LinkedBlockingQueue源码分析
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几乎JDK中所有以linked开头的类(LinkedBlockingDeque、LinkedList、LinkedHashMap)等,其内部都是以链表做支撑。
现在我们来看看LinkedBlockingQueue的源码,了解其如何去实现阻塞队列的。
LinkedBlockingQueue在内部维护了一个Node对象,对象存放这当前节点跟下一个节点
static class Node<E> { //当前节点存放的 E item; /** * - 指向下一个节点 * - 指向当前节点,意味着这事头节点的下一个节点 * - 如果为空,意味着当前节点为尾节点 */ Node<E> next; Node(E x) { item = x; } }
//队列容量 private final int capacity; //当前队列长度 private final AtomicInteger count = new AtomicInteger();
队列里面维护着一个头节点(方便出队操作)跟最后一个节点(方便入队操作)
/** * 头节点, head.item 固定为 null */ transient Node<E> head; /** * 尾节点,last.next固定 null */ private transient Node<E> last;
LinkedBlockingQueue里面最核心的莫过于出队跟入队这两个锁,以及其所对应的Condition,其阻塞都是通过这两个取实现的
/**元素出队列的锁 */ private final ReentrantLock takeLock = new ReentrantLock(); /** 队列为空时,notEmpty进入await状态,此时从队列取元素的操作阻塞着,知道其他地方往队列存如元素。存放时会调用signal */ private final Condition notEmpty = takeLock.newCondition(); /** 元素进队列的锁 */ private final ReentrantLock putLock = new ReentrantLock(); /** 队列满时,notEmpty进入await状态,此时从队列存元素的操作阻塞着,直到其他地方从队列取元素。取元素时会调用signal */ private final Condition notFull = putLock.newCondition();
/** * 唤醒一个取元素的线程,只有在put或offer方法中调用(往队列里放了元素,此时队列肯定不为空,就可以唤醒取元素的线程) * 用于一开始队列为空,首次往里面放入元素的时候 */ private void signalNotEmpty() { final ReentrantLock takeLock = this.takeLock; takeLock.lock(); try { notEmpty.signal();//用锁保护是为了在唤醒取元素线程时,不会有其他线程从队列取走元素,导致队列为空 } finally { takeLock.unlock(); } } /** * 唤醒存放元素线程. 只能由take/poll调用(从队列取走元素,此时队列肯定不是满的,就可以唤醒存放元素的线程) * 用于当前队列满,取走一个元素之后调用 */ private void signalNotFull() { final ReentrantLock putLock = this.putLock; putLock.lock(); try { notFull.signal();//用锁保护是为了在唤醒存元素线程时,不会有其他线程从队列存元素,导致队列满了 } finally { putLock.unlock(); } }
接着就是入队跟出队的操作,都是通过操作指针来达到目的,后面结合图解可能比较好理解些
/** * 入队操作,当前尾指针指向新增的节点,之后最后节点指向新增节点(新增的节点就是最后的节点) * * @param node the node */ private void enqueue(Node<E> node) { // assert putLock.isHeldByCurrentThread(); // assert last.next == null; last = last.next = node; } /** * 出队列操作 * * @return the node */ private E dequeue() { // assert takeLock.isHeldByCurrentThread(); // assert head.item == null; Node<E> h = head; Node<E> first = h.next; h.next = h; // help GC head = first; E x = first.item; first.item = null; return x; }
下面这两个方法主要是某些特殊操作(清空队列,移除元素等)时控制整个队列不能出队入队
/** * put、take操作全锁住 */ void fullyLock() { putLock.lock(); takeLock.lock(); } /** * put、take操作全解锁 */ void fullyUnlock() { takeLock.unlock(); putLock.unlock(); }
/** * 当我们new一个LinkedBlockingQueue,默认队列容量的int的最大值 */ public LinkedBlockingQueue() { this(Integer.MAX_VALUE); } /** *初始化的时候,头、尾节点是同一个节点,且节点元素为空 */ public LinkedBlockingQueue(int capacity) { if (capacity <= 0) throw new IllegalArgumentException(); this.capacity = capacity; last = head = new Node<E>(null); } /** * 实例化的时候传入Collection,这种比较少用 */ 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(); } }
/** * 返回队列长度 */ public int size() { return count.get(); } /** * 剩余容量 */ public int remainingCapacity() { return capacity - count.get(); }
入队相关的操作,有阻塞方法,也有非阻塞方法
/** * 尾插入(队列先进先出),阻塞的方法 */ 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 { //队列已满,循环等待 while (count.get() == capacity) { notFull.await(); } //入队 enqueue(node); c = count.getAndIncrement();//注意此处getAndIncrement是先取当前值,之后count再+1 if (c + 1 < capacity) notFull.signal(); } finally { putLock.unlock(); } if (c == 0)//一开始队列空的时候,c == 0 signalNotEmpty();//唤醒取线程 } /** * 跟上面put差不多,只是多了超时插入失败操作,阻塞的方法(timeout时间内) */ 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);//awaitNanos若指定时间内收到signal()或signalALL()则返回nanos减去已经等待的时间; } enqueue(new Node<E>(e)); c = count.getAndIncrement(); if (c + 1 < capacity) notFull.signal(); } finally { putLock.unlock(); } if (c == 0) signalNotEmpty(); return true; } /** * 跟put差不多,只是offer方法如果存元素的时候队列满,立即失败,非阻塞的方法 */ 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 { //如果队列没满,node入队,c得到赋值 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;//队列满的时候,此处c=-1,返回false }
出队操作
/** * 取元素,阻塞的方法 */ 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) { //队列是空的,循环等待 notEmpty.await(); } x = dequeue(); c = count.getAndDecrement(); if (c > 1) notEmpty.signal(); } finally { takeLock.unlock(); } if (c == capacity) signalNotFull(); return x; } /** * 同样是取元素,多了超时失败操作,阻塞的方法(timeout时间内) */ 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; } /** * 同样是取元素,只是队列空的时候,返回的是null,非阻塞的方法 */ 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; }
/** * 返回队列中的第一个元素,但是第一个元素不会出队,非阻塞的方法 */ 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(); } }
移除节点的操作
/** * 用p的前一个节点来移除内部节点p(画解) */ void unlink(Node<E> p, Node<E> trail) { // assert isFullyLocked(); // p.next is not changed, to allow iterators that are // traversing p to maintain their weak-consistency guarantee. p.item = null; trail.next = p.next; if (last == p) last = trail; if (count.getAndDecrement() == capacity) notFull.signal(); } /** * 移除一个节点(移除的时候不允许从队列存、取元素) */ 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(); } }
/** * 判断队列是否存在元素o,(操作时不允许从队列存、取元素) */ public boolean contains(Object o) { if (o == null) return false; fullyLock(); try { for (Node<E> p = head.next; p != null; p = p.next) if (o.equals(p.item)) return true; return false; } finally { fullyUnlock(); } } /** * 队列转数组(操作时不允许从队列存、取元素) */ public Object[] toArray() { fullyLock(); try { int size = count.get(); Object[] a = new Object[size]; int k = 0; for (Node<E> p = head.next; p != null; p = p.next) a[k++] = p.item; return a; } finally { fullyUnlock(); } } /** * 转成特定类型数组 */ @SuppressWarnings("unchecked") public <T> T[] toArray(T[] a) { fullyLock(); try { int size = count.get(); if (a.length < size) a = (T[])java.lang.reflect.Array.newInstance (a.getClass().getComponentType(), size); int k = 0; for (Node<E> p = head.next; p != null; p = p.next) a[k++] = (T)p.item; if (a.length > k) a[k] = null; return a; } finally { fullyUnlock(); } } public String toString() { fullyLock(); try { Node<E> p = head.next; if (p == null) return "[]"; StringBuilder sb = new StringBuilder(); sb.append('['); for (;;) { E e = p.item; sb.append(e == this ? "(this Collection)" : e); p = p.next; if (p == null) return sb.append(']').toString(); sb.append(',').append(' '); } } finally { fullyUnlock(); } } /** * 原子的清空队列(画图) */ public void clear() { fullyLock(); try { for (Node<E> p, h = head; (p = h.next) != null; h = p) { h.next = h; p.item = null; } head = last; // assert head.item == null && head.next == null; if (count.getAndSet(0) == capacity) notFull.signal(); } finally { fullyUnlock(); } }
一般常用的方法就是上面的代码,余下的方法不做解读。接着用图来看看队列是如何处理的。
LinkedBlockingQueue里面的数据结构是这样的,Node对象保存着当前元素,还有下一个元素
当我们去实例化一个LinkedBlockingQueue的时候,队列中head、last指向同一个元素为空的Node节点
当往队列里放元素的时候,head位置不变,之后将head的next节点指向新增的节点,同时last也指向新增的节点
当往队列取元素的时候,是这样一个过程
到这,队列里面的代码以及出队入队的操作应该就相对清晰很多了,有些人可能会有疑问,为什么出队、入队操作里面都会有一个while循环,之后是await
while (count.get() == capacity) { notFull.await(); }
其实是这样的:await方法调用之后,当前线程会释放锁,知道其他线程调signal方法。但是signal方法每次只是随机的唤醒一个线程。那么,如果唤醒之后依然不满足条件,那么它还得继续等,知道条件满足(此处对应:队列满的时候,多个入队线程阻塞着,之后队列取走一个元素,有一个入队线程被唤醒,但它还没执行的时候,又有另一个线程入队,这时队列又满了,被唤醒的队列不满足 条件,还得继续等待)
signalNotEmpty()跟signalNotFull()为什么都在特殊情况下使用?
以signalNotEmpty为例:
队列一开始为空的时候,有多个线程来取元素,那么这些线程就都阻塞着,当往队列里面存入元素时,如果不去唤醒这些线程,在没有薪线程来取元素的情况下,这些线程就永远阻塞着,无论之后队列里又新增了多少个元素
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