你真的了解LinkedBlockingQueue的put,add和offer的区别吗

概述

LinkedBlockingQueue的put,add和offer这三个方法功能很类似，都是往队列尾部添加一个元素。既然都是一样的功能，为啥要有有三个方法呢？

这三个方法的区别在于：

• put方法添加元素，若是队列已满，会阻塞直到有空间能够放
• add方法在添加元素的时候，若超出了度列的长度会直接抛出异常
• offer方法添加元素，若是队列已满，直接返回false

索引这三种不一样的方法在队列满时，插入失败会有不一样的表现形式，咱们能够在不一样的应用场景中选择合适的方法。

用法示例

先看看add方法，

public class LinkedBlockingQueueTest {

    public static void main(String[] args) throws InterruptedException {
        LinkedBlockingQueue<String> fruitQueue = new LinkedBlockingQueue<>(2);
        fruitQueue.add("apple");
        fruitQueue.add("orange");
        fruitQueue.add("berry");
    }

当咱们执行这个方法的时候，会报下面的异常，

Exception in thread "main" java.lang.IllegalStateException: Queue full
    at java.util.AbstractQueue.add(AbstractQueue.java:98)
    at com.pony.app.LinkedBlockingQueueTest.testAdd(LinkedBlockingQueueTest.java:23)
    at com.pony.app.LinkedBlockingQueueTest.main(LinkedBlockingQueueTest.java:16)

而后再来看看put用法，

public class LinkedBlockingQueueTest implements Runnable {

    static LinkedBlockingQueue<String> fruitQueue = new LinkedBlockingQueue<>(2);


    public static void main(String[] args) throws InterruptedException {
        new Thread(new LinkedBlockingQueueTest()).start();

        fruitQueue.put("apple");
        fruitQueue.put("orange");
        fruitQueue.put("berry");

        System.out.println(fruitQueue.toString());

    }

    @Override
    public void run() {

        try {
            Thread.sleep(3000);
        } catch (InterruptedException e) {
            e.printStackTrace();
        }

        fruitQueue.poll();
    }
}

运行这段代码，你会发现首先程序会卡住（队列阻塞）3秒左右，而后打印队列的orange和berry两个元素。

由于我在程序的启动的时候顺便启动了一个线程，这个线程会在3秒后从队列头部移除一个元素。

最后看看offer的用法，

public static void main(String[] args) throws InterruptedException {
        LinkedBlockingQueue<String> fruitQueue = new LinkedBlockingQueue<>(2);
        
        System.out.println(fruitQueue.offer("apple"));
        System.out.println(fruitQueue.offer("orange"));
        System.out.println(fruitQueue.offer("berry"));

    }

运行结果：

true
true
false

源码分析

先来看看add方法的实现，

public boolean add(E e) {
        if (offer(e))
            return true;
        else
            throw new IllegalStateException("Queue full");
    }

因此add实际上是包装了一下offer，没什么能够说的。

而后来看看put和offer的实现，两个放在一块儿说。

put方法源码，

public void put(E e) throws InterruptedException {
        if (e == null) throw new NullPointerException();
        // Note: convention in all put/take/etc is to preset local var
        // holding count negative to indicate failure unless set.
        int c = -1;
        Node<E> node = new Node<E>(e);
        final ReentrantLock putLock = this.putLock;
        final AtomicInteger count = this.count;
        putLock.lockInterruptibly();
        try {
            /*
             * Note that count is used in wait guard even though it is
             * not protected by lock. This works because count can
             * only decrease at this point (all other puts are shut
             * out by lock), and we (or some other waiting put) are
             * signalled if it ever changes from capacity. Similarly
             * for all other uses of count in other wait guards.
             */
            while (count.get() == capacity) {
                notFull.await();
            }
            enqueue(node);
            c = count.getAndIncrement();
            if (c + 1 < capacity)
                notFull.signal();
        } finally {
            putLock.unlock();
        }
        if (c == 0)
            signalNotEmpty();
    }

offer方法源码，

public boolean offer(E e, long timeout, TimeUnit unit)
        throws InterruptedException {

        if (e == null) throw new NullPointerException();
        long nanos = unit.toNanos(timeout);
        int c = -1;
        final ReentrantLock putLock = this.putLock;
        final AtomicInteger count = this.count;
        putLock.lockInterruptibly();
        try {
            while (count.get() == capacity) {
                if (nanos <= 0)
                    return false;
                nanos = notFull.awaitNanos(nanos);
            }
            enqueue(new Node<E>(e));
            c = count.getAndIncrement();
            if (c + 1 < capacity)
                notFull.signal();
        } finally {
            putLock.unlock();
        }
        if (c == 0)
            signalNotEmpty();
        return true;
    }

咱们重点关注他们的区别，offer方法在插入的时候会等一个超时时间timeout，若是时间到了队列仍是满的（count.get() == capacity），就会返回false。

而put方法是无限期等待，

while (count.get() == capacity) {
                notFull.await();
            }

因此咱们在应用层使用的时候，若是队列满再插入会阻塞。

实际场景应用

在早期版本的kafka中，生产者端发送消息使用了阻塞队列，代码以下：

private def asyncSend(messages: Seq[KeyedMessage[K,V]]) {
    for (message <- messages) {
      val added = config.queueEnqueueTimeoutMs match {
        case 0  =>
          queue.offer(message)
        case _  =>
          try {
            if (config.queueEnqueueTimeoutMs < 0) {
              queue.put(message)
              true
            } else {
              queue.offer(message, config.queueEnqueueTimeoutMs, TimeUnit.MILLISECONDS)
            }
          }
          catch {
            case _: InterruptedException =>
              false
          }
      }
      if(!added) {
        producerTopicStats.getProducerTopicStats(message.topic).droppedMessageRate.mark()
        producerTopicStats.getProducerAllTopicsStats.droppedMessageRate.mark()
        throw new QueueFullException("Event queue is full of unsent messages, could not send event: " + message.toString)
      }else {
        trace("Added to send queue an event: " + message.toString)
        trace("Remaining queue size: " + queue.remainingCapacity)
      }
    }
  }

能够看到，config.queueEnqueueTimeoutMs是0的时候，使用的是offer方法，小于0的时候则使用put方法。

咱们在使用kafka的时候，能够经过queue.enqueue.timeout.ms来决定使用哪一种方式。好比某些应用场景下，好比监控，物联网等场景，容许丢失一些消息，能够把queue.enqueue.timeout.ms配置成0，这样就kafka底层就不会出现阻塞了。

新版的kafka(我印象中是2.0.0版本开始？)用java重写了，再也不使用阻塞队列，因此没有上面说的问题。