System.out.println("begin"+Thread.currentThread().getName());
                userInfo.setUsrname("aaaa");
                Thread.sleep(2000);
                System.out.println("end"+ System.currentTimeMillis());
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
        }
    }
}

package com.cky.daemon;

/**
 * Created by edison on 2017/12/9.
 */
public class UserInfo {
    private String usrname;

    public String getUsrname() {
        return usrname;
    }

    public void setUsrname(String usrname) {
        this.usrname = usrname;
    }
}

package com.cky.daemon;

import com.cky.thread.ThreadA;
import com.cky.thread.ThreadB;

/**
 * Created by edison on 2017/12/9.
 */
public class Test {
    public static void main(String[] args) {
        try {
            Service service = new Service();
            UserInfo userInfo = new UserInfo();
            ThreadA threadA = new ThreadA(service, userInfo);
            threadA.setName("a");
            threadA.start();
            Thread.sleep(50);
            ThreadB threadB = new ThreadB(service, userInfo);
            threadB.setName("b");
            threadB.start();
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
    }
}

package com.cky.thread;

import com.cky.daemon.Service;
import com.cky.daemon.UserInfo;

/**
 * Created by edison on 2017/12/8.
 */
public class ThreadA extends  Thread{
    private Service service;
    private UserInfo userInfo;

    public ThreadA(Service service, UserInfo userInfo) {

        this.service = service;
        this.userInfo = userInfo;
    }

    @Override
    public void run() {
        super.run();
        service.methodA(userInfo);
    }
}

package com.cky.thread;

import com.cky.daemon.Service;
import com.cky.daemon.UserInfo;

/**
 * Created by edison on 2017/12/8.
 */
public class ThreadB extends  Thread{


    private Service service;
    private UserInfo userInfo;

    public ThreadB(Service service, UserInfo userInfo) {

        this.service = service;
        this.userInfo = userInfo;
    }

    @Override
    public void run() {
        super.run();
        service.methodA(userInfo);
    }
}

begina
end1512804642120
beginb
end1512804644121