最近在看Java Threads第三版，收获颇多。全英文阅读，感觉真的是爽歪歪。推荐大家都看看。
这一篇想系统的讲一讲，线程之间通信的2种模式，wait-notify 和 Condition。
先上一个生产者和消费者的例子
package waitnotify; import java.util.ArrayList; import java.util.List; import java.util.Random; public class Data { private List<Integer> data = new ArrayList<Integer>(5); private Random random = new Random(); public void put() { synchronized (data) { if (data.size() >= 5) { try { data.wait(); } catch (InterruptedException e) { e.printStackTrace(); } } else { data.add(random.nextInt(100)); data.notify(); } } } public void get() { synchronized (data) { if (data.size() < 1) { try { data.wait(); } catch (InterruptedException e) { e.printStackTrace(); } } else { data.remove(0); data.notify(); } } } }
package waitnotify; public class Producer extends Thread{ private Data data; public Producer(Data data, String name) { this.data = data; this.setName(name); } @Override public void run() { for (int i=0; i<3; i++) { data.put(); } } }
package waitnotify; public class Consumer extends Thread { private Data data; public Consumer(Data data, String name) { this.data = data; this.setName(name); } @Override public void run() { for (int i=0; i<3; i++) { data.get(); } } }
package waitnotify; public class Test { public static void main (String args[]) { Data data = new Data(); new Producer(data, "put---1").start(); new Consumer(data, "get---1").start(); new Consumer(data, "get---2").start(); new Producer(data, "put---2").start(); } }
这个例子简单、易懂、易读。首先基于synchronized锁定共享资源(data)，然后生产者和消费者通过wait和notify相互通信，实现互斥的访问共享资源。这里的关键是互斥，只要用了synchronized，所有的线程都只能互斥的执行。
1、那我们思考一个有关数据库的问题，写操作和读操作、写操作和写操作都是需要互斥的，读操作和读操作需要互斥吗？不需要，因为两个读操作同时进行，不会产生脏数据。这种情况，synchronized就无从下手。
2、还有，当多个线程访问synchronized代码，只有一个线程在执行，其它的线程只能死等。这里也是可以增加一些灵活性的，比如说，其它的线程可以等到一定时间后，中断自己，去干别的事情。
3、notify的作用是唤醒除了自己以外的其它线程，至于是哪一个，它不管。试想一个场景，有多个生产者线程和消费者线程。如果一个生产者线程发现data被装满，自己就等待，然后唤醒一个线程，那唤醒的那个线程是生产者线程，还是消费者线程呢？如果又唤醒了一个生产者线程，它还是等待，虽然不会产生脏数据，但是浪费了性能和时间啊。此处也可以优化。
下面就一起看看，如何解决上面的问题。
先来看看Lock这个接口，定义了哪些行为
public interface Lock { void lock(); void lockInterruptibly() throws InterruptedException; boolean tryLock(); boolean tryLock(long time, TimeUnit unit) throws InterruptedException; void unlock(); Condition newCondition(); }
其中2个抛出中断异常的方法可以解决第二个问题；newCondition方法生成多个Condition可以解决第三个问题；而另一个锁接口ReadWriteLock解决了第一个问题，实现类里面用到了共享锁。
再来一个Condition版本的生产者和消费者（主要是修改了Data类）
package condition; import java.util.ArrayList; import java.util.List; import java.util.Random; import java.util.concurrent.locks.Condition; import java.util.concurrent.locks.Lock; import java.util.concurrent.locks.ReentrantLock; public class Data { private List<Integer> data = new ArrayList<Integer>(5); private Random random = new Random(); Lock lock = new ReentrantLock(); Condition full = lock.newCondition(); Condition empty = lock.newCondition(); public void put() { lock.lock(); try { if (data.size() >= 5) { full.await(); } else { data.add(random.nextInt(100)); empty.signal(); } } catch (InterruptedException e) { e.printStackTrace(); } finally { lock.unlock(); } } public void get() { lock.lock(); try { if (data.size() < 1) { empty.await(); } else { data.remove(0); full.signal(); } } catch (InterruptedException e) { e.printStackTrace(); } finally { lock.unlock(); } } }
这里用一个lock生成了2个Condition，然后分别进行await和signal。假设一个生产者线程发现data装满，然后full.await，然后之前的empty.signal会唤醒一个线程，此线程必定是消费者线程。其实讲到这里，读者应该能猜到，full和empty分别维护了各自的等待队列，full的队列里全是生产者线程，empty队列里全是消费者线程。另外发现，代码似乎变复杂了，必须在finally里面释放锁。因为synchronized是JVM实现的锁语义，JVM会自动的帮你释放锁。而Lock是Java代码层面的语义，如果有异常，需要自己释放。
讲了半天，好像只是描述了一个表面现象，而没有接触到实质。没办法，源码走起。
从最重要的一个类ReentrantLock的实现看起，发现几乎所有的操作，兜兜转转都是调用了另一个核心类AbstractQueuedSynchronizer。这个类是Java并发大师的杰作，是Lock机制的灵魂所在，我至今还有些地方没能够完全领悟。哎，大师就是大师。
当然，大师也是站在巨人的肩膀上。AbstractQueuedSynchronizer的核心思想是CLH队列，说白了，就是将同一个锁上等待的所有线程看成一个个节点，每一个节点都在自旋，等待前驱节点的信号并且尝试获取锁。而且，AbstractQueuedSynchronizer将同一个Condition上等待的所有线程也封装成了节点队列。
那就先看看这个节点是个什么数据结构
static final class Node { /** Marker to indicate a node is waiting in shared mode */ static final Node SHARED = new Node(); /** Marker to indicate a node is waiting in exclusive mode */ static final Node EXCLUSIVE = null; /** waitStatus value to indicate thread has cancelled */ static final int CANCELLED = 1; /** waitStatus value to indicate successor's thread needs unparking */ static final int SIGNAL = -1; /** waitStatus value to indicate thread is waiting on condition */ static final int CONDITION = -2; /** * waitStatus value to indicate the next acquireShared should * unconditionally propagate */ static final int PROPAGATE = -3; /** * Status field, taking on only the values: * SIGNAL: The successor of this node is (or will soon be) * blocked (via park), so the current node must * unpark its successor when it releases or * cancels. To avoid races, acquire methods must * first indicate they need a signal, * then retry the atomic acquire, and then, * on failure, block. * CANCELLED: This node is cancelled due to timeout or interrupt. * Nodes never leave this state. In particular, * a thread with cancelled node never again blocks. * CONDITION: This node is currently on a condition queue. * It will not be used as a sync queue node * until transferred, at which time the status * will be set to 0. (Use of this value here has * nothing to do with the other uses of the * field, but simplifies mechanics.) * PROPAGATE: A releaseShared should be propagated to other * nodes. This is set (for head node only) in * doReleaseShared to ensure propagation * continues, even if other operations have * since intervened. * 0: None of the above * * The values are arranged numerically to simplify use. * Non-negative values mean that a node doesn't need to * signal. So, most code doesn't need to check for particular * values, just for sign. * * The field is initialized to 0 for normal sync nodes, and * CONDITION for condition nodes. It is modified using CAS * (or when possible, unconditional volatile writes). */ volatile int waitStatus; /** * Link to predecessor node that current node/thread relies on * for checking waitStatus. Assigned during enqueuing, and nulled * out (for sake of GC) only upon dequeuing. Also, upon * cancellation of a predecessor, we short-circuit while * finding a non-cancelled one, which will always exist * because the head node is never cancelled: A node becomes * head only as a result of successful acquire. A * cancelled thread never succeeds in acquiring, and a thread only * cancels itself, not any other node. */ volatile Node prev; /** * Link to the successor node that the current node/thread * unparks upon release. Assigned during enqueuing, adjusted * when bypassing cancelled predecessors, and nulled out (for * sake of GC) when dequeued. The enq operation does not * assign next field of a predecessor until after attachment, * so seeing a null next field does not necessarily mean that * node is at end of queue. However, if a next field appears * to be null, we can scan prev's from the tail to * double-check. The next field of cancelled nodes is set to * point to the node itself instead of null, to make life * easier for isOnSyncQueue. */ volatile Node next; /** * The thread that enqueued this node. Initialized on * construction and nulled out after use. */ volatile Thread thread; /** * Link to next node waiting on condition, or the special * value SHARED. Because condition queues are accessed only * when holding in exclusive mode, we just need a simple * linked queue to hold nodes while they are waiting on * conditions. They are then transferred to the queue to * re-acquire. And because conditions can only be exclusive, * we save a field by using special value to indicate shared * mode. */ Node nextWaiter; /** * Returns true if node is waiting in shared mode. */ final boolean isShared() { return nextWaiter == SHARED; } /** * Returns previous node, or throws NullPointerException if null. * Use when predecessor cannot be null. The null check could * be elided, but is present to help the VM. * * @return the predecessor of this node */ final Node predecessor() throws NullPointerException { Node p = prev; if (p == null) throw new NullPointerException(); else return p; } Node() { // Used to establish initial head or SHARED marker } Node(Thread thread, Node mode) { // Used by addWaiter this.nextWaiter = mode; this.thread = thread; } Node(Thread thread, int waitStatus) { // Used by Condition this.waitStatus = waitStatus; this.thread = thread; } }
这个Node是AbstractQueuedSynchronizer的一个内部类，大部分的英文注释说的很明白了。Node就是对线程的包装，Node有两种模式，独占（EXCLUSIVE）和共享（SHARED），前面提到的读-读操作就是利用了共享锁。对于一个节点的等待状态，CANCELLED表示当前节点被取消，比如说线程中断导致的；SIGNAL表示后继节点可以执行；CONDITION表示此节点在等待一个Condition，在Condition等待队列里面。
再看看Condition等待队列的结构
public class ConditionObject implements Condition, java.io.Serializable { private static final long serialVersionUID = 1173984872572414699L; /** First node of condition queue. */ private transient Node firstWaiter; /** Last node of condition queue. */ private transient Node lastWaiter; /** * Creates a new {@code ConditionObject} instance. */ public ConditionObject() { } // Internal methods /** * Adds a new waiter to wait queue. * @return its new wait node */ private Node addConditionWaiter() { Node t = lastWaiter; // If lastWaiter is cancelled, clean out. if (t != null && t.waitStatus != Node.CONDITION) { unlinkCancelledWaiters(); t = lastWaiter; } Node node = new Node(Thread.currentThread(), Node.CONDITION); if (t == null) firstWaiter = node; else t.nextWaiter = node; lastWaiter = node; return node; } /** * 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); } /** * 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); } /** * Unlinks cancelled waiter nodes from condition queue. * Called only while holding lock. This is called when * cancellation occurred during condition wait, and upon * insertion of a new waiter when lastWaiter is seen to have * been cancelled. This method is needed to avoid garbage * retention in the absence of signals. So even though it may * require a full traversal, it comes into play only when * timeouts or cancellations occur in the absence of * signals. It traverses all nodes rather than stopping at a * particular target to unlink all pointers to garbage nodes * without requiring many re-traversals during cancellation * storms. */ private void unlinkCancelledWaiters() { Node t = firstWaiter; Node trail = null; while (t != null) { Node next = t.nextWaiter; if (t.waitStatus != Node.CONDITION) { t.nextWaiter = null; if (trail == null) firstWaiter = next; else trail.nextWaiter = next; if (next == null) lastWaiter = trail; } else trail = t; t = next; } } // public methods /** * 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); } /** * 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); } /** * Implements uninterruptible condition wait. * <ol> * <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. * <li> Reacquire by invoking specialized version of * {@link #acquire} with saved state as argument. * </ol> */ public final void awaitUninterruptibly() { Node node = addConditionWaiter(); int savedState = fullyRelease(node); boolean interrupted = false; while (!isOnSyncQueue(node)) { LockSupport.park(this); if (Thread.interrupted()) interrupted = true; } if (acquireQueued(node, savedState) || interrupted) selfInterrupt(); } /* * For interruptible waits, we need to track whether to throw * InterruptedException, if interrupted while blocked on * condition, versus reinterrupt current thread, if * interrupted while blocked waiting to re-acquire. */ /** Mode meaning to reinterrupt on exit from wait */ private static final int REINTERRUPT = 1; /** Mode meaning to throw InterruptedException on exit from wait */ private static final int THROW_IE = -1; /** * Checks for interrupt, returning THROW_IE if interrupted * before signalled, REINTERRUPT if after signalled, or * 0 if not interrupted. */ private int checkInterruptWhileWaiting(Node node) { return Thread.interrupted() ? (transferAfterCancelledWait(node) ? THROW_IE : REINTERRUPT) : 0; } /** * Throws InterruptedException, reinterrupts current thread, or * does nothing, depending on mode. */ private void reportInterruptAfterWait(int interruptMode) throws InterruptedException { if (interruptMode == THROW_IE) throw new InterruptedException(); else if (interruptMode == REINTERRUPT) selfInterrupt(); } /** * 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(); 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); } /** * Implements timed 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, interrupted, or timed out. * <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 long awaitNanos(long nanosTimeout) throws InterruptedException { if (Thread.interrupted()) throw new InterruptedException(); Node node = addConditionWaiter(); int savedState = fullyRelease(node); final long deadline = System.nanoTime() + nanosTimeout; int interruptMode = 0; while (!isOnSyncQueue(node)) { if (nanosTimeout <= 0L) { transferAfterCancelledWait(node); break; } if (nanosTimeout >= spinForTimeoutThreshold) LockSupport.parkNanos(this, nanosTimeout); if ((interruptMode = checkInterruptWhileWaiting(node)) != 0) break; nanosTimeout = deadline - System.nanoTime(); } if (acquireQueued(node, savedState) && interruptMode != THROW_IE) interruptMode = REINTERRUPT; if (node.nextWaiter != null) unlinkCancelledWaiters(); if (interruptMode != 0) reportInterruptAfterWait(interruptMode); return deadline - System.nanoTime(); } /** * Implements absolute timed 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, interrupted, or timed out. * <li> Reacquire by invoking specialized version of * {@link #acquire} with saved state as argument. * <li> If interrupted while blocked in step 4, throw InterruptedException. * <li> If timed out while blocked in step 4, return false, else true. * </ol> */ public final boolean awaitUntil(Date deadline) throws InterruptedException { long abstime = deadline.getTime(); if (Thread.interrupted()) throw new InterruptedException(); Node node = addConditionWaiter(); int savedState = fullyRelease(node); boolean timedout = false; int interruptMode = 0; while (!isOnSyncQueue(node)) { if (System.currentTimeMillis() > abstime) { timedout = transferAfterCancelledWait(node); break; } LockSupport.parkUntil(this, abstime); if ((interruptMode = checkInterruptWhileWaiting(node)) != 0) break; } if (acquireQueued(node, savedState) && interruptMode != THROW_IE) interruptMode = REINTERRUPT; if (node.nextWaiter != null) unlinkCancelledWaiters(); if (interruptMode != 0) reportInterruptAfterWait(interruptMode); return !timedout; } /** * Implements timed 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, interrupted, or timed out. * <li> Reacquire by invoking specialized version of * {@link #acquire} with saved state as argument. * <li> If interrupted while blocked in step 4, throw InterruptedException. * <li> If timed out while blocked in step 4, return false, else true. * </ol> */ public final boolean await(long time, TimeUnit unit) throws InterruptedException { long nanosTimeout = unit.toNanos(time); if (Thread.interrupted()) throw new InterruptedException(); Node node = addConditionWaiter(); int savedState = fullyRelease(node); final long deadline = System.nanoTime() + nanosTimeout; boolean timedout = false; int interruptMode = 0; while (!isOnSyncQueue(node)) { if (nanosTimeout <= 0L) { timedout = transferAfterCancelledWait(node); break; } if (nanosTimeout >= spinForTimeoutThreshold) LockSupport.parkNanos(this, nanosTimeout); if ((interruptMode = checkInterruptWhileWaiting(node)) != 0) break; nanosTimeout = deadline - System.nanoTime(); } if (acquireQueued(node, savedState) && interruptMode != THROW_IE) interruptMode = REINTERRUPT; if (node.nextWaiter != null) unlinkCancelledWaiters(); if (interruptMode != 0) reportInterruptAfterWait(interruptMode); return !timedout; } // support for instrumentation /** * Returns true if this condition was created by the given * synchronization object. * * @return {@code true} if owned */ final boolean isOwnedBy(AbstractQueuedSynchronizer sync) { return sync == AbstractQueuedSynchronizer.this; } /** * Queries whether any threads are waiting on this condition. * Implements {@link AbstractQueuedSynchronizer#hasWaiters(ConditionObject)}. * * @return {@code true} if there are any waiting threads * @throws IllegalMonitorStateException if {@link #isHeldExclusively} * returns {@code false} */ protected final boolean hasWaiters() { if (!isHeldExclusively()) throw new IllegalMonitorStateException(); for (Node w = firstWaiter; w != null; w = w.nextWaiter) { if (w.waitStatus == Node.CONDITION) return true; } return false; } /** * Returns an estimate of the number of threads waiting on * this condition. * Implements {@link AbstractQueuedSynchronizer#getWaitQueueLength(ConditionObject)}. * * @return the estimated number of waiting threads * @throws IllegalMonitorStateException if {@link #isHeldExclusively} * returns {@code false} */ protected final int getWaitQueueLength() { if (!isHeldExclusively()) throw new IllegalMonitorStateException(); int n = 0; for (Node w = firstWaiter; w != null; w = w.nextWaiter) { if (w.waitStatus == Node.CONDITION) ++n; } return n; } /** * Returns a collection containing those threads that may be * waiting on this Condition. * Implements {@link AbstractQueuedSynchronizer#getWaitingThreads(ConditionObject)}. * * @return the collection of threads * @throws IllegalMonitorStateException if {@link #isHeldExclusively} * returns {@code false} */ protected final Collection<Thread> getWaitingThreads() { if (!isHeldExclusively()) throw new IllegalMonitorStateException(); ArrayList<Thread> list = new ArrayList<Thread>(); for (Node w = firstWaiter; w != null; w = w.nextWaiter) { if (w.waitStatus == Node.CONDITION) { Thread t = w.thread; if (t != null) list.add(t); } } return list; } }