将ReentrantLock放入HashMap时，锁丢失

Question

我试图让多个消费者线程侦听一个生产者线程，直到生产者有东西要发布。我认为可以工作的代码在放入和取出共享类时“丢失”了锁。

在控制器类中，我启动运行Server server=new Server（）的线程；

    Thread serverThread = new Thread(server,"Server");
    serverThread.start(); 

    Consumer consumer = new Consumer();
    Thread consumerThread;
    for (int i =0;i<6;i++){
        consumerThread = new Thread(consumer,"Consumer No:"+i);
    consumerThread.start();
    server.threadRefs[i]= consumerThread;
    }

consumer类将线程的详细信息放入映射中，如下所示：

public class Consumer implements Runnable {

private ReentrantLock lock = new ReentrantLock();
private Condition cond = lock.newCondition();

@Override
public void run() {

    long threadID = Thread.currentThread().getId();
    while (true) {
        try {

            lock.lock();

            MDRequest.threadLocks.put(threadID, lock);
            System.out.println("Thread " + threadID + "  lock = " + lock.toString());

            cond.await();
            System.out.println("Thread " + threadID + "  done waiting");
        } catch (InterruptedException ex) {
            System.out.println("Interruped " + threadID);
        } finally {
            lock.unlock();
        }

        System.out.println("Finished " + threadID);
    }

}

共享类仅为：

public class MDRequest {

    protected static ConcurrentHashMap<Long, ReentrantLock> threadLocks = new ConcurrentHashMap<Long, ReentrantLock>();

服务器有以下run（）方法：

public void run() {
    try {
        Thread.sleep(5000);
    } catch (InterruptedException ex) {
        Logger.getLogger(Server.class.getName()).log(Level.SEVERE, null, ex);
    }
    Set<Long> keys = MDRequest.threadLocks.keySet();
    Long[] threadIDs = keys.toArray(new Long[1]);

    // generates a random series of numbers for each thread and notifies threads about them 
    while (true) {


        Random random = new Random();
        int threadRef = random.nextInt(5);

        System.out.println("About to signal thread " + threadIDs[threadRef].toString());

        // notify the thread of the data 
        ReentrantLock lock = MDRequest.threadLocks.get(threadIDs[threadRef]);
        System.out.println("Thread " + threadIDs[threadRef].toString() + "  lock = " + lock.toString());

        Condition cond = lock.newCondition();
        cond.signal();
        lock.unlock();

    }

输出如下：

Thread 11  lock = java.util.concurrent.locks.ReentrantLock@272d7a10[Locked by thread Consumer No:0]
Thread 12  lock = java.util.concurrent.locks.ReentrantLock@272d7a10[Locked by thread Consumer No:1]
Thread 13  lock = java.util.concurrent.locks.ReentrantLock@272d7a10[Locked by thread Consumer No:2]
Thread 14  lock = java.util.concurrent.locks.ReentrantLock@272d7a10[Locked by thread Consumer No:3]
Thread 15  lock = java.util.concurrent.locks.ReentrantLock@272d7a10[Locked by thread Consumer No:4]
Thread 16  lock = java.util.concurrent.locks.ReentrantLock@272d7a10[Locked by thread Consumer No:5]
About to signal thread 14
Thread 14  lock = java.util.concurrent.locks.ReentrantLock@272d7a10[Unlocked]
Exception in thread "Price Server" java.lang.IllegalMonitorStateException
    at java.util.concurrent.locks.AbstractQueuedSynchronizer$ConditionObject.signal(AbstractQueuedSynchronizer.java:1885)
    at faster.Server.run(Server.java:46)
    at java.lang.Thread.run(Thread.java:695)

从服务器类的输出int中，我可以看到，当我从映射中读取锁时，它现在的状态为“Unlocked”。当它被放入时，它的状态为锁定在线程14上。

为什么对ReentrantLock的引用会“丢失”锁？

是否有一种方法可以在多个消费者线程和服务器线程之间以不丢失锁的方式共享锁？

Answer 1

问题是Server类中的线程尝试解锁但没有锁定Lock

lock.unlock();

请参阅ReentrantLock文档，其中明确说明：

如果当前线程不是此锁的持有者，则会引发非法监视器状态异常（IllegalMonitorStateException）。

Answer 2

您面临的问题可能是由于服务器中的以下行

Condition cond = lock.newCondition();
cond.signal();
lock.unlock();

您是否需要从Server调用lock（）以获取被消费者锁定的Lock？我认为调用信号（）就足够了。

想想这个。