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ReentrantLock源码之二unlock�Ҏ(gu��)��解析(锁的释放)

zhangxl — Fri, 19 Aug 2011 07:24:00 GMT

�?a href="http://m.tkk7.com/zhanglongsr/articles/356782.html">前面一�?/a>中，我们分析了ReentrantLock.lock()�Ҏ(gu��)��Q�接下来我们接着分析ReentrantLock.unlock()�Ҏ(gu��)��?br />

1.ReentrantLock.unlock()分析

(1)首先��试释放锁，如果要求释放数等于锁状态数�Q�那么将锁状态位�?�Q�清除锁所有者，�q�回true�Q�否则返回false;
(2)如果(1)�q�回的是true�Q�说明锁完全释放。接下来��检查等待队列，�q��择一个waitStatus处于�{�待状态的节点下的�U�程unpark(恢复)�Q�选择的依据是从尾节点开始，选取最靠近头节点的�{�待节点,同时清理队列中线�E�被取消的节点；
(3)如果(1)�q�回false�Q�说明锁只是部分释放�Q�当前线�E�仍旧持有该锁；

1java.util.concurrent.locks.ReentrantLock
2 public void unlock() {
3        sync.release(1);
4 }
5
6java.util.concurrent.locks.AbstractQueuedSynchronizer
7public final boolean release(int arg) {
8        if (tryRelease(arg)) {
9            Node h = head;
10            if (h != null && h.waitStatus != 0)
11                unparkSuccessor(h);
12            return true;
13        }
14        return false;
15    }
16
17
18 protected final boolean tryRelease(int releases) {
19            int c = getState() - releases;   //重入锁加锁的�ơ数-释放数量
20            if (Thread.currentThread() != getExclusiveOwnerThread())   //判断独占锁是否�ؓ当前�U�程所�?/span>
21                throw new IllegalMonitorStateException();
22            boolean free = false;
23            if (c == 0) {       //加锁�ơ数=释放数量
24                free = true;
25                setExclusiveOwnerThread(null);     //清除锁拥有者标�?/span>
26            }
27            setState(c);        //讄��加锁状�?/span>
28            return free;
29        }
30
31
32 /**
33     * Wakes up node's successor, if one exists.
34     *
35     * @param node the node
36     */
37    private void unparkSuccessor(Node node) {
38        /*
39         * Try to clear status in anticipation of signalling.  It is
40         * OK if this fails or if status is changed by waiting thread.
41         */
42        compareAndSetWaitStatus(node, Node.SIGNAL, 0);    //清除头节点signal状�?/span>
43
44        /*
45         * Thread to unpark is held in successor, which is normally
46         * just the next node.  But if cancelled or apparently null,
47         * traverse backwards from tail to find the actual
48         * non-cancelled successor.
49         */
50        Node s = node.next;
51        if (s == null || s.waitStatus > 0) {    //�{�待队列唤醒的竞争满��FIFO�Q�本�D�代码主要是��L��最靠近头节点的�Q�且waitStatus为signal、condition的链表节�?/span>
52            s = null;
53            for (Node t = tail; t != null && t != node; t = t.prev)
54                if (t.waitStatus <= 0)
55                    s = t;
56        }
57        if (s != null)
58            LockSupport.unpark(s.thread);
59    }

zhangxl 2011-08-19 15:24 发表评论

ReentrantLock源码之一lock�Ҏ(gu��)��解析(锁的获取)

zhangxl — Thu, 18 Aug 2011 06:12:00 GMT

摘要: 一、前�a� ReentrantLock是JDK1.5引入的，它拥有与synchronized相同的�ƈ发性和内存语义�Q��ƈ提供了超出synchonized的其他高�U�功�?例如�Q�中断锁�{�候、条件变量等)�Q��ƈ且��用ReentrantLock比synchronized能获得更好的可�׾~�性�? ReentrantL... 阅读全文

zhangxl 2011-08-18 14:12 发表评论

zhangxl — Thu, 28 Jul 2011 04:11:00 GMT

java�?.2版本引入wait-and-notify机制�Q�用于线�E�间通信�Q�它能够让某个线�E�与其他�U�程在特定条件符合时�q�行通信�Q�而条件变量是�?.5版本中concurrency包中提供的，条�g变量是一�U�有其多其他多线�E�系�l�所提供的同步类型，它与java的wai-and-notify机制非常�c�M��Q�下面我们通过实例的方式来探讨他们之间的区别�?br /> wai-and-notify�C�Z��代码�Q?

package com.zxl.thread.notify;
/**
*
* @author zhangxl
*
*/
public class SynThread1 extends Thread {

    private Object lockObj;
    private volatile int i = 0;
    public SynThread1(Object lockObj){
        this.lockObj = lockObj;
    }

    public void run(){

        synchronized(lockObj){
            while(true){
                if(i==0){
                    try {
                        System.out.println(Thread.currentThread().getName()+" will be waiting.");
                        lockObj.wait();
                    } catch (InterruptedException e) {
                        // TODO Auto-generated catch block
                        e.printStackTrace();
                    }
                }

                System.out.println(Thread.currentThread().getName()+": i="+i);
                //i++;

                if(i>=1) return;
            }
        }
    }

    public static void  main(String[] arg){

        Object lock = new Object();

        SynThread1 t1 = new SynThread1(lock);
        t1.setName("SynThread_1");
        SynThread1 t2 = new SynThread1(lock);
        t2.setName("SynThread_2");
        t2.start();
        t1.start();



        try {
            Thread.sleep(1000);

            synchronized(lock){
                lock.notify();    //通过�l�果观察�Q�sun JVM(hotspot)的通知机制极有可能是FIFO的策略，原因是t1、t2只要谁先启动�Q�谁��将被唤醒�?/span>
            }
        } catch (InterruptedException e) {
            // TODO Aut o-generated catch block            e.printStackTrace();
        }
    }
}

条�g变量�C�Z��Q?br />

package com.zxl.thread.notify;

import java.util.concurrent.locks.Condition;
import java.util.concurrent.locks.ReentrantLock;

public class ConditionThread implements Runnable {

private ReentrantLock lock;

private Condition c;

private volatile int i=0;

public ConditionThread(ReentrantLock lock){
this.lock = lock;
c = lock.newCondition();
}

public void run() {

  lock.lock();

  try{

   while(true){
    if(i==0){
     System.out.println(Thread.currentThread().getName()+" will be waiting...");
     c.await();
    }

    System.out.println(Thread.currentThread().getName()+" is already waked up,i="+i);

    if(i>=1) return;
   }

  } catch (InterruptedException e) {
   // TODO Auto-generated catch block
   e.printStackTrace();
  }
  finally{
   lock.unlock();
  }



}

public Condition getCondition(){
  return c;
}

public static void main(String[] args){



  ReentrantLock lock = new ReentrantLock();
  ConditionThread ct1 = new ConditionThread(lock);
  ConditionThread ct2 = new ConditionThread(lock);
  Thread t1 = new Thread(ct1);
  t1.setName("C_Thread_1");
  Thread t2 = new Thread(ct2);
  t2.setName("C_Thread_2");

  t1.start();
  t2.start();

  try {
   Thread.sleep(1000);
  } catch (InterruptedException e) {
   // TODO Auto-generated catch block
   e.printStackTrace();
  }

  lock.lock();
  ct1.getCondition().signal();   //唤醒�W�一个线�E?br />  lock.unlock();
}

}

zhangxl 2011-07-28 12:11 发表评论