java并发锁ReentrantLock源码分析二之Condition实现原理
1、java.util.concurrent.locks.Condition接口一览
void await() throws InterruptedException;
void awaitUninterruptibly();
long awaitNanos(long nanosTimeout) throws InterruptedException;
boolean await(long time, TimeUnit unit) throws InterruptedException;
boolean awaitUntil(Date deadline) throws InterruptedException;
void signal();
void signalAll();Condition 实现的语义为 Object.wait 与 Object.notify。
关于Condition 的实现类为 AbstractQueuedSynchronizer.ConditionObject 内部类。
首先在讲解源码之前，我重点罗列出ConditionObject的关键数据结构：
private transient Node fristWaiter;
private transient Node lastWaiter;从这里看出，每个CondtionObject,都维护着自己的条件等待等待队列，并且是一个双端链表。
1.1 void await() throws InterruptedException
/**
* Implements interruptible condition wait.
*
* - 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.
*
*/
public final void await() throws InterruptedException {
if (Thread.interrupted()) // @1
throw new InterruptedException();
Node node = addConditionWaiter(); //@2
int savedState = fullyRelease(node); // @3
int interruptMode = 0;
while (!isOnSyncQueue(node)) { //@4
LockSupport.park(this);
if ((interruptMode = checkInterruptWhileWaiting(node)) != 0) // @5
break;
}
if (acquireQueued(node, savedState) && interruptMode != THROW_IE) //@6
interruptMode = REINTERRUPT;
if (node.nextWaiter != null) // clean up if cancelled //@7
unlinkCancelledWaiters();
if (interruptMode != 0) //@8
reportInterruptAfterWait(interruptMode);
}代码@1：检测当前线程的中断标记，如果中断位为1，则抛出异常。
代码@2：添加等待节点。就是一个简单的链表维护节点的操作，具体参照addConditionWaiter讲解。
代码@3：释放占有的锁，并获取当前锁的state,因为await实现的语意为Object.wait,释放锁并并等待条件的发生。当条件满足后，线程被唤醒后，第一步是需要获取锁，然后在上次await的下一条指令处继续执行。代码3就是实现上述语义的释放锁。
代码@4：isOnSyncQueue 当前节点是否在同步队列中，如果在同步阻塞队列中，则申请锁，去执行；如果不在同步队列中（在条件队列中），阻塞，等待满足条件，新增的节点，默认在条件队列中(Conditon)。isOnSyncQueue 源码解读在下文中；
代码@5：线程从条件等待被唤醒,唤醒后，线程要从条件队列移除，进入到同步等待队列，被唤醒有有如下两种情况，一是条件满足，收到singal信号，二是线程被取消（中断），该步骤是从条件队列移除，加入到同步等待队列，返回被唤醒的原因，如果是被中断，需要根据不同模式，处理中断。处理中断，也有两种方式：1.继续设置中断位；2：直接抛出InterruptedException。请看下文关于checkInterruptWhileWaiting的源码解读。
代码@6：运行到代码6时，说明线程已经结束了释放锁，从条件队列移除，线程运行，在继续执行业务逻辑之前，必须先获取锁。只有成功获取锁后，才会去判断线程的中断标志，才能在中断标志为真时，抛出InterruptException。
代码@7，执行一些收尾工作，清理整个条件队列：
代码@8，处理中断，是设置中断位，还是抛出InterruptException。
那我们先关注一下addConditionWaiter方法：
/**
* 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) { //@1
unlinkCancelledWaiters();
t = lastWaiter;
}
Node node = new Node(Thread.currentThread(), Node.CONDITION); //@2
if (t == null)
firstWaiter = node;
else
t.nextWaiter = node;
lastWaiter = node;
return node;
}添加条件等待节点，根据链表的特征，直接在尾部节点的nextWaiter指向新建的节点，并将新建的节点设置为整个链表的尾部，首先要知道如下数据结构：
object {
Node firstWaiter;
Node lastWaiter;
node {
node nextWaiter;
该节点承载的业务数据，比如这里的Thread t;等
}
}知道上述结构，其实整个链的数据维护，基本一目了然，自己都可以实现下面的逻辑。
代码@1,如果最后一个等待节点的状态不是Node.CONDITION,则，则先删除等待链中节点状态不为Node.CONDITION的节点。具体代码分析请参照下文unlinkCancelledWaiters的解读。
代码@2开始，就是普通链表的节点添加的基本方法。
清除等待节点方法。
/**
* 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; //@1
while (t != null) {
Node next = t.nextWaiter;
if (t.waitStatus != Node.CONDITION) { // @3
t.nextWaiter = null;
if (trail == null) // @4
firstWaiter = next;
else
trail.nextWaiter = next; //@5
if (next == null) // @6
lastWaiter = trail;
}
else // @4
trail = t;
t = next;
}
}该方法的思路为，从第一节点开始，将不等于Node.CONDITION的节点。
代码@1,设置尾部节点临时变量，用来记录最终的尾部节点。代码@1 第一次循环，是循环第一个节点，如果它的状态为Node.CONDITION, 则该链的头节点保持不变，设置临时尾节点为t,然后进行一个节点的判断，如果节点不为Node.CONDITION, 重置头节点的下一个节点，或尾部节点的下一个节点(@4,@5)。代码@6代表整个循环结束，设置 ConditionObject对象的lastWaiter为trail的值；
await步骤中，释放锁过程源码解析。释放锁的过程，逻辑为unlock,但该方法，返回当前锁的state,因为释放锁后，该方法在条件没有满足前提下，自身需要阻塞。被唤醒后，需要先尝试获取锁，然后才能执行接下来的逻辑。
/**
* Invokes release with current state value; returns saved state.
* Cancels node and throws exception on failure.
* @param node the condition node for this wait
* @return previous sync state
*/
final int fullyRelease(Node node) {
boolean failed = true;
try {
int savedState = getState();
if (release(savedState)) {
failed = false;
return savedState;
} else {
throw new IllegalMonitorStateException();
}
} finally {
if (failed)
node.waitStatus = Node.CANCELLED;
}
}await，@4步骤中，isOnSyncQueue 源码解读：
/**
* Returns true if a node, always one that was initially placed on
* a condition queue, is now waiting to reacquire on sync queue.
* @param node the node
* @return true if is reacquiring
*/
final boolean isOnSyncQueue(Node node) {
if (node.waitStatus == Node.CONDITION || node.prev == null) // @1
return false;
if (node.next != null) // If has successor, it must be on queue // @2
return true;
/*
* node.prev can be non-null, but not yet on queue because
* the CAS to place it on queue can fail. So we have to
* traverse from tail to make sure it actually made it. It
* will always be near the tail in calls to this method, and
* unless the CAS failed (which is unlikely), it will be
* there, so we hardly ever traverse much.
*/
return findNodeFromTail(node);
}代码@1,如果节点的状态为Node.CONDITION 或 node.prev == null,表明该节点在条件队列中，并没有加入同步阻塞队列（同步阻塞队列为申请锁等待的队列），await方法中，新增的节点，默认满足上述条件，所以返回false,表示在条件队列中，等待条件的发生，条件满足之前，当前线程应该阻塞。这里，先预留一个疑问，那node.prev在什么时候会改变呢？
代码@2,如果node.next不为空，说明在同步阻塞队列中。这个我想毫无疑问。当然也说明next域肯定是在进入同步队列过程中会设置值。
代码@3, 上面的注释也说的比较清楚，node.prev不为空，但也不在同步队列中，这个是由于CAS可能会失败，为了不丢失信号，从同步队列中再次选择该节点，如果找到则返回true,否则返回false,在这里，我就更加对node.prev在什么时候会设置值感兴趣了，请继续await方法向下看，总有水落石出的时候。
await @5 checkInterruptWhileWaiting 代码解读：
/*
* 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; // 重新设置中断位，中断由上层处理
/** Mode meaning to throw InterruptedException on exit from wait */
private static final int THROW_IE = -1; // 直接抛出 InterruptedException 0:正常
/**
* 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;
}
/**
* Transfers node, if necessary, to sync queue after a cancelled
* wait. Returns true if thread was cancelled before being
* signalled.
* @param current the waiting thread
* @param node its node
* @return true if cancelled before the node was signalled
*/
final boolean transferAfterCancelledWait(Node node) {
if (compareAndSetWaitStatus(node, Node.CONDITION, 0)) { //@1
enq(node);
return true;
}
/*
* If we lost out to a signal(), then we can't proceed
* until it finishes its enq(). Cancelling during an
* incomplete transfer is both rare and transient, so just
* spin.
*/
while (!isOnSyncQueue(node))
Thread.yield();
return false;
}
/**
* 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;
}
}
}
}重点关注@1,首先需要知道一点，如果收到正常的singal()信号而被唤醒的节点【这个再singal方法时重点分析】，状态为Node.SINGAL,不会是Node.CONDITION状态，所以如果代码1compareAndSetWaitStatus设置成功，说明线程是调用了t.interrupt方法而使得LockSupport.park解除阻塞的，然后将该节点加入到同步队列中，使得 while( ! isOnSyncQueue(node)) 的条件为真，结束 await的等待条件触发语义，，进入到 抢占锁阶段。【再次重申Object wait语义，释放当前锁，然后等待条件的触发【条件队列】，，条件发生后，要先重新去抢占锁，获取锁则继续执行，否则阻塞在获取锁【同步队列】】，所以当 线程阻塞在 await 方法时，调用 t.interrupt方法时只是中断条件队列的等待，并不能马上取消执行，马上抛出InterrupterException。
await方法流程图：
1.2 signal()方法详解
/**
* 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() { // @1
if (!isHeldExclusively()) //如果当前线程不是锁的持有者，直接抛出异常。
throw new IllegalMonitorStateException();
Node first = firstWaiter;
if (first != null)
doSignal(first); //通知第一个等待者 //@2
}
/**
* 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) // @3
lastWaiter = null;
first.nextWaiter = null;
} while (!transferForSignal(first) &&
(first = firstWaiter) != null); // @4
}
/**
* 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) { // @5
/*
* If cannot change waitStatus, the node has been cancelled.
*/
if (!compareAndSetWaitStatus(node, Node.CONDITION, 0)) //@6
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); // @7
int ws = p.waitStatus;
if (ws > 0 || !compareAndSetWaitStatus(p, ws, Node.SIGNAL)) //@8
LockSupport.unpark(node.thread);
return true;
}signal的具体实现，是从doSignal方法开始。
@代码3，首先将要被通知的节点的下一个节点设置为等待队列的head节点，如果当前节点的下一个节点为空，则设在等待队列的尾节点（lastWaiter）设置为空，然后将当前被通知的节点的下一个节点设为空；该步骤核心思想就是将被通知节点移除条件等待队列，然后重新维护条件等待对的firstWaiter和lastWaiter。
@代码4，!transferForSignal(first) && (first = firstWaiter) != null，根据后文的解析可以知道，如果被通知节点没有进入到同步阻塞队列(transferForSignal返回false)并且条件等待队列还有不为空的节点，则继续循环通知。
@代码5，transferForSignal该方法，将被通知的节点放入同步等待队列。
@代码6，首先判断，尝试将节点状态设置为0,如果设置失败，则说明该节点的状态已经不是Node.CONDITION,进一步说明该节点在没有等到通知信号时，被取消，直接返回false,通知下一个等待者。（回到代码@3,@4）
@代码7，将节点放入到同步队列中。个人认为信号通知，主要是将节点从条件等待队列移入到同步等待队列，主要是防止sinal信号的丢失。
@代码8，如果前置节点取消，或者在设置前置节点状态为Node.SIGNAL状态失败时，唤醒被通知节点代表的线程，@8设置失败发送的情况也就是前置节点状态发送改变（被取消等），所以直接唤醒被通知节点的线程，也就是说，sinal方法，只有在入队列后，前置节点被取消时，才会执行LockSupport.unpark方法唤醒线程，通常该方法，只是将节点从条件等待队列放入同步队列，然后该方法执行完毕，释放持有的锁。
整个通知sinal方法的流程如下：
