ArrayBlockingQueue 源码解析

BlockingQueue也是JUC的重要知识，而且包含的BlockingQueue的种类还比较多，本篇先分析ArrayBlockingQueue ，主要分为三部分

1 创建

2 放数据

3 取数据

一 创建

public ArrayBlockingQueue(int capacity) {
        this(capacity, false);
    }

　　

public ArrayBlockingQueue(int capacity, boolean fair) {
        if (capacity <= 0)
            throw new IllegalArgumentException();
        this.items = new Object[capacity];
        lock = new ReentrantLock(fair);
        notEmpty = lock.newCondition();
        notFull =  lock.newCondition();
    }

　　BlockingQueue和HashMap可不一样啊，必须指定好初始值的大小，且可不能扩容

　　可以看内部，其实就是通过重入锁和Condition实现的

　　看看还有哪些重要的属性，这样的数据结构不难理解，底层存储是一个数组，要从数据取值就要有下标。而BlockingQueue是支持并发的，所以必须得有重入锁进行同步

/** The queued items */
    final Object[] items;

    /** items index for next take, poll, peek or remove */
    int takeIndex;

    /** items index for next put, offer, or add */
    int putIndex;

    /** Number of elements in the queue */
    int count;

    /*
     * Concurrency control uses the classic two-condition algorithm
     * found in any textbook.
     */

    /** Main lock guarding all access */
    final ReentrantLock lock;

    /** Condition for waiting takes */
    private final Condition notEmpty;

    /** Condition for waiting puts */
    private final Condition notFull;

    /**
     * Shared state for currently active iterators, or null if there
     * are known not to be any.  Allows queue operations to update
     * iterator state.
     */
    transient Itrs itrs = null;

二 放数据

　　

操作	抛出异常	特殊值	阻塞	超时
插入	add(e)	offer(e)	put(e)	offer(e, time, unit)
移除	remove()	poll()	take()	poll(time, unit)
检查	element()	peek()	-	-

　　offer的特殊值表示操作成功返回true，失败了返回false

　　poll也是的

　　我们首先分析三个放数据的方法，按照offer put add的顺序，因为add就我看源码的经验看用的真心不多

　　1 offer

public boolean offer(E e) {
        checkNotNull(e);
        final ReentrantLock lock = this.lock;
        lock.lock();
        try {
            if (count == items.length)
                return false;
            else {
                enqueue(e);
                return true;
            }
        } finally {
            lock.unlock();
        }
    }

private void enqueue(E x) {
        // assert lock.getHoldCount() == 1;
        // assert items[putIndex] == null;
        final Object[] items = this.items;
        items[putIndex] = x;
        if (++putIndex == items.length)
            putIndex = 0;//到头了就返回为0
        count++;
        notEmpty.signal();
    }

　　2 put

public void put(E e) throws InterruptedException {//具有抛中断异常的能力
        checkNotNull(e);
        final ReentrantLock lock = this.lock;
        lock.lockInterruptibly();
        try {
            while (count == items.length)
                notFull.await();//用condition的await方法阻塞线程
            enqueue(e);
        } finally {
            lock.unlock();
        }
    }

　　3 add

　　add方法定义在 AbstracQueue里面

public boolean add(E e) {
        if (offer(e))
            return true;
        else
            throw new IllegalStateException("Queue full");//抛异常的逻辑
    }

三 取数据

　　其实取数据的逻辑相比放数据要复杂好多，还是按照 poll take remove的方式分析

　　1 poll

public E poll() {
        final ReentrantLock lock = this.lock;
        lock.lock();
        try {
            return (count == 0) ? null : dequeue();
        } finally {
            lock.unlock();
        }
    }

private E dequeue() {
        // assert lock.getHoldCount() == 1;
        // assert items[takeIndex] != null;
        final Object[] items = this.items;
        @SuppressWarnings("unchecked")
        E x = (E) items[takeIndex];//纯数组操作
        items[takeIndex] = null;//原位置放null
        if (++takeIndex == items.length)//到头了就返回0
            takeIndex = 0;
        count--;//数量减一
        if (itrs != null)
            itrs.elementDequeued();
        notFull.signal();//唤醒Condition中的等待线程
        return x;
    }

　　2 take() 

public E take() throws InterruptedException {
        final ReentrantLock lock = this.lock;
        lock.lockInterruptibly();
        try {
            while (count == 0)
                notEmpty.await();//靠的就是这个阻塞当前线程
            return dequeue();
        } finally {
            lock.unlock();
        }
    }

　　3 remove

　　定义在 AbstracQueue里面，里面调用poll，如果返回值是null，直接抛异常

 public E remove() {
        E x = poll();
        if (x != null)
            return x;
        else
            throw new NoSuchElementException();
    }

四 查看数据

　　查看数据有两个方法 element和peek，先看peek

　　1 peek

public E peek() {
        final ReentrantLock lock = this.lock;
        lock.lock();
        try {
            return itemAt(takeIndex); // null when queue is empty，就是从数组拿值
        } finally {
            lock.unlock();
        }
    }

　　2 element

　　element定义在AbstracQueue

public E element() {
        E x = peek();
        if (x != null)
            return x;
        else
            throw new NoSuchElementException();
    }

五 总结

　　从上面的源码看，凡是定义在AbstractQueue里的都抛异常

　　阻塞都是通过conditioin的await方法