ReenTrantLock源码解析
可重入锁在实现上基于AQS框架，内部维护线程的state以及队列（CLH）的waitStatus,大量采用非阻塞算法，在中低量的并发上效率是非常高的。
下面分多种情况分析源码，这里只分析非公平锁。
1.简单应用，不使用条件队列
final ReentrantLock lock=new ReentrantLock();
for (int i = 0; i <2; i++) {
final int task=i;
new Thread(new Runnable() {
@Override
public void run() {
lock.lock();
try{
System.out.println(Thread.currentThread().getName()+" is task "+task);
}finally {
lock.unlock();
}
}
}).start();
}->加锁
final void lock() {
if (compareAndSetState(0, 1))
setExclusiveOwnerThread(Thread.currentThread());
else
acquire(1);
}这里采用cas并发原语，只有一个线程可以将state状态设置为1，然后设置排他线程，后续的线程都会进入acquire的CLH的队列中。
public final void acquire(int arg) {
if (!tryAcquire(arg) &&
acquireQueued(addWaiter(Node.EXCLUSIVE), arg))
selfInterrupt();
} 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.acquire调用tryAcquire接口，最终调用的是nonfairTryAcquire。这里重复了之前的操作，考虑到大量并发的情况下，会出现未进入队列的线程和已进入队列的线程产生竞争，重新检测条件可以避免这种情况，这里是公平锁与非公平锁的主要区别。else if里面的代码就是可重入的意思，如果当前线程二次进入加锁状态，这里会增加计数，并将线程的状态设置为计数值；其它的情况统统返回false,然后进入等待队列中。
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;
}
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);
}
} private static boolean shouldParkAfterFailedAcquire(Node pred, Node node) {
int ws = pred.waitStatus;
if (ws == Node.SIGNAL)
/*
* This node has already set status asking a release
* to signal it, so it can safely park.
*/
return true;
if (ws > 0) {
/*
* Predecessor was cancelled. Skip over predecessors and
* indicate retry.
*/
do {
node.prev = pred = pred.prev;
} while (pred.waitStatus > 0);
pred.next = node;
} else {
/*
* waitStatus must be 0 or PROPAGATE. Indicate that we
* need a signal, but don't park yet. Caller will need to
* retry to make sure it cannot acquire before parking.
*/
compareAndSetWaitStatus(pred, ws, Node.SIGNAL);
}
return false;
}private final boolean parkAndCheckInterrupt() {
LockSupport.park(this);
return Thread.interrupted();
}2.设置等待队列同样存在大量并发的情况，先是调用addWaiter()将该节点加入队尾并返回当前节点，接着采用自旋操作。if (p == head && tryAcquire(arg)) ，从head节点开始，再次获取条件tryAcquire,如果当前线程没有释放锁，这个条件显然进不去。再来看看 if (shouldParkAfterFailedAcquire(p, node) && parkAndCheckInterrupt())这个判断，前者设置其waitStatus状态，如果线程没有取消，最终的等待状态为-1，最后调用LockSupport阻塞库阻塞线程。
->unlock
public void unlock() {
sync.release(1);
} 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;
}重入是一个一个的进入的，显然释放也要一个一个的释放，release(1)。
if (c == 0) {
free = true;
setExclusiveOwnerThread(null);
}
只有当线程的持有锁的计数变为0时，线程才释放锁。接着是唤醒等待队列，同样是从head节点开始，节点不为null并且其等待状态不为0时，才进入唤醒。
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);
同样，这段代码并不是再同步中完成的，可能会出现已经取消但没有从队列中清除的现象。如果出现这种情况，循环找到该节点后续的第一个等待被唤醒的节点，唤醒对应的等待线程。
2.使用条件队列
测试代码：第一个线程会在加锁后进入警戒条件中，接着线程进入条件队列并释放锁，等待被唤醒；第二个线程起到唤醒的作用。
注：这两个线程并没有存在偏序关系，但是我在测试的时候可以指定线程的触发和运行状态，使其顺序触发或者按特定的顺序触发。
final ReentrantLock lock=new ReentrantLock();
Condition condition=lock.newCondition();
for (int i = 0; i <1 ; i++) {
final int task=i;
new Thread(new Runnable() {
@Override
public void run() {
lock.lock();
try{
while(state==0){
condition.await();
}
System.out.println(Thread.currentThread().getName()+" is "+task);
}catch (java.lang.InterruptedException e){
e.printStackTrace();
}finally {
lock.unlock();
}
}
}).start();
}
Thread.sleep(3000);
new Thread(new Runnable() {
@Override
public void run() {
lock.lock();
try{
System.out.println(Thread.currentThread().getName()+" is noticing");
state=1;
condition.signalAll();
}finally {
lock.unlock();
}
}
}).start();->lock
这个同上
->await
public final void await() throws InterruptedException {
if (Thread.interrupted())
throw new InterruptedException();
Node node = addConditionWaiter();
int savedState = fullyRelease(node);
int interruptMode = 0;
while (!isOnSyncQueue(node)) {
LockSupport.park(this);
if ((interruptMode = checkInterruptWhileWaiting(node)) != 0)
break;
}
if (acquireQueued(node, savedState) && interruptMode != THROW_IE)
interruptMode = REINTERRUPT;
if (node.nextWaiter != null) // clean up if cancelled
unlinkCancelledWaiters();
if (interruptMode != 0)
reportInterruptAfterWait(interruptMode);
} private Node addConditionWaiter() {
Node t = lastWaiter;
// If lastWaiter is cancelled, clean out.
if (t != null && t.waitStatus != Node.CONDITION) {
unlinkCancelledWaiters();
t = lastWaiter;
}
Node node = new Node(Thread.currentThread(), Node.CONDITION);
if (t == null)
firstWaiter = node;
else
t.nextWaiter = node;
lastWaiter = node;
return node;
}1.首先检测线程是否中断，接着添加一个等待者到等待队列的队尾，这段代码并没有使用任何同步，在高并发的环境下，可能会报错，所以await方法一定要在同步块中。
final int fullyRelease(Node node) {
boolean failed = true;
try {
int savedState = getState();
if (release(savedState)) {
failed = false;
return savedState;
} else {
throw new IllegalMonitorStateException();
}
} finally {
if (failed)
node.waitStatus = Node.CANCELLED;
}
}2.这里释放全部的锁，调用release方法，将持有线程设置为null,唤醒head阻塞节点的线程。
final boolean isOnSyncQueue(Node node) {
if (node.waitStatus == Node.CONDITION || node.prev == null)
return false;
if (node.next != null) // If has successor, it must be on queue
return true;
/*
* node.prev can be non-null, but not yet on queue because
* the CAS to place it on queue can fail. So we have to
* traverse from tail to make sure it actually made it. It
* will always be near the tail in calls to this method, and
* unless the CAS failed (which is unlikely), it will be
* there, so we hardly ever traverse much.
*/
return findNodeFromTail(node);
}3.这里可以存在多种并发情况，跟等待队列有关的是signalAllfang方法，这里会将等待队列中的线程加入阻塞队列。例如下面这段代码：
private Node enq(final Node node) {
for (;;) {
Node t = tail;
if (t == null) { // Must initialize
if (compareAndSetHead(new Node()))
tail = head;
} else {
node.prev = t;
if (compareAndSetTail(t, node)) {
t.next = node;
return t;
}
}
}
}
设置一个节点时首先会设置这个节点的prev节点，接着使用cas添加队尾节点，如果失败，if (node.waitStatus == Node.CONDITION || node.prev == null)不满足，节点不在阻塞队列中，所以必须再遍历一遍节点。
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);
}
}如果isOnSyncQueue返回false,线程进入阻塞状态；如果这里返回true,表示该线程已经存在阻塞队列中。之前释放了几个锁，这里就获取几个锁acquireQueued(node, savedState)。
3.1.先讨论返回为false的情况，线程进入阻塞后，线程本身无法唤醒自己，只有等待被其它线程唤醒，这种情况下，该线程先检测是否为head节点,然后尝试获取锁，如果这里获取成功，循环退出，然后继续执行
if (node.nextWaiter != null)
unlinkCancelledWaiters();
这块代码是在枷锁之后的，目的为了清除之前取消的线程，帮助垃圾回收。
if (interruptMode != 0)
reportInterruptAfterWait(interruptMode);
线程的中断状态有三种，0=正常，1=再次中断，-1中断异常。为1时，再次设置线程的中断状态；为-1时，抛出异常。
3.2.如果这里返回true,表示已经被唤醒过并写入同步队列中，接下来尝试获取锁，剩下的操作与上面一样。
3.接下来分析下中断策略