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annotate src/EDU/oswego/cs/dl/util/concurrent/FIFOSemaphore.java @ 27:511648fa4d64 Version 2.1

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Version to 2.1
author Dan Creswell <dan.creswell@gmail.com>
date Mon, 04 Jan 2010 13:00:40 +0000
parents 3dc0c5604566
children
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1 /*
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2 File: FIFOSemaphore.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
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15 package EDU.oswego.cs.dl.util.concurrent;
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16
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17 /**
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18 * A First-in/First-out implementation of a Semaphore.
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19 * Waiting requests will be satisified in
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20 * the order that the processing of those requests got to a certain point.
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21 * If this sounds vague it is meant to be. FIFO implies a
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22 * logical timestamping at some point in the processing of the
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23 * request. To simplify things we don't actually timestamp but
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24 * simply store things in a FIFO queue. Thus the order in which
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25 * requests enter the queue will be the order in which they come
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26 * out. This order need not have any relationship to the order in
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27 * which requests were made, nor the order in which requests
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28 * actually return to the caller. These depend on Java thread
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29 * scheduling which is not guaranteed to be predictable (although
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30 * JVMs tend not to go out of their way to be unfair).
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31 * <p>[<a href="http://gee.cs.oswego.edu/dl/classes/EDU/oswego/cs/dl/util/concurrent/intro.html"> Introduction to this package. </a>]
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32 **/
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33
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34 public class FIFOSemaphore extends QueuedSemaphore {
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35
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36 /**
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37 * Create a Semaphore with the given initial number of permits.
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38 * Using a seed of one makes the semaphore act as a mutual exclusion lock.
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39 * Negative seeds are also allowed, in which case no acquires will proceed
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40 * until the number of releases has pushed the number of permits past 0.
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41 **/
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42
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43 public FIFOSemaphore(long initialPermits) {
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44 super(new FIFOWaitQueue(), initialPermits);
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45 }
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46
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47 /**
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48 * Simple linked list queue used in FIFOSemaphore.
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49 * Methods are not synchronized; they depend on synch of callers
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50 **/
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51
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52 protected static class FIFOWaitQueue extends WaitQueue {
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53 protected WaitNode head_ = null;
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54 protected WaitNode tail_ = null;
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55
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56 protected void insert(WaitNode w) {
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57 if (tail_ == null)
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58 head_ = tail_ = w;
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59 else {
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60 tail_.next = w;
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61 tail_ = w;
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62 }
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63 }
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64
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65 protected WaitNode extract() {
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66 if (head_ == null)
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67 return null;
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68 else {
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69 WaitNode w = head_;
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70 head_ = w.next;
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71 if (head_ == null) tail_ = null;
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72 w.next = null;
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73 return w;
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74 }
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75 }
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76 }
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77
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78 }