ArrayBlockingQueue源码分析

Posted 2022-03-10 本站大佬

构造方法：

方法	作用
add	增加数据，如果阻塞队列为满，则返回false
put	增加数据，如果阻塞队列为满，则等待
poll无超时时间	如果队列为空，则返回null
poll有超时时间	如果队列为空，则等待时间，超过等待时间，仍然为空，则返回null
take	在取出数据时，如果队列为空，则会等待
remainingCapacity	返回队列剩余容量
size	返回队列中数据的长度
remove	取出队列中指定的Object
contains	返回队列中是否含有某个对象

//一次性写入一个集合的数据
    public ArrayBlockingQueue(int capacity, boolean fair,
                              Collection<? extends E> c) 
        this(capacity, fair);
//final ReentrantLock lock;获取到这把锁
        final ReentrantLock lock = this.lock;
        lock.lock(); // Lock only for visibility, not mutual exclusion
        try 
            int i = 0;
            try 
                for (E e : c) 
                //检查元素是否为空。
                    checkNotNull(e);
                    items[i++] = e;
                
             catch (ArrayIndexOutOfBoundsException ex) 
                throw new IllegalArgumentException();
            
            count = i;
            //查看队列是否满了，如果满了的话就将putIndex为0.
            putIndex = (i == capacity) ? 0 : i;
         finally 
            lock.unlock();
        
    

    public ArrayBlockingQueue(int capacity, boolean fair) 
        if (capacity <= 0)
            throw new IllegalArgumentException();
        this.items = new Object[capacity];
        lock = new ReentrantLock(fair);
        //不为空条件变量
        notEmpty = lock.newCondition();
        //不满条件变量
        notFull =  lock.newCondition();
    

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

添加元素add

    public boolean add(E e) 
        return super.add(e);
    

    public boolean add(E e) 
    //在添加元素时调用offer方法。
        if (offer(e))
            return true;
        else
            throw new IllegalStateException("Queue full");
    

    public boolean offer(E e) 
        checkNotNull(e);
        //获取全局锁
        final ReentrantLock lock = this.lock;
        lock.lock();
        try 
            if (count == items.length)
                return false;
            else 
            //将满足条件的数据存入阻塞队列。
                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;
        count++;
        //通知不为空锁解开。
        notEmpty.signal();
    

添加元素put（分析源码可知，put遇到满队列时，会进入等待状态。而add不会。）

    public void put(E e) throws InterruptedException 
        checkNotNull(e);
        final ReentrantLock lock = this.lock;
        lock.lockInterruptibly();
        try 
            while (count == items.length)
                notFull.await();
            enqueue(e);
         finally 
            lock.unlock();
        
    

取出数据

    public E poll(long timeout, TimeUnit unit) throws InterruptedException 
        long nanos = unit.toNanos(timeout);
        final ReentrantLock lock = this.lock;
        //这个方法设置了超时时间，当获得锁后会进行判断，队列中是否有数据。如果没有则等待所设置的时间。
        lock.lockInterruptibly();
        try 
            while (count == 0) 
                if (nanos <= 0)
                    return null;
                nanos = notEmpty.awaitNanos(nanos);
            
            return dequeue();
         finally 
            lock.unlock();
        
    

    public E poll() 
        final ReentrantLock lock = this.lock;
        lock.lock();
        try 
        //如果阻塞队列中没有数据的话则返回null。
            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];
        items[takeIndex] = null;
        if (++takeIndex == items.length)
            takeIndex = 0;
        count--;
        if (itrs != null)
            itrs.elementDequeued();
        notFull.signal();
        return x;
    

take方法在取出数据时，如果队列为空，则会等待。
    public E take() throws InterruptedException 
        final ReentrantLock lock = this.lock;
        lock.lockInterruptibly();
        try 
            while (count == 0)
                notEmpty.await();
            return dequeue();
         finally 
            lock.unlock();