概述
- 对象锁
- 一般指synchronized，和对象有关
- 每个对象都有个隐形的监视器，用于线程的同步
- 线程状态：创建(new)->就绪(start)->运行(run)->阻塞(lock/wait/join/sleep)->销毁
- ReentrantLock
- Condition
- 实现wait，notify，notifyAll的功能
- ReadWriteLock - ReentrantReadWriteLock
ReentrantLock
- synchronized的功能增强版；JDK 1.5之前性能优于synchronized；现在性能相差不多
// 简单用法
public static ReentrantLock lock = new ReentrantLock();
try {
lock.lock();
i++;
} finally {
lock.unlock(); // 保证异常情况下也释放；synchronized由JVM释放
}
- 可重入
- 可中断
- lock.lock()
- 不能响应中断
- 在park时被中断，park会解除，设置interrupt标识，但是不抛异常，线程继续
- lock.lockInterruptibly()
- 能响应中断
- 被lock.lockInterruptibly() block的线程，收到interrupt信号时，会抛出InterruptedException，进而退出
- 可限时
- lock.tryLock(long timeout, TimeUnit unit)，超时返回false
- 公平锁
- 默认非公平
- 公平
- 直接排队
- public ReentrantLock(boolean fair)
- synchronized是非公平锁
Condition
public static ReentrantLock lock = new ReentrantLock();
public static Condition condition = lock.newCondition();
- lock内部维护一个同步队列
- 同步队列头的类型是AbstractQueuedSynchronizer
- condition内部维护一个条件队列
- 条件队列头的类型为AbstractQueuedSynchronizer$ConditionObject
- 一个lock可以有多个condition，即多个条件队列
- 持锁者才可以执行await
- 因为await中有release的逻辑，会抛IllegalMonitorStateException
- condition.await()：类似Object.wait()
- 持锁线程将自己加入条件队列
- release同步队列中后继节点
- unparkSuccessor()，这步一定要做，不然同步队列里的线程就死在那里了
- 调用LockSupport.park()将自己阻塞
- 当有别的线程调用了signal()，实际就是调用LockSupport.unpark()把该线程唤醒
- 被唤醒线程继续await()方法中的剩余逻辑
- acquireQueued，这是lock.lock()中的逻辑，即获取锁
- condition.signal()：Object.notify()
- condition.signalAll()：Object.notifyAll()
- awaitUninterruptibly()
Semaphore
概述
Semaphore semaphore = new Semaphore(5);
public Semaphore(int permits) {
sync = new NonfairSync(permits);
}
NonfairSync(int permits) {
super(permits);
}
Sync(int permits) {
setState(permits);
}
AbstractQueuedSynchronizer：
protected final void setState(int newState) {
state = newState;
}
// 公平锁
public Semaphore(int permits, boolean fair) {
sync = fair ? new FairSync(permits) : new NonfairSync(permits);
}
- 默认非公平
- 构造方法参数即许可数，被设置到了AbstractQueuedSynchronizer的state属性
- 线程先尝试获得许可，如果成功，许可数-1；线程运行结束后释放许可，许可数+1
- 许可数为0，则获取失败；线程进入AQS的同步队列，被其他释放许可的线程唤醒
acquire
public void acquire() throws InterruptedException {
sync.acquireSharedInterruptibly(1);
}
public final void acquireSharedInterruptibly(int arg)
throws InterruptedException {
if (Thread.interrupted())
throw new InterruptedException();
if (tryAcquireShared(arg) < 0)
doAcquireSharedInterruptibly(arg);
}
protected int tryAcquireShared(int acquires) {
return nonfairTryAcquireShared(acquires);
}
final int nonfairTryAcquireShared(int acquires) {
for (;;) {
int available = getState();
int remaining = available - acquires;
if (remaining < 0 || compareAndSetState(available, remaining))
return remaining;
}
}
- 比较available（其实就是state属性）和acquires
- 如果拿掉许可之后不小于0，就可以拿许可
- 若是小于0，则表示拿许可失败，在doAcquireSharedInterruptibly(arg)中将自己阻塞，即进入同步队列
- 阻塞：在parkAndCheckInterrupt中通过LockSupport.park(this)进行自我阻塞
release
public void release() {
sync.releaseShared(1);
}
public final boolean releaseShared(int arg) {
if (tryReleaseShared(arg)) {
doReleaseShared();
return true;
}
return false;
}
protected final boolean tryReleaseShared(int releases) {
for (;;) {
int current = getState();
int next = current + releases;
if (next < current) // overflow
throw new Error("Maximum permit count exceeded");
if (compareAndSetState(current, next))
return true;
}
}
- 将AQS中的state值+1
- 通过doReleaseShared()唤醒同步队列中的第一个节点
场景
CountDownLatch
概述
CountDownLatch countDownLatch = new CountDownLatch(10);
- 有点高级join的感觉，主线程调用countDownLatch.await()，进入等待
- countDownLatch.countDown()执行10次，把那个10消耗光
- 主线程继续
机制
- CountDownLatch持有一个内部类Sync，这个10会传给AbstractQueuedSynchronizer的state属性
- 执行await时，只要state不等于0，线程就会进入阻塞
- 队列头是AbstractQueuedSynchronizer类型的，貌似只有Condition例外，其余的队列头应该都是AQS类型
- countDown方法中，当state变为0时（一定要是变为0哦，不能本来就是0）
- 执行doReleaseShared把同步队列里的第一个有效Node给unpark，其实就是上面被阻塞的那个主线程
CyclicBarrier
public CyclicBarrier(int parties) {
this(parties, null);
}
// parties：栅栏放开前需要调用的线程数
// count：当前剩余需要调用的线程数
// barrierCommand：调用线程数满足后，栅栏放开前执行的命令
public CyclicBarrier(int parties, Runnable barrierAction) {
if (parties <= 0) throw new IllegalArgumentException();
this.parties = parties;
this.count = parties;
this.barrierCommand = barrierAction;
}
// 用法，每个线程调用await，耗完parties即可往下走
CyclicBarrier.await
final ReentrantLock lock = this.lock; // this就是CyclicBarrier
lock.lock(); // 先拿锁
int index = --count;
if (index == 0) { // 判断index，为0表示线程数已达到
boolean ranAction = false;
try {
final Runnable command = barrierCommand;
if (command != null) // 可能是空的，看构造函数了
command.run();
ranAction = true;
nextGeneration(); // signalAll，重置count，重置generation
return 0; // await结束
} finally {
if (!ranAction) // command.run异常时，打破栅栏，释放线程
breakBarrier(); // signalAll，重置count，设置broken属性
}
}
trip.await // 自我阻塞
private void nextGeneration() {
// signal completion of last generation
trip.signalAll(); // trip：lock生成的Condition
// set up next generation
count = parties;
generation = new Generation();
}
private void breakBarrier() {
generation.broken = true;
count = parties;
trip.signalAll();
}
LockSupport
/**
* Makes available the permit for the given thread, if it
* was not already available. If the thread was blocked on
* {@code park} then it will unblock. Otherwise, its next call
* to {@code park} is guaranteed not to block. This operation
* is not guaranteed to have any effect at all if the given
* thread has not been started.
*
* @param thread the thread to unpark, or {@code null}, in which case
* this operation has no effect
*/
public static void unpark(Thread thread) {
if (thread != null)
UNSAFE.unpark(thread);
}
// 从上面注释可以看出，unpack可以释放park状态线程,或者将执行park的线程；不过对于还没开始执行的线程，unpark并不保证效果
/**
* Disables the current thread for thread scheduling purposes unless the
* permit is available.
*
* <p>If the permit is available then it is consumed and the call
* returns immediately; otherwise the current thread becomes disabled
* for thread scheduling purposes and lies dormant until one of three
* things happens:
*
* <ul>
*
* <li>Some other thread invokes {@link #unpark unpark} with the
* current thread as the target; or
*
* <li>Some other thread {@linkplain Thread#interrupt interrupts}
* the current thread; or
*
* <li>The call spuriously (that is, for no reason) returns.
* </ul>
*
* <p>This method does <em>not</em> report which of these caused the
* method to return. Callers should re-check the conditions which caused
* the thread to park in the first place. Callers may also determine,
* for example, the interrupt status of the thread upon return.
*/
public static void park() {
UNSAFE.park(false, 0L);
}
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