摘要:今天在群上抛出来一个问题,如下我以自带的数据结构为例,用源码的形式说明,如何阻塞线程通知线程的。一以可重入锁和两个对象来控制并发。四使用来控制并发,同时也使用的对象来与线程交互。
今天在QQ群上抛出来一个问题,如下
我以Java自带的数据结构为例,用源码的形式说明,如何阻塞线程、通知线程的。
一、Lock & Condition
ArrayBlockingQueue以可重入锁和两个Condition对象来控制并发。
/* * Concurrency control uses the classic two-condition algorithm * found in any textbook. */ /** Main lock guarding all access */ private final ReentrantLock lock; /** Condition for waiting takes */ private final Condition notEmpty; /** Condition for waiting puts */ private final Condition notFull;
构造函数中初始化了notEmpty和notFull.
/** * Creates an ArrayBlockingQueue with the given (fixed) * capacity and the specified access policy. * @param capacity the capacity of this queue * @param fair if true then queue accesses for threads blocked * on insertion or removal, are processed in FIFO order; * if false the access order is unspecified. * @throws IllegalArgumentException if capacity is less than 1 */ 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(); }
二、线程阻塞
当ArrayBlockingQueue存储的元素是0个的时候,take()方法会阻塞.
public Object take() throws InterruptedException { final ReentrantLock lock = this.lock; lock.lockInterruptibly(); try { try { while (count == 0) notEmpty.await(); } catch (InterruptedException ie) { notEmpty.signal(); // propagate to non-interrupted thread throw ie; } Object x = extract(); return x; } finally { lock.unlock(); } }
这里take方法首先获得可重入锁lock,然后判断如果元素为空就执行notEmpty.await(); 这个时候线程挂起。
三、通知线程
比如使用put放入一个新元素,
/** * Inserts the specified element at the tail of this queue, waiting * for space to become available if the queue is full. * * @throws InterruptedException {@inheritDoc} * @throws NullPointerException {@inheritDoc} */ 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(); } }
在enqueue方法中,
/** * Inserts element at current put position, advances, and signals. * Call only when holding lock. */ 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++; notEmpty.signal(); }
对刚才的notEmptyCondition进行通知。
四、ReentrantLock vs AbstractQueuedSynchronizer
ArrayBlockingQueue使用ReentrantLock来控制并发,同时也使用ArrayBlockingQueue的Condition对象来与线程交互。notEmpty和notFull都是由
ReentrantLock的成员变量sync生成的,
public Condition newCondition() { return sync.newCondition(); }
sync可以认为是一个抽象类类型,Sync,它是在ReentrantLock内部定义的静态抽象类,抽象类实现了newCondition方法,
final ConditionObject newCondition() { return new ConditionObject(); }
返回的类型是实现了Condition接口的ConditionObject类,这是在AbstractQueuedSynchronizer内部定义的类。在ArrayBlockingQueue中的notEmpty就是ConditionObject实例。
阻塞:
当ArrayBlockingQueue为空时,notEmpty.await()将自己挂起,如ConditionObject的await方法,
/** * Implements interruptible condition wait. **
*/ public final void await() throws InterruptedException { if (Thread.interrupted()) throw new InterruptedException(); Node node = addConditionWaiter(); int savedState = fullyRelease(node); int interruptMode = 0; while (!isOnSyncQueue(node)) { LockSupport.park(this); if ((interruptMode = checkInterruptWhileWaiting(node)) != 0) break; } if (acquireQueued(node, savedState) && interruptMode != THROW_IE) interruptMode = REINTERRUPT; if (node.nextWaiter != null) // clean up if cancelled unlinkCancelledWaiters(); if (interruptMode != 0) reportInterruptAfterWait(interruptMode); }- If current thread is interrupted, throw InterruptedException. *
- Save lock state returned by {@link #getState}. *
- Invoke {@link #release} with saved state as argument, * throwing IllegalMonitorStateException if it fails. *
- Block until signalled or interrupted. *
- Reacquire by invoking specialized version of * {@link #acquire} with saved state as argument. *
- If interrupted while blocked in step 4, throw InterruptedException. *
addConditionWaiter是将当前线程作为一个node加入到ConditionObject的队列中,队列是用链表实现的。
如果是初次加入队列的情况,node.waitStatus == Node.CONDITION成立,方法isOnSyncQueue返回false,那么就将线程park。
while (!isOnSyncQueue(node)) { LockSupport.park(this); .... }
至此线程被挂起,LockSupport.park(this);这里this是指ConditionObject,是notEmpty.
通知:
当新的元素put进入ArrayBlockingQueue后,notEmpty.signal()通知在这上面等待的线程,如ConditionObject的signal方法,
/** * Moves the longest-waiting thread, if one exists, from the * wait queue for this condition to the wait queue for the * owning lock. * * @throws IllegalMonitorStateException if {@link #isHeldExclusively} * returns {@code false} */ public final void signal() { if (!isHeldExclusively()) throw new IllegalMonitorStateException(); Node first = firstWaiter; if (first != null) doSignal(first); }
doSignal方法,
/** * Removes and transfers nodes until hit non-cancelled one or * null. Split out from signal in part to encourage compilers * to inline the case of no waiters. * @param first (non-null) the first node on condition queue */ private void doSignal(Node first) { do { if ( (firstWaiter = first.nextWaiter) == null) lastWaiter = null; first.nextWaiter = null; } while (!transferForSignal(first) && (first = firstWaiter) != null); }
doSignal一开始接收到的参数就是firstWaiter这个参数,在内部实现中用了do..while的形式,首先将first的的nextWaiter找出来保存到firstWaiter此时(first和firstWaiter不是一回事),在while的比较条件中可调用了transferForSignal方法,
整个while比较条件可以看着短路逻辑,如果transferForSignal结果为true,后面的first = firstWaiter就不执行了,整个while循环就结束了。
参照注释,看
transferForSignal方法,
/** * Transfers a node from a condition queue onto sync queue. * Returns true if successful. * @param node the node * @return true if successfully transferred (else the node was * cancelled before signal) */ final boolean transferForSignal(Node node) { /* * If cannot change waitStatus, the node has been cancelled. */ if (!compareAndSetWaitStatus(node, Node.CONDITION, 0)) return false; /* * Splice onto queue and try to set waitStatus of predecessor to * indicate that thread is (probably) waiting. If cancelled or * attempt to set waitStatus fails, wake up to resync (in which * case the waitStatus can be transiently and harmlessly wrong). */ Node p = enq(node); int ws = p.waitStatus; if (ws > 0 || !compareAndSetWaitStatus(p, ws, Node.SIGNAL)) LockSupport.unpark(node.thread); return true; }
首先确保想要被signal的等待node还是处于Node.CONDITION状态,然后调整状态为Node.SIGNAL,这两个都是采用CAS方法,最后调用的是
LockSupport.unpark(node.thread);
五、LockSupport
至此,我们已经知道了线程的挂起和通知都是使用LockSupport来完成的,并发数据结构与线程直接的交互最终也是需要LockSupport。那么关于LockSupport,我们又可以了解多少呢?
Ref:
Java中的ReentrantLock和synchronized两种锁定机制的对比
Java的LockSupport.park()实现分析
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