java BlockingQueue does not have a blocking peek?

Question

I have a blocking queue of objects.

I want to write a thread that blocks till there is a object on the queue. Similar to the functionality provided by BlockingQueue.

Answer 1

The 'simplest' solution

Do not process the next element until the previous element is processed succesfully.

public void run() {

Object lastSuccessfullyProcessedElement = null;

    while (!exit) {
        Object obj =  lastSuccessfullyProcessedElement == null ? queue.take() : lastSuccessfullyProcessedElement; // blocking
        
        boolean successful = process(obj);
        
        if(!successful) {
            lastSuccessfullyProcessedElement = obj;
        } else {
            lastSuccessfullyProcessedElement = null;
        }
    }
}

1. Calling peek() and checking if the value is null is not CPU efficient.

I have seen CPU usage going to 10% on my system when the queue is empty for the following program.

while (true) {
   Object o = queue.peek();
   if(o == null) continue;
   // omitted for the sake of brevity
}

2. Adding sleep() adds slowness.

3. Adding it back to the queue using putLast will disturb the order. Moreover, it is a blocking operation which requires locks.