SynchronousQueue和LinkedBlockingQueue区别

Posted 2021-01-26 持续在更新的博客

在多线程开发中，会用到SynchronousQueue(new CachedThreadPool())和LinkedBlockingQueue(new FixedThreadPoll())

我们来简单分析一下这两个队列的区别

SynchronousQueue:

　　offer()：当线程offer操作时，当same mode时，加入队列失败，即时返回 （如果是put操作，元素会存储到队列中，并且阻塞等待）；

当complimentary mode时，立即把元素transfer给等待的take线程

        take()：线程take操作，当same mode时，该线程把元素存储到队列中，并且阻塞等待（如果是poll操作，元素会加入队列失败，即时返回）；

当complimentary mode时，立即在队列中找到等待的put线程关联的元素，取出来，返回

 

LinkedBlockingQueue

　　offer(): 线程把元素放入队列中（多线程并发竞争），返回，超过bound，返回失败

 1 /** Lock held by take, poll, etc */
 2     private final ReentrantLock takeLock = new ReentrantLock();
 3 
 4     /** Wait queue for waiting takes */
 5     private final Condition notEmpty = takeLock.newCondition();
 6 
 7     /** Lock held by put, offer, etc */
 8     private final ReentrantLock putLock = new ReentrantLock();
 9 
10     /** Wait queue for waiting puts */
11     private final Condition notFull = putLock.newCondition();
12 
13 
14 public boolean offer(E e) {
15         if (e == null) throw new NullPointerException();
16         final AtomicInteger count = this.count;
17         if (count.get() == capacity)
18             return false;
19         int c = -1;
20         Node<E> node = new Node<E>(e);
21         final ReentrantLock putLock = this.putLock;
22         putLock.lock();
23         try {
24             if (count.get() < capacity) {
25                 enqueue(node);
26                 c = count.getAndIncrement();
27                 if (c + 1 < capacity)
28                     notFull.signal();
29             }
30         } finally {
31             putLock.unlock();
32         }
33         if (c == 0)
34             signalNotEmpty();
35         return c >= 0;
36     }

offer方法

 

       take(); 队列不为空时，获取元素（多线程并发竞争），为空时，阻塞等待

 1 public E take() throws InterruptedException {
 2         E x;
 3         int c = -1;
 4         final AtomicInteger count = this.count;
 5         final ReentrantLock takeLock = this.takeLock;
 6         takeLock.lockInterruptibly();
 7         try {
 8             while (count.get() == 0) {
 9                 notEmpty.await();
10             }
11             x = dequeue();
12             c = count.getAndDecrement();
13             if (c > 1)
14                 notEmpty.signal();
15         } finally {
16             takeLock.unlock();
17         }
18         if (c == capacity)
19             signalNotFull();
20         return x;
21     }

View Code

 

设想一种场景：

　　当有大量线程在offer和take时，

　　1、LinkedBlockingQueue在入队和出队时，并发竞争激烈，cpu线程切换频繁，性能较低；

 　　2、SynchronousQueue中每一个元素对应一个put线程、一个take线程，不会存在锁竞争

　　但是反过来 SynchronousQueue需要的线程数较多，如果take消费不及时，会导致put线程阻塞（如果是使用offer的话，会加入队列失败）