Interview Focused: BlockingQueue in Java? 10 Points

Interview Focused: BlockingQueue in Java? 10 Points

1) The Java BlockingQueue interface in the java.util.concurrent package represents a queue which is thread safe to put and take objects from.

2) BlockingQueue implementations are designed to be used primarily for producer-consumer queues, but additionally support the Collection interface. So, for example, it is possible to remove an arbitrary element from a queue using remove(x). However, such operations are in general not performed very efficiently, and are intended for only occasional use, such as when a queued message is cancelled.

3) BlockingQueue methods come in four forms, with different ways of handling operations that cannot be satisfied immediately, but may be satisfied at some point in the future: one throws an exception, the second returns a special value (either null or false, depending on the operation), the third blocks the current thread indefinitely until the operation can succeed, and the fourth blocks for only a given maximum time limit before giving up. These methods are summarized in the following table:

4) A BlockingQueue does not accept null elements. Implementations throw NullPointerException on attempts to add, put or offer a null. A null is used as a sentinel value to indicate failure of poll operations.

5) The “blocking” nature of the queue has a couple of advantages. First, on adding elements, if the queue capacity is limited, memory consumption is limited as well. Also, if the queue consumers get too far behind producers, the producers are naturally throttled since they have to wait to add elements. When taking elements from the queue, the main advantage is simplicity; waiting forever is trivial, and correctly waiting for a specified time-out is only a little more complicated. –http://stackoverflow.com/users/3474/erickson

6) A limited capacity BlockingQueue is helpful if you want to throttle requests. With an unbounded queue, a producers can get far ahead of the consumers.With an unbounded queue, a producers can get far ahead of the consumers. The tasks will eventually be performed (unless there are so many that they cause an OutOfMemoryError), but the producer may long since have given up, so the effort is wasted. –http://stackoverflow.com/users/3474/erickson

7) A ConcurrentLinkedQueue is an appropriate choice when many threads will share access to a common collection. This queue does not permit null elements.

8) ArrayBlockingQueue is a classic “bounded buffer”, in which a fixed-sized array holds elements inserted by producers and extracted by consumers. This class supports an optional fairness policy for ordering waiting producer and consumer threads.

9) LinkedBlockingQueue typically have higher throughput than array-based queues but less predictable performance in most concurrent applications.

10) BlockingQueue implementations are thread-safe. All queuing methods achieve their effects atomically using internal locks or other forms of concurrency control. However, the bulk Collection operations addAll, containsAll, retainAll and removeAll are not necessarily performed atomically unless specified otherwise in an implementation. So it is possible, for example, for addAll(c) to fail (throwing an exception) after adding only some of the elements in c.