How to wait for data with ReentrantReadWriteLock?

Question

It is said, that ReentrantReadWriteLock is intended for one writer and multiple readers.

Nevertheless, readers should wait until some data is present in the buffer.

So, what to lock?

I created concurrency objects like follows:

private final ReentrantReadWriteLock rwl = new ReentrantReadWriteLock();
protected final Lock readLock = rwl.readLock();
protected final Lock writeLock = rwl.writeLock();
protected final Condition hasData = writeLock.newCondition();

now in write method I do:

writeLock.lock();

// writing first portion and updating variables

hasData.signalAll();

// if required then writing second portion and updating variables

hasData.signalAll();

But how to write a reader? Should it acquire only readLock? But how it can wait for a signal then? If it aquires also a writeLock then where is the supremacy fo read/write locking?

How to ensure required variables will not change during reading if they are protected only by writeLock?

QUEUES DON'T MATCH THE TASK

This is the question about ReentrantReadWriteLock.

Answer 1

The ReentrantReadWriteLock is indeed a bit confusing because the readLock doesn't have a condition. You have to upgrade to a writeLock in your reader only to wait for the condition.

In the writer.

writeLock.lock(); //locks all readers and writers
// do write data
hasData.signalAll();
writeLock.unlock();

In reader you do:

readLock.lock(); //blocks writers only
try{
 if(!checkData()) //check if there's data, don't modify shared variables
 {
  readLock.unlock();
  writeLock.lock(); // need to lock the writeLock to allow to use the condition.
                    // only one reader will get the lock, other readers will wait here      
  try{
   while(!checkData()) // check if there' still no data
   {
     hasData.await(); //will unlock and re-lock after writer has signalled and unlocked.
   }
   readLock.lock();    // continue blocking writer
  }
  finally
  {
    writeLock.unlock(); //let other readers in
  }
 }
 //there should be data now
 readData(); // don't modify variables shared by readers.
}
finally
{
  readlock.unlock(); //let writers in
}

For completeness, each unlock() should be in a finally block, of course.

Answer 2

But how to write a reader? Should it acquire only readLock? But how it can wait for a signal then? If it aquires also a writeLock then where is the supremacy fo read/write locking?

I'd switch to using a BlockingQueue that will take care of all of this for you. Your readers can call queue.take() which blocks waiting for there to be elements in the queue.

Your writer is a bit more complicated. What I'd do is something like the following:

// initially try to put an element into the queue
if (!queue.offer(element)) {
   // if the queue is full then take an element off the head and just drop it
   // this won't block and may not remove anything due to race conditions
   queue.poll();
   // this put will never block because now there will be space in the queue
   queue.put(element);
}

This won't work if there are multiple writers. You'd need a synchronized lock then. If you are dealing with a fixed size queue then the ArrayBlockingQueue should work well.