一个关于wait/notify与锁关系的探究
wait/notify 机制是解决生产者消费者问题的良药。它的核心逻辑是基于条件变量的锁机制处理。所以，它们到底是什么关系？wait()时是否需要持有锁？ notify()是否需要持有锁？先说答案：都需要持有锁。
wait需要持有锁的原因是，你肯定需要知道在哪个对象上进行等待，如果不持有锁，将无法做到对象变更时进行实时感知通知的作用。与此同时，为了让其他线程可以操作该值的变化，它必须要先释放掉锁，然后在该节点上进行等待。不持有锁而进行wait，可能会导致长眠不起。而且，如果不持有锁，则当wait之后的操作，都可能是错的，因为可能这个数据已经过时，其实也叫线程不安全了。总之，一切为了安全，单独的wait做不成这事。
notify需要持有锁的原因是，它要保证线程的安全，只有它知道数据变化了，所以它有权力去通知其他线程数据变化。而且通知完之后，不能立即释放锁，即必须在持有锁的情况下进行通知，否则notify后续的工作的线程安全性将无法保证，尽量它是在lock的范围内，但却因为锁释放，将导致不可预期的结果。而且在notify的时候，并不能真正地将对应的线程唤醒，即不能从操作系统层面唤醒线程，因为此时当前通知线程持有锁，而此时如果将其他等待线程唤醒，它们将立即参与到锁的竞争中来，而这时的竞争是一定会失败的，这可能会导致被唤醒的线程立即又进入等待队列，更糟糕的是它可能再也不会被唤醒 了。所以不能将在持有锁的时，将对应的线程真正唤醒，我们看到的notify只是从语言上下文级别，将它从等待队列转移到同步队列而已，对此操作系统一无所知。
1. 实验验证
我们通过一个实验来看一下，wait/和notify是否会在持有锁的情况下进行。
private ReentrantLock mainLock = new ReentrantLock(); @Test public void testWaitNotify() throws InterruptedException { Condition c1 = mainLock.newCondition(); Condition c3 = mainLock.newCondition(); CountDownLatch t1StartLatch = new CountDownLatch(2); Thread t1 = new Thread(() -> { mainLock.lock(); try { System.out.println(LocalDateTime.now() + " - t1 start"); c1.await(); System.out.println(LocalDateTime.now() + " - t1 c1 await out"); // 过早通知问题，导致无法测试下一步 // c3.await(); // System.out.println(LocalDateTime.now() + " - t1 c2 await out"); t1StartLatch.await(); System.out.println(LocalDateTime.now() + " - t1 sleeping"); SleepUtil.sleepMillis(10_000L); c1.signalAll(); c3.signalAll(); System.out.println(LocalDateTime.now() + " - t1 notified, sleeping again"); SleepUtil.sleepMillis(10_000L); System.out.println(LocalDateTime.now() + " - t1 out"); } catch (Exception e) { System.err.println("t1 exception "); e.printStackTrace(); } finally { mainLock.unlock(); } }, "t1"); Thread t2 = new Thread(() -> { mainLock.lock(); try { t1StartLatch.countDown(); System.out.println(LocalDateTime.now() + " - t2 c1 signal"); c1.signalAll(); System.out.println(LocalDateTime.now() + " - t2 wait"); c1.await(); System.out.println(LocalDateTime.now() + " - t2 out"); } catch (Exception e) { System.err.println("t2 exception "); e.printStackTrace(); } finally { mainLock.unlock(); } }, "t2"); Thread t3 = new Thread(() -> { mainLock.lock(); try { t1StartLatch.countDown(); System.out.println(LocalDateTime.now() + " - t3 c3 signal"); c3.signalAll(); System.out.println(LocalDateTime.now() + " - t3 wait"); c3.await(); System.out.println(LocalDateTime.now() + " - t3 out"); } catch (Exception e) { System.err.println("t2 exception "); e.printStackTrace(); } finally { mainLock.unlock(); } }, "t3"); t1.start(); t2.start(); t3.start(); t1.join(); System.out.println(LocalDateTime.now() + " - main t1 out"); t2.join(); System.out.println(LocalDateTime.now() + " - main t2 out"); t3.join(); System.out.println(LocalDateTime.now() + " - main t3 out"); }
大概意思是，针对同一个锁，wait之后，是否可以被其他线程进入临界区？如果wait之前不通知进入，wait之后能进入，说明wait依赖于锁，而且会释放当前锁。notify之后，wait()是否会立即执行，如果必须等到notify的模块完成后，才执行，说明notify是必须要依赖于锁的。
结果如下：
2022-03-27T20:09:43.588 - t1 start 2022-03-27T20:09:43.603 - t2 c1 signal 2022-03-27T20:09:43.603 - t2 wait 2022-03-27T20:09:43.603 - t3 c3 signal 2022-03-27T20:09:43.603 - t3 wait 2022-03-27T20:09:43.603 - t1 c1 await out 2022-03-27T20:09:43.603 - t1 sleeping 2022-03-27T20:09:53.605 - t1 notified, sleeping again 2022-03-27T20:10:03.612 - t1 out 2022-03-27T20:10:03.612 - t2 out 2022-03-27T20:10:03.612 - main t1 out 2022-03-27T20:10:03.612 - t3 out 2022-03-27T20:10:03.612 - main t2 out 2022-03-27T20:10:03.612 - main t3 out 2022-03-27T20:11:39.982 - t1 start 2022-03-27T20:11:39.982 - t2 c1 signal 2022-03-27T20:11:39.982 - t2 wait 2022-03-27T20:11:39.982 - t3 c3 signal 2022-03-27T20:11:39.982 - t3 wait 2022-03-27T20:11:39.982 - t1 c1 await out 2022-03-27T20:11:39.982 - t1 sleeping 2022-03-27T20:11:49.989 - t1 notified, sleeping again 2022-03-27T20:11:59.990 - t1 out 2022-03-27T20:11:59.990 - t2 out 2022-03-27T20:11:59.990 - main t1 out 2022-03-27T20:11:59.990 - t3 out 2022-03-27T20:11:59.990 - main t2 out 2022-03-27T20:11:59.990 - main t3 out
2. wait/notify 的实现机制
我们以AQS的实现机制为线索，探索wait/notify机制。它在唤醒操作队列时，设置状态为 SIGNAL , 但它实际不执行操作系统唤醒。
// java.util.concurrent.locks.AbstractQueuedSynchronizer.ConditionObject#signalAll /** * Moves all threads 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 signalAll() { if (!isHeldExclusively()) throw new IllegalMonitorStateException(); Node first = firstWaiter; if (first != null) doSignalAll(first); } // java.util.concurrent.locks.AbstractQueuedSynchronizer.ConditionObject#doSignalAll /** * Removes and transfers all nodes. * @param first (non-null) the first node on condition queue */ private void doSignalAll(Node first) { lastWaiter = firstWaiter = null; do { Node next = first.nextWaiter; first.nextWaiter = null; transferForSignal(first); first = next; } while (first != null); } // java.util.concurrent.locks.AbstractQueuedSynchronizer#transferForSignal /** * 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; // 不到万不得已，不会真正唤醒等待中的队列，从而满足notify无法将线程唤醒的作用，或者说线程仍然在操作系统的等待队列上 // 它只是将当前线程移动到本语文的同步队列中，以下线程下次运行过来时可以通过该限制 if (ws > 0 || !compareAndSetWaitStatus(p, ws, Node.SIGNAL)) LockSupport.unpark(node.thread); return true; } /** * Inserts node into queue, initializing if necessary. See picture above. * @param node the node to insert * @return node's predecessor */ private Node enq(final Node node) { for (;;) { Node t = tail; if (t == null) { // Must initialize if (compareAndSetHead(new Node())) tail = head; } else { node.prev = t; if (compareAndSetTail(t, node)) { t.next = node; return t; } } } } // java.util.concurrent.locks.AbstractQueuedSynchronizer.ConditionObject#await() /** * Implements interruptible condition wait. * <ol> * <li> If current thread is interrupted, throw InterruptedException. * <li> Save lock state returned by {@link #getState}. * <li> Invoke {@link #release} with saved state as argument, * throwing IllegalMonitorStateException if it fails. * <li> Block until signalled or interrupted. * <li> Reacquire by invoking specialized version of * {@link #acquire} with saved state as argument. * <li> If interrupted while blocked in step 4, throw InterruptedException. * </ol> */ public final void await() throws InterruptedException { if (Thread.interrupted()) throw new InterruptedException(); Node node = addConditionWaiter(); // 进来等待队列，先释放锁，此时进入线程不安全状态 int savedState = fullyRelease(node); int interruptMode = 0; // 此判断只是本语文级别的等待队列限制 // notify 时只能满足这个条件，而不会将线程从操作系统挂起队列中唤醒，即不会进行 LockSupport.unpark() while (!isOnSyncQueue(node)) { // 交由操作系统进行线程挂起 LockSupport.park(this); if ((interruptMode = checkInterruptWhileWaiting(node)) != 0) break; } // 重新进行锁的获取，尝试 if (acquireQueued(node, savedState) && interruptMode != THROW_IE) interruptMode = REINTERRUPT; if (node.nextWaiter != null) // clean up if cancelled unlinkCancelledWaiters(); if (interruptMode != 0) reportInterruptAfterWait(interruptMode); } // java.util.concurrent.locks.AbstractQueuedSynchronizer#acquireQueued /** * Acquires in exclusive uninterruptible mode for thread already in * queue. Used by condition wait methods as well as acquire. * * @param node the node * @param arg the acquire argument * @return {@code true} if interrupted while waiting */ final boolean acquireQueued(final Node node, int arg) { boolean failed = true; try { boolean interrupted = false; for (;;) { final Node p = node.predecessor(); // 获取当锁，则替换head后返回 // 而 tryAcquire() 则由各自策略实现 if (p == head && tryAcquire(arg)) { setHead(node); p.next = null; // help GC failed = false; return interrupted; } // 如果获取不到锁，则重新进入操作系统等待队列 if (shouldParkAfterFailedAcquire(p, node) && parkAndCheckInterrupt()) interrupted = true; } } finally { if (failed) cancelAcquire(node); } }
所以，总结：
1. wait将会释放持有的锁;
2. wait将会加入到语言级别的等待队列，同时也会提交给操作系统的等待队列，做到真正的线程挂起;
3. wait将会在被操作系统唤醒后，重新进行新一轮的锁获取尝试，返回时已携带回原有的锁，从外部看起来就像锁一直都在一样;
4. notify不会真正的唤醒等待的线程，而只是将各等待线程从语言级别的等待队列移出，到语言级别的同步队列;
5. notify只有在极端情况下，才会做到线程的真正唤醒作用，比如中断，但这被唤醒的线程将无法正常进行业务操作，所以也是安全的;
6. 只有在整体的锁在进行 unlock() 的时候，才会唤醒线程，使其重新参与锁的竞争;
3. lock/unlock 流程
同样的AQS的实现为线索，lock/unlock 流程如下:
// java.util.concurrent.locks.ReentrantLock#lock /** * Acquires the lock. * * <p>Acquires the lock if it is not held by another thread and returns * immediately, setting the lock hold count to one. * * <p>If the current thread already holds the lock then the hold * count is incremented by one and the method returns immediately. * * <p>If the lock is held by another thread then the * current thread becomes disabled for thread scheduling * purposes and lies dormant until the lock has been acquired, * at which time the lock hold count is set to one. */ public void lock() { sync.lock(); } // java.util.concurrent.locks.ReentrantLock.NonfairSync#lock /** * Performs lock. Try immediate barge, backing up to normal * acquire on failure. */ final void lock() { if (compareAndSetState(0, 1)) setExclusiveOwnerThread(Thread.currentThread()); else acquire(1); } // java.util.concurrent.locks.AbstractQueuedSynchronizer#acquire /** * Acquires in exclusive mode, ignoring interrupts. Implemented * by invoking at least once {@link #tryAcquire}, * returning on success. Otherwise the thread is queued, possibly * repeatedly blocking and unblocking, invoking {@link * #tryAcquire} until success. This method can be used * to implement method {@link Lock#lock}. * * @param arg the acquire argument. This value is conveyed to * {@link #tryAcquire} but is otherwise uninterpreted and * can represent anything you like. */ public final void acquire(int arg) { if (!tryAcquire(arg) && // 同上wait时的锁争抢操作 acquireQueued(addWaiter(Node.EXCLUSIVE), arg)) selfInterrupt(); } // java.util.concurrent.locks.ReentrantLock#unlock /** * Attempts to release this lock. * * <p>If the current thread is the holder of this lock then the hold * count is decremented. If the hold count is now zero then the lock * is released. If the current thread is not the holder of this * lock then {@link IllegalMonitorStateException} is thrown. * * @throws IllegalMonitorStateException if the current thread does not * hold this lock */ public void unlock() { sync.release(1); } // java.util.concurrent.locks.AbstractQueuedSynchronizer#release /** * Releases in exclusive mode. Implemented by unblocking one or * more threads if {@link #tryRelease} returns true. * This method can be used to implement method {@link Lock#unlock}. * * @param arg the release argument. This value is conveyed to * {@link #tryRelease} but is otherwise uninterpreted and * can represent anything you like. * @return the value returned from {@link #tryRelease} */ public final boolean release(int arg) { if (tryRelease(arg)) { Node h = head; // 直接唤醒头节点（真正的唤醒） if (h != null && h.waitStatus != 0) unparkSuccessor(h); return true; } return false; } // java.util.concurrent.locks.AbstractQueuedSynchronizer#unparkSuccessor /** * Wakes up node's successor, if one exists. * * @param node the node */ private void unparkSuccessor(Node node) { /* * If status is negative (i.e., possibly needing signal) try * to clear in anticipation of signalling. It is OK if this * fails or if status is changed by waiting thread. */ int ws = node.waitStatus; if (ws < 0) compareAndSetWaitStatus(node, ws, 0); /* * Thread to unpark is held in successor, which is normally * just the next node. But if cancelled or apparently null, * traverse backwards from tail to find the actual * non-cancelled successor. */ Node s = node.next; if (s == null || s.waitStatus > 0) { s = null; for (Node t = tail; t != null && t != node; t = t.prev) if (t.waitStatus <= 0) s = t; } // 真正唤醒线程，只有一个线程将被唤醒 if (s != null) LockSupport.unpark(s.thread); }
总结： lock/unlock 是一个真正的上锁解锁操作，上锁时如未成功，则进行park()进行操作系统挂起，解锁时将头节点unpark()交由操作系统调度。
4. 唤醒多个等待线程
如何唤醒多个等待线程？共享锁有这个需求，其实也是notifyAll 的表面语义所在。
// java.util.concurrent.locks.AbstractQueuedSynchronizer#releaseShared /** * Releases in shared mode. Implemented by unblocking one or more * threads if {@link #tryReleaseShared} returns true. * * @param arg the release argument. This value is conveyed to * {@link #tryReleaseShared} but is otherwise uninterpreted * and can represent anything you like. * @return the value returned from {@link #tryReleaseShared} */ public final boolean releaseShared(int arg) { if (tryReleaseShared(arg)) { doReleaseShared(); return true; } return false; } // java.util.concurrent.locks.AbstractQueuedSynchronizer#doReleaseShared /** * Release action for shared mode -- signals successor and ensures * propagation. (Note: For exclusive mode, release just amounts * to calling unparkSuccessor of head if it needs signal.) */ private void doReleaseShared() { /* * Ensure that a release propagates, even if there are other * in-progress acquires/releases. This proceeds in the usual * way of trying to unparkSuccessor of head if it needs * signal. But if it does not, status is set to PROPAGATE to * ensure that upon release, propagation continues. * Additionally, we must loop in case a new node is added * while we are doing this. Also, unlike other uses of * unparkSuccessor, we need to know if CAS to reset status * fails, if so rechecking. */ for (;;) { Node h = head; if (h != null && h != tail) { int ws = h.waitStatus; if (ws == Node.SIGNAL) { if (!compareAndSetWaitStatus(h, Node.SIGNAL, 0)) continue; // loop to recheck cases // 唤醒头节点 unparkSuccessor(h); } // 因为上一头节点刚刚被设置为0，说明正在执行中，设置当前head为 PROPAGATE else if (ws == 0 && !compareAndSetWaitStatus(h, 0, Node.PROPAGATE)) continue; // loop on failed CAS } // 即尽量只设置一个 head 节点即可 // 除非在这期间发生变更 if (h == head) // loop if head changed break; } } // java.util.concurrent.locks.AbstractQueuedSynchronizer#acquireSharedInterruptibly /** * Acquires in shared mode, aborting if interrupted. Implemented * by first checking interrupt status, then invoking at least once * {@link #tryAcquireShared}, returning on success. Otherwise the * thread is queued, possibly repeatedly blocking and unblocking, * invoking {@link #tryAcquireShared} until success or the thread * is interrupted. * @param arg the acquire argument. * This value is conveyed to {@link #tryAcquireShared} but is * otherwise uninterpreted and can represent anything * you like. * @throws InterruptedException if the current thread is interrupted */ public final void acquireSharedInterruptibly(int arg) throws InterruptedException { if (Thread.interrupted()) throw new InterruptedException(); if (tryAcquireShared(arg) < 0) doAcquireSharedInterruptibly(arg); } // java.util.concurrent.locks.AbstractQueuedSynchronizer#doAcquireSharedInterruptibly /** * Acquires in shared interruptible mode. * @param arg the acquire argument */ private void doAcquireSharedInterruptibly(int arg) throws InterruptedException { final Node node = addWaiter(Node.SHARED); boolean failed = true; try { for (;;) { final Node p = node.predecessor(); if (p == head) { int r = tryAcquireShared(arg); if (r >= 0) { // 共享式锁的传播性质实现 setHeadAndPropagate(node, r); p.next = null; // help GC failed = false; return; } } if (shouldParkAfterFailedAcquire(p, node) && parkAndCheckInterrupt()) throw new InterruptedException(); } } finally { if (failed) cancelAcquire(node); } } // java.util.concurrent.locks.AbstractQueuedSynchronizer#setHeadAndPropagate /** * Sets head of queue, and checks if successor may be waiting * in shared mode, if so propagating if either propagate > 0 or * PROPAGATE status was set. * * @param node the node * @param propagate the return value from a tryAcquireShared */ private void setHeadAndPropagate(Node node, int propagate) { Node h = head; // Record old head for check below setHead(node); /* * Try to signal next queued node if: * Propagation was indicated by caller, * or was recorded (as h.waitStatus either before * or after setHead) by a previous operation * (note: this uses sign-check of waitStatus because * PROPAGATE status may transition to SIGNAL.) * and * The next node is waiting in shared mode, * or we don't know, because it appears null * * The conservatism in both of these checks may cause * unnecessary wake-ups, but only when there are multiple * racing acquires/releases, so most need signals now or soon * anyway. */ if (propagate > 0 || h == null || h.waitStatus < 0 || (h = head) == null || h.waitStatus < 0) { Node s = node.next; // 递归进行唤醒下一线程节点，从而级联唤醒 if (s == null || s.isShared()) doReleaseShared(); } } /** * Release action for shared mode -- signals successor and ensures * propagation. (Note: For exclusive mode, release just amounts * to calling unparkSuccessor of head if it needs signal.) */ private void doReleaseShared() { /* * Ensure that a release propagates, even if there are other * in-progress acquires/releases. This proceeds in the usual * way of trying to unparkSuccessor of head if it needs * signal. But if it does not, status is set to PROPAGATE to * ensure that upon release, propagation continues. * Additionally, we must loop in case a new node is added * while we are doing this. Also, unlike other uses of * unparkSuccessor, we need to know if CAS to reset status * fails, if so rechecking. */ for (;;) { Node h = head; if (h != null && h != tail) { int ws = h.waitStatus; if (ws == Node.SIGNAL) { if (!compareAndSetWaitStatus(h, Node.SIGNAL, 0)) continue; // loop to recheck cases unparkSuccessor(h); } else if (ws == 0 && !compareAndSetWaitStatus(h, 0, Node.PROPAGATE)) continue; // loop on failed CAS } if (h == head) // loop if head changed break; } }
总结： 多个线程的唤醒，主要是使用了级联唤醒的机制，在做共享锁时，根据现有的情况，进行唤醒下一线程。而当线程调度很快或算法不确定时，就会给人一种所有线程一起被唤醒工作的效果。
不要害怕今日的苦，你要相信明天，更苦！
