在多线程访问的时候，同一时刻只能有一个线程能够用 synchronized 修饰的方法或者代码块，解决了资源共享。下面代码示意三个窗口购5张火车票：
1 package com.jikexueyuan.thread;
2 /*
3 * 未使用synchronized，存在并发
4 */
5 class RunnableDemo implements Runnable{
6 private int tickets = 5;
7 @Override
8 public void run() {
9 for (int i = 0; i < 10; i++) {
10 try {
11 Thread.sleep(500);
12 } catch (InterruptedException e) {
13 e.printStackTrace();
14 }
15 if (tickets>0) {
16 System.out.println("车票: "+tickets--);
17 }
18 }
19
20 }
21 }
22
23 public class ThreadTest {
24
25 public static void main(String[] args) {
26 RunnableDemo r = new RunnableDemo();
27 Thread t1 = new Thread(r);
28 Thread t2 = new Thread(r);
29 Thread t3 = new Thread(r);
30 t1.start();
31 t2.start();
32 t3.start();
33 }
34
35 }
其中一次的运行结果：
车票: 5
车票: 4
车票: 3
车票: 2
车票: 1
车票: 2
使用synchronized同步块后：
1 package com.jikexueyuan.thread;
2 /*
3 * 使用synchronized块
4 */
5 class RunnableDemo implements Runnable{
6 private int tickets = 5;
7 @Override
8 public void run() {
9 for (int i = 0; i < 10; i++) {
10 try {
11 Thread.sleep(500);
12 } catch (InterruptedException e) {
13 e.printStackTrace();
14 }
15 synchronized (this) {
16 if (tickets>0) {
17 System.out.println("车票: "+tickets--);
18 }
19 }
20 }
21 }
22 }
23
24 public class ThreadTest {
25
26 public static void main(String[] args) {
27 RunnableDemo r = new RunnableDemo();
28 Thread t1 = new Thread(r);
29 Thread t2 = new Thread(r);
30 Thread t3 = new Thread(r);
31 t1.start();
32 t2.start();
33 t3.start();
34 }
35
36 }
使用synchronized同步方法：
1 package com.jikexueyuan.thread;
2 /*
3 * 使用synchronized同步方法
4 */
5 class RunnableDemo implements Runnable{
6 private int tickets = 5;
7 @Override
8 public void run() {
9 for (int i = 0; i < 10; i++) {
10 show();
11 }
12 }
13 public synchronized void show() {
14 if (tickets>0) {
15 System.out.println("车票: "+tickets--);
16 }
17 }
18 }
19
20 public class ThreadTest {
21
22 public static void main(String[] args) {
23 RunnableDemo r = new RunnableDemo();
24 Thread t1 = new Thread(r);
25 Thread t2 = new Thread(r);
26 Thread t3 = new Thread(r);
27 t1.start();
28 t2.start();
29 t3.start();
30 }
31
32 }
无论使用synchronized同步块还是同步方法，运行结果均为合理结果：
车票: 5
车票: 4
车票: 3
车票: 2
车票: 1
思考：volatile是另一种同步机制，是否可以呢？参考链接文章：Java理论与实践:正确使用Volatile变量 http://www.ibm.com/developerworks/cn/java/j-jtp06197.html