本文基于jdk1.8進行分析。
ReentrantLock是一個可重入鎖，在ConcurrentHashMap中使用了ReentrantLock。
首先看一下源碼中對ReentrantLock的介紹。如下圖。ReentrantLock是一個可重入的排他鎖，它和synchronized的方法和代碼有著相同的行為和語義，但有更多的功能。ReentrantLock是被最后一個成功lock鎖并且還沒有unlock的線程擁有著。如果鎖沒有被別的線程擁有，那么一個線程調用lock方法，就會成功獲取鎖并返回。如果當前線程已經擁有該鎖，那么lock方法會立刻返回。這個可以通過isHeldByCurrentThread方法和getHoldCount方法進行驗證。除了這部分介紹外，類前面的javadoc文檔很長，就不在這里全部展開。隨著后面介紹源碼，會一一涉及到。
/**
* A reentrant mutual exclusion {@link Lock} with the same basic
* behavior and semantics as the implicit monitor lock accessed using
* {@code synchronized} methods and statements, but with extended
* capabilities.
*
A {@code ReentrantLock} is owned by the thread last
* successfully locking, but not yet unlocking it. A thread invoking
* {@code lock} will return, successfully acquiring the lock, when
* the lock is not owned by another thread. The method will return
* immediately if the current thread already owns the lock. This can
* be checked using methods {@link #isHeldByCurrentThread}, and {@link
* #getHoldCount}.
首先看一下成員變量，如下圖。ReentrantLock只有一個成員變量sync，即同步器，這個同步器提供所有的機制。Sync是AbstractQueuedSynchronizer的子類，同時，Sync有2個子類，NonfairSync和FairSync，分別是非公平鎖和公平鎖。Sync，NonfaireSync和FairSync的具體實現后面再講。
/** Synchronizer providing all implementation mechanics **/
private final Sync sync;
下面看一下構造函數。如下圖。可以看到，ReentrantLock默認是非公平鎖，它可以通過參數，指定初始化為公平鎖或非公平鎖。
/**
* Creates an instance of {@code ReentrantLock}.
* This is equivalent to using {@code ReentrantLock(false)}.
**/
public ReentrantLock() {
sync = new NonfairSync();
}
/**
* Creates an instance of {@code ReentrantLock} with the
* given fairness policy.
* @param fair {@code true} if this lock should use a fair ordering policy
**/
public ReentrantLock(boolean fair) {
sync = fair ? new FairSync() : new NonfairSync();
}
下面看一下ReentrantLock的主要方法。首先是lock方法。如下圖。lock方法的實現很簡單，就是調用Sync的lock方法。而Sync的lock方法是個抽象的，具體實現在NonfairSync和FairSync中。這里我們先不展開講，而是先讀一下lock方法的注釋，看看它的作用。lock方法的作用是獲取該鎖。分為3種情況。
1，如果鎖沒有被別的線程占有，那么當前線程就可以獲取到鎖并立刻返回，并把鎖計數設置為1。
2，如果當前線程已經占有該鎖了，那么就會把鎖計數加1，立刻返回。
3，如果鎖被另一個線程占有了，那么當前線程就無法再被線程調度，并且開始睡眠，直到獲取到鎖，在獲取到到鎖時，會把鎖計數設置為1。
lockInterruptibly方法與lock功能類似，但lockInterruptibly方法在等待的過程中，可以響應中斷。
/**
* Acquires the lock.
*
Acquires the lock if it is not held by another thread and returns
* immediately, setting the lock hold count to one.
*
If the current thread already holds the lock then the hold
* count is incremented by one and the method returns immediately.
*
If the lock is held by another thread then the
* current thread becomes disabled for thread scheduling
* purposes and lies dormant until the lock has been acquired,
* at which time the lock hold count is set to one.
**/
public void lock() {
sync.lock();
}
public void lockInterruptibly() throws InterruptedException {
sync.acquireInterruptibly(1);
}
下面，詳細看一下非公平鎖和公平鎖中對lock函數的實現。如下圖。下圖同時列出了公平鎖和非公平鎖中lock的實現邏輯。從注釋和代碼邏輯中，都可以看出，非公平鎖進行lock時，先嘗試立刻闖入(搶占)，如果成功，則獲取到鎖，如果失敗，再執行通常的獲取鎖的行為，即acquire(1)。
/**
* 非公平鎖中的lock
* Performs lock. Try immediate barge, backing up to normal
* acquire on failure.
**/
final void lock() {
if (compareAndSetState(0, 1))
setExclusiveOwnerThread(Thread.currentThread());
else
acquire(1);
}
//公平鎖中的lock
final void lock() {
acquire(1);
}
那么，我們首先了解下，非公平鎖“嘗試立刻闖入”，究竟做了什么。稍后再繼續講解通常的獲取鎖的行為。下圖是立即闖入行為compareAndSetState(0, 1)的實現。從compareAndSetState函數的注釋中，可以知道，如果同步狀態值與期望值相等，那么就把它的值設置為updated值。否則同步狀態值與期望值不相等，則返回false。這個操作和volatile有著相同的內存語義，也就是說，這個操作對其他線程是可見的。compareAndSetState函數注釋里描述的功能，是通過unsafe.compareAndSwapInt方法實現的，而unsafe.compareAndSwapInt是一個native方法，是用c++實現的。那么繼續追問，c++底層是怎么實現的？C++底層是通過CAS指令來實現的。什么是CAS指令呢？來自維基百科的解釋是，CAS，比較和交換，Compare and Swap，是用用于實現多線程原子同步的指令。它將內存位置的內容和給定值比較，只有在相同的情況下，將該內存的值設置為新的給定值。這個操作是原子操作。那么繼續追問，CAS指令的原子性，是如何實現的呢？我們都知道指令時CPU來執行的，在多CPU系統中，內存是共享的，內存和多個cpu都掛在總線上，當一個CPU執行CAS指令時，它會先將總線LOCK位點設置為高電平。如果別的CPU也要執行CAS執行，它會發現總線LOCK位點已經是高電平了，則無法執行CAS執行。CPU通過LOCK保證了指令的原子執行。
現在來看一下非公平鎖的lock行為，compareAndSetState(0, 1)，它期望鎖狀態為0，即沒有別的線程占用，并把新狀態設置為1，即標記為占用狀態。如果成功，則非公平鎖成功搶到鎖，之后setExclusiveOwnerThread，把自己設置為排他線程。非公平鎖這小子太壞了。如果搶占失敗，則執行與公平鎖相同的操作。
/**
* Atomically sets synchronization state to the given updated
* value if the current state value equals the expected value.
* This operation has memory semantics of a {@code volatile} read
* and write.
* @param expect the expected value
* @param update the new value
* @return {@code true} if successful. False return indicates that the actual
* value was not equal to the expected value.
**/
protected final boolean compareAndSetState(int expect, int update) {
// See below for intrinsics setup to support this
return unsafe.compareAndSwapInt(this, stateOffset, expect, update);
}
public final native boolean compareAndSwapInt(Object var1, long var2, int var4, int var5);
下面看一下公平鎖獲取鎖時的行為。如下圖。這部分的邏輯有些多，請閱讀代碼中的注釋進行理解。
/**
* 公平鎖的lock
**/
final void lock() {
acquire(1);
}
/**
* Acquires in exclusive mode, ignoring interrupts. Implemented
* by invoking at least once {@link #tryAcquire},
* returning on success. Otherwise the thread is queued, possibly
* repeatedly blocking and unblocking, invoking {@link
* #tryAcquire} until success. This method can be used
* to implement method {@link Lock#lock}.
* @param arg the acquire argument. This value is conveyed to
* {@link #tryAcquire} but is otherwise uninterpreted and
* can represent anything you like.
**/
public final void acquire(int arg) {
/**
* acquire首先進行tryAcquire()操作。如果tryAcquire()成功時則獲取到鎖，即刻返回。
* 如果tryAcquire()false時，會執行acquireQueued(addWaiter(Node.EXCLUSIVE), arg)
* 操作。如果acquireQueued(addWaiter(Node.EXCLUSIVE), arg)true時，則當前線程中斷自己。
* 如果acquireQueued(addWaiter(Node.EXCLUSIVE), arg)false，則返回。
* 其中tryAcquire()操作在NonfairSync中和FairSync中實現又有所區別。
**/
if (!tryAcquire(arg) &&
acquireQueued(addWaiter(Node.EXCLUSIVE), arg))
selfInterrupt();
}
/**
* NonfairSync中的tryAcquire。
* @param acquires
* @return
**/
protected final boolean tryAcquire(int acquires) {
return nonfairTryAcquire(acquires);
}
/**
* Performs non-fair tryLock. tryAcquire is implemented in
* subclasses, but both need nonfair try for trylock method.
**/
final boolean nonfairTryAcquire(int acquires) {
final Thread current = Thread.currentThread();
//首先獲取當前同步狀態值
int c = getState();
if (c == 0) {
//c為0，表示目前沒有線程占用鎖。沒有線程占用鎖時，當前線程嘗試搶鎖，如果搶鎖成功，則返回true。
if (compareAndSetState(0, acquires)) {
setExclusiveOwnerThread(current);
return true;
}
}
else if (current == getExclusiveOwnerThread()) {
//c不等于0時表示鎖被線程占用。如果是當前線程占用了，則將鎖計數加上acquires，并返回true。
int nextc = c + acquires;
if (nextc < 0) // overflow
throw new Error("Maximum lock count exceeded");
setState(nextc);
return true;
}
//以上情況都不是時，返回false，表示非公平搶鎖失敗。
return false;
}
/**
* Fair version of tryAcquire. Don't grant access unless
* recursive call or no waiters or is first.
* 這個是公平版本的tryAcquire
**/
protected final boolean tryAcquire(int acquires) {
final Thread current = Thread.currentThread();
int c = getState();
if (c == 0) {
//c=0時表示鎖未被占用。這里是先判斷隊列中前面是否有別的線程。沒有別的線程時，才進行CAS操作。
//公平鎖之所以公平，正是因為這里。它發現鎖未被占用時，首先判斷等待隊列中是否有別的線程已經在等待了。
//而非公平鎖，發現鎖未被占用時，根本不管隊列中的排隊情況，上來就搶。
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;
}
/**
* Acquires in exclusive uninterruptible mode for thread already in
* queue. Used by condition wait methods as well as acquire.
* 當搶鎖失敗時，先執行addWaiter(Node.EXCLUSIVE)，將當前線程加入等待隊列，再執行該方法。
* 該方法的作用是中斷當前線程，并進行檢查，知道當前線程是隊列中的第一個線程，并且搶鎖成功時，
* 該方法返回。
* @param node the node
* @param arg the acquire argument
* @return {@code true} if interrupted while waiting
**/
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);
}
}
接下來是tryLock方法。代碼如下。從注釋中我們可以理解到，只有當調用tryLock時鎖沒有被別的線程占用，tryLock才會獲取鎖。如果鎖沒有被另一個線程占用，那么就獲取鎖，并立刻返回true，并把鎖計數設置為1. 甚至在鎖被設置為公平排序的情況下，若果鎖可用，調用tryLock會立刻獲取鎖，而不管有沒有別的線程在等待鎖了。從這里我們總結出，不管可重入鎖是公平鎖還是非公平鎖，tryLock方法只會是非公平的。
/**
* Acquires the lock only if it is not held by another thread at the time
* of invocation.
*
Acquires the lock if it is not held by another thread and
* returns immediately with the value {@code true}, setting the
* lock hold count to one. Even when this lock has been set to use a
* fair ordering policy, a call to {@code tryLock()} will
* immediately acquire the lock if it is available, whether or not
* other threads are currently waiting for the lock.
* This "barging" behavior can be useful in certain
* circumstances, even though it breaks fairness. If you want to honor
* the fairness setting for this lock, then use
* {@link #tryLock(long, TimeUnit) tryLock(0, TimeUnit.SECONDS) }
* which is almost equivalent (it also detects interruption).
*
If the current thread already holds this lock then the hold
* count is incremented by one and the method returns {@code true}.
*
If the lock is held by another thread then this method will return
* immediately with the value {@code false}.
* @return {@code true} if the lock was free and was acquired by the
* current thread, or the lock was already held by the current
* thread; and {@code false} otherwise
**/
public boolean tryLock() {
return sync.nonfairTryAcquire(1);
}
public boolean tryLock(long timeout, TimeUnit unit)
throws InterruptedException {
return sync.tryAcquireNanos(1, unit.toNanos(timeout));
}
接下來是釋放鎖的方法unlock。代碼如下。unlock方式的實現，是以參數1來調用sync.release方法。而release方法是如何實現的呢？release方法首先會調用tryRelease方法，如果tryRelease成功，則喚醒后繼者線程。而tryRelease的實現過程十分清晰，首先獲取鎖狀態，鎖狀態減去參數(放鎖次數)，得到新狀態。然后判斷持有鎖的線程是否為當前線程，如果不是當前線程，則拋出IllegalMonitorStateException。然后判斷，如果新狀態為0，說明放鎖成功，則把持有鎖的線程設置為null，并返回true。如果新狀態不為0，則返回false。從tryRelease的返回值來看，它返回的true或false，指的是否成功的釋放了該鎖。成功的釋放該鎖的意思是徹底釋放鎖，別的線程就可以獲取鎖了。這里要認識到，即便tryRelease返回false，它也只是說明了鎖沒有完全釋放，本次調用的這個釋放次數值，依然是釋放成功的。
/**
* Attempts to release this lock.
*
If the current thread is the holder of this lock then the hold
* count is decremented. If the hold count is now zero then the lock
* is released. If the current thread is not the holder of this
* lock then {@link IllegalMonitorStateException} is thrown.
* @throws IllegalMonitorStateException if the current thread does not
* hold this lock
**/
public void unlock() {
sync.release(1);
}
/**
* Releases in exclusive mode. Implemented by unblocking one or
* more threads if {@link #tryRelease} returns true.
* This method can be used to implement method {@link Lock#unlock}.
* @param arg the release argument. This value is conveyed to
* {@link #tryRelease} but is otherwise uninterpreted and
* can represent anything you like.
* @return the value returned from {@link #tryRelease}
**/
public final boolean release(int arg) {
if (tryRelease(arg)) {
Node h = head;
if (h != null && h.waitStatus != 0)
unparkSuccessor(h);
return true;
}
return false;
}
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;
}
/**
* Wakes up node's successor, if one exists.
* @param node the 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);
}
接下來是newCondition方法。關于Condition這里不展開介紹，只是了解下該方法的作用。如下圖。該方法返回一個和這個鎖實例一起使用的Condition實例。返回的Condition實例支持和Object的監控方法例如wait-notify和notifyAll相同的用法。
1，如果沒有獲取鎖，調用Condition的await，signal，signalAll方法的任何一個時，會拋出IllegalMonitorStateException異常。
2，調用Condition的await方法時，鎖也會釋放，在await返回之前，鎖會被重新獲取，并且鎖計數會恢復到調用await方法時的值。
3，如果一個線程在等待的過程中被中斷了，那么等待就會結束，并拋出InterruptedException異常，線程的中斷標志位會被清理。
4，等待的線程以FIFO的順序被喚醒。
5，從await方法返回的線程們的獲取到鎖的順序，和線程最開始獲取鎖的順序相同，這是未指定情況下的默認實現。但是，公平鎖更鐘愛那些已經等待了最長時間的線程。
/**
* Returns a {@link Condition} instance for use with this
* {@link Lock} instance.
*
The returned {@link Condition} instance supports the same
* usages as do the {@link Object} monitor methods ({@link
* Object#wait() wait}, {@link Object#notify notify}, and {@link
* Object#notifyAll notifyAll}) when used with the built-in
* monitor lock.
*
*
If this lock is not held when any of the {@link Condition}* {@linkplain Condition#await() waiting} or {@linkplain
* Condition#signal signalling} methods are called, then an {@link
* IllegalMonitorStateException} is thrown.
*
When the condition {@linkplain Condition#await() waiting}* methods are called the lock is released and, before they
* return, the lock is reacquired and the lock hold count restored
* to what it was when the method was called.
*
If a thread is {@linkplain Thread#interrupt interrupted}* while waiting then the wait will terminate, an {@link
* InterruptedException} will be thrown, and the thread's
* interrupted status will be cleared.
*
Waiting threads are signalled in FIFO order.*
The ordering of lock reacquisition for threads returning* from waiting methods is the same as for threads initially
* acquiring the lock, which is in the default case not specified,
* but for fair locks favors those threads that have been
* waiting the longest.
*
* @return the Condition object
**/
public Condition newCondition() {
return sync.newCondition();
}
可重入鎖還有一些其他的方法，這里就不一一介紹了。This is the end.
總結
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