java thread 之Lock
concurrent包里面有很多Lock的具体实现，其具体的实现都是基于AQS实现的
ReentrantLock
ReentrantLock是可重入的互斥锁，重点是重入和互斥，ReentrantLock 将由最近成功获得锁的线程所持有，当这个线程再次尝试拥有这个Lock时就是重入。互斥就是
在某一时间只有一个线程能持有Lock。
public void lock() {
sync.lock();
}
获得锁方法，Sync是AQS的抽象子类，实现可重入和互斥的大部分功能。在Sync的子类中有FairSync和NonfairSync两种代表公平锁策略和非公平锁策略
Sync lock方法留给子类去实现，NonfairSync的实现：
final void lock() {
if (compareAndSetState(0, 1))
setExclusiveOwnerThread(Thread.currentThread());
else
acquire(1);
}
不管是不是有其他线程在AQS的阻塞的队列里面，如果当前线程能获得线程，就直接获得线程，不行的话执行AQS的acquire()方法，基本就是进入阻塞队列的命了。
关键点:AQS的acquire()中调用的tryAcquire(int arg)是留给子类实现的，是在进入阻塞队列前再尝试一次获取锁(lock()方法到这个点上面可能其他线程已经释放锁)
NonfairSync的tryAcquire(int arg)调用的nonfairTryAcquire()实现:
final boolean nonfairTryAcquire(int acquires) {
final Thread current = Thread.currentThread();
int c = getState();
if (c == 0) {//关键点1
if (compareAndSetState(0, acquires)) {
setExclusiveOwnerThread(current);
return true;
}
}
else if (current == getExclusiveOwnerThread()) {
int nextc = c + acquires;//关键点2
if (nextc < 0) // overflow
throw new Error("Maximum lock count exceeded");
setState(nextc);
return true;
}
return false;
}
关键点1：state为0，没有线程请求锁，直接分配给本线程
关键点2. 如果本线程已经得到锁，state加1，即重入。
FairSync的实现:
final void lock() {
acquire(1);//关键点1
}
protected final boolean tryAcquire(int acquires) {
final Thread current = Thread.currentThread();
int c = getState();
if (c == 0) {
if (isFirst(current) && //关键点2
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;
}
关键点1：公平策略，没有给当前线程优惠
关键点2：判断当前线程有没有在阻塞队列的第一位，没在第一位不往下继续执行，这样先给阻塞队列的线程机会，这样做速度比较慢，吞吐量比较小
public boolean tryLock() {
return sync.nonfairTryAcquire(1);
}
tryLock尝试获得锁，不能获得话就直接返回，没有公平策略一说。
unlock
public void unlock() {
sync.release(1);
}
public final boolean release(int arg) {
if (tryRelease(arg)) {
Node h = head;
if (h != null && h.waitStatus != 0)
unparkSuccessor(h);
return true;
}
return false;
}
tryRelease()留给AQS子类执行：
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;
}
如果state被减后还不为0，表示这个锁被一个线程重入后还没有完全释放，release()方法后面不会执行，也就是不会执行unpark阻塞队列中的线程.
ReentrantReadWriteLock
ReentrantReadWriteLock内部有两个锁：互斥可重入的WriteLock和共享的ReadLock
内部实现中state字段的高16位代表的是read count，低16位代表的是write count
AQS的实现也有NonfairSync和FairSync之分：
主要区别是同时有读写线程时候对待读写线程策略不同。
NonfairSync:
final boolean writerShouldBlock(Thread current) {
return false; // writers can always barge
}
final boolean readerShouldBlock(Thread current) {
/* As a heuristic to avoid indefinite writer starvation,
* block if the thread that momentarily appears to be head
* of queue, if one exists, is a waiting writer. This is
* only a probablistic effect since a new reader will not
* block if there is a waiting writer behind other enabled
* readers that have not yet drained from the queue.
*/
return apparentlyFirstQueuedIsExclusive();
}
write永远都不阻塞，read的话看阻塞队列header之后的node是不是write的，如果是就阻塞，可以避免read一直运行导致的write饥饿。
FairSync：
final boolean writerShouldBlock(Thread current) {
// only proceed if queue is empty or current thread at head
return !isFirst(current);
}
final boolean readerShouldBlock(Thread current) {
// only proceed if queue is empty or current thread at head
return !isFirst(current);
}
write和read都有判断是不是阻塞队列header后的第一个node。
对于ReadLock
public void lock() {
sync.acquireShared(1);
}
调用AQS的acquireShared()，其中会执行留给AQS子类实现的tryAcquireShared():
protected final int tryAcquireShared(int unused) {
/*
* Walkthrough:
* 1. If write lock held by another thread, fail
* 2. If count saturated, throw error
* 3. Otherwise, this thread is eligible for
* lock wrt state, so ask if it should block
* because of queue policy. If not, try
* to grant by CASing state and updating count.
* Note that step does not check for reentrant
* acquires, which is postponed to full version
* to avoid having to check hold count in
* the more typical non-reentrant case.
* 4. If step 3 fails either because thread
* apparently not eligible or CAS fails,
* chain to version with full retry loop.
*/
Thread current = Thread.currentThread();
int c = getState();
if (exclusiveCount(c) != 0 &&
getExclusiveOwnerThread() != current)
return -1;
if (sharedCount(c) == MAX_COUNT)
throw new Error("Maximum lock count exceeded");
if (!readerShouldBlock(current) &&
compareAndSetState(c, c + SHARED_UNIT)) {
HoldCounter rh = cachedHoldCounter;
if (rh == null || rh.tid != current.getId())
cachedHoldCounter = rh = readHolds.get();
rh.count++;
return 1;
}
return fullTryAcquireShared(current);
}
策略是以下的几点：
1.如果当前write持有锁，直接返回-1.
2.没有write持有锁，判断是否需要阻塞读请求，之后原子更新read count
3.上面成功后就返回1，不成功的话在循环中试着读取：
final int fullTryAcquireShared(Thread current) {
/*
* This code is in part redundant with that in
* tryAcquireShared but is simpler overall by not
* complicating tryAcquireShared with interactions between
* retries and lazily reading hold counts.
*/
HoldCounter rh = cachedHoldCounter;
if (rh == null || rh.tid != current.getId())
rh = readHolds.get();
for (;;) {
int c = getState();
int w = exclusiveCount(c);
if ((w != 0 && getExclusiveOwnerThread() != current) ||
((rh.count | w) == 0 && readerShouldBlock(current)))
return -1;
if (sharedCount(c) == MAX_COUNT)
throw new Error("Maximum lock count exceeded");
if (compareAndSetState(c, c + SHARED_UNIT)) {
cachedHoldCounter = rh; // cache for release
rh.count++;
return 1;
}
}
}
unLock方法就是将read count 减1，然后查看阻塞队列有没有线程需要unpark
WriteLock的实现和ReetrantLock的实现是一致的
Semaphore
Semaphore是信号量的概念，主要控制同一时间内对访问资源的线程数的控制。底层实现是AQS的
acquireShared()和releaseShared().
CountDownLatch
CountDownLatch在完成一组正在其他线程中执行的操作之前，允许线程等待直到他们完成操作
await()方法一直等到state=0，初始时CountDownLatch初始化state为传入的数值，一个线程
执行操作完成的话执行countDown()方法将state减1，直到为0，await()方法不再阻塞。
CyclicBarrier
当一组线程调用CyclicBarrier.await()方法，就会阻塞在那里，当最后一个线程进入调用此方法时，就执行
一个公共的Runnable.原理是：初始化时有count属性，当前面count-1个线程到来都condition.await()，第count个
线程来就执行公共Runnable，然后执行condition.signAll()方法来使得其他线程从阻塞中解脱出来.
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