Java 7之多线程并发容器 - LinkedBlockingQueue

发布时间:2014-10-22 19:24:34编辑:www.fx114.net 分享查询网我要评论
本篇文章主要介绍了"Java 7之多线程并发容器 - LinkedBlockingQueue",主要涉及到Java 7之多线程并发容器 - LinkedBlockingQueue方面的内容,对于Java 7之多线程并发容器 - LinkedBlockingQueue感兴趣的同学可以参考一下。

LinkedBlockingQueue是一个单向链表实现的阻塞队列。该队列按 FIFO(先进先出)排序元素,新元素插入到队列的尾部,并且队列获取操作会获得位于队列头部的元素。链接队列的吞吐量通常要高于基于数组的队列,但是在大多数并发应用程序中,其可预知的性能要低。 此外,LinkedBlockingQueue还是可选容量的(防止过度膨胀),即可以指定队列的容量。如果不指定,默认容量大小等于Integer.MAX_VALUE。 看一下LinkedBlockingQueue类中定义的重要变量和Node节点,如下: // 链表的节点定义 static class Node<E> { E item; Node<E> next; Node(E x) { item = x; } } private final int capacity; // 链表的容量,可以在创建链表时指定 private final AtomicInteger count = new AtomicInteger(0);// 记录队列元素个数 private transient Node<E> head; // 链表的表头。取出数据时,都是从表头head处插入 private transient Node<E> last; // 链表的表尾。新增数据时,都是从表尾last处插入 // putLock是插入锁,takeLock是取出锁;notEmpty是非空条件,notFull是未满条件。通过它们对链表进行并发控制 private final ReentrantLock takeLock = new ReentrantLock(); private final Condition notEmpty = takeLock.newCondition(); private final ReentrantLock putLock = new ReentrantLock(); private final Condition notFull = putLock.newCondition(); 看一下主要的两个构造函数,如下: public LinkedBlockingQueue(int capacity) { if (capacity <= 0) throw new IllegalArgumentException(); this.capacity = capacity; last = head = new Node<E>(null); } 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(); } } 将c容器中的元素做为Node节点的值添加到链表中,如下: private void enqueue(Node<E> node) { // assert putLock.isHeldByCurrentThread(); // assert last.next == null; last = last.next = node; } 将node节点添加到last.next节点后面,并将last.next节点的值赋给last. 1、添加元素 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); 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(); } enqueue(node); c = count.getAndIncrement(); if (c + 1 < capacity) notFull.signal(); } finally { putLock.unlock(); } if (c == 0) signalNotEmpty(); } /** * Inserts the specified element at the tail of this queue, waiting if * necessary up to the specified wait time for space to become available. */ public boolean offer(E e, long timeout, TimeUnit unit) throws InterruptedException { if (e == null) throw new NullPointerException(); long nanos = unit.toNanos(timeout); int c = -1; final ReentrantLock putLock = this.putLock; final AtomicInteger count = this.count; putLock.lockInterruptibly(); try { while (count.get() == capacity) { if (nanos <= 0) return false; nanos = notFull.awaitNanos(nanos); } enqueue(new Node<E>(e)); c = count.getAndIncrement(); if (c + 1 < capacity) notFull.signal(); } finally { putLock.unlock(); } if (c == 0) signalNotEmpty(); return true; } /** * Inserts the specified element at the tail of this queue if it is * possible to do so immediately without exceeding the queue's capacity, */ 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); 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; } 2、删除元素 public boolean remove(Object o) { if (o == null) return false; fullyLock(); try { for (Node<E> trail = head, p = trail.next; p != null; trail = p, p = p.next) { if (o.equals(p.item)) { unlink(p, trail); return true; } } return false; } finally { fullyUnlock(); } } 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(); } x = dequeue(); c = count.getAndDecrement(); if (c > 1) notEmpty.signal(); } finally { takeLock.unlock(); } if (c == capacity) signalNotFull(); return x; } public E poll(long timeout, TimeUnit unit) throws InterruptedException { E x = null; int c = -1; long nanos = unit.toNanos(timeout); final AtomicInteger count = this.count; final ReentrantLock takeLock = this.takeLock; takeLock.lockInterruptibly(); try { while (count.get() == 0) { if (nanos <= 0) return null; nanos = notEmpty.awaitNanos(nanos); } x = dequeue(); c = count.getAndDecrement(); if (c > 1) notEmpty.signal(); } finally { takeLock.unlock(); } if (c == capacity) signalNotFull(); return x; } 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; } 3、删除元素 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(); } } 4、迭代元素 public Iterator<E> iterator() { return new Itr(); } private class Itr implements Iterator<E> { // 当前节点 private Node<E> current; // 上一次返回的节点 private Node<E> lastRet; // 当前节点对应的值 private E currentElement; Itr() { // 同时获取“插入锁putLock” 和 “取出锁takeLock” fullyLock(); try { // 设置“当前元素”为“队列表头的下一节点”,即为队列的第一个有效节点 current = head.next; if (current != null) currentElement = current.item; } finally { // 释放“插入锁putLock” 和 “取出锁takeLock” fullyUnlock(); } } // 返回“下一个节点是否为null” public boolean hasNext() { return current != null; } private Node<E> nextNode(Node<E> p) { for (;;) { Node<E> s = p.next; if (s == p) return head.next; if (s == null || s.item != null) return s; p = s; } } // 返回下一个节点 public E next() { fullyLock(); try { if (current == null) throw new NoSuchElementException(); E x = currentElement; lastRet = current; current = nextNode(current); currentElement = (current == null) ? null : current.item; return x; } finally { fullyUnlock(); } } // 删除下一个节点 public void remove() { if (lastRet == null) throw new IllegalStateException(); fullyLock(); try { Node<E> node = lastRet; lastRet = null; for (Node<E> trail = head, p = trail.next; p != null; trail = p, p = p.next) { if (p == node) { unlink(p, trail); break; } } } finally { fullyUnlock(); } } }


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