ReetrantLock是一个可重入的独占锁,主要有两个特性,一个是支持公平锁和非公平锁,一个是可重入。 ReetrantLock实现依赖于AQS(AbstractQueuedSynchronizer) ReetrantLock主要依靠AQS维护一个阻塞队列,多个线程对加锁时,失败则会进入阻塞队列。等待唤醒,重新尝试加锁。下图是其类图
在new ReetrantLock对象的时候,可以指定其支持公平锁还是非公平锁
public ReentrantLock() {
sync = new NonfairSync();
}
public ReentrantLock(boolean fair) {
sync = fair ? new FairSync() : new NonfairSync();
}
FairSync继承Sync,而Sync继承AbstractQueuedSynchronizer。ReentrantLock调用lock方法,最终会调用sync的tryAcquire函数,获取资源。FairSync的tryAcquire函数,**当前线程只有在队列为空或者时队首节点的时候,才能获取资源,否则会被加入到阻塞队列中。**下面的hasQueuedPredecessors函数用于判断队列是否为空,或者当前元素是否为队首元素。
static final class FairSync extends Sync {
private static final long serialVersionUID = -3000897897090466540L;
final void lock() {
acquire(1);
}
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;
}
}
hasQueuedPredecessors函数,如果h==t(队列为空),或者h!=t && s.thread == Thread.currentThread()(队首节点),则返回false,否则返回true
public final boolean hasQueuedPredecessors() {
Node t = tail; // Read fields in reverse initialization order
Node h = head;
Node s;
return h != t &&
((s = h.next) == null || s.thread != Thread.currentThread());
}
NoFairSync同样继承Sync,ReentrantLock调用lock方法,最终会调用sync的tryAcquire函数,获取资源。而NoFairSync的tryAcquire函数,会调用父类Sync的方法nofairTryAcquire函数。通过对比可以发现,如果资源释放时,新的线程会尝试CAS操作获取资源,而不管阻塞队列中时候有先于其申请的线程。
static final class NonfairSync extends Sync {
private static final long serialVersionUID = 7316153563782823691L;
final void lock() {
if (compareAndSetState(0, 1))
setExclusiveOwnerThread(Thread.currentThread());
else
acquire(1);
}
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;
}
可重入性:获取独占资源的线程,可以重复得获取该独占资源,不需要重复请求。
ReentrantLock在申请资源的时候,都会判断当前持有独占资源的线程是不是当前线程,如果是的话,只是简单得将state值加1.记录当前线程的重入次数。
else if (current == getExclusiveOwnerThread()) {
int nextc = c + acquires;
if (nextc < 0)
throw new Error("Maximum lock count exceeded");
setState(nextc);
return true;
}
ReentrantLock在释放资源的时候,都会调用tryRelease,只有state值为0的时候,才会释放资源。换句话说就是,重入多少次,就必须释放多少次
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;
}