东平的笔记仓库

ReentrantLock, Semaphore, CountDownLatch实现

2018/11/20 Java Java多线程

JUC中ReentrantLock, Semaphore, CountDownLatch, ReentrantReadWriteLock, FutureTask都是基于AQS构建。但是没有直接扩展AQS，而是都将它们的相应功能委托给私有的AQS子类Sync实现。

ReentrantLock

源代码

方法 方法
ReentrantLock() getHoldCount()
ReentrantLock(boolean fair) getOwner()
void lock() getQueuedThreads()
void lockinterruptibly() getQueueLength()
boolean tryLock() getWaitingThreads(Condition condition)
boolean tryLock(long timeout, TimeUnit unit) getWaitQueueLength(Condition condition)
void unlock() hasQueuedThread(thread thread)
Condition newCondition() hasQueuedThreads()
isFair() hasWaiters(Condition condition)

https://github.com/unofficial-openjdk/openjdk/blob/jdk/jdk/src/java.base/share/classes/java/util/concurrent/locks/ReentrantLock.java

  1. ReentrantLock的state为可重入锁的重入次数，当state=0时，表示未锁定。state>0时，表示占有锁的线程重入的次数。
  2. ReentrantLock有公平和非公平两种模式。非公平模式先直接调用compareAndSetState尝试获取锁，失败后再调用acquire。公平模式先调用hasQueuedPredecessors()判断同步队列没有前序结点后再尝试获取锁。
  3. ReentrantLock中有两个子类fairSync和nonFairSync分别继承了Sync并提供了tryAquire和Lock实现。
  4. ReentrantLock的Condition队列直接返回AQS中的ConditionObject实现。

image

Semaphore

方法 | 方法 — | — void acquire() | getQueuedThreads() void acquire(int permits) | getQueueLength() void acquireUninterruptibly() | hasQueuedThreads() void acquireUninterruptibly(int permits) | reducePermits(int reduction) int availablePermits() | void release() boolean tryAcquire() | void release(int permits) boolean tryAcquire(int permits) | boolean isFair() boolean tryAcquire(int permits, long timeout, Timeunit unit)

源代码

https://github.com/unofficial-openjdk/openjdk/blob/jdk/jdk/src/java.base/share/classes/java/util/concurrent/Semaphore.java

  1. Semaphore的state表示permit的总数目。

  2. Semaphore允许多个线程同时持有锁，因此Semaphore为==共享锁SHARED==。

  3. Semaphore和ReentrantLock一样也有公平和非公平两种模式。公平模式同样也是先调用hasQueuedPredecessors()判断同步队列没有前序结点后再尝试获取锁。

  4. 和ReentrantLock一样提供fairSync和nonFairSync两个子类继承Sync实现不同的tryAcquireShared。

image

CountDownLatch

方法 方法
await()  
await(long timeout, TimeUnit unit)  
countDown()  
getCount()  

源代码

https://github.com/unofficial-openjdk/openjdk/blob/jdk/jdk/src/java.base/share/classes/java/util/concurrent/CountDownLatch.java

  1. CountDownLatch的state表示计数数目。
  2. CountDownLatch只有一个Sync实现。当计数减至0时，需要释放所有阻塞的线程，因此使用SHARED模式，复写tryAcquireShared和tryReleaseShared。 
    private static final class Sync extends AbstractQueuedSynchronizer {
 private static final long serialVersionUID = 4982264981922014374L;

 Sync(int count) {
     setState(count);
 }

 int getCount() {
     return getState();
 }

 protected int tryAcquireShared(int acquires) {
     return (getState() == 0) ? 1 : -1;
 }

 protected boolean tryReleaseShared(int releases) {
     // Decrement count; signal when transition to zero
     for (;;) {
         int c = getState();
         if (c == 0)
             return false;
         int nextc = c-1;
         if (compareAndSetState(c, nextc))
             return nextc == 0;
     }
 }
}
  3. await实现 由于state不等于0时，tryAcquireShared一直返回-1，线程调用await会在doAcquireShared中被park阻塞。 
    public void await() throws InterruptedException {
 sync.acquireSharedInterruptibly(1);
}
  4. countDown实现 调用countDown会调用tryReleaseShared使得state减一，当state不等于0时，tryReleaseShared返回false，不会释放阻塞的线程。 
    public void countDown() {
 sync.releaseShared(1);
}

image

ReentrantReadWriteLock

见笔记 ReentrantReadWriteLock实现分析。

源代码

https://github.com/unofficial-openjdk/openjdk/blob/jdk/jdk/src/java.base/share/classes/java/util/concurrent/locks/ReentrantReadWriteLock.java

本文地址：https://cheng-dp.github.io/2018/11/20/reentrant-lock-semaphore-countdownlatch/

Search

    Table of Contents