Mercurial > hg > blitz_stable

annotate src/EDU/oswego/cs/dl/util/concurrent/Semaphore.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: Semaphore.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 5Aug1998 dl replaced int counters with longs
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13 24Aug1999 dl release(n): screen arguments
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14 */
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15
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16 package EDU.oswego.cs.dl.util.concurrent;
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17
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18 /**
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19 * Base class for counting semaphores.
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20 * Conceptually, a semaphore maintains a set of permits.
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21 * Each acquire() blocks if necessary
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22 * until a permit is available, and then takes it.
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23 * Each release adds a permit. However, no actual permit objects
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24 * are used; the Semaphore just keeps a count of the number
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25 * available and acts accordingly.
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26 * <p>
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27 * A semaphore initialized to 1 can serve as a mutual exclusion
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28 * lock.
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29 * <p>
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30 * Different implementation subclasses may provide different
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31 * ordering guarantees (or lack thereof) surrounding which
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32 * threads will be resumed upon a signal.
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33 * <p>
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34 * The default implementation makes NO
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35 * guarantees about the order in which threads will
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36 * acquire permits. It is often faster than other implementations.
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37 * <p>
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38 * <b>Sample usage.</b> Here is a class that uses a semaphore to
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39 * help manage access to a pool of items.
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40 * <pre>
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41 * class Pool {
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42 * static final MAX_AVAILABLE = 100;
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43 * private final Semaphore available = new Semaphore(MAX_AVAILABLE);
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44 *
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45 * public Object getItem() throws InterruptedException { // no synch
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46 * available.acquire();
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47 * return getNextAvailableItem();
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48 * }
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49 *
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50 * public void putItem(Object x) { // no synch
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51 * if (markAsUnused(x))
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52 * available.release();
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53 * }
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54 *
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55 * // Not a particularly efficient data structure; just for demo
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56 *
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57 * protected Object[] items = ... whatever kinds of items being managed
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58 * protected boolean[] used = new boolean[MAX_AVAILABLE];
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59 *
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60 * protected synchronized Object getNextAvailableItem() {
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61 * for (int i = 0; i < MAX_AVAILABLE; ++i) {
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62 * if (!used[i]) {
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63 * used[i] = true;
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64 * return items[i];
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65 * }
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66 * }
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67 * return null; // not reached
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68 * }
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69 *
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70 * protected synchronized boolean markAsUnused(Object item) {
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71 * for (int i = 0; i < MAX_AVAILABLE; ++i) {
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72 * if (item == items[i]) {
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73 * if (used[i]) {
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74 * used[i] = false;
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75 * return true;
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76 * }
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77 * else
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78 * return false;
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79 * }
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80 * }
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81 * return false;
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82 * }
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83 *
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84 * }
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85 *</pre>
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86 * <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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87 **/
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88
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89
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90 public class Semaphore implements Sync {
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91 /** current number of available permits **/
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92 protected long permits_;
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93
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94 /**
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95 * Create a Semaphore with the given initial number of permits.
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96 * Using a seed of one makes the semaphore act as a mutual exclusion lock.
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97 * Negative seeds are also allowed, in which case no acquires will proceed
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98 * until the number of releases has pushed the number of permits past 0.
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99 **/
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100 public Semaphore(long initialPermits) { permits_ = initialPermits; }
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101
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102
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103 /** Wait until a permit is available, and take one **/
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104 public void acquire() throws InterruptedException {
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105 if (Thread.interrupted()) throw new InterruptedException();
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106 synchronized(this) {
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107 try {
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108 while (permits_ <= 0) wait();
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109 --permits_;
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110 }
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111 catch (InterruptedException ex) {
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112 notify();
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113 throw ex;
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114 }
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115 }
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116 }
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117
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118 /** Wait at most msecs millisconds for a permit. **/
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119 public boolean attempt(long msecs) throws InterruptedException {
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120 if (Thread.interrupted()) throw new InterruptedException();
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121
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122 synchronized(this) {
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123 if (permits_ > 0) {
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124 --permits_;
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125 return true;
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126 }
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127 else if (msecs <= 0)
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128 return false;
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129 else {
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130 try {
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131 long startTime = System.currentTimeMillis();
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132 long waitTime = msecs;
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133
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134 for (;;) {
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135 wait(waitTime);
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136 if (permits_ > 0) {
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137 --permits_;
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138 return true;
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139 }
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140 else {
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141 waitTime = msecs - (System.currentTimeMillis() - startTime);
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142 if (waitTime <= 0)
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143 return false;
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144 }
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145 }
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146 }
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147 catch(InterruptedException ex) {
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148 notify();
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149 throw ex;
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150 }
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151 }
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152 }
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153 }
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154
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155 /** Release a permit **/
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156 public synchronized void release() {
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157 ++permits_;
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158 notify();
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159 }
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160
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161
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Dan Creswell <dan.creswell@gmail.com>
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162 /**
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163 * Release N permits. <code>release(n)</code> is
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164 * equivalent in effect to:
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165 * <pre>
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166 * for (int i = 0; i < n; ++i) release();
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167 * </pre>
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168 * <p>
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169 * But may be more efficient in some semaphore implementations.
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170 * @exception IllegalArgumentException if n is negative.
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171 **/
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172 public synchronized void release(long n) {
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173 if (n < 0) throw new IllegalArgumentException("Negative argument");
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174
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175 permits_ += n;
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176 for (long i = 0; i < n; ++i) notify();
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177 }
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178
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179 /**
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180 * Return the current number of available permits.
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181 * Returns an accurate, but possibly unstable value,
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182 * that may change immediately after returning.
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183 **/
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184 public synchronized long permits() {
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185 return permits_;
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186 }
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187
3dc0c5604566 Initial checkin of blitz 2.0 fcs - no installer yet.
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188 }
3dc0c5604566 Initial checkin of blitz 2.0 fcs - no installer yet.
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189