Michael_user3015308 Michael_user3015308 - 1 month ago 6
Java Question

Lock is lost when putting ReentrantLock into HashMap

I am trying to make several consumer threads that listen to one producer thread wait until the producer has something to publish. The code that I thought would work "loses" the lock on being put into and taken out of a shared class.

In a controller class I start the thread running with
Server server = new Server();

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;
}


The consumer classes put details of threads into the Map as follows:

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);
}

}


The shared class is simply:

public class MDRequest {

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


The Server has the following run() method:

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();

}


The output is as follows:

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)


From the output int the Server class I can see that when I read the lock out of the Map it now has a status of "Unlocked". When it was put in it had a status of Locked on thread 14.

Why does the reference to the ReentrantLock "lose" the lock?

Is there a way of sharing the lock between the multiple consumer threads and the server thread in a way that the lock is not lost?

Answer

The problem is thread in Server class tries to unlock but has not lock the Lock

lock.unlock();

Please see ReentrantLock documentation where is clearly stated:

If the current thread is not the holder of this lock then IllegalMonitorStateException is thrown.