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comparison src/EDU/oswego/cs/dl/util/concurrent/FIFOSemaphore.java @ 0:3dc0c5604566

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Initial checkin of blitz 2.0 fcs - no installer yet.
author Dan Creswell <dan.creswell@gmail.com>
date Sat, 21 Mar 2009 11:00:06 +0000
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1 /*
2 File: FIFOSemaphore.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
15 package EDU.oswego.cs.dl.util.concurrent;
16
17 /**
18 * A First-in/First-out implementation of a Semaphore.
19 * Waiting requests will be satisified in
20 * the order that the processing of those requests got to a certain point.
21 * If this sounds vague it is meant to be. FIFO implies a
22 * logical timestamping at some point in the processing of the
23 * request. To simplify things we don't actually timestamp but
24 * simply store things in a FIFO queue. Thus the order in which
25 * requests enter the queue will be the order in which they come
26 * out. This order need not have any relationship to the order in
27 * which requests were made, nor the order in which requests
28 * actually return to the caller. These depend on Java thread
29 * scheduling which is not guaranteed to be predictable (although
30 * JVMs tend not to go out of their way to be unfair).
31 * <p>[<a href="http://gee.cs.oswego.edu/dl/classes/EDU/oswego/cs/dl/util/concurrent/intro.html"> Introduction to this package. </a>]
32 **/
33
34 public class FIFOSemaphore extends QueuedSemaphore {
35
36 /**
37 * Create a Semaphore with the given initial number of permits.
38 * Using a seed of one makes the semaphore act as a mutual exclusion lock.
39 * Negative seeds are also allowed, in which case no acquires will proceed
40 * until the number of releases has pushed the number of permits past 0.
41 **/
42
43 public FIFOSemaphore(long initialPermits) {
44 super(new FIFOWaitQueue(), initialPermits);
45 }
46
47 /**
48 * Simple linked list queue used in FIFOSemaphore.
49 * Methods are not synchronized; they depend on synch of callers
50 **/
51
52 protected static class FIFOWaitQueue extends WaitQueue {
53 protected WaitNode head_ = null;
54 protected WaitNode tail_ = null;
55
56 protected void insert(WaitNode w) {
57 if (tail_ == null)
58 head_ = tail_ = w;
59 else {
60 tail_.next = w;
61 tail_ = w;
62 }
63 }
64
65 protected WaitNode extract() {
66 if (head_ == null)
67 return null;
68 else {
69 WaitNode w = head_;
70 head_ = w.next;
71 if (head_ == null) tail_ = null;
72 w.next = null;
73 return w;
74 }
75 }
76 }
77
78 }