锁持有者管理器AbstractOwnableSynchronizer： http://donald-draper.iteye.com/blog/2360109
AQS线程挂起辅助类LockSupport： http://donald-draper.iteye.com/blog/2360206
AQS详解-CLH队列，线程等待状态： http://donald-draper.iteye.com/blog/2360256
AQS-Condition详解： http://donald-draper.iteye.com/blog/2360381
/*
* ORACLE PROPRIETARY/CONFIDENTIAL. Use is subject to license terms.
* Written by Doug Lea with assistance from members of JCP JSR-166
* Expert Group and released to the public domain, as explained at
* http://creativecommons.org/publicdomain/zero/1.0/
*/
前面的文章中，我们看了CAS原理和AQS机制，今天我们来看已下可重入锁ReentrantLock。
ReentrantLock本质上一种独占锁，获取锁方式有公平与非公平获取锁方式。
import java.util.*;
import java.util.concurrent.*;
import java.util.concurrent.atomic.*;
/**
* 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.
*
一个与implicit monitor lock具有相同功能的可扩展的可重入互质锁。
* A {@code ReentrantLock} is [i]owned[/i] 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}.
可重入锁，被上次成功获取锁，还没释放的线程，所拥有的；当锁没有被其他线程所
持有，线程可以调用lock函数，获取锁；当锁的持有者为当前线程，当前线程调用lock函数，
立刻返回，并获取锁；可用isHeldByCurrentThread方法，判断锁是否被当前单线程所持有，
用getHoldCount获取当前线程，持有锁的次数（在线程持有锁，再次调用lock，成功获取锁的次数）。
*
*
The constructor for this class accepts an optional
* [i]fairness[/i] parameter. When set {@code true}, under
* contention, locks favor granting access to the longest-waiting
* thread. Otherwise this lock does not guarantee any particular
* access order. Programs using fair locks accessed by many threads
* may display lower overall throughput (i.e., are slower; often much
* slower) than those using the default setting, but have smaller
* variances in times to obtain locks and guarantee lack of
* starvation. Note however, that fairness of locks does not guarantee
* fairness of thread scheduling. Thus, one of many threads using a
* fair lock may obtain it multiple times in succession while other
* active threads are not progressing and not currently holding the
* lock.
* Also note that the untimed {@link #tryLock() tryLock} method does not
* honor the fairness setting. It will succeed if the lock
* is available even if other threads are waiting.
*
ReentrantLock的构造函数有一个公平性参数boolean，来确定，可重入锁是公平锁，还是非公平锁。
如果是公平锁，当锁没有持有者时，将锁授予，最早等待获取锁的线程；非公平锁，不能保证按照
获取锁的顺序，将锁的授予线程；非公平锁在性能上，更优一些，但在获取锁的尝试次数和保证
lack of starvation（锁的饥渴性，暂时这么翻译）上，两种锁没有太多的差别。公平锁，也不能
绝对的保证公平性，比如，当其他的线程等待锁的时候，一个线程持有锁，也许在持有锁的过程中，
多次获取锁。tryLock也不能保证公平性，即使其他的线程在等待锁，一个线程持有锁，调用tryLock
如果锁可利用，则线程获取锁成功。
为什么，非公平锁的性能比公平锁要高呢？假设现在有一些线程在等待锁，当锁被持有者释放时，
这时，正好有一个线程获取锁，非公平锁则获取成功，公平锁则要从锁等待队列线程中，唤醒一个线程
，进入就绪运行状态，切换上下文，倒不如，让正在请求锁的线程，直接获取锁。
*
It is recommended practice to [i]always[/i] immediately
* follow a call to {@code lock} with a {@code try} block, most
* typically in a before/after construction such as:
*
在使用ReentrantLock时候，强烈建议在获取锁后面使用try语句块，以便在
finally中释放锁，如下
*
* class X {
* private final ReentrantLock lock = new ReentrantLock();
* // ...
*
* public void m() {
* lock.lock(); // block until condition holds
* try {
* // ... method body
* } finally {
* lock.unlock()
* }
* }
* }
*
*
* In addition to implementing the {@link Lock} interface, this
* class defines methods {@code isLocked} and
* {@code getLockQueueLength}, as well as some associated
* {@code protected} access methods that may be useful for
* instrumentation and monitoring.
对于实现可重入锁ReentrantLock，除了Lock中的方法外，还可以调用
ReentrantLock锁的isLocked和getLockQueueLength方法，和一些protected的
方法，以便监视锁的状态
*
*
Serialization of this class behaves in the same way as built-in
* locks: a deserialized lock is in the unlocked state, regardless of
* its state when serialized.
*
序列化可重入锁ReentrantLock，则忽略锁状态,反序列化时，锁处于unlocked state；
*
This lock supports a maximum of 2147483647 recursive locks by
* the same thread. Attempts to exceed this limit result in
* {@link Error} throws from locking methods.
*一个线程可以持有锁的次数为2147483647(2^31-1),当尝试次数，超过最大限制时，则
抛出异常，如果线程在持有2147483647次的情况下，再TryAcquire，则锁的持有数为-1；
超过整数范围溢出；
* @since 1.5
* @author Doug Lea
*/
public class ReentrantLock implements Lock, java.io.Serializable {
private static final long serialVersionUID = 7373984872572414699L;
//同步器，提供所有锁机制
/** Synchronizer providing all implementation mechanics */
private final Sync sync;
/**
* Base of synchronization control for this lock. Subclassed
* into fair and nonfair versions below. Uses AQS state to
* represent the number of holds on the lock.
*/
//可重入锁，依赖于同步Sync，同步是基于AQS的实现；同步Sync
//有两种实现一种是公平锁，一种是非公平锁；用AQS state表示，锁的状态。
abstract static class Sync extends AbstractQueuedSynchronizer {
private static final long serialVersionUID = -5179523762034025860L;
/**
* Performs {@link Lock#lock}. The main reason for subclassing
* is to allow fast path for nonfair version.
*/
在非公平锁实现中，允许快速获取锁
abstract void lock();
/**
* Performs non-fair tryLock. tryAcquire is
* implemented in subclasses, but both need nonfair
* try for trylock method.
*/
//在非公平锁的尝试获取锁方法中，会调用nonfairTryAcquire
//acquires为尝试获取次数，一般为1
final boolean nonfairTryAcquire(int acquires) {
//获取当前线程
final Thread current = Thread.currentThread();
int c = getState();//获取锁状态
if (c == 0) {//如果没有线程持有锁
if (compareAndSetState(0, acquires)) {
//尝试获取锁，如果获取成功，则设置锁的持有者，为当前线程，返回ture
setExclusiveOwnerThread(current);
return true;
}
}
else if (current == getExclusiveOwnerThread()) {
//如果锁被线程持有，判断持有者是不是当前线程；
//如果当前线程是锁的持有者，则锁被当前线程持有的次数+获取次数acquires
int nextc = c + acquires;
if (nextc < 0) // overflow
//如果锁被线程连续持有次数，小于0，则超出，一个线程可以连续持有锁的最大次数
//抛出异常
throw new Error("Maximum lock count exceeded");
//否则，设置锁状态，返回true
setState(nextc);
return true;
}
//锁被持有，且持有者非当前线程，返回false，获取锁失败。
return false;
}
//尝试释放锁，releases释放次数
protected final boolean tryRelease(int releases) {
//获取释放releases次锁后的锁状态。
int c = getState() - releases;
//如果当前线程非锁持有者，抛出状态监控异常
if (Thread.currentThread() != getExclusiveOwnerThread())
throw new IllegalMonitorStateException();
boolean free = false;
//如果锁持有者线程释放，releases次后，锁状态为打开
if (c == 0) {
//释放锁成功
free = true;
//设置锁持有者为NULL
setExclusiveOwnerThread(null);
}
//如果，释放releases次后，线程仍持有锁，设置锁状态，释放失败。
setState(c);
return free;
}
//检查锁持有者是否为当前线程
protected final boolean isHeldExclusively() {
// While we must in general read state before owner,
// we don't need to do so to check if current thread is owner
return getExclusiveOwnerThread() == Thread.currentThread();
}
//创建条件，这个我们在前面讲过
final ConditionObject newCondition() {
return new ConditionObject();
}
// Methods relayed from outer class
//获取锁持有者线程，无持有者，则为null
final Thread getOwner() {
return getState() == 0 ? null : getExclusiveOwnerThread();
}
//获取线程连续持有锁的次数，如果是当前线程持有锁，则返回state，否则为0
final int getHoldCount() {
return isHeldExclusively() ? getState() : 0;
}
//锁是否被持有
final boolean isLocked() {
return getState() != 0;
}
/**
* Reconstitutes this lock instance from a stream.
* @param s the stream
*/
//反序列化锁，设置锁为打开状态
private void readObject(java.io.ObjectInputStream s)
throws java.io.IOException, ClassNotFoundException {
s.defaultReadObject();
setState(0); // reset to unlocked state
}
}
/**
* Creates an instance of {@code ReentrantLock}.
* This is equivalent to using {@code ReentrantLock(false)}.
*/
//创建可重入锁，默认为非公平锁
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
*/
//根据公平锁与非公平锁标志，创建相应的锁
public ReentrantLock(boolean fair) {
sync = fair ? new FairSync() : new NonfairSync();
}
static final class NonfairSync extends Sync {}
static final class FairSync extends Sync {}
}
从上面可以看出ReentrantLock关联一个同步锁SYNC，内部的SYNC是基于AQS实现的。
同步锁SYNC有两种实现，公平锁与非公平锁；ReentrantLock默认创建的是非公平锁。
下面再来看一下公平锁与非公平锁，先看非公平锁
/**
* Sync object for non-fair locks
*/
static final class NonfairSync extends Sync {
private static final long serialVersionUID = 7316153563782823691L;
/**
* Performs lock. Try immediate barge, backing up to normal
* acquire on failure.
*/
final void lock() {
//先以CAS方式获取锁，如果获取成功，设置当前线程为锁，持有者
if (compareAndSetState(0, 1))
setExclusiveOwnerThread(Thread.currentThread());
else
//否则，这一步我们单看
acquire(1);
}
//尝试获取锁，acquires次，一般为1
protected final boolean tryAcquire(int acquires) {
//以非公平的方式获取锁
return nonfairTryAcquire(acquires);
}
}
//AQS
/**
* 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}.
*尝试以独占模式，获取锁，忽略中断。至少尝试一次，获取锁，成功则返回，
获取失败，添加到同步等待队列，可能重复的blocking and unblocking，
尝试获取锁，直到成功，用于lock方法
* @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) {
//如果获取锁失败，则添加独占模式节点，到队列中，自旋，队列头部节点尝试获取锁，
如果尝试获取失败，检查是否可以中断当前线程，如果可以，则中断当前线程。
if (!tryAcquire(arg) &&
acquireQueued(addWaiter(Node.EXCLUSIVE), arg))
selfInterrupt();
}
//待父类扩展
protected boolean tryAcquire(int arg) {
throw new UnsupportedOperationException();
}
先看
addWaiter(Node.EXCLUSIVE)
再看
acquireQueued(addWaiter(Node.EXCLUSIVE), arg))
最后再看
selfInterrupt();
//添加独占模式等待节点
addWaiter(Node.EXCLUSIVE)
/**
* Creates and enqueues node for current thread and given mode.
*
* @param mode Node.EXCLUSIVE for exclusive, Node.SHARED for shared
* @return the new node
*/
创建独占或共享模式节点到同步等待队列中
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;
}
再看
acquireQueued(addWaiter(Node.EXCLUSIVE), arg))
这个方法我们在Condition那篇文章中说过，
首次查看节点的前驱节点线程，是否是头节点，如果时，则尝试获取
锁，如果成功，则设置节点为头节点；否则检查当是否应该再获取锁的时候
，唤醒后继节点；如果尝试获取锁失败，则park当前线程，如果失败，则整个
过程失败，从队列中移除当前线程节点。
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);
}
}
自旋请求锁，如果可能的话挂起线程，直到得到锁，返回当前线程是否中断过
（如果park()过并且中断过的话有一个interrupted中断位）。
。acquireQueued过程是这样的：
1. 如果当前节点是AQS队列的头结点（如果第一个节点是DUMP节点也就是傀儡节点，
那么第二个节点实际上就是头结点了），就尝试在此获取锁tryAcquire(arg)。
如果成功就将头结点设置为当前节点（不管第一个结点是否是DUMP节点），返回中断位。否则进行2。
2. 检测当前节点是否应该park()，如果应该park()就挂起当前线程并且返回当前线程中断位。进行操作1。
最后再看
selfInterrupt();
/**
* Convenience method to interrupt current thread.
*/
private static void selfInterrupt() {
Thread.currentThread().interrupt();
}
再看公平锁
/**
* Sync object for fair locks
*/
static final class FairSync extends Sync {
private static final long serialVersionUID = -3000897897090466540L;
final void lock() {
//这个过程，前面说过//如果获取锁失败，则添加独占模式节点，
到队列中，自旋，队列头部节点尝试获取锁，
如果尝试获取失败，检查是否可以中断当前线程，如果可以，则中断当前线程。
acquire(1);
}
/**
* Fair version of tryAcquire. Don't grant access unless
* recursive call or no waiters or is first.
*/
这个函数与SYNC的nonfairTryAcquire方法基本相同
protected final boolean tryAcquire(int acquires) {
final Thread current = Thread.currentThread();
int c = getState();
if (c == 0) {
//首先检查是否有前继节点，如果没有，则获取锁
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;
}
}
我们再回到SYNC的nonfairTryAcquire方法，非公平尝试获取锁；
//SYNC
/**
* Performs non-fair tryLock. tryAcquire is
* implemented in subclasses, but both need nonfair
* try for trylock method.
*/
//在非公平锁的尝试获取锁方法中，会调用nonfairTryAcquire
//acquires为尝试获取次数，一般为1
final boolean nonfairTryAcquire(int acquires) {
//获取当前线程
final Thread current = Thread.currentThread();
int c = getState();//获取锁状态
if (c == 0) {//如果没有线程持有锁
if (compareAndSetState(0, acquires)) {
//尝试获取锁，如果获取成功，则设置锁的持有者，为当前线程，返回ture
setExclusiveOwnerThread(current);
return true;
}
}
else if (current == getExclusiveOwnerThread()) {
//如果锁被线程持有，判断持有者是不是当前线程；
//如果当前线程是锁的持有者，则锁被当前线程持有的次数+获取次数acquires
int nextc = c + acquires;
if (nextc < 0) // overflow
//如果锁被线程连续持有次数，小于0，则超出，一个线程可以连续持有锁的最大次数
//抛出异常
throw new Error("Maximum lock count exceeded");
//否则，设置锁状态，返回true
setState(nextc);
return true;
}
//锁被持有，且持有者非当前线程，返回false，获取锁失败。
return false;
}
比较非公平锁的尝试获取锁nonfairTryAcquire与公平锁TryAcquire的区别在与
非公平尝试获取锁时，如果锁为打开状态，则锁住锁；而公平锁，则先看有没有前驱节点
，有前驱，则不能锁住锁，没有则可锁住。
我们再对比一下公平锁和非公平锁的lock方法
static final class FairSync extends Sync {
private static final long serialVersionUID = -3000897897090466540L;
final void lock() {
//这个过程，前面说过
//如果获取锁失败，则添加独占模式节点，
到队列中，自旋，队列头部节点尝试获取锁，如果获取成功，设置当前节点为头节点；
如果尝试获取失败，检查是否可以中断当前线程，如果可以，则中断当前线程。
acquire(1);
}
}
static final class NonfairSync extends Sync {
private static final long serialVersionUID = 7316153563782823691L;
/**
* Performs lock. Try immediate barge, backing up to normal
* acquire on failure.
*/
final void lock() {
//先以CAS方式获取锁，如果获取成功，设置当前线程为锁，持有者
if (compareAndSetState(0, 1))
setExclusiveOwnerThread(Thread.currentThread());
else
acquire(1);
}
从上面可以看出，非公平锁与公平锁lock的时候，最大的不同是非公平锁，
先以CAS的方式锁住锁，在进行acquire操作，而公平锁，直接acquire操作。
再来看可重入锁的其他方法
//ReentrantLock
/**
* Acquires the lock.
*
* Acquires the lock if it is not held by another thread and returns
* immediately, setting the lock hold count to one.
* 如果锁没有被其他线程锁持有，则立即返回，锁持有锁为1
*
If the current thread already holds the lock then the hold
* count is incremented by one and the method returns immediately.
*如果当前线程已经持有锁，则所持有锁自增1
*
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.
*/
如果锁被其他线程所持有，则当前线程自旋，知道获取锁
public void lock() {
//委托给同步器
sync.lock();
}
/**
* Acquires the lock only if it is not held by another thread at the time
* of invocation.
* 如果锁没有被其他线程所持有，则获取锁成功
*
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()} [i]will[/i]
* 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).
*如果锁没有被其他线程持有，则获取锁成功，立即返回true，锁持有数设为1.
当我们获取锁的策略为公平策略时，尝试获取锁时，如果锁可用，则获取成功，
无论其他线程和当前线程是否在等待锁。如果必须要保持公平可以用tryLock(long, TimeUnit)
方法。
*
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
*/
//以非公平方式尝试获取锁
public boolean tryLock() {
return sync.nonfairTryAcquire(1);
}
//以公平方式获取锁，其实公平方式，也不一定能保证绝对的公平，前面讲AQS说过
public boolean tryLock(long timeout, TimeUnit unit)
throws InterruptedException {
return sync.tryAcquireNanos(1, unit.toNanos(timeout));
}
/**
* Acquires the lock unless the current thread is
* {@linkplain Thread#interrupt interrupted}.
*以可中断方式获取锁，当线程获取锁失败，则中断，当线程中断状态被消除时，
可以尝试获取锁。
*
Acquires the lock if it is not held by another thread and returns
* immediately, setting the lock hold count to one.
* 如果锁没有被其他线程锁持有，则立即返回，锁持有锁为1
*
If the current thread already holds this lock then the hold count
* is incremented by one and the method returns immediately.
*如果当前线程已经持有锁，则所持有锁自增1
*
If the lock is held by another thread then the
* current thread becomes disabled for thread scheduling
* purposes and lies dormant until one of two things happens:
*
* [list]
当锁被其他线程持有，则自旋，尝试获取锁，直到以下两种条件发生
*
*
If the lock is acquired by the current thread then the lock hold * count is set to one. * *
If the current thread: * * [list] * *
In this implementation, as this method is an explicit * interruption point, preference is given to responding to the * interrupt over normal or reentrant acquisition of the lock. * * @throws InterruptedException if the current thread is interrupted */ public void lockInterruptibly() throws InterruptedException { sync.acquireInterruptibly(1); }
//AQS
/**
* Acquires in exclusive mode, aborting if interrupted.
* Implemented by first checking interrupt status, then invoking
* at least once {@link #tryAcquire}, returning on
* success. Otherwise the thread is queued, possibly repeatedly
* blocking and unblocking, invoking {@link #tryAcquire}
* until success or the thread is interrupted. This method can be
* used to implement method {@link Lock#lockInterruptibly}.
*
以独占模式获取锁，如线程被中断，则aborting。
* @param arg the acquire argument. This value is conveyed to
* {@link #tryAcquire} but is otherwise uninterpreted and
* can represent anything you like.
* @throws InterruptedException if the current thread is interrupted
*/
public final void acquireInterruptibly(int arg)
throws InterruptedException {
if (Thread.interrupted())
//检查线程是否处于中断状态，是，则抛出中断异常
throw new InterruptedException();
if (!tryAcquire(arg))
//如果尝试获取锁，失败，则
doAcquireInterruptibly(arg);
}
/**
* Acquires in exclusive interruptible mode.
* @param arg the acquire argument
*/
private void doAcquireInterruptibly(int arg)
throws InterruptedException {
//添加独占模式节点，到同步等待队列
final Node node = addWaiter(Node.EXCLUSIVE);
boolean failed = true;
try {
for (;;) {
//以自旋方式，这个过程与acquireQueued相似
final Node p = node.predecessor();
if (p == head && tryAcquire(arg)) {
setHead(node);
p.next = null; // help GC
failed = false;
return;
}
if (shouldParkAfterFailedAcquire(p, node) &&
parkAndCheckInterrupt())
throw new InterruptedException();
}
} finally {
if (failed)
//如果失败，移除取消等待的线程节点。
cancelAcquire(node);
}
}
自旋请求锁，如果可能的话挂起线程，直到得到锁；在这一过程中，如果获取失败，且可park当前线程，则park当前线程，再判断是否可以中断，可以则抛出中断异常。
总结：
可重入自旋锁，当线程持有锁，可以多次获取锁，但最多只有2^31-1次；获取失败时，添加到同步等待队列自旋，直到获取锁成功；ReentrantLock关联一个同步锁SYNC，内部的SYNC是基于AQS实现的。同步锁SYNC有两种实现，公平锁与非公平锁；ReentrantLock默认创建的是非公平锁。比较非公平锁的尝试获取锁nonfairTryAcquire与公平锁TryAcquire的区别在于，非公平尝试获取锁时，如果锁为打开状态，则锁住锁；而公平锁，则先看有没有前驱节点，有前驱，则不能锁住锁，没有则可锁住。公平锁与公平锁lock，最大的不同是非公平锁，先以CAS的方式锁住锁，在进行acquire操作，而公平锁，直接acquire操作。
acquire操作主要过程为，自旋，检查节点的前驱节点是否为头节点，如果是，当前节点为同步队列的第一个节点，则尝试获取锁，如果成功，设置头结点为当前节点，否则判断尝试获取锁失败，是否应该park，如果需要park，则park当前线程，park后，检查是否可中断当前线程，如果可，则中断当前线程。
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