Java中利用synchronized关键字实现多线程同步问题

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    Java 中多线程的同步依靠的是对象锁机制,synchronized关键字就是利用了封装对象锁来实现对共享资源的互斥访问。
      下面以一个简单例子来说明多线程同步问题，我们希望在run()方法里加入synchronized关键字来实现互斥访问。

package com.clark.thread;public class MyThread implements Runnable{    private int threadId;        public MyThread(int id){        this.threadId = id;    }    @Override    public synchronized void run() {        //此时关键字synchronized锁住的是this对象,即当前运行线程对象本身        for (int i = 0; i < 100; i++) {            if(i % 10 ==0){                System.out.println();            }            System.out.print("Thread ID:"+this.threadId+":"+i+" ");        }    }}

测试类：

package com.clark.thread;public class ThreadDemo {public static void main(String[] args) throws InterruptedException {                 //代码中创建了10个线程，而每个线程都持有this对象的对象锁，这不能实现线程的同步。                 for (int i = 0; i < 10; i++) {new Thread(new MyThread(i)).start();Thread.sleep(1); }}}

打印结果部分如下:其线程不是互斥访问的。因此没有达到资源多线程同步效果。。。

Thread ID:1:60 Thread ID:1:61 Thread ID:1:62 Thread ID:1:63 Thread ID:1:64 Thread ID:1:65 Thread ID:1:66 Thread ID:1:67 Thread ID:1:68 Thread ID:1:69 Thread ID:1:70 Thread ID:1:71 Thread ID:1:72 Thread ID:1:73 Thread ID:1:74 Thread ID:1:75 Thread ID:1:76 Thread ID:1:77 Thread ID:1:78 Thread ID:1:79 Thread ID:1:80 Thread ID:1:81 Thread ID:2:0 Thread ID:1:82 Thread ID:1:83 Thread ID:1:84 Thread ID:1:85 Thread ID:1:86 Thread ID:1:87 Thread ID:1:88 Thread ID:1:89 Thread ID:1:90 Thread ID:1:91 Thread ID:1:92 Thread ID:1:93 Thread ID:1:94 Thread ID:1:95 Thread ID:2:1 Thread ID:2:2 Thread ID:2:3 Thread ID:2:4 Thread ID:2:5 Thread ID:2:6 Thread ID:2:7 Thread ID:2:8 Thread ID:2:9

从上述得知,要想实现线程同步，必须让这些线程去竞争一个唯一的共享对象锁。

package com.clark.thread;public class MyThread implements Runnable{private int threadId;private Object object;//线程之间竞争使用的对象锁public MyThread(int id,Object object){this.threadId = id;this.object = object;}@Overridepublic  void run() {/** * 将这个object对象的引用传递给每一个线程对象的lock成员变量 * 从而每个线程的lock成员都指向同一个Object对象 * 这样就可以让线程去竞争这个唯一的共享的对象锁，从而实现同步。 */synchronized(object){for (int i = 0; i < 100; i++) {if(i % 10 ==0){System.out.println();}System.out.print("Thread ID:"+this.threadId+":"+i+" ");}}}}

package com.clark.thread;public class ThreadDemo {public static void main(String[] args) throws InterruptedException {Object obj = new Object();for (int i = 0; i < 10; i++) {new Thread(new MyThread(i,obj)).start();Thread.sleep(1);}}}

测试结果如下:

Thread ID:5:60 Thread ID:5:61 Thread ID:5:62 Thread ID:5:63 Thread ID:5:64 Thread ID:5:65 Thread ID:5:66 Thread ID:5:67 Thread ID:5:68 Thread ID:5:69 Thread ID:5:70 Thread ID:5:71 Thread ID:5:72 Thread ID:5:73 Thread ID:5:74 Thread ID:5:75 Thread ID:5:76 Thread ID:5:77 Thread ID:5:78 Thread ID:5:79 Thread ID:5:80 Thread ID:5:81 Thread ID:5:82 Thread ID:5:83 Thread ID:5:84 Thread ID:5:85 Thread ID:5:86 Thread ID:5:87 Thread ID:5:88 Thread ID:5:89 Thread ID:5:90 Thread ID:5:91 Thread ID:5:92 Thread ID:5:93 Thread ID:5:94 Thread ID:5:95 Thread ID:5:96 Thread ID:5:97 Thread ID:5:98 Thread ID:5:99 Thread ID:6:0 Thread ID:6:1 Thread ID:6:2 Thread ID:6:3 Thread ID:6:4 Thread ID:6:5 Thread ID:6:6 Thread ID:6:7 Thread ID:6:8 Thread ID:6:9 Thread ID:6:10 Thread ID:6:11 Thread ID:6:12 Thread ID:6:13 Thread ID:6:14 Thread ID:6:15 Thread ID:6:16 Thread ID:6:17 Thread ID:6:18 Thread ID:6:19 Thread ID:6:20 Thread ID:6:21 Thread ID:6:22 Thread ID:6:23 Thread ID:6:24 Thread ID:6:25 Thread ID:6:26 Thread ID:6:27 Thread ID:6:28 Thread ID:6:29

从第二段代码可知，同步的关键是多个线程对象竞争同一个共享资源即可，上面的代码中是通过外部创建共享资源，然后传递到线程中来实现。我们也可以利用类成员变量被所有类的实例所共享这一特性，因此可以将lock用静态成员对象来实现，代码如下所示：修改MyThread.java类如下

package com.clark.thread;public class MyThread implements Runnable{private int threadId;private static Object object = new Object();//线程之间竞争使用的对象锁public MyThread(int id){this.threadId = id;}@Overridepublic  void run() {<strong>/** * 将这个object对象的引用传递给每一个线程对象的lock成员变量 * 从而每个线程的lock成员都指向同一个Object对象 * 这样就可以让线程去竞争这个唯一的共享的对象锁，从而实现同步。 */</strong>synchronized(object){for (int i = 0; i < 100; i++) {if(i % 10 ==0){System.out.println();}System.out.print("Thread ID:"+this.threadId+":"+i+" ");}}}}

结果如下：

Thread ID:0:0 Thread ID:0:1 Thread ID:0:2 Thread ID:0:3 Thread ID:0:4 Thread ID:0:5 Thread ID:0:6 Thread ID:0:7 Thread ID:0:8 Thread ID:0:9 Thread ID:0:10 Thread ID:0:11 Thread ID:0:12 Thread ID:0:13 Thread ID:0:14 Thread ID:0:15 Thread ID:0:16 Thread ID:0:17 Thread ID:0:18 Thread ID:0:19 Thread ID:0:20 Thread ID:0:21 Thread ID:0:22 Thread ID:0:23 Thread ID:0:24 Thread ID:0:25 Thread ID:0:26 Thread ID:0:27 Thread ID:0:28 Thread ID:0:29 Thread ID:0:30 Thread ID:0:31 Thread ID:0:32 Thread ID:0:33 Thread ID:0:34 Thread ID:0:35 Thread ID:0:36 Thread ID:0:37 Thread ID:0:38 Thread ID:0:39

再来看第一段代码，实例方法中加入sychronized关键字封锁的是this对象本身，而在静态方法中加入sychronized关键字封锁的就是类本身。静态方法是所有类实例对象所共享的，因此线程对象在访问此静态方法时是互斥访问的，从而可以实现线程的同步，代码如下所示：

package com.clark.thread;public class MyThread implements Runnable{private int threadId;public MyThread(int id){this.threadId = id;}@Overridepublic  void run() {start(this.threadId);}private static synchronized void start(int threadId2) {for (int i = 0; i < 100; i++) {if(i % 10 == 0){System.out.println();}System.out.print("Thread ID:"+threadId2+":"+i+" ");}}}

结果如下=========

Thread ID:0:0 Thread ID:0:1 Thread ID:0:2 Thread ID:0:3 Thread ID:0:4 Thread ID:0:5 Thread ID:0:6 Thread ID:0:7 Thread ID:0:8 Thread ID:0:9 Thread ID:0:10 Thread ID:0:11 Thread ID:0:12 Thread ID:0:13 Thread ID:0:14 Thread ID:0:15 Thread ID:0:16 Thread ID:0:17 Thread ID:0:18 Thread ID:0:19 Thread ID:0:20 Thread ID:0:21 Thread ID:0:22 Thread ID:0:23 Thread ID:0:24 Thread ID:0:25 Thread ID:0:26 Thread ID:0:27 Thread ID:0:28 Thread ID:0:29 Thread ID:0:30 Thread ID:0:31 Thread ID:0:32 Thread ID:0:33 Thread ID:0:34 Thread ID:0:35 Thread ID:0:36 Thread ID:0:37 Thread ID:0:38 Thread ID:0:39 Thread ID:0:40 Thread ID:0:41 Thread ID:0:42 Thread ID:0:43 Thread ID:0:44 Thread ID:0:45 Thread ID:0:46 Thread ID:0:47 Thread ID:0:48 Thread ID:0:49 Thread ID:0:50 Thread ID:0:51 Thread ID:0:52 Thread ID:0:53 Thread ID:0:54 Thread ID:0:55 Thread ID:0:56 Thread ID:0:57 Thread ID:0:58 Thread ID:0:59

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