源码入口
ReentrantLock reentrantLock = new ReentrantLock();
reentrantLock.lock();
reentrantLock.unlock();
一.构造方法
/** Synchronizer providing all implementation mechanics */
private final Sync sync;
/**
* Creates an instance of {@code ReentrantLock}.
* This is equivalent to using {@code ReentrantLock(false)}.
*/
public ReentrantLock() {
sync = new NonfairSync();
}
/**
* Creates an instance of {@code ReentrantLock} with the
* given fairness policy.
*
* @param fair {@code true} if this lock should use a fair ordering policy
*/
public ReentrantLock(boolean fair) {
sync = fair ? new FairSync() : new NonfairSync();
}
1.无参构造方法默认初始化非公平的Sync实现，Sync继承AbstractQueuedSynchronizer。AbstractQueuedSynchronizer -- 为实现依赖于先进先出 (FIFO) 等待队列的阻塞锁和相关同步器（信号量、事件，等等）提供了一个框架。
2.传入一个boolean值的有参构造方法，根据传入的boolean来初始化公平或非公平的Sync实现。
二.reentrantLock.lock()
lock()方法流程图
1.由reentrantLock.lock()点进ReentrantLock类的lock()方法
public void lock() {
sync.lock();
}
2.sync.lock()进入Sync内部类的lock抽象方法，找到非公平实现。
/**
* 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);
}
lock方法首先用CAS操作将state从0改为1：
如果成功，设置独占线程为当前线程，lock()结束。（对应流程图‘state==0’）
3.如果CAS操作失败，进入acquire(1)方法。
/**
* 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) &&
acquireQueued(addWaiter(Node.EXCLUSIVE), arg))
selfInterrupt();
}
4.首先if()条件里面，进入tryAcquire(1)方法，找到非公平实现。进入nonfairTryAcquire(1)方法
protected final boolean tryAcquire(int acquires) {
return nonfairTryAcquire(acquires);
}
/**
* Performs non-fair tryLock. tryAcquire is implemented in
* subclasses, but both need nonfair try for trylock method.
*/
final boolean nonfairTryAcquire(int acquires) {
final Thread current = Thread.currentThread();
int c = getState();
if (c == 0) {
if (compareAndSetState(0, acquires)) {
setExclusiveOwnerThread(current);
return true;
}
}
else if (current == getExclusiveOwnerThread()) {
int nextc = c + acquires;
if (nextc < 0) // overflow
throw new Error("Maximum lock count exceeded");
setState(nextc);
return true;
}
return false;
}
获取当前线程，判断state == 0，如果相等，和上一步操作一样。如果不等，判断当前线程是否是独占线程。如果是，state+1，lock()结束。
如果不是，返回false。（对应流程图‘当前线程是否是独占线程’）
5.再次回到acquire(1)方法里面
/**
* 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) &&
acquireQueued(addWaiter(Node.EXCLUSIVE), arg))
selfInterrupt();
}
if()判断里面，第一步tryAcquire(1)返回false以后，进入第二个条件判断里面，首先将当前线程添加到等待队列里面addWaiter(Node.EXCLUSIVE)。
6.进入addWaiter(Node.EXCLUSIVE)方法
/**
* Creates and enqueues node for current thread and given mode.
*
* @param mode Node.EXCLUSIVE for exclusive, Node.SHARED for shared
* @return the new node
*/
private Node addWaiter(Node mode) {
Node node = new Node(Thread.currentThread(), mode);
// Try the fast path of enq; backup to full enq on failure
Node pred = tail;
if (pred != null) {
node.prev = pred;
if (compareAndSetTail(pred, node)) {
pred.next = node;
return node;
}
}
enq(node);
return node;
}
从第五步传来的mode为Node.EXCLUSIVE，即为独占模式。首先new一个新的节点，然后获取当前队列的尾节点。这里的Node是一个双向链表，就不展开看了。
如果尾节点不为空，则将new出来的节点设为尾节点，返回新的节点。
如果为空，进行入队操作，然后再将新的节点返回。
7.addWaiter(Node.EXCLUSIVE)返回Node之后，进入第5步if()判断里面的 acquireQueued(Node, arg)方法
/**
* 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();
if (p == head && tryAcquire(arg)) {
setHead(node);
p.next = null; // help GC
failed = false;
return interrupted;
}
if (shouldParkAfterFailedAcquire(p, node) &&
parkAndCheckInterrupt())
interrupted = true;
}
} finally {
if (failed)
cancelAcquire(node);
}
}
/**
* Sets head of queue to be node, thus dequeuing. Called only by
* acquire methods. Also nulls out unused fields for sake of GC
* and to suppress unnecessary signals and traversals.
*
* @param node the node
*/
private void setHead(Node node) {
head = node;
node.thread = null;
node.prev = null;
}
首先，将failed = true，然后try代码块里面，进入一个死循环。
通过 node.predecessor()，获取第六步addWaiter(Node.EXCLUSIVE)返回Node的前节点p。
如果p是头节点，进行第4步的tryAcquire(1)操作，操作成功，将当前节点设为头节点(头节点是空的)。lock()方法结束。
上面的操作失败以后，首先判断是否应该将此节点挂起，传入参数为当前节点的前节点p和当前节点node，第一次操作即为head和node。
8.进入if()判断里面的shouldParkAfterFailedAcquire(p, node) 方法，
/**
* Checks and updates status for a node that failed to acquire.
* Returns true if thread should block. This is the main signal
* control in all acquire loops. Requires that pred == node.prev.
*
* @param pred node's predecessor holding status
* @param node the node
* @return {@code true} if thread should block
*/
private static boolean shouldParkAfterFailedAcquire(Node pred, Node node) {
int ws = pred.waitStatus;
if (ws == Node.SIGNAL)
/*
* This node has already set status asking a release
* to signal it, so it can safely park.
*/
return true;
if (ws > 0) {
/*
* Predecessor was cancelled. Skip over predecessors and
* indicate retry.
*/
do {
node.prev = pred = pred.prev;
} while (pred.waitStatus > 0);
pred.next = node;
} else {
/*
* waitStatus must be 0 or PROPAGATE. Indicate that we
* need a signal, but don't park yet. Caller will need to
* retry to make sure it cannot acquire before parking.
*/
compareAndSetWaitStatus(pred, ws, Node.SIGNAL);
}
return false;
}
第一次进来此方法，前节点p的waitStatus没有被初始化过，所以默认是0。
进入else代码块里面，然后将前节点p的waitStatus设为Node.SIGNAL，即-1。此方法返回false。
9.shouldParkAfterFailedAcquire(p, node) 返回false之后，for()代码块执行结束，开始进行第二次循环。
for (;;) {
final Node p = node.predecessor();
if (p == head && tryAcquire(arg)) {
setHead(node);
p.next = null; // help GC
failed = false;
return interrupted;
}
if (shouldParkAfterFailedAcquire(p, node) &&
parkAndCheckInterrupt())
interrupted = true;
}
重复进行第7步操作，然后再次进入shouldParkAfterFailedAcquire(p, node)方法，在第8步中，已经将p的waitStatus设为Node.SIGNAL了。所以直接返回true，说明此时线程应该被挂起。
int ws = pred.waitStatus;
if (ws == Node.SIGNAL)
/*
* This node has already set status asking a release
* to signal it, so it can safely park.
*/
return true;
10.进入parkAndCheckInterrupt()方法，执行挂起当前线程操作。
/**
* Convenience method to park and then check if interrupted
*
* @return {@code true} if interrupted
*/
private final boolean parkAndCheckInterrupt() {
LockSupport.park(this);
return Thread.interrupted();
}
reentrantLock.lock()方法结束。
三.reentrantLock.unLock()
unLock()方法流程图
1.进入ReentrantLock的unlock()方法
/**
* 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);
}
2.进入sync.release(1)方法
/**
* 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;
if (h != null && h.waitStatus != 0)
unparkSuccessor(h);
return true;
}
return false;
}
3.在if()判断里面，进入tryRelease(1)方法，找到非公平实现
protected final boolean tryRelease(int releases) {
int c = getState() - releases;
if (Thread.currentThread() != getExclusiveOwnerThread())
throw new IllegalMonitorStateException();
boolean free = false;
if (c == 0) {
free = true;
setExclusiveOwnerThread(null);
}
setState(c);
return free;
}
获取state，然后将state-1，如果当前线程不是独占线程，抛出异常。
如果state != 0，返回false，unlock()方法结束。
如果state == 0，将独占线程设为null，然后返回true。
4.第2步中，if()判断里面的tryRelease(1)返回true以后，进入if()代码块
public final boolean release(int arg) {
if (tryRelease(arg)) {
Node h = head;
if (h != null && h.waitStatus != 0)
unparkSuccessor(h);
return true;
}
return false;
}
获取等待队列的头节点head，如果head不为null，且有线程在等待(ps:将线程添加到等待队列时，会将head的waitStatus设为Node.EXCLUSIVE)，执行唤醒操作。
5.进入unparkSuccessor(1)方法
/**
* 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;
}
if (s != null)
LockSupport.unpark(s.thread);
}
首先把head的waitStatus设为0，然后获取head的下一节点s。
如果s不为null，则唤醒s。
如果s为null，然后从链表尾部开始遍历链表，找到第一个waitStatus<=0的节点，然后唤醒它。
