CODEWITHSUNDEEP
javaqueueblocking
Mike Q
Mike Q

Reputation: 23229

BlockingQueue.peekWait()

I have a situation where I would like a method peekWait on BlockingQueue. I would describe this method as

retrieves but not remove the head of the queue, waiting if necessary until an element becomes available

I cannot see a corresponding method on BlockingQueue, LinkedBlockingQueue. Does this exist or can I do this somehow? I considered doing poll() + addFirst() but the queue may fill up in between leaving me stuck.

Upvotes: 4

Views: 2196

Answers (1)

Boris Pavlović
Boris Pavlović

Reputation: 64632

Wrap the call to BlockingQueue#peek() in a Callable, execute it and wait on Future<T>.get(long, TimeUnit):

final BlockingQueue<String> queue = new ArrayBlockingQueue<String>(10);
ExecutorService executor = Executors.newCachedThreadPool();
ScheduledExecutorService scheduler = Executors.newScheduledThreadPool(3);

scheduler.schedule(new Runnable()
{
  @Override
  public void run()
  {
    String e = UUID.randomUUID().toString();
    System.out.println("adding = " + e);
    queue.add(e);
  }
}, 2, TimeUnit.SECONDS);

Callable<String> task = new Callable<String>()
{
  @Override
  public String call() throws Exception
  {
    String result = null;
    while ((result = queue.peek()) == null)
    {
      Thread.sleep(100L);
    }
    return result;
  }
};

String peeked = null;
try
{
  peeked = executor.submit(task).get(1, TimeUnit.SECONDS);
  System.out.println("this should never be printed");
  queue.poll();
}
catch (TimeoutException e)
{
  System.out.println("null: peeked = " + peeked);
  e.printStackTrace();
}

try
{
  peeked = executor.submit(task).get(2, TimeUnit.SECONDS);
  System.out.println("not null: peeked = " + peeked);
}
catch (TimeoutException e)
{
  e.printStackTrace();
  System.out.println("should not throw an exception");
}

Upvotes: 1

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