ReentrantLock分为公平锁和非公平锁，默认的为非公平锁
public ReentrantLock() {
sync = new NonfairSync();
}
可以手动指定
public ReentrantLock(boolean fair) {
sync = fair ? new FairSync() : new NonfairSync();
}
先从非公平锁讲起
1 lock
final void lock() {
if (compareAndSetState(0, 1))
setExclusiveOwnerThread(Thread.currentThread());
else
acquire(1);
}
首先尝试cas把state设置为1，如果成功则获取独占锁，失败执行acquire
2 acquire
public final void acquire(int arg) {
if (!tryAcquire(arg) &&
acquireQueued(addWaiter(Node.EXCLUSIVE), arg))
selfInterrupt();
}
2.1 tryAcquire
protected final boolean tryAcquire(int acquires) {
return nonfairTryAcquire(acquires);
}
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;
}
先获取当前独占锁的数量：
1.如果为0，则cas获取锁，成功则获取独占锁。
2.如果不为0，则检查当前线程是否是重入锁，是就独占锁数量加一并返回。
2.2.1 addWaiter
如果tryAcquire都失败，那么就执行入队操作。
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;
}
如果尾节点不为空，执行一次快速入队操作，如果cas成功，就入队成功。
尾节点为空或者快速入队不成功，for循环执行入队直到成功
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;
}
}
}
}
这里有个生成空节点的操作，因为在存在队列的情况下，队列头节点表示的是当前拥有锁的线程
2.2.1 acquireQueued
节点入队成功之后再执行线程挂起
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);
}
}
注意 p == head && tryAcquire(arg)这个条件是这个函数的唯一出口，先看线程挂起部分p == head && tryAcquire(arg)
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;
}
判断前置节点的状态，在这里分为三种情况：
1.Node.SIGNAL,可以唤醒后继节点，表示可以挂起单钱节点线程，所以直接返回
2.大于0，表示取消状态，则往前遍历删除节点直至非取消状态
3.如果不是前两者，将前置节点置为Node.SIGNAL
线程挂起
private final boolean parkAndCheckInterrupt() {
LockSupport.park(this);
return Thread.interrupted();
}
2.3 selfInterrupt
static void selfInterrupt() {
Thread.currentThread().interrupt();
}
这是一个比较细节的地方，结合挂起部分看。
线程挂起的方式是采用LockSupport.park，能够响应中断，但是不会抛异常。也就是说挂起的线程有可能被ReentrantLock之外的线程唤醒，这时候就需要重置中断状态，并且保证线程正确被唤醒并获取锁的时候，保持中断状态。
3 unlock
采用LockSupport.unpark唤醒队列下一个线程。
再来看公平锁
其实差别就两点：
1.没有直接cas获取锁的操作
2.在获取锁数量为0的时候，不再直接进行cas操作，要先判断下队列中是否还有节点未执行
protected final boolean tryAcquire(int acquires) {
final Thread current = Thread.currentThread();
int c = getState();
if (c == 0) {
if (!hasQueuedPredecessors() &&
compareAndSetState(0, acquires)) {
setExclusiveOwnerThread(current);
return true;
}
}
else if (current == getExclusiveOwnerThread()) {
int nextc = c + acquires;
if (nextc < 0)
throw new Error("Maximum lock count exceeded");
setState(nextc);
return true;
}
return false;
}
}