我理解的CLH
学而时习之，不亦说乎！
--《论语》
原创，转载请附原文链接，谢谢。
CLH
思路
- 保持时序的锁基本思路就是将等待获取锁的线程放入集合，锁释放后，等待线程之一获取到锁。
问题
如何排队？
- CLH使用反向链表的形式进行排队。也就是后继节点主动询问，而不是前继节点主动通知。
排队是否公平？
- CLH是公平锁。即后申请获取锁的排在队列末尾。
如何唤醒？
- CLH通过每个线程自旋。每个等待线程通过不断自旋前继节点状态判断是否能获取到锁。
实现
- CLH排队使用公平方式，锁内部需要保存【尾节点】的引用，每次排队线程加入到队列最末尾。
- CLH锁使用反向链表方式进行排队，那么每个线程就需要维护【自己的状态】和保持一个【前向节点的引用】。
- CLH使用一个【boolean值】表示当前线程获取锁状态。false表示当前线程释放锁；true表示当前线程等待获取锁或已经获取到锁。
代码
Lock.java
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package com.zby.clh;
public interface Lock {
void lock();
void unlock();
}
CLHLock.java
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package com.zby.clh;
import java.util.concurrent.atomic.AtomicReference;
public class CLHLock implements Lock {
private final AtomicReference<QNode> tail;
private final ThreadLocal<QNode> myPred;
private final ThreadLocal<QNode> myNode;
public CLHLock() {
tail = new AtomicReference<QNode>(new QNode());
myNode = new ThreadLocal<QNode>() {
@Override
protected QNode initialValue() {
return new QNode();
}
};
myPred = new ThreadLocal<QNode>() {
@Override
protected QNode initialValue() {
return null;
}
};
}
@Override
public void lock() {
QNode qnode = myNode.get();
qnode.locked = true;
QNode pred = tail.getAndSet(qnode);
myPred.set(pred);
while (pred.locked) {
}
}
@Override
public void unlock() {
QNode qnode = myNode.get();
qnode.locked = false;
// myNode.set(myPred.get());这一行是否有必要？？？
}
private static class QNode {
public volatile boolean locked;
}
}
测试
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package com.zby.clh;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;
import java.util.concurrent.TimeUnit;
public class CLHMain {
private Lock lock = new CLHLock();
public void say(String msg) {
System.out.println(Thread.currentThread().getName() + " ： 尝试获取锁");
lock.lock();
System.out.println(Thread.currentThread().getName() + " ： 获取到锁");
try {
System.out.println(Thread.currentThread().getName() + msg + "?");
TimeUnit.SECONDS.sleep(1);
System.out.println(Thread.currentThread().getName() + msg + "!");
TimeUnit.SECONDS.sleep(1);
System.out.println(Thread.currentThread().getName() + msg + ".");
TimeUnit.SECONDS.sleep(1);
} catch (Exception e) {
e.printStackTrace();
} finally {
lock.unlock();
System.out.println(Thread.currentThread().getName() + " ： 释放锁");
}
}
public static void main(String[] args) throws Exception {
String msg = "Hello,World";
CLHMain clhMain = new CLHMain();
ExecutorService executorService = Executors.newCachedThreadPool();
for (int i = 0; i < 5; i++) {
executorService.submit(new Runnable() {
@Override
public void run() {
clhMain.say(msg);
}
});
}
executorService.awaitTermination(20, TimeUnit.SECONDS);
}
}
结果
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pool-1-thread-1 ： 尝试获取锁
pool-1-thread-2 ： 尝试获取锁
pool-1-thread-1 ： 获取到锁
pool-1-thread-1Hello,World?
pool-1-thread-3 ： 尝试获取锁
pool-1-thread-4 ： 尝试获取锁
pool-1-thread-5 ： 尝试获取锁
pool-1-thread-1Hello,World!
pool-1-thread-1Hello,World.
pool-1-thread-1 ： 释放锁
pool-1-thread-2 ： 获取到锁
pool-1-thread-2Hello,World?
pool-1-thread-2Hello,World!
pool-1-thread-2Hello,World.
pool-1-thread-2 ： 释放锁
pool-1-thread-3 ： 获取到锁
pool-1-thread-3Hello,World?
pool-1-thread-3Hello,World!
pool-1-thread-3Hello,World.
pool-1-thread-3 ： 释放锁
pool-1-thread-4 ： 获取到锁
pool-1-thread-4Hello,World?
pool-1-thread-4Hello,World!
pool-1-thread-4Hello,World.
pool-1-thread-4 ： 释放锁
pool-1-thread-5 ： 获取到锁
pool-1-thread-5Hello,World?
pool-1-thread-5Hello,World!
pool-1-thread-5Hello,World.
pool-1-thread-5 ： 释放锁
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