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annotate src/EDU/oswego/cs/dl/util/concurrent/Latch.java @ 24:28c84687bdb8

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author Dan Creswell <dan.creswell@gmail.com>
date Fri, 28 Aug 2009 21:01:04 +0100
parents 3dc0c5604566
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1 /*
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2 File: Latch.java
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3
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4 Originally written by Doug Lea and released into the public domain.
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5 This may be used for any purposes whatsoever without acknowledgment.
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6 Thanks for the assistance and support of Sun Microsystems Labs,
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7 and everyone contributing, testing, and using this code.
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8
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9 History:
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10 Date Who What
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11 11Jun1998 dl Create public version
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12 */
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13
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14 package EDU.oswego.cs.dl.util.concurrent;
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15
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16 /**
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17 * A latch is a boolean condition that is set at most once, ever.
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18 * Once a single release is issued, all acquires will pass.
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19 * <p>
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20 * <b>Sample usage.</b> Here are a set of classes that use
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21 * a latch as a start signal for a group of worker threads that
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22 * are created and started beforehand, and then later enabled.
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23 * <pre>
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24 * class Worker implements Runnable {
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25 * private final Latch startSignal;
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26 * Worker(Latch l) { startSignal = l; }
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27 * public void run() {
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28 * startSignal.acquire();
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29 * doWork();
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30 * }
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31 * void doWork() { ... }
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32 * }
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33 *
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34 * class Driver { // ...
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35 * void main() {
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36 * Latch go = new Latch();
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37 * for (int i = 0; i < N; ++i) // make threads
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38 * new Thread(new Worker(go)).start();
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39 * doSomethingElse(); // don't let run yet
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40 * go.release(); // let all threads proceed
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41 * }
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42 * }
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43 *</pre>
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44 * [<a href="http://gee.cs.oswego.edu/dl/classes/EDU/oswego/cs/dl/util/concurrent/intro.html"> Introduction to this package. </a>] <p>
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45 **/
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46
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47 public class Latch implements Sync {
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48 protected boolean latched_ = false;
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49
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50 /*
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51 This could use double-check, but doesn't.
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52 If the latch is being used as an indicator of
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53 the presence or state of an object, the user would
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54 not necessarily get the memory barrier that comes with synch
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55 that would be needed to correctly use that object. This
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56 would lead to errors that users would be very hard to track down. So, to
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57 be conservative, we always use synch.
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58 */
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59
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60 public void acquire() throws InterruptedException {
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61 if (Thread.interrupted()) throw new InterruptedException();
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62 synchronized(this) {
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63 while (!latched_)
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64 wait();
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65 }
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66 }
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67
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68 public boolean attempt(long msecs) throws InterruptedException {
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69 if (Thread.interrupted()) throw new InterruptedException();
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70 synchronized(this) {
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71 if (latched_)
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72 return true;
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73 else if (msecs <= 0)
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74 return false;
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75 else {
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76 long waitTime = msecs;
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77 long start = System.currentTimeMillis();
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78 for (;;) {
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79 wait(waitTime);
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80 if (latched_)
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81 return true;
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82 else {
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83 waitTime = msecs - (System.currentTimeMillis() - start);
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84 if (waitTime <= 0)
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85 return false;
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86 }
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87 }
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88 }
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89 }
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90 }
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91
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92 /** Enable all current and future acquires to pass **/
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93 public synchronized void release() {
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94 latched_ = true;
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95 notifyAll();
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96 }
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97
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98 }
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99