1. ReentrantLock定义

PS:要想理解ReentrantLock原理，需要先了解AQS，不了解AQS的可以看先之前的文章->aqs源码解析

ReentrantLock是jdk提供的可中断, 可重入获取, 支持超时, 支持尝试获取锁。它与synchronized锁主要有以下几点不同之处：

1. 可重入, 一个线程获取独占锁后, 可多次获取, 多次释放(synchronized也一样, 只是synchronized内的代码执行异常后会自动释放到monitor上的锁)
2. 支持中断(synchronized不支持)
3. 支持超时机制, 支持尝试获取lock, 支持公不公平获取lock(主要区别在 判断 AQS 中的 Sync Queue 里面是否有其他线程等待获取 lock)
4. 支持调用 Condition 提供的 await(释放lock, 并等待), signal(将线程节点从 Condition Queue 转移到 Sync Queue 里面)
5. 在运行 synchronized 里面的代码若抛出异常, 则会自动释放监视器上的lock, 而 ReentrantLock 是需要显示的调用 unlock方法

下面我们先来看看reentrantLock如何使用的：

public class ReentrantLockDemo extends Thread {

public static Lock lock = new ReentrantLock();
public static int i = 0;

public ReentrantLockDemo(String name) {
super(name);
}

public static void main(String[] args) {
new ReentrantLockDemo("thread1").start();
new ReentrantLockDemo("thread2").start();
new ReentrantLockDemo("thread3").start();
}

@Override
public void run(){
try {
lock.lock();
System.out.println(Thread.currentThread().getName() + "获取lock锁");
TimeUnit.SECONDS.sleep(1);

}catch (Exception e){
e.printStackTrace();
}finally {
// 因为程序发生异常，lock锁不会自动释放，所以放在finally中释放锁
lock.unlock();
}
}
}

同synchronized一样，对于同一把锁，在同一时刻只会有一个线程获取到锁

2. ReentrantLock特性

2.1 公平锁和非公平锁

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
*/
public ReentrantLock(boolean fair) {
sync = fair ? new FairSync() : new NonfairSync();
}

ReentrantLock内部提供两种锁实现，分别是FairSync（公平锁）和NonfairSync（非公平锁）。通过上面的构造方法可以看出，默认是非公平锁，这样的好处是吞吐量高。可以通过有参构造传值，决定使用公平锁或者非公平锁。

2.2 重入锁

和Synchronized一样，ReentrantLock也支持重入锁。因为ReentrantLock是基于AQS实现的（了解aqs可以看我之前的文章，aqs源码解析），当获取锁的线程再次尝试获取锁的时候，通过state++标记锁的获取次数。

3. 源码分析

公平锁和非公平锁的父类：

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.
*/
abstract void lock();

/**
* 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();
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;
}

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;
}

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();
}

final ConditionObject newCondition() {
return new ConditionObject();
}

// Methods relayed from outer class

final Thread getOwner() {
return getState() == 0 ? null : getExclusiveOwnerThread();
}

final int getHoldCount() {
return isHeldExclusively() ? getState() : 0;
}

final boolean isLocked() {
return getState() != 0;
}

/**
* Reconstitutes the instance from a stream (that is, deserializes it).
*/
private void readObject(java.io.ObjectInputStream s)
throws java.io.IOException, ClassNotFoundException {
s.defaultReadObject();
setState(0); // reset to unlocked state
}
}

公平锁：

static final class FairSync extends Sync {
private static final long serialVersionUID = -3000897897090466540L;

final void lock() {
acquire(1);
}

/**
* Fair version of tryAcquire.  Don't grant access unless
* recursive call or no waiters or is first.
*/
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;
}
}

非公平锁：

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() {
if (compareAndSetState(0, 1))
setExclusiveOwnerThread(Thread.currentThread());
else
acquire(1);
}

protected final boolean tryAcquire(int acquires) {
return nonfairTryAcquire(acquires);
}
}

3.1 lock（）分析

我们可以看到FairSync和NoFairSync都继承自Sync，而Sync继承自AbstractQueuedSynchronizer，下面我们来分析获取锁的过程：

final void lock() {
acquire(1);
}

当调用lock方法时，实际调用的是AQS中的acquire方法

public final void acquire(int arg) {
if (!tryAcquire(arg) &&
acquireQueued(addWaiter(Node.EXCLUSIVE), arg))
selfInterrupt();
}

AQS中的tryAcquire方法是抽象方法，具体的实现是在FairSync和NoFairSync中的，下面是Sync中tryAcquire的具体实现：

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;
}

当tryAcquire方法调用后，会继续走aqs的逻辑去尝试获取锁，如果没有获取到锁，会加入到aqs队列中。

3.2 unlock（）分析

public void unlock() {
sync.release(1);
}

当调用unlock方法时，调用的也是aqs的release方法：

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方法同上，在aqs中也是抽象方法，具体实现也是在Sync中：

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;
}

后续会继续走aqs逻辑进行释放锁操作。

3.3 公平锁和非公平锁分析

3.3.1 非公平锁FairSync

非公平锁中的lock逻辑：

final void lock() {
// 会直接尝试获取锁
if (compareAndSetState(0, 1))
setExclusiveOwnerThread(Thread.currentThread());
else
acquire(1);
}

当线程调用lock方法时，不管aqs中等待队列中是否有线程等待，会直接调用compareAndSetState方法尝试获取锁，所以获取锁操作时非公平的。

3.3.2 公平锁NoFairSync

公平锁中的lock逻辑：

final void lock() {
acquire(1);
}

当线程调用lock方法时，直接走aqs的acquire逻辑，判断队列中是否有线程等待，如果有线程等待，不会尝试获取锁，而是加入到等待队列的尾部，服从先到先得的逻辑。