首先先分析下reentrantLock的类的基本结构关系，下面是关系图
ReentrantLock.png
从图中可以看出，reentrantlock类实现了Lock接口，并且包含了Sync抽象内部类；而Sync类又继承了AbstractQueuedSynchronizer抽象类，并且Sync的两个具体实现类NonfairSync和FairSync均是reentrantLock的内部类；下面我们就从reentrantlock的lock和unlock方法切入，分析reentrantlock加锁的具体实现流程。
public ReentrantLock() {
sync = new NonfairSync();
}
当我们new一个reentrantLock对象时，实际上是new一个Sync的NonfairSync对象，当调用了reentrantLock.lock方法时，也是调用Sync抽象类的lock方法。
public void lock() {
sync.lock();
}
由于new对象是NonfairSync对象，我们直接看NonfairSync类的lock方法
final void lock() {
if (compareAndSetState(0, 1))
setExclusiveOwnerThread(Thread.currentThread());
else
acquire(1);
}
进入NonfairSync的lock方法时，首先会先调用compareAndSetState方法，此方法主要是用于比较并置换state变量值，当传入的期望值0等于state变量值时，则会将state变量值变为置换值1，并且返回true，具体的内部实现方法是native方法，这里就不再深究；回到代码上问题，假设有两个线程A和线程B，线程A先进入lock方法，并且进入if判断逻辑，此时state值默认为0，与期望值0一致，所以会将state值变为1，并进入setExclusiveOwnerThread方法，此方法作用是将exclusiveOwnerThread设置为当前线程，可理解为当前线程可以优先执行；而线程B进入lock方法时，此时state值已经变为1，与期望值0不一致，则会走acquire方法
public final void acquire(int arg) {
if (!tryAcquire(arg) &&
acquireQueued(addWaiter(Node.EXCLUSIVE), arg))
selfInterrupt();
}
进入acquire方法时，会先调用tryAcquire，最终调用的是Sync.nonfairTryAcquire方法，我们直接看nonfairTryAcquire方法逻辑
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;
}
nonfairTryAcquire方法会先尝试获取当前state变量的值，如果state=0，则会重新调用compareAndSetState方法判断并置换，获得锁优先执行权；或者当前线程为优先执行权线程时，也可以获得锁优先执行权；如果两者都不满足条件，则会返回true，执行addWaiter方法。
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;
}
addWaiter方法会生成一个包含当前线程对象的Node节点对象，并把新生成的node节点对象设置在链表的尾部，把原尾部node节点的next node设置为当前节点；当不存在tail node尾部节点时，把当前节点node设置为尾部tail节点和head节点（enq方法）；大概逻辑理解可以用以下图解释。
未命名文件.png
当执行完addWaiter方法后，会返回新生成的node对象进入acquireQueued方法
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);
}
}
此时又会尝试调用tryAcquire方法，因为被线程A占用着，所以tryAcquire返回false,进入shouldParkAfterFailedAcquire方法
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;
}
因为pred.waitStatus=0，所以会进入compareAndSetWaitStatus方法设置waitStatus=-1,并返回false执行parkAndCheckInterrupt方法
private final boolean parkAndCheckInterrupt() {
LockSupport.park(this);
return Thread.interrupted();
}
此时当前线程B就被中断挂起了，需要等到线程A执行完成后调用unlock方法释放锁后再唤醒线程B才能执行；
线程A执行完后，释放锁需要调用unlock方法，调用reentrantlock.unlock方法最终调用的是Sync.release方法，下面来看下release方法具体实现：
public final boolean release(int arg) {
if (tryRelease(arg)) {
Node h = head;
if (h != null && h.waitStatus != 0)
unparkSuccessor(h);
return true;
}
return false;
}
线程A进入release方法会先调用tryRelease方法,将state值设置为state-1
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;
}
设置完state后，进入if逻辑获取head node节点对象，进入unparkSuccessor方法设置下一个执行线程节点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);
}
获取头部head node节点的next node节点，从next node节点获取到下一个执行的线程对象s.thread（B线程）并调用LockSupport.unpark(s.thread)将其唤醒，此时B线程将获得执行权限，执行之前被挂起的死循环逻辑（上面讲到的acquireQueued方法）获得锁，至此逻辑结束。
网友评论