java - Java中无法同步线程(使用信号量)

Question

美好的一天!

我遇到了同步线程的问题。我正在编写类似于哲学家晚餐的程序。我的进程(例如 3 个)和资源(例如 4 个)很少。每个进程只能使用 2 个可用资源。这意味着第一个进程只能使用第一个和第二个资源等。

我决定使用信号量来实现我的目标。问题是仍然没有同步。例如，如果第一个和第三个进程正在使用资源，那么第二个进程必须等待，直到他的资源不会被释放。在我的程序中有时会发生...有时不会。

有什么问题吗？我该如何解决这个问题？

代码在这里:

public class Sem
{
    public Sem()
    {
        available = new ConcurrentHashMap< Integer, Semaphore >();//Resources.

    for ( int i = 1; i <= 4; i++)
    {
        available.put( i, new Semaphore(1, true) );//Each resource contains semaphore.
    }
}

public void start( final int id )
{
    thisThread = new Thread()
    {
         public void run()
         {
            try
            {
                work( id );                                                 //Try to take resourses.
                Thread.currentThread().sleep(1000);
                release( id );                                              //Release resources.
            } catch (InterruptedException ex) {
                Logger.getLogger(Sem.class.getName()).log(Level.SEVERE, null, ex);
            }

         }
    };
    thisThread.start();
}

public synchronized void work( int id ) throws InterruptedException
{
    available.get(id).acquire();                                            //Try to take resourse[id] and resourse[id+1]
    available.get(id+1).acquire();                                          //Thread is blocking till it will be possible.

    System.out.printf("Acquired [%s], id = %d\n",Thread.currentThread().getName(), id);

}

public void release( int id )
{
    available.get(id).release();                                            //Release resources which hava been captured by thread.
    available.get(id+1).release();                                          //After this other thread can take these resourses.

    System.out.printf("Released [%s], id = %d\n",Thread.currentThread().getName(), id);
}


private ConcurrentHashMap< Integer, Semaphore > available;                  //Integer - id of thread[1..4]; Semaphore - is gate with param (1)
                                                                            //Available - map of resources which can be busy by processes.
Thread thisThread;
}

我这样启动这个程序:

Sem sem = new Sem();

        sem.start(1);
        sem.start(2);
        sem.start(3);

我的输出消息很少，但我最喜欢的是:

Acquired [Thread-1], id = 1
Acquired [Thread-3], id = 3
Released [Thread-1], id = 1
Acquired [Thread-2], id = 2
Released [Thread-3], id = 3
Released [Thread-2], id = 2 

流程 2 开始工作，而他却做不到!!

Answer 1

我认为您应该以与获取锁相反的顺序释放锁。