Latch


1   /*
2     File: Latch.java
3   
4     Originally written by Doug Lea and released into the public domain.
5     This may be used for any purposes whatsoever without acknowledgment.
6     Thanks for the assistance and support of Sun Microsystems Labs,
7     and everyone contributing, testing, and using this code.
8   
9     History:
10    Date       Who                What
11    11Jun1998  dl               Create public version
12  */
13  
14  package javax.concurrent;
15  
16  /**
17   * A latch is a boolean condition that is set at most once, ever.
18   * Once a single release is issued, all acquires will pass.
19   * <p>
20   * <b>Sample usage.</b> Here are a set of classes that use
21   * a latch as a start signal for a group of worker threads that
22   * are created and started beforehand, and then later enabled.
23   * <pre>
24   * class Worker implements Runnable {
25   *   private final Latch startSignal;
26   *   Worker(Latch l) { startSignal = l; }
27   *    public void run() {
28   *      startSignal.acquire();
29   *      doWork();
30   *   }
31   *   void doWork() { ... }
32   * }
33   *
34   * class Driver { // ...
35   *   void main() {
36   *     Latch go = new Latch();
37   *     for (int i = 0; i < N; ++i) // make threads
38   *       new Thread(new Worker(go)).start();
39   *     doSomethingElse();         // don't let run yet 
40   *     go.release();              // let all threads proceed
41   *   } 
42   * }
43   *</pre>
44   * [<a HREF="http://gee.cs.oswego.edu/dl/classes/EDU/oswego/cs/dl/util/concurrent/intro.html"> Introduction to this package. </a>] <p>
45  **/  
46  
47  public class Latch implements Sync {
48    protected boolean latched_ = false;
49  
50    /*
51      This could use double-check, but doesn't.
52      If the latch is being used as an indicator of
53      the presence or state of an object, the user would
54      not necessarily get the memory barrier that comes with synch
55      that would be needed to correctly use that object. This
56      would lead to errors that users would be very hard to track down. So, to
57      be conservative, we always use synch.
58    */
59  
60    public void acquire() throws InterruptedException   {
61      if (Thread.interrupted()) throw new InterruptedException  ();
62      synchronized(this) {
63        while (!latched_) 
64          wait(); 
65      }
66    }
67  
68    public boolean attempt(long msecs) throws InterruptedException   {
69      if (Thread.interrupted()) throw new InterruptedException  ();
70      synchronized(this) {
71        if (latched_) 
72          return true;
73        else if (msecs <= 0) 
74          return false;
75        else {
76          long waitTime = msecs;
77          long start = System.currentTimeMillis();
78          for (;;) {
79            wait(waitTime);
80            if (latched_) 
81              return true;
82            else {
83              waitTime = msecs - (System.currentTimeMillis() - start);
84              if (waitTime <= 0) 
85                return false;
86            }
87          }
88        }
89      }
90    }
91  
92    /** Enable all current and future acquires to pass **/
93    public synchronized void release() {
94      latched_ = true;
95      notifyAll();
96    }
97  
98  }
99  
100
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