ArrayBlockingQueue源码解读

//先看构造函数
//初始化一个给定容量的ArrayBlockingQueue
public ArrayBlockingQueue(int capacity) {
        this(capacity, false);
    }
//通过给定的容量初始化内部的数组和锁以及条件。
public ArrayBlockingQueue(int capacity, boolean fair) {
        if (capacity <= 0)
            throw new IllegalArgumentException();
	//初始化一个给定容量的数组
        this.items = new Object[capacity];
        lock = new ReentrantLock(fair);
        notEmpty = lock.newCondition();
        notFull =  lock.newCondition();
    }
//通过给定的集合初始化数组超出大小会抛出异常这里为什么要上锁？源码注释写的是这里不会有互斥,只是为了保证可见性。防止指令重排？
public ArrayBlockingQueue(int capacity, boolean fair,
                              Collection<? extends E> c) {
        this(capacity, fair);

        final ReentrantLock lock = this.lock;
        lock.lock(); 
        try {
            int i = 0;
            try {
                for (E e : c) {
                    checkNotNull(e);
                    items[i++] = e;
                }
            } catch (ArrayIndexOutOfBoundsException ex) {
                throw new IllegalArgumentException();
            }
	    //将数量设置为i
            count = i;
	    //设置putIndex的位置如果数组已满的话设置为0
            putIndex = (i == capacity) ? 0 : i;
        } finally {
            lock.unlock();
        }
    }

//add方法 调用offer方法新增元素如果成功返回true失败抛出异常
 public boolean add(E e) {
        return super.add(e);
    }

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

  public boolean offer(E e) {
        //不能加入null元素
        checkNotNull(e);
        final ReentrantLock lock = this.lock;
        lock.lock();
        try {
	    //如果数组已经满了
            if (count == items.length)
                return false;
            else {
                insert(e);
                return true;
            }
        } finally {
            lock.unlock();
        }
    }

    private void insert(E x) {
        
        items[putIndex] = x;
	//设置putIndex位置
        putIndex = inc(putIndex);
	//总数+1
        ++count;
	//因为数组已经非空所以唤醒notEmpty条件上的等待队列
        notEmpty.signal();
    }

    final int inc(int i) {
        //队列已满put设置为0否则设置为i+1
        return (++i == items.length) ? 0 : i;
    }

//poll方法没有元素获取null
    public E poll() {
        final ReentrantLock lock = this.lock;
        lock.lock();
        try {
	  //如果当前没有元素了返回null。
            return (count == 0) ? null : extract();
        } finally {
            lock.unlock();
        }
    }


    private E extract() {
        final Object[] items = this.items;
	//获取E的实际类型
        E x = this.<E>cast(items[takeIndex]);
	//将当前位置的值设置为null防止内存溢出。
        items[takeIndex] = null;
	//将takeIndex加1如果数组已满设置为0
        takeIndex = inc(takeIndex);
	//数组元素数量减少一个
        --count;
	//当前数组已经是非满状态所以唤醒在notFull条件上等待的队列。
        notFull.signal();
        return x;
    }

    @SuppressWarnings("unchecked")
    static <E> E cast(Object item) {
        return (E) item;
    }

//poll(long timeout, TimeUnit unit)在该时间内一直尝试去获取元素超时返回null。
/**这个方法实现的很精妙,直接在notEmpty条件上挂起当前线程，如果该条件被唤醒,
他就去获取一次值。
*/
public E poll(long timeout, TimeUnit unit) throws InterruptedException {
        long nanos = unit.toNanos(timeout);
        final ReentrantLock lock = this.lock;
        lock.lockInterruptibly();
        try {
            while (count == 0) {
                if (nanos <= 0)
                    return null;
		//在notEmpty条件上等待nanos时间
                nanos = notEmpty.awaitNanos(nanos);
            }
            return extract();
        } finally {
            lock.unlock();
        }
    }

//offer(E e, long timeout, TimeUnit unit)在指定时间内插入元素超时返回false
  public boolean offer(E e, long timeout, TimeUnit unit)
        throws InterruptedException {

        checkNotNull(e);
        long nanos = unit.toNanos(timeout);
        final ReentrantLock lock = this.lock;
        lock.lockInterruptibly();
        try {
            while (count == items.length) {
                if (nanos <= 0)
                    return false;
		//如果当前队列是满的那么在notFull条件上等待唤醒
                nanos = notFull.awaitNanos(nanos);
            }
            insert(e);
            return true;
        } finally {
            lock.unlock();
        }
    }

//remove方法成功返回true失败返回false
public boolean remove(Object o) {
        if (o == null) return false;
        final Object[] items = this.items;
        final ReentrantLock lock = this.lock;
        lock.lock();
        try {
	    //从takeIndex开始遍历查找o
            for (int i = takeIndex, k = count; k > 0; i = inc(i), k--) {
                if (o.equals(items[i])) {
                    removeAt(i);
                    return true;
                }
            }
            return false;
        } finally {
            lock.unlock();
        }
    }
//如果删除的是头元素直接删除takeIndex+1,否则滑动整个数组,重新构建整个数组。
void removeAt(int i) {
        final Object[] items = this.items;
        //如果移除正好位于takeindex上。
        if (i == takeIndex) {
            items[takeIndex] = null;
            takeIndex = inc(takeIndex);
        } else {
            // slide over all others up through putIndex.
            for (;;) {
                int nexti = inc(i);
		//移动整个元素列表
                if (nexti != putIndex) {
                    items[i] = items[nexti];
                    i = nexti;
                } else {
                    items[i] = null;
		    //将putindex往前移
                    putIndex = i;
                    break;
                }
            }
        }
        --count;
	//当前队列已经非满唤醒在非满条件上等待的线程
        notFull.signal();
    }

//peek:获取头元素如果没有获取到则返回null。仅仅是获取不移除。
 public E peek() {
        final ReentrantLock lock = this.lock;
        lock.lock();
        try {
            return (count == 0) ? null : itemAt(takeIndex);
        } finally {
            lock.unlock();
        }
    }

    final E itemAt(int i) {
        return this.<E>cast(items[i]);
    }


//put()如果队列已满。则阻塞。直到notFull条件被唤醒。
 public void put(E e) throws InterruptedException {
        checkNotNull(e);
        final ReentrantLock lock = this.lock;
        lock.lockInterruptibly();
        try {
            while (count == items.length)
                notFull.await();
            insert(e);
        } finally {
            lock.unlock();
        }
    }

//take()如果队列为空则阻塞.直到notEmpty条件被唤醒。
 public E take() throws InterruptedException {
        final ReentrantLock lock = this.lock;
        lock.lockInterruptibly();
        try {
            while (count == 0)
                notEmpty.await();
            return extract();
        } finally {
            lock.unlock();
        }
    }


//remainingCapacity()返回当前还能够容下的元素个数
public int remainingCapacity() {
        final ReentrantLock lock = this.lock;
        lock.lock();
        try {
            return items.length - count;
        } finally {
            lock.unlock();
        }
    }


//contains返回是否包含当前元素
   public boolean contains(Object o) {
        if (o == null) return false;
        final Object[] items = this.items;
        final ReentrantLock lock = this.lock;
        lock.lock();
        try {
            for (int i = takeIndex, k = count; k > 0; i = inc(i), k--)
                if (o.equals(items[i]))
                    return true;
            return false;
        } finally {
            lock.unlock();
        }
    }

//返回一个包含当前元素的新数组Object类型
   public Object[] toArray() {
        final Object[] items = this.items;
        final ReentrantLock lock = this.lock;
        lock.lock();
        try {
            final int count = this.count;
            Object[] a = new Object[count];
            for (int i = takeIndex, k = 0; k < count; i = inc(i), k++)
                a[k] = items[i];
            return a;
        } finally {
            lock.unlock();
        }
    }

//返回指定类型的新数组
 public <T> T[] toArray(T[] a) {
        final Object[] items = this.items;
        final ReentrantLock lock = this.lock;
        lock.lock();
        try {
            final int count = this.count;
            final int len = a.length;
	    //如果传入的数组长度<当前的元素个数就重新创建一个count长度的数组
            if (len < count)
                a = (T[])java.lang.reflect.Array.newInstance(
                    a.getClass().getComponentType(), count);
            for (int i = takeIndex, k = 0; k < count; i = inc(i), k++)
                a[k] = (T) items[i];
	    //如果长度>count
            if (len > count)
                a[count] = null;
            return a;
        } finally {
            lock.unlock();
        }
    }

//移除此队列中的所有元素
public void clear() {
        final Object[] items = this.items;
        final ReentrantLock lock = this.lock;
        lock.lock();
        try {
            for (int i = takeIndex, k = count; k > 0; i = inc(i), k--)
                items[i] = null;
       //将这几个变量设置为初始值。
            count = 0;
            putIndex = 0;
            takeIndex = 0;
            notFull.signalAll();
        } finally {
            lock.unlock();
        }
    }



//返回迭代器一个内部类实现这里就不写了
 public Iterator<E> iterator() {
        return new Itr();
    }

//移除队列中的所有元素并封装到collection中
public int drainTo(Collection<? super E> c) {
        checkNotNull(c);
        if (c == this)
            throw new IllegalArgumentException();
        final Object[] items = this.items;
        final ReentrantLock lock = this.lock;
        lock.lock();
        try {
            int i = takeIndex;
            int n = 0;
            int max = count;
            while (n < max) {
                c.add(this.<E>cast(items[i]));
		//帮助垃圾回收器回收,防止内存泄漏
                items[i] = null;
                i = inc(i);
                ++n;
            }
            if (n > 0) {
	    //将当前集合置为初始化状态
                count = 0;
                putIndex = 0;
                takeIndex = 0;
		//唤醒非满条件上的等待。
                notFull.signalAll();
            }
            return n;
        } finally {
            lock.unlock();
        }
    }

//从此队列中最多移出maxElements个元素。
public int drainTo(Collection<? super E> c, int maxElements) {
        checkNotNull(c);
        if (c == this)
            throw new IllegalArgumentException();
        if (maxElements <= 0)
            return 0;
        final Object[] items = this.items;
        final ReentrantLock lock = this.lock;
        lock.lock();
        try {
            int i = takeIndex;
            int n = 0;
	    //如果maxElements<count就移出maxElements个否则全部移出。
            int max = (maxElements < count) ? maxElements : count;
            while (n < max) {
                c.add(this.<E>cast(items[i]));
                items[i] = null;
                i = inc(i);
                ++n;
            }
            if (n > 0) {
                count -= n;
                takeIndex = i;
                notFull.signalAll();
            }
            return n;
        } finally {
            lock.unlock();
        }
    }



/**
总结：ArrayBlockingQueue低层是用数组+锁实现。是一种可以用于生产者消费者模式的可阻塞的队列。但是插入和删除这些操作都是用的一把锁。这可能会导致效率不高？
*/