ReentrantLock是一个可重入的独占锁，内部使用AQS实现。state记录着持有锁的线程的进入同步代码块的次数。
1. AQS的基本实现Sync
Sync是一个抽象类，他没有重写tryAcquire方法，但是他多了一个nonfairTryAcquire方法，该方法是一个非公平获取锁资源的方法。提供了一个抽象的lock方法，这是一个统一的加锁方法。
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
}
}
1.1 nonfairTryAcquire
因为在ReentrantLock是支持公平锁和非公平锁的，这里的nonfairTryAcquire方法就是非公平获取锁资源的实现。我们可以看到分两种情况，一是当没有线程持有锁时，直接cas修改state，修改成功就获取到了锁，并将当前持有独占锁的线程设置为自己。二是判断当前自己是不是持有锁的线程(可能是重入)，如果是就只需增加锁的重入次数。最后就是有其他线程占用了锁，返回false。
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;
}
1.2 tryRelease
进行锁资源的释放，该方法是线程安全的，因为独占模式只有一个线程只有锁，也就只有一个线程可以释放锁资源。释放锁刚好更获取锁资源反着来，减少锁的重入次数，设置当前持有锁的线程为null，当state为0时，也就说明当前线程释放了锁资源，就返回true，从而唤醒后继节点。
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;
}
2. 公平和非公平模式
可以通过有参构造方法，指定使用公平锁还是非公平锁，默认无参构造方法是非公平锁。
public ReentrantLock(boolean fair) {
sync = fair ? new FairSync() : new NonfairSync();
}
2.1 非公平模式
实现了上面我们说的Sync类型，补上对应两个需要实现的方法lock和tryAcquire，这里的tryAcquire直接调用我们上面说的非公平获取锁资源的方法nonfairTryAcquire。这里lock方法本来可以直接调用acquire方法，但是他在前面还是做了一次快速获取锁资源的尝试，直接casstate，如果成功就设置持有锁的线程是当前线程，如果失败就再走一次正常的逻辑。
非公平性：代码我们可以看到，主要线程获取到了锁资源，就表示该线程持有了锁，不关系队列中释放有排在其前面的线程在等待。优点很明显，就是吞吐高；缺点也很明显，会导致线程饥渴，一直等待无法获取到锁资源(因为会被其他线程抢)。
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);
}
}
2.2 公平模式
也是实现了lock和tryAcquire方法，这里的lock方法就有了一点变化了，没有在lock里面之前获取锁资源了，而是全部交给了acquire来处理，这体现了公平性的一点，因为如果直接跟上面的非公平锁中lock一样获取锁资源的话，就没办法办证公平性了。tryAcquire中相比非公平锁，多了hasQueuedPredecessors方法的判断，该方法用来判断当前节点是否有前驱节点，如果有，就获取锁失败，也就是说来获取锁资源的线程必须保证他没有前驱节点，其他代码基本和非公平锁一样。
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;
}
}
public final boolean hasQueuedPredecessors() {
Node t = tail; // Read fields in reverse initialization order
Node h = head;
Node s;
return h != t &&
((s = h.next) == null || s.thread != Thread.currentThread());
}
3. 实现了lock类
实现了统一的lock接口，我们在使用时，一般是使用lock接口中的方法进行加锁和解锁。这里的sync就是构造方法中创建的NonfairSync或FairSync，另外newCondition是创建一个条件队列对象。
public class ReentrantLock implements Lock, java.io.Serializable {
...
public void lock() {
sync.lock();
}
public void lockInterruptibly() throws InterruptedException {
sync.acquireInterruptibly(1);
}
public boolean tryLock() {
return sync.nonfairTryAcquire(1);
}
public boolean tryLock(long timeout, TimeUnit unit)
throws InterruptedException {
return sync.tryAcquireNanos(1, unit.toNanos(timeout));
}
public void unlock() {
sync.release(1);
}
public Condition newCondition() {
return sync.newCondition();
}
}
4. 总结
ReentrantLock其实很简单，如果你看过AQS的源码分析，那ReentrantLock就是基于AQS实现一种独占(排他)可重入的锁。他提供了公平和非公平模式，公平性是在获取锁资源时，检查当前节点是否有前驱节点。最后是实现了标准的Lock接口，方便使用统一的lock和unlock进行加锁和解锁。