BlockingQueue.peekWait()

Question

有一种情况是，我希望在BlockingQueue上使用一种方法BlockingQueue。我把这个方法描述为

检索但不移除队列的头，必要时等待元素可用。

在BlockingQueue，LinkedBlockingQueue上，我看不到相应的方法。这是存在的还是我可以这样做？我考虑过执行poll() + addFirst()，但是队列可能会在中间填满，使我陷入困境。

Answer 1

将对BlockingQueue#peek()的调用包装在一个Callable中，执行它并等待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");
}