java 併發工具包 BlockingQueue-ArrayBlockingQueue

簡介

ArrayBlockingQueue 字義理解就是 ： 數組阻塞隊列；看名字就很好理解！！
ArrayBlockingQueue 是有界隊列，意思是隊列個數不能超出某個數，超出入隊阻塞，初始化就需要定義好個數，不能自動擴增或者修改。
上源碼：

初始化

//初始化，定義臨界值及是否公平兩參數與
  public ArrayBlockingQueue(int capacity, boolean fair) {
  //臨界值必須>0
        if (capacity <= 0)
            throw new IllegalArgumentException();
            //初始化數組
        this.items = new Object[capacity];

        lock = new ReentrantLock(fair);
        //不空的標誌
        notEmpty = lock.newCondition();
        //不滿的標誌
        notFull =  lock.newCondition();
    }

入隊

因爲實現BlockingQueue 所以 ArrayBlockingQueue 也有add,put,offer入隊方法

  public boolean add(E e) {
      //其實就是調用offer 超出個數 報異常
        return super.add(e);
    }
     public boolean offer(E e) {
        checkNotNull(e);
        final ReentrantLock lock = this.lock;
        lock.lock();
        try {
        //不能超出數組個數
            if (count == items.length)
                return false;
            else {
            //入隊
                enqueue(e);
                return true;
            }
        } finally {
            lock.unlock();
        }
    }
      private void enqueue(E x) {

        final Object[] items = this.items;
        //直接賦值
        items[putIndex] = x;
        //如果數組已經滿了 ，重置putIndex 因爲出隊都是從0開始，
        if (++putIndex == items.length)
            putIndex = 0;
            //入隊次數
        count++;
        //通知不空阻塞 即 我有數據入隊了
        notEmpty.signal();
    }
      public void put(E e) throws InterruptedException {
        checkNotNull(e);
        final ReentrantLock lock = this.lock;
        //可打斷鎖
        lock.lockInterruptibly();
        try {
        //如果 已經滿了，則等待 等通知繼續入隊
            while (count == items.length)
                notFull.await();
            enqueue(e);
        } finally {
            lock.unlock();
        }
    }
      public boolean offer(E e, long timeout, TimeUnit unit)
        throws InterruptedException {

        checkNotNull(e);
        //獲取超時時間
        long nanos = unit.toNanos(timeout);
        final ReentrantLock lock = this.lock;
        lock.lockInterruptibly();
        try {
        //如果 已經滿了 則等待直到過期，繼續入隊
            while (count == items.length) {
                if (nanos <= 0)
                    return false;
                nanos = notFull.awaitNanos(nanos);
            }
            enqueue(e);
            return true;
        } finally {
            lock.unlock();
        }
    }

出隊

    //沒有則返回空
  public E poll() {
        final ReentrantLock lock = this.lock;
        lock.lock();
        try {
        //如果不空，則出隊
            return (count == 0) ? null : dequeue();
        } finally {
            lock.unlock();
        }
    }
 private E dequeue() {
        // assert lock.getHoldCount() == 1;
        // assert items[takeIndex] != null;
        final Object[] items = this.items;
        //直接根據takeIndex 獲取元素 將數組位置賦值爲空
        @SuppressWarnings("unchecked")
        E x = (E) items[takeIndex];
        items[takeIndex] = null;
        //超出循環 賦值爲0
        if (++takeIndex == items.length)
            takeIndex = 0;
           //減少個數，對應count++
        count--;
        if (itrs != null)
            itrs.elementDequeued();
            //通知不滿條件 因爲出隊 肯定不滿了
        notFull.signal();
        return x;
    }
    //必須等到元素出現
    public E take() throws InterruptedException {
        final ReentrantLock lock = this.lock;
        lock.lockInterruptibly();
        try {
        //如果爲空，則 一直等到不空的通知
            while (count == 0)
                notEmpty.await();
                //出隊
            return dequeue();
        } finally {
            lock.unlock();
        }
    }
    //在一定時間內獲取元素，過期返回null
    public E poll(long timeout, TimeUnit unit) throws InterruptedException {
    //生成時間
        long nanos = unit.toNanos(timeout);
        final ReentrantLock lock = this.lock;
        lock.lockInterruptibly();
        try {
            //如果爲空 則等你個nanos時間，還沒有？返回null
            while (count == 0) {
                if (nanos <= 0)
                    return null;
                nanos = notEmpty.awaitNanos(nanos);
            }
            //如果有數據 則出隊
            return dequeue();
        } finally {
            lock.unlock();
        }
    }
    //獲取元素，但不移除，即當前待獲取元素不變
    public E peek() {
        final ReentrantLock lock = this.lock;
        lock.lock();
        try {
            return itemAt(takeIndex); // null when queue is empty
        } finally {
            lock.unlock();
        }
    }
    //移除某元素 效率差，儘量避免使用
      public boolean remove(Object o) {
        if (o == null) return false;
        final Object[] items = this.items;
        final ReentrantLock lock = this.lock;
        lock.lock();
        try {
            if (count > 0) {
                final int putIndex = this.putIndex;
                int i = takeIndex;
                從//takeIndex 開始循環equals 找到
                do {
                    if (o.equals(items[i])) {
                    //找到就移除
                        removeAt(i);
                        return true;
                    }
                    if (++i == items.length)
                        i = 0;
                } while (i != putIndex);
            }
            return false;
        } finally {
            lock.unlock();
        }
    }
    //移除某位置元素 
      void removeAt(final int removeIndex) {
        // assert lock.getHoldCount() == 1;
        // assert items[removeIndex] != null;
        // assert removeIndex >= 0 && removeIndex < items.length;
        final Object[] items = this.items;
        if (removeIndex == takeIndex) {
            // removing front item; just advance
            //賦爲null
            items[takeIndex] = null;
            if (++takeIndex == items.length)
                takeIndex = 0;
            count--;
            if (itrs != null)
                itrs.elementDequeued();
        } else {
            // an "interior" remove

            // slide over all others up through putIndex.
            final int putIndex = this.putIndex;
            //設置putIndex爲removeIndex 即下次插入爲這個位置，並且循環往前移動知道原先的putIndex位置的前一個位置
            for (int i = removeIndex;;) {
                int next = i + 1;
                if (next == items.length)
                    next = 0;
                if (next != putIndex) {
                    items[i] = items[next];
                    i = next;
                } else {
                    items[i] = null;
                    this.putIndex = i;
                    break;
                }
            }
            //減少個數
            count--;
            if (itrs != null)
                itrs.removedAt(removeIndex);
        }
        //不滿條件通知
        notFull.signal();
    }

總結

ArrayBlockingQueue 是有界隊列，不能自動擴容，當時性能相較其他隊列要稍微好點；
適用於確定隊列個數或者某範圍的隊列！