JDK1.8 LinkedBlockingQueue类说明

Posted 2022-12-05 zhujm320

介绍

LinkedBlockingQueue是一种基于链表实现的阻塞队列，它实现了BlockingQueue的接口，线程安全。LinkedBlockingQueue初始化时，可以不指定大小，不指定大小默认大小为Integer.MAX_VALUE，属于无界队列。使用时建议初始化一个大小，避免LinkedBlockingQueue元素过大，将系统内存耗光。

关于队列的实现原理请参考 队列实现原理和JDK1.8 BlockingQueue接口说明，链表原理参考

设计思想

 

在队头出队，在队尾入队，出队和入队分别采用了各自的锁，这样入队和出队可以同时操作，提供队列的吞吐量，提高了效率。实现了生产者与消费者模型。

基本功能解读

节点类(内部类)

static class Node<E> 
        E item;
        Node<E> next;
        Node(E x)  item = x; 
    

队列元素节点，该元素有一个指针指向后继节点，单向链表节点。

成员属性

队列容量，如果不设置，默认大小为Integer.MAX_VALUE

private final int capacity;

队列实际大小

private final AtomicInteger count = new AtomicInteger();

采用原子操作，由于入队和出队可以同时操作，队列实际大小可能同时会变化，则需要一个原子操作来保障数据的同步。

队列头

transient Node<E> head;

队列尾

private transient Node<E> last;

出队锁

private final ReentrantLock takeLock = new ReentrantLock();

队列非空条件

private final Condition notEmpty = takeLock.newCondition();

入队锁

private final ReentrantLock putLock = new ReentrantLock();

队列非满条件

private final Condition notFull = putLock.newCondition();

构造函数

默认构造函数

public LinkedBlockingQueue() 
	this(Integer.MAX_VALUE);

默认队列大小为Integer.MAX_VALUE

需要设置队列大小的构造函数

public LinkedBlockingQueue(int capacity) 
        if (capacity <= 0) throw new IllegalArgumentException();
        this.capacity = capacity;
        last = head = new Node<E>(null);
    

集合初始化队列构造函数，队列大小为Integer.MAX_VALUE

public LinkedBlockingQueue(Collection<? extends E> c) 
        this(Integer.MAX_VALUE);			
        final ReentrantLock putLock = this.putLock;
        putLock.lock(); // Never contended, but necessary for visibility
        try 
            int n = 0;
            for (E e : c) 
                if (e == null)
                    throw new NullPointerException();
                if (n == capacity)
                    throw new IllegalStateException("Queue full");
                enqueue(new Node<E>(e));
                ++n;
            
            count.set(n);
         finally 
            putLock.unlock();
        
    

基本功能介绍

通知队列非空函数

private void signalNotEmpty() 
        final ReentrantLock takeLock = this.takeLock;
        takeLock.lock();
        try 
            notEmpty.signal();
         finally 
            takeLock.unlock();
        
    

如果有线程进行出队操作，由于队列没有元素，该线程被阻塞了，那么下次有元素入队时，则通过该函数进行唤醒

通知队列非满函数

private void signalNotFull() 
        final ReentrantLock putLock = this.putLock;
        putLock.lock();
        try 
            notFull.signal();
         finally 
            putLock.unlock();
        
    

如果有线程进行入队操作，由于队列满了，则该线程阻塞，那么下次有元素出队时，这时队列不是满的，则通过该函数进行唤醒

入队操作

阻塞式入队

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();			//队列满, 线程阻塞
            
            enqueue(node);
            c = count.getAndIncrement();
            if (c + 1 < capacity)
                notFull.signal();
         finally 
            putLock.unlock();
        
        if (c == 0)
            signalNotEmpty();
    

如果队列满，则线程阻塞（notFull.await()调用, 线程阻塞），反之如果队列不是满的，则通过enqueue进行入队操作

private void enqueue(Node<E> node) 
        // assert putLock.isHeldByCurrentThread();
        // assert last.next == null;
        last = last.next = node;
    

尾部入队操作

非阻塞入队

 public boolean offer(E e) 
        if (e == null) throw new NullPointerException();
        final AtomicInteger count = this.count;
        if (count.get() == capacity)
            return false;
        int c = -1;
        Node<E> node = new Node<E>(e);
        final ReentrantLock putLock = this.putLock;
        putLock.lock();
        try 
            if (count.get() < capacity) 
                enqueue(node);
                c = count.getAndIncrement();
                if (c + 1 < capacity)
                    notFull.signal();
            
         finally 
            putLock.unlock();
        
        if (c == 0)
            signalNotEmpty();
        return c >= 0;
    

非阻塞入队，如果队列满，直接返回false

出队操作

阻塞式出队

public E take() throws InterruptedException 
        E x;
        int c = -1;
        final AtomicInteger count = this.count;
        final ReentrantLock takeLock = this.takeLock;
        takeLock.lockInterruptibly();
        try 
            while (count.get() == 0) 
                notEmpty.await();		//队列为空, 则线程阻塞
            
            x = dequeue();
            c = count.getAndDecrement();
            if (c > 1)
                notEmpty.signal();
         finally 
            takeLock.unlock();
        
        if (c == capacity)
            signalNotFull();
        return x;
    

空队列时，调用该函数进行出队操作，则线程直接阻塞

非阻塞出队操作

public E poll() 
        final AtomicInteger count = this.count;
        if (count.get() == 0)
            return null;
        E x = null;
        int c = -1;
        final ReentrantLock takeLock = this.takeLock;
        takeLock.lock();
        try 
            if (count.get() > 0) 
                x = dequeue();
                c = count.getAndDecrement();
                if (c > 1)
                    notEmpty.signal();
            
         finally 
            takeLock.unlock();
        
        if (c == capacity)
            signalNotFull();
        return x;
    

非阻塞出队，队列为空时，直接返回null

查看对头元素

public E peek() 
        if (count.get() == 0)
            return null;
        final ReentrantLock takeLock = this.takeLock;
        takeLock.lock();
        try 
            Node<E> first = head.next;
            if (first == null)
                return null;
            else
                return first.item;
         finally 
            takeLock.unlock();
        
    

查看对头元素，如果队列是空的，直接返回null

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