使用Runnable实现数据共享

使用Runnable实现数据共享，供大家参考，具体内容如下

先上代码：

public class TestThread {
  private static final Logger logger = LoggerFactory.getLogger(TestThread.class);

  private final class MyRunnable implements Runnable {
    private int i;
    public MyRunnable() {
      this.i = 10;
    }
    public void run() {
      while(i > 0) {
        synchronized (this) {
          if (i > 0) {
            i--;
            logger.debug("{} buy one ticket, {} left. ", Thread.currentThread().getName(), i);
          }
        }
      }
    }
  }

  @Test
  public void testRunable() throws InterruptedException{
    MyRunnable myRunnable = new MyRunnable();
    Thread th1 = new Thread(myRunnable);
    Thread th2 = new Thread(myRunnable);
    th1.start();
    th2.start();
    th1.join();
    th2.join();
  }
}

楼上的代码很简单，模拟一个售票系统。通过两个Thread对象开启两条线程同时运行一个MyRunnable实例。

几个注意点：

1. 没有加上synchronised关键词的话，即

public void run() {
      while(i > 0) {
        if (i > 0) {
          i--;
          logger.debug("{} buy one ticket, {} left. ", Thread.currentThread().getName(), i);
        }
      }
    }

系统的运行结果：

Thread-1 buy one ticket, 8 left.
Thread-2 buy one ticket, 8 left.
Thread-2 buy one ticket, 6 left.
Thread-1 buy one ticket, 6 left.
Thread-2 buy one ticket, 5 left.
Thread-1 buy one ticket, 4 left.
Thread-2 buy one ticket, 3 left.
Thread-1 buy one ticket, 2 left.
Thread-2 buy one ticket, 1 left.
Thread-1 buy one ticket, 0 left. 

可以看到，缺少同步的程序输出明显有问题。

2. 在进入同步代码块之后，还需要对i的值再进行一次判断，即，如果不加if判断：

public void run() {
      while(i > 0) {
        synchronized (this) {
          i--;
          logger.debug("{} buy one ticket, {} left. ", Thread.currentThread().getName(), i);

        }
      }
    }

程序的运行结果为：

Thread-2 buy one ticket, 9 left.
Thread-2 buy one ticket, 8 left.
Thread-2 buy one ticket, 7 left.
Thread-2 buy one ticket, 6 left.
Thread-2 buy one ticket, 5 left.
Thread-2 buy one ticket, 4 left.
Thread-2 buy one ticket, 3 left.
Thread-2 buy one ticket, 2 left.
Thread-2 buy one ticket, 1 left.
Thread-2 buy one ticket, 0 left.
Thread-1 buy one ticket, -1 left. 

可以看出，出现了“多卖”的现象， 所以需要在进入同步代码块中再进行一次if判断。

总结

synchronised用于互斥访问共享变量， 并在同步代码块中使用if判断更新共享变量。

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