在將任何數(shù)據(jù)類型作為同步鎖時(shí)，需要注意的是，是否有多個(gè)線程同時(shí)持有鎖對(duì)象，如果同時(shí)持有相同的鎖對(duì)象，則這些線程之間就是同步的；如果分別獲得鎖對(duì)象，這些線程之間就是異步的。

/**
 * @author wuyoushan
 * @date 2017/4/19.
 */
public class MyService {
    private String lock="123";

    public void testMethod(){
        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();
        }
    }
}

/**
 * @author wuyoushan
 * @date 2017/4/4.
 */
public class ThreadA extends Thread{

    private MyService service;

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

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

/**
 * @author wuyoushan
 * @date 2017/4/4.
 */
public class ThreadB extends Thread{

   private MyService service;

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

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

/**
 * @author wuyoushan
 * @date 2017/3/20.
 */
public class Run {
    public static void main(String[] args) throws InterruptedException {
        MyService service=new MyService();
        ThreadA a=new ThreadA(service);
        a.setName("A");
        ThreadB b=new ThreadB(service);
        b.setName("B");
        a.start();
        Thread.sleep(50);
        b.start();
    }
}

程序運(yùn)行后的結(jié)果為:

A begin 1493684592096
B begin 1493684592146
A end 1493684594096
B end 1493684594146

程序的結(jié)果是異步輸出的。因?yàn)?0毫秒過后B取得的鎖是“456”

/**
 * @author wuyoushan
 * @date 2017/3/20.
 */
public class Run {
    public static void main(String[] args) throws InterruptedException {
        MyService service=new MyService();
        ThreadA a=new ThreadA(service);
        a.setName("A");
        ThreadB b=new ThreadB(service);
        b.setName("B");
        a.start();
//        Thread.sleep(50);
        b.start();
    }
}

去掉Thread.sleep(50)，程序的運(yùn)行結(jié)果如下:

A begin 1493685002103
A end 1493685004104
B begin 1493685004104
B end 1493685006104

線程A和B持有的鎖都是“123”，雖然將鎖改成了“456”，但是結(jié)果還是同步的，因?yàn)锳和B共同爭搶的鎖是“123”。

還需要提示一下，只要對(duì)象不變，即使對(duì)象的屬性被改變，運(yùn)行的結(jié)果還是同步。

/**
 * @author wuyoushan
 * @date 2017/4/10.
 */
public class Service {
   public void serviceMethodA(UserInfo userInfo){
        synchronized (userInfo){
            try {
                System.out.println(Thread.currentThread().getName());
                userInfo.setUserName("abcabcabc");
                Thread.sleep(3000);
                System.out.println("end! time="+System.currentTimeMillis());
            }catch (InterruptedException e){
                e.printStackTrace();
            }
        }
   }
}

/**
 * @author wuyoushan
 * @date 2017/4/4.
 */
public class ThreadA extends Thread{

    private Service service;
    private UserInfo userInfo;

    public ThreadA(Service service,UserInfo userInfo) {
        super();
        this.service = service;
        this.userInfo=userInfo;
    }

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

/**
 * @author wuyoushan
 * @date 2017/4/4.
 */
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.serviceMethodA(userInfo);
    }
}

/**
 * @author wuyoushan
 * @date 2017/3/20.
 */
public class Run {
    public static void main(String[] args){
        try {
            Service service = new Service();
            UserInfo userInfo = new UserInfo();
            ThreadA a = new ThreadA(service, userInfo);
            a.setName("a");
            a.start();
            Thread.sleep(50);

            ThreadB b = new ThreadB(service, userInfo);
            b.setName("b");
            b.start();
        }catch(InterruptedException e){
            e.printStackTrace();
        }
    }
}

程序的運(yùn)行結(jié)果為：

a
end! time=1493685934399
b
end! time=1493685937399

摘選自 java多線程核心編程技術(shù)-2.2.16