Java源码分析系列笔记-17.ReentrantReadWriteLock
目录[*]1. ReentrantReadWriteLock是什么
[*]2. 非公平ReentrantReadWriteLock
[*]2.1. 是什么
[*]2.2. 怎么使用
[*]2.3. 源码分析
[*]2.3.1. uml
[*]2.3.2. 构造方法
[*]2.3.3. 读锁加锁
[*]2.3.3.1. 使用AQS加共享锁
[*]2.3.3.1.1. 使用Sync尝试加共享锁
[*]2.3.3.1.1.1. 判断是否需要阻塞读【非公平锁】
[*]2.3.3.1.1.2. 快速尝试加锁失败，那么改用死循环加锁
[*]2.3.3.1.2. 尝试加锁失败，那么入AQS队列并阻塞，等待唤醒继续抢占锁
[*]2.3.3.1.2.1. 加入AQS队列并阻塞
[*]2.3.4. 读锁解锁
[*]2.3.4.1. 使用AQS解共享锁
[*]2.3.4.1.1. 使用Sync尝试解锁
[*]2.3.4.1.2. 所有共享锁都被释放，唤醒AQS队列中头节点的下一个节点
[*]2.3.4.1.2.1. 唤醒AQS队列中头节点的下一个节点
[*]2.3.5. 写锁加锁
[*]2.3.5.1. 调用AQS加互斥锁
[*]2.3.5.1.1. 使用Sync尝试加互斥锁
[*]2.3.5.1.1.1. 判断写是否需要阻塞【非公平锁】
[*]2.3.5.1.2. 尝试加锁失败，那么入AQS队列并阻塞，等待唤醒继续抢占锁
[*]2.3.6. 写锁解锁
[*]2.3.6.1. 调用AQS解互斥锁
[*]2.3.6.1.1. 调用Sync尝试解互斥锁
[*]2.3.6.1.2. 尝试解锁成功后，唤醒AQS队列中头节点的下一个节点
[*]3. 公平ReentrantReadWriteLock
[*]3.1. 是什么
[*]3.2. 怎么使用
[*]3.3. 源码分析
[*]3.3.1. uml
[*]3.3.2. 构造方法
[*]3.3.3. 读锁加锁
[*]3.3.3.1. 使用AQS加共享锁
[*]3.3.3.1.1. 使用Sync尝试加共享锁
[*]3.3.3.1.1.1. 判断是否需要阻塞读【公平】
[*]3.3.3.1.1.2. 快速尝试加锁失败，那么改用死循环加锁
[*]3.3.3.1.2. 尝试加锁失败，那么入AQS队列并阻塞，等待唤醒继续抢占锁
[*]3.3.3.1.2.1. 加入AQS队列并阻塞
[*]3.3.4. 读锁解锁
[*]3.3.4.1. 使用AQS解共享锁
[*]3.3.4.1.1. 使用Sync尝试解锁
[*]3.3.4.1.2. 所有共享锁都被释放，唤醒AQS队列中头节点的下一个节点
[*]3.3.4.1.2.1. 唤醒AQS队列中头节点的下一个节点
[*]3.3.5. 写锁加锁
[*]3.3.5.1. 调用AQS加互斥锁
[*]3.3.5.1.1. 使用Sync尝试加互斥锁
[*]3.3.5.1.1.1. 判断写是否需要阻塞【公平】
[*]3.3.5.1.2. 尝试加锁失败，那么入AQS队列并阻塞，等待唤醒继续抢占锁
[*]3.3.6. 写锁解锁
[*]3.3.6.1. 调用AQS解互斥锁
[*]3.3.6.1.1. 调用Sync尝试解互斥锁
[*]3.3.6.1.2. 尝试解锁成功后，唤醒AQS队列中头节点的下一个节点
[*]4. 参考
1. ReentrantReadWriteLock是什么
ReentrantLock保证了同一时间只有一个线程可以在临界区读或者写数据，这意味着如果有两个读线程同时读取数据，ReentrantLock也只允许其中一个通过，但我们想要的是读可以并发执行，一旦有写则其他线程等待。如下表：
是否可以同时进行读写读√×写××因此，ReentrantReadWriteLock就诞生了
2. 非公平ReentrantReadWriteLock
2.1. 是什么
无论队列前面是否有人排队等待锁，我直接去抢
2.2. 怎么使用
public class ReadWriteLockTest
{
private static ReadWriteLock lock = new ReentrantReadWriteLock();//默认非公平
private static Lock readLock = lock.readLock();
private static Lock writeLock = lock.writeLock();
private static List<Integer> data = new ArrayList<>();
public static void main(String[] args) throws InterruptedException
{
Thread readThread = new Thread(() -> {
while (true)
{
try
{
TimeUnit.MILLISECONDS.sleep(500);
readLock.lock();
System.out.println(Thread.currentThread().getName() + " read: " + data);
}
catch (InterruptedException e)
{
e.printStackTrace();
}
finally
{
readLock.unlock();
}
}
});
Thread readThread2 = new Thread(() -> {
while (true)
{
try
{
TimeUnit.MILLISECONDS.sleep(300);
readLock.lock();
System.out.println(Thread.currentThread().getName() + " read: " + data);
}
catch (InterruptedException e)
{
e.printStackTrace();
}
finally
{
readLock.unlock();
}
}
});
Thread writeThread = new Thread(() -> {
int i = 0;
while (true)
{
try
{
TimeUnit.MILLISECONDS.sleep(200);
writeLock.lock();
if (i % 2 == 0)
{
data.add(i);
}else
{
data.remove(0);
}
i++;
}
catch (InterruptedException e)
{
e.printStackTrace();
}
finally
{
writeLock.unlock();
}
}
});
readThread.start();
readThread2.start();
writeThread.start();
readThread.join();
readThread2.join();
writeThread.join();
}
}2.3. 源码分析
2.3.1. uml
@startuml
skinparam classAttributeIconSize 0
interface AQS{
}
class Sync{
}
interface Lock{
}
interface ReadWriteLock{
}
class ReentrantReadWriteLock{
}
class WriteLock{
}
class ReadLock{
}
class WriteLock{
}
Lock <|-- ReadLock
Lock <|-- WriteLock
ReadWriteLock <|-- ReentrantReadWriteLock
AQS <|-- Sync
ReentrantReadWriteLock --> ReadLock
ReentrantReadWriteLock --> WriteLock
ReadLock --> Sync
WriteLock --> Sync
@enduml2.3.2. 构造方法
[*]ReentrantReadWriteLock
public ReentrantReadWriteLock() {
//默认是false
this(false);
}
public ReentrantReadWriteLock(boolean fair) {
//初始化了Sync
//false的话使用的是NonfairSync
sync = fair ? new FairSync() : new NonfairSync();
//初始化读写锁
readerLock = new ReadLock(this);
writerLock = new WriteLock(this);
}
[*]ReentrantReadWriteLock.ReadLock
protected ReadLock(ReentrantReadWriteLock lock) {
//其实就是保存了ReentrantReadWriteLock的Sync
sync = lock.sync;
}
[*]ReentrantReadWriteLock.WriteLock
protected WriteLock(ReentrantReadWriteLock lock) {
//其实就是保存了ReentrantReadWriteLock的Sync
sync = lock.sync;
}2.3.3. 读锁加锁
[*]ReentrantReadWriteLock.readLock
//返回就是读锁
public ReentrantReadWriteLock.ReadLockreadLock(){ return readerLock; }
[*]ReadLock.lock
public void lock() {
//使用AQS加共享锁
sync.acquireShared(1);
}2.3.3.1. 使用AQS加共享锁
[*]AQS acquireShared
public final void acquireShared(int arg) {
//ReentrantReadWriteLock的Sync重写了tryAcquireShared
//所以这里调用的是Sync的tryAcquireShared
//如果返回<0说明尝试加锁失败，执行doAcquireShared入AQS队列并阻塞，等待唤醒
//返回>=0说明加锁成功，执行后续的业务逻辑
if (tryAcquireShared(arg) < 0)
doAcquireShared(arg);
}2.3.3.1.1. 使用Sync尝试加共享锁
[*]ReentrantReadWriteLock.Sync.tryAcquireShared
protected final int tryAcquireShared(int unused) {
//当前线程
Thread current = Thread.currentThread();
//当前的state数目（或者说加锁【既包括读锁也包括写锁】的数量）
int c = getState();
//从state中获取互斥锁的数目（写锁的数量）
//如果数量不为0（即>0）说明已经加了写锁
if (exclusiveCount(c) != 0 &&
//判断加锁的线程是否当前线程
getExclusiveOwnerThread() != current)
//不是的话返回-1表示已经有其他线程加了写锁（后面就需要入AQS队列阻塞等待唤醒）
return -1;
//走到这里说明没有线程加写锁或者加写锁的就是本线程
//从state中获取共享锁的数目（读锁的数目）
int r = sharedCount(c);
//非公平锁：这里调用NonfairSync的readerShouldBlock判断是否需要阻塞读
//返回false的话不需要阻塞，接着执行&&后面的逻辑
if (!readerShouldBlock() &&
//判断读锁数量是否小于最大数目
r < MAX_COUNT &&
//使用CAS加读锁
compareAndSetState(c, c + SHARED_UNIT)) {
//上面的if中的逻辑执行完毕后，锁state已经加好了读锁
//下面的逻辑负责处理其他需要修改的属性
//第一次加读锁
if (r == 0) {
//设置第一次加锁的线程以及初始化读锁数量
firstReader = current;
firstReaderHoldCount = 1;
//第n次加锁的仍旧是第一次加锁的线程
} else if (firstReader == current) {
//修改读锁数量即可
firstReaderHoldCount++;
//第n次加锁的不是第一次加锁的线程
} else {
//cachedHoldCounter是缓存
HoldCounter rh = cachedHoldCounter;
if (rh == null || rh.tid != getThreadId(current))
cachedHoldCounter = rh = readHolds.get();
else if (rh.count == 0)
readHolds.set(rh);
rh.count++;
}
//返回1（正数）表示加读锁成功
return 1;
}
//走到这里说明发生了以下几种情况：
//1.需要阻塞读
//2.读锁数目已经超过最大值
//3.CAS加读锁失败
return fullTryAcquireShared(current);
}2.3.3.1.1.1. 判断是否需要阻塞读【非公平锁】
[*]ReentrantReadWriteLock.NonfairSync#readerShouldBlock
final boolean readerShouldBlock() {
//队头是互斥节点（加写锁的节点）情况下才需要阻塞读
//这就是非公平锁的特点
//即使队列前面已经有其他加读锁的线程等待我也不管
return apparentlyFirstQueuedIsExclusive();
}
[*]apparentlyFirstQueuedIsExclusive
final boolean apparentlyFirstQueuedIsExclusive() {
Node h, s;
//队列不为空
return (h = head) != null &&
//并且 队列的实际头节点【之所以说实际队列的头节点是个占位符】不为空
(s = h.next)!= null &&
//并且 队列的实际头节点不是共享的头节点（即加的不是读锁）
!s.isShared() &&
//并且 队列的实际头节点的线程不为空
s.thread != null;
//满足以上所有情况才需要阻塞当前加读锁的线程
}2.3.3.1.1.2. 快速尝试加锁失败，那么改用死循环加锁
[*]ReentrantReadWriteLock.Sync#fullTryAcquireShared
final int fullTryAcquireShared(Thread current) {
HoldCounter rh = null;
//死循环
for (;;) {
int c = getState();
//写锁的数量不为0
if (exclusiveCount(c) != 0) {
//且加写锁的不是本线程
if (getExclusiveOwnerThread() != current)
//返回-1
return -1;
// else we hold the exclusive lock; blocking here
// would cause deadlock.
//没有人加写锁，如果读需要阻塞
} else if (readerShouldBlock()) {
// Make sure we're not acquiring read lock reentrantly
if (firstReader == current) {
// assert firstReaderHoldCount > 0;
} else {
if (rh == null) {
rh = cachedHoldCounter;
if (rh == null || rh.tid != getThreadId(current)) {
rh = readHolds.get();
if (rh.count == 0)
readHolds.remove();
}
}
if (rh.count == 0)
return -1;
}
}
//走到这里说明没有加写锁，读也不需要阻塞
if (sharedCount(c) == MAX_COUNT)
//加锁已超过最大值
throw new Error("Maximum lock count exceeded");
//加读锁，同上面的tryAcquireShared
if (compareAndSetState(c, c + SHARED_UNIT)) {
if (sharedCount(c) == 0) {
firstReader = current;
firstReaderHoldCount = 1;
} else if (firstReader == current) {
firstReaderHoldCount++;
} else {
if (rh == null)
rh = cachedHoldCounter;
if (rh == null || rh.tid != getThreadId(current))
rh = readHolds.get();
else if (rh.count == 0)
readHolds.set(rh);
rh.count++;
cachedHoldCounter = rh; // cache for release
}
return 1;
}
}
}2.3.3.1.2. 尝试加锁失败，那么入AQS队列并阻塞，等待唤醒继续抢占锁
private void doAcquireShared(int arg) {
//构造SHARE节点加入AQS队列阻塞等待唤醒
final Node node = addWaiter(Node.SHARED);
boolean failed = true;
try {
boolean interrupted = false;
//死循环抢占锁
for (;;) {
//当前节点的前一个节点是头节点
final Node p = node.predecessor();
if (p == head) {
//继续尝试加共享锁
int r = tryAcquireShared(arg);
if (r >= 0) {
setHeadAndPropagate(node, r);
p.next = null; // help GC
if (interrupted)
selfInterrupt();
failed = false;
return;
}
}
//判断是否需要阻塞
if (shouldParkAfterFailedAcquire(p, node) &&
//需要的话进行阻塞
parkAndCheckInterrupt())
interrupted = true;
}
} finally {
if (failed)
cancelAcquire(node);
}
}2.3.3.1.2.1. 加入AQS队列并阻塞
[*]addWaiter
private Node addWaiter(Node mode) {
Node node = new Node(Thread.currentThread(), mode);
// Try the fast path of enq; backup to full enq on failure
//快速尝试加入队尾
Node pred = tail;
if (pred != null) {
node.prev = pred;
if (compareAndSetTail(pred, node)) {
pred.next = node;
return node;
}
}
//快速尝试加入队尾失败，那么改用enq加入队尾
enq(node);
return node;
}参考：5.AQS.md
2.3.4. 读锁解锁
[*]ReentrantReadWriteLock.ReadLock#unlock
public void unlock() {
//使用AQS解共享锁
sync.releaseShared(1);
}2.3.4.1. 使用AQS解共享锁
[*]AbstractQueuedSynchronizer#releaseShared
public final boolean releaseShared(int arg) {
//调用Sync尝试解共享锁
//如果共享锁【读锁】已经全部释放完，那么执行doReleaseShared
if (tryReleaseShared(arg)) {
doReleaseShared();
return true;
}
return false;
}2.3.4.1.1. 使用Sync尝试解锁
[*]ReentrantReadWriteLock.Sync#tryReleaseShared
protected final boolean tryReleaseShared(int unused) {
Thread current = Thread.currentThread();
//当前线程就是第一次加读锁的线程
if (firstReader == current) {
//所有读锁都释放完毕
if (firstReaderHoldCount == 1)
//那么置加锁线程为空
firstReader = null;
//没有释放完那么减读锁数量
else
firstReaderHoldCount--;
//当前线程不是第一次加读锁的线程
} else {
HoldCounter rh = cachedHoldCounter;
if (rh == null || rh.tid != getThreadId(current))
rh = readHolds.get();
int count = rh.count;
if (count <= 1) {
readHolds.remove();
if (count <= 0)
throw unmatchedUnlockException();
}
--rh.count;
}
//死循环修改state数量
for (;;) {
int c = getState();
int nextc = c - SHARED_UNIT;
//CAS修改state数量
if (compareAndSetState(c, nextc))
return nextc == 0;//减为0了那么返回true
}
}3.3. 源码分析
3.3.1. uml
@startuml
skinparam classAttributeIconSize 0
interface AQS{
}
class Sync{
}
interface Lock{
}
interface ReadWriteLock{
}
class ReentrantReadWriteLock{
}
class WriteLock{
}
class ReadLock{
}
class WriteLock{
}
Lock <|-- ReadLock
Lock <|-- WriteLock
ReadWriteLock <|-- ReentrantReadWriteLock
AQS <|-- Sync
ReentrantReadWriteLock --> ReadLock
ReentrantReadWriteLock --> WriteLock
ReadLock --> Sync
WriteLock --> Sync
@enduml3.3.2. 构造方法
[*]ReentrantReadWriteLock
private void unparkSuccessor(Node node) {
int ws = node.waitStatus;
//如果当前节点的状态是正常的？
if (ws < 0)
//当前节点的状态<0,则把状态改为0
//0是空的状态，因为node这个节点的线程释放了锁后续不需要做任何
compareAndSetWaitStatus(node, ws, 0);
//获取当前节点的下一个节点
Node s = node.next;
//如果下一个节点是空（即当前节点是尾节点）或者下一个节点的状态>0（取消）
if (s == null || s.waitStatus > 0) {
s = null;
//从尾节点往前遍历至当前节点
for (Node t = tail; t != null && t != node; t = t.prev)
//找到最靠近当前节点的状态<=0（非取消）的节点
if (t.waitStatus <= 0)
s = t;
}
//唤醒当前节点的下一个节点
if (s != null)
LockSupport.unpark(s.thread);
}
[*]ReentrantReadWriteLock.ReadLock
protected ReadLock(ReentrantReadWriteLock lock) {
//其实就是保存了ReentrantReadWriteLock的Sync
sync = lock.sync;
}
[*]ReentrantReadWriteLock.WriteLock
protected WriteLock(ReentrantReadWriteLock lock) {
//其实就是保存了ReentrantReadWriteLock的Sync
sync = lock.sync;
}3.3.3. 读锁加锁
[*]ReentrantReadWriteLock.readLock
//返回就是读锁
public ReentrantReadWriteLock.ReadLockreadLock(){ return readerLock; }
[*]ReadLock.lock
public void lock() {
//使用AQS加共享锁
sync.acquireShared(1);
}3.3.3.1. 使用AQS加共享锁
[*]AQS acquireShared
public final void acquireShared(int arg) {
//ReentrantReadWriteLock的Sync重写了tryAcquireShared
//所以这里调用的是Sync的tryAcquireShared
//如果返回<0说明尝试加锁失败，执行doAcquireShared入AQS队列并阻塞，等待唤醒
//返回>=0说明加锁成功，执行后续的业务逻辑
if (tryAcquireShared(arg) < 0)
doAcquireShared(arg);
}3.3.3.1.1. 使用Sync尝试加共享锁
[*]ReentrantReadWriteLock.Sync.tryAcquireShared
protected final int tryAcquireShared(int unused) {
//当前线程
Thread current = Thread.currentThread();
//当前的state数目（或者说加锁【既包括读锁也包括写锁】的数量）
int c = getState();
//从state中获取互斥锁的数目（写锁的数量）
//如果数量不为0（即>0）说明已经加了写锁
if (exclusiveCount(c) != 0 &&
//判断加锁的线程是否当前线程
getExclusiveOwnerThread() != current)
//不是的话返回-1表示已经有其他线程加了写锁（后面就需要入AQS队列阻塞等待唤醒）
return -1;
//走到这里说明没有线程加写锁或者加写锁的就是本线程
//从state中获取共享锁的数目（读锁的数目）
int r = sharedCount(c);
//非公平锁：这里调用NonfairSync的readerShouldBlock判断是否需要阻塞读
//返回false的话不需要阻塞，接着执行&&后面的逻辑
if (!readerShouldBlock() &&
//判断读锁数量是否小于最大数目
r < MAX_COUNT &&
//使用CAS加读锁
compareAndSetState(c, c + SHARED_UNIT)) {
//上面的if中的逻辑执行完毕后，锁state已经加好了读锁
//下面的逻辑负责处理其他需要修改的属性
//第一次加读锁
if (r == 0) {
//设置第一次加锁的线程以及初始化读锁数量
firstReader = current;
firstReaderHoldCount = 1;
//第n次加锁的仍旧是第一次加锁的线程
} else if (firstReader == current) {
//修改读锁数量即可
firstReaderHoldCount++;
//第n次加锁的不是第一次加锁的线程
} else {
//cachedHoldCounter是缓存
HoldCounter rh = cachedHoldCounter;
if (rh == null || rh.tid != getThreadId(current))
cachedHoldCounter = rh = readHolds.get();
else if (rh.count == 0)
readHolds.set(rh);
rh.count++;
}
//返回1（正数）表示加读锁成功
return 1;
}
//走到这里说明发生了以下几种情况：
//1.需要阻塞读
//2.读锁数目已经超过最大值
//3.CAS加读锁失败
return fullTryAcquireShared(current);
}3.3.3.1.1.1. 判断是否需要阻塞读【公平】
[*]ReentrantReadWriteLock.FairSync#readerShouldBlock
//...
[*]AQS hasQueuedPredecessors
public void unlock() {
//AQS解锁
sync.release(1);
}3.3.3.1.1.2. 快速尝试加锁失败，那么改用死循环加锁
[*]ReentrantReadWriteLock.Sync#fullTryAcquireShared
final int fullTryAcquireShared(Thread current) {
HoldCounter rh = null;
//死循环
for (;;) {
int c = getState();
//写锁的数量不为0
if (exclusiveCount(c) != 0) {
//且加写锁的不是本线程
if (getExclusiveOwnerThread() != current)
//返回-1
return -1;
// else we hold the exclusive lock; blocking here
// would cause deadlock.
//没有人加写锁，如果读需要阻塞
} else if (readerShouldBlock()) {
// Make sure we're not acquiring read lock reentrantly
if (firstReader == current) {
// assert firstReaderHoldCount > 0;
} else {
if (rh == null) {
rh = cachedHoldCounter;
if (rh == null || rh.tid != getThreadId(current)) {
rh = readHolds.get();
if (rh.count == 0)
readHolds.remove();
}
}
if (rh.count == 0)
return -1;
}
}
//走到这里说明没有加写锁，读也不需要阻塞
if (sharedCount(c) == MAX_COUNT)
//加锁已超过最大值
throw new Error("Maximum lock count exceeded");
//加读锁，同上面的tryAcquireShared
if (compareAndSetState(c, c + SHARED_UNIT)) {
if (sharedCount(c) == 0) {
firstReader = current;
firstReaderHoldCount = 1;
} else if (firstReader == current) {
firstReaderHoldCount++;
} else {
if (rh == null)
rh = cachedHoldCounter;
if (rh == null || rh.tid != getThreadId(current))
rh = readHolds.get();
else if (rh.count == 0)
readHolds.set(rh);
rh.count++;
cachedHoldCounter = rh; // cache for release
}
return 1;
}
}
}3.3.3.1.2. 尝试加锁失败，那么入AQS队列并阻塞，等待唤醒继续抢占锁
private void doAcquireShared(int arg) {
//构造SHARE节点加入AQS队列阻塞等待唤醒
final Node node = addWaiter(Node.SHARED);
boolean failed = true;
try {
boolean interrupted = false;
//死循环抢占锁
for (;;) {
//当前节点的前一个节点是头节点
final Node p = node.predecessor();
if (p == head) {
//继续尝试加共享锁
int r = tryAcquireShared(arg);
if (r >= 0) {
setHeadAndPropagate(node, r);
p.next = null; // help GC
if (interrupted)
selfInterrupt();
failed = false;
return;
}
}
//判断是否需要阻塞
if (shouldParkAfterFailedAcquire(p, node) &&
//需要的话进行阻塞
parkAndCheckInterrupt())
interrupted = true;
}
} finally {
if (failed)
cancelAcquire(node);
}
}3.3.3.1.2.1. 加入AQS队列并阻塞
[*]addWaiter
private Node addWaiter(Node mode) {
Node node = new Node(Thread.currentThread(), mode);
// Try the fast path of enq; backup to full enq on failure
//快速尝试加入队尾
Node pred = tail;
if (pred != null) {
node.prev = pred;
if (compareAndSetTail(pred, node)) {
pred.next = node;
return node;
}
}
//快速尝试加入队尾失败，那么改用enq加入队尾
enq(node);
return node;
}参考：5.AQS.md
3.3.4. 读锁解锁
[*]ReentrantReadWriteLock.ReadLock#unlock
public void unlock() {
//使用AQS解共享锁
sync.releaseShared(1);
}3.3.4.1. 使用AQS解共享锁
[*]AbstractQueuedSynchronizer#releaseShared
public final boolean releaseShared(int arg) {
//调用Sync尝试解共享锁
//如果共享锁【读锁】已经全部释放完，那么执行doReleaseShared
if (tryReleaseShared(arg)) {
doReleaseShared();
return true;
}
return false;
}3.3.4.1.1. 使用Sync尝试解锁
[*]ReentrantReadWriteLock.Sync#tryReleaseShared
protected final boolean tryReleaseShared(int unused) { Thread current = Thread.currentThread(); //当前线程就是第一次加读锁的线程 if (firstReader == current) { //所有读锁都释放完毕 if (firstReaderHoldCount == 1) //那么置加锁线程为空 firstReader = null; //没有释放完那么减读锁数量 else firstReaderHoldCount--; //当前线程不是第一次加读锁的线程 } else { HoldCounter rh = cachedHoldCounter; if (rh == null || rh.tid != getThreadId(current)) rh = readHolds.get(); int count = rh.count; if (count
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