ReentrantLock默认使用非公平锁
public ReentrantLock() {
sync = new NonfairSync();
}
所谓公平锁就是先阻塞的程序先获得锁。非公平锁则有操作系统的调度系统来调度。
NonfairSync就是一个同步器
final static 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);
}
}
而Sync则是这样实现的
static abstract class Sync extends AbstractQueuedSynchronizer {
现在来关注下ReentrantLock的lock方法
public void lock() {
sync.lock();
}
实际就是调用的NonFairSync的lock方法
final void lock() {
if (compareAndSetState(0, 1))
setExclusiveOwnerThread(Thread.currentThread());
else
acquire(1);
}
而这个用的AbstractQueueSynchronizer中的方法来实现的
protected final boolean compareAndSetState(int expect, int update) {
// See below for intrinsics setup to support this
return unsafe.compareAndSwapInt(this, stateOffset, expect, update);
}
意思就是如果内存中的state这个字段是0，则将它置为1，否则执行acquire这个函数。注意compareAndSwapInt这句话是原子性的。所以保证只能有一个线程能成功将state置为1，从而获得锁。
同时执行
protected final void setExclusiveOwnerThread(Thread t) {
exclusiveOwnerThread = t;
}
这个函数，置当前获得锁的函数。
那执行compareAndSetState失败的函数则执行acquire这个函数。看一下这个函数都干嘛了呢
<pre name="code" class="java"> public final void acquire(int arg) {
if (!tryAcquire(arg) &&
acquireQueued(addWaiter(Node.EXCLUSIVE), arg))
selfInterrupt();
}
看一个tryAcquire这个函数
protected final boolean tryAcquire(int acquires) {
return nonfairTryAcquire(acquires);
}
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;
}
此时会先尝试着compareAndSetState这个函数，若又失败return false
public final void acquire(int arg) {
if (!tryAcquire(arg) &&
acquireQueued(addWaiter(Node.EXCLUSIVE), arg))
selfInterrupt();
}
那么会先执行addWaiter这个函数。这个函数就是将当前线程生成一个节点对象，然后放入队列里头
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;
}
如果队列不是空则放在队列尾部
否则初始化一个队列
public final void acquire(int arg) {
if (!tryAcquire(arg) &&
acquireQueued(addWaiter(Node.EXCLUSIVE), arg))
selfInterrupt();
}
在看下acquireQueued都干了些什么呢
final boolean acquireQueued(final Node node, int arg) {
try {
boolean interrupted = false;
for (;;) {
final Node p = node.predecessor();
if (p == head && tryAcquire(arg)) {
setHead(node);
p.next = null; // help GC
return interrupted;
}
if (shouldParkAfterFailedAcquire(p, node) &&
parkAndCheckInterrupt())
interrupted = true;
}
} catch (RuntimeException ex) {
cancelAcquire(node);
throw ex;
}
}
就是如果当前线程的前驱是头节点的话，就试着去再次获得锁，因为head节点是当前获得锁的线程。
若失败，则执行这个方法shouldParkAfterFailedAcquire，parkAndCheckInterrupt
意思就是如果获取失败，就挂起当前线程，进去这个方法里面，看一看
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;
}
/**
* Convenience method to park and then check if interrupted
*
* @return {@code true} if interrupted
*/
private final boolean parkAndCheckInterrupt() {
LockSupport.park(this);
return Thread.interrupted();
}
就是调用了LockSupport的park方法来挂起当前线程。interrupted的作用是干这个用的
- //清除中断标记，并返回上一次中断标记的值
- public static boolean interrupted() { ... }
若当成的中断是true的话，下面就将执行这句话selfInterrupt方法
public final void acquire(int arg) {
if (!tryAcquire(arg) &&
acquireQueued(addWaiter(Node.EXCLUSIVE), arg))
selfInterrupt();
}
/**
* Convenience method to interrupt current thread.
*/
private static void selfInterrupt() {
Thread.currentThread().interrupt();
}
设置当前线程的中断标记，这里只是在线程上设置了一个中断标记，具体怎么处理这个中断标记，由应用程序来确定。
时间： 2024-10-08 10:08:49