Mercurial > hg > blitz_stable

annotate src/EDU/oswego/cs/dl/util/concurrent/Sync.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: Sync.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 Added some convenient time constants
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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 * Main interface for locks, gates, and conditions.
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19 * <p>
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20 * Sync objects isolate waiting and notification for particular
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21 * logical states, resource availability, events, and the like that are
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22 * shared across multiple threads. Use of Syncs sometimes
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23 * (but by no means always) adds flexibility and efficiency
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24 * compared to the use of plain java monitor methods
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25 * and locking, and are sometimes (but by no means always)
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26 * simpler to program with.
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27 * <p>
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28 *
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29 * Most Syncs are intended to be used primarily (although
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30 * not exclusively) in before/after constructions such as:
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31 * <pre>
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32 * class X {
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33 * Sync gate;
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34 * // ...
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35 *
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36 * public void m() {
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37 * try {
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38 * gate.acquire(); // block until condition holds
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39 * try {
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40 * // ... method body
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41 * }
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42 * finally {
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43 * gate.release()
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44 * }
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45 * }
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46 * catch (InterruptedException ex) {
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47 * // ... evasive action
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48 * }
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49 * }
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50 *
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51 * public void m2(Sync cond) { // use supplied condition
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52 * try {
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53 * if (cond.attempt(10)) { // try the condition for 10 ms
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54 * try {
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55 * // ... method body
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56 * }
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57 * finally {
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58 * cond.release()
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59 * }
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60 * }
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61 * }
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62 * catch (InterruptedException ex) {
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63 * // ... evasive action
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64 * }
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65 * }
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66 * }
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67 * </pre>
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68 * Syncs may be used in somewhat tedious but more flexible replacements
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69 * for built-in Java synchronized blocks. For example:
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70 * <pre>
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71 * class HandSynched {
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72 * private double state_ = 0.0;
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73 * private final Sync lock; // use lock type supplied in constructor
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74 * public HandSynched(Sync l) { lock = l; }
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75 *
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76 * public void changeState(double d) {
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77 * try {
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78 * lock.acquire();
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79 * try { state_ = updateFunction(d); }
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80 * finally { lock.release(); }
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81 * }
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82 * catch(InterruptedException ex) { }
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83 * }
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84 *
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85 * public double getState() {
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86 * double d = 0.0;
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87 * try {
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88 * lock.acquire();
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89 * try { d = accessFunction(state_); }
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90 * finally { lock.release(); }
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91 * }
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92 * catch(InterruptedException ex){}
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93 * return d;
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94 * }
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95 * private double updateFunction(double d) { ... }
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96 * private double accessFunction(double d) { ... }
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97 * }
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98 * </pre>
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99 * If you have a lot of such methods, and they take a common
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100 * form, you can standardize this using wrappers. Some of these
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101 * wrappers are standardized in LockedExecutor, but you can make others.
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102 * For example:
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103 * <pre>
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104 * class HandSynchedV2 {
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105 * private double state_ = 0.0;
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106 * private final Sync lock; // use lock type supplied in constructor
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107 * public HandSynchedV2(Sync l) { lock = l; }
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108 *
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109 * protected void runSafely(Runnable r) {
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110 * try {
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111 * lock.acquire();
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112 * try { r.run(); }
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113 * finally { lock.release(); }
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114 * }
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115 * catch (InterruptedException ex) { // propagate without throwing
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116 * Thread.currentThread().interrupt();
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117 * }
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118 * }
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119 *
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120 * public void changeState(double d) {
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121 * runSafely(new Runnable() {
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122 * public void run() { state_ = updateFunction(d); }
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123 * });
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124 * }
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125 * // ...
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126 * }
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127 * </pre>
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128 * <p>
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129 * One reason to bother with such constructions is to use deadlock-
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130 * avoiding back-offs when dealing with locks involving multiple objects.
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131 * For example, here is a Cell class that uses attempt to back-off
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132 * and retry if two Cells are trying to swap values with each other
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133 * at the same time.
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134 * <pre>
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135 * class Cell {
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136 * long value;
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137 * Sync lock = ... // some sync implementation class
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138 * void swapValue(Cell other) {
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139 * for (;;) {
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140 * try {
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141 * lock.acquire();
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142 * try {
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143 * if (other.lock.attempt(100)) {
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144 * try {
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145 * long t = value;
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146 * value = other.value;
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147 * other.value = t;
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148 * return;
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149 * }
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150 * finally { other.lock.release(); }
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151 * }
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Dan Creswell <dan.creswell@gmail.com>
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152 * }
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153 * finally { lock.release(); }
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Dan Creswell <dan.creswell@gmail.com>
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154 * }
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155 * catch (InterruptedException ex) { return; }
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156 * }
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157 * }
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158 * }
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159 *</pre>
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160 * <p>
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161 * Here is an even fancier version, that uses lock re-ordering
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162 * upon conflict:
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Dan Creswell <dan.creswell@gmail.com>
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163 * <pre>
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164 * class Cell {
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165 * long value;
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166 * Sync lock = ...;
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167 * private static boolean trySwap(Cell a, Cell b) {
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168 * a.lock.acquire();
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169 * try {
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170 * if (!b.lock.attempt(0))
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171 * return false;
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172 * try {
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173 * long t = a.value;
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174 * a.value = b.value;
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Dan Creswell <dan.creswell@gmail.com>
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175 * b.value = t;
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176 * return true;
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177 * }
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178 * finally { other.lock.release(); }
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179 * }
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180 * finally { lock.release(); }
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181 * return false;
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182 * }
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Dan Creswell <dan.creswell@gmail.com>
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183 *
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184 * void swapValue(Cell other) {
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185 * try {
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186 * while (!trySwap(this, other) &&
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187 * !tryswap(other, this))
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188 * Thread.sleep(1);
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189 * }
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190 * catch (InterruptedException ex) { return; }
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191 * }
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Dan Creswell <dan.creswell@gmail.com>
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192 *}
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Dan Creswell <dan.creswell@gmail.com>
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193 *</pre>
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194 * <p>
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195 * Interruptions are in general handled as early as possible.
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196 * Normally, InterruptionExceptions are thrown
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197 * in acquire and attempt(msec) if interruption
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198 * is detected upon entry to the method, as well as in any
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199 * later context surrounding waits.
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200 * However, interruption status is ignored in release();
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Dan Creswell <dan.creswell@gmail.com>
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201 * <p>
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202 * Timed versions of attempt report failure via return value.
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203 * If so desired, you can transform such constructions to use exception
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204 * throws via
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205 * <pre>
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206 * if (!c.attempt(timeval)) throw new TimeoutException(timeval);
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207 * </pre>
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208 * <p>
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209 * The TimoutSync wrapper class can be used to automate such usages.
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210 * <p>
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211 * All time values are expressed in milliseconds as longs, which have a maximum
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212 * value of Long.MAX_VALUE, or almost 300,000 centuries. It is not
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213 * known whether JVMs actually deal correctly with such extreme values.
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parents:
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214 * For convenience, some useful time values are defined as static constants.
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215 * <p>
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216 * All implementations of the three Sync methods guarantee to
3dc0c5604566 Initial checkin of blitz 2.0 fcs - no installer yet.
Dan Creswell <dan.creswell@gmail.com>
parents:
diff changeset
217 * somehow employ Java <code>synchronized</code> methods or blocks,
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Dan Creswell <dan.creswell@gmail.com>
parents:
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218 * and so entail the memory operations described in JLS
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Dan Creswell <dan.creswell@gmail.com>
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219 * chapter 17 which ensure that variables are loaded and flushed
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Dan Creswell <dan.creswell@gmail.com>
parents:
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220 * within before/after constructions.
3dc0c5604566 Initial checkin of blitz 2.0 fcs - no installer yet.
Dan Creswell <dan.creswell@gmail.com>
parents:
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221 * <p>
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Dan Creswell <dan.creswell@gmail.com>
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222 * Syncs may also be used in spinlock constructions. Although
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223 * it is normally best to just use acquire(), various forms
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Dan Creswell <dan.creswell@gmail.com>
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224 * of busy waits can be implemented. For a simple example
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225 * (but one that would probably never be preferable to using acquire()):
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Dan Creswell <dan.creswell@gmail.com>
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226 * <pre>
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227 * class X {
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228 * Sync lock = ...
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229 * void spinUntilAcquired() throws InterruptedException {
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230 * // Two phase.
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231 * // First spin without pausing.
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232 * int purespins = 10;
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233 * for (int i = 0; i < purespins; ++i) {
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234 * if (lock.attempt(0))
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Dan Creswell <dan.creswell@gmail.com>
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diff changeset
235 * return true;
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236 * }
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237 * // Second phase - use timed waits
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238 * long waitTime = 1; // 1 millisecond
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Dan Creswell <dan.creswell@gmail.com>
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239 * for (;;) {
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Dan Creswell <dan.creswell@gmail.com>
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diff changeset
240 * if (lock.attempt(waitTime))
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Dan Creswell <dan.creswell@gmail.com>
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241 * return true;
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242 * else
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243 * waitTime = waitTime * 3 / 2 + 1; // increase 50%
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244 * }
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245 * }
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Dan Creswell <dan.creswell@gmail.com>
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246 * }
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Dan Creswell <dan.creswell@gmail.com>
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247 * </pre>
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248 * <p>
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249 * In addition pure synchronization control, Syncs
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Dan Creswell <dan.creswell@gmail.com>
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250 * may be useful in any context requiring before/after methods.
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251 * For example, you can use an ObservableSync
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252 * (perhaps as part of a LayeredSync) in order to obtain callbacks
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253 * before and after each method invocation for a given class.
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254 * <p>
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255
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256 * <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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257 **/
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258
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259
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260 public interface Sync {
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261
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262 /**
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Dan Creswell <dan.creswell@gmail.com>
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263 * Wait (possibly forever) until successful passage.
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264 * Fail only upon interuption. Interruptions always result in
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265 * `clean' failures. On failure, you can be sure that it has not
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266 * been acquired, and that no
3dc0c5604566 Initial checkin of blitz 2.0 fcs - no installer yet.
Dan Creswell <dan.creswell@gmail.com>
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267 * corresponding release should be performed. Conversely,
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Dan Creswell <dan.creswell@gmail.com>
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268 * a normal return guarantees that the acquire was successful.
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269 **/
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270
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271 public void acquire() throws InterruptedException;
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272
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273 /**
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274 * Wait at most msecs to pass; report whether passed.
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275 * <p>
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276 * The method has best-effort semantics:
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277 * The msecs bound cannot
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278 * be guaranteed to be a precise upper bound on wait time in Java.
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279 * Implementations generally can only attempt to return as soon as possible
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280 * after the specified bound. Also, timers in Java do not stop during garbage
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281 * collection, so timeouts can occur just because a GC intervened.
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282 * So, msecs arguments should be used in
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283 * a coarse-grained manner. Further,
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284 * implementations cannot always guarantee that this method
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285 * will return at all without blocking indefinitely when used in
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286 * unintended ways. For example, deadlocks may be encountered
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287 * when called in an unintended context.
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288 * <p>
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289 * @param msecs the number of milleseconds to wait.
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290 * An argument less than or equal to zero means not to wait at all.
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291 * However, this may still require
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292 * access to a synchronization lock, which can impose unbounded
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293 * delay if there is a lot of contention among threads.
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294 * @return true if acquired
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295 **/
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296
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297 public boolean attempt(long msecs) throws InterruptedException;
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298
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299 /**
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300 * Potentially enable others to pass.
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301 * <p>
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302 * Because release does not raise exceptions,
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303 * it can be used in `finally' clauses without requiring extra
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304 * embedded try/catch blocks. But keep in mind that
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305 * as with any java method, implementations may
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306 * still throw unchecked exceptions such as Error or NullPointerException
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307 * when faced with uncontinuable errors. However, these should normally
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308 * only be caught by higher-level error handlers.
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309 **/
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310
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311 public void release();
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312
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313 /** One second, in milliseconds; convenient as a time-out value **/
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314 public static final long ONE_SECOND = 1000;
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315
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316 /** One minute, in milliseconds; convenient as a time-out value **/
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317 public static final long ONE_MINUTE = 60 * ONE_SECOND;
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318
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319 /** One hour, in milliseconds; convenient as a time-out value **/
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320 public static final long ONE_HOUR = 60 * ONE_MINUTE;
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321
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322 /** One day, in milliseconds; convenient as a time-out value **/
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323 public static final long ONE_DAY = 24 * ONE_HOUR;
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324
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325 /** One week, in milliseconds; convenient as a time-out value **/
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326 public static final long ONE_WEEK = 7 * ONE_DAY;
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327
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328 /** One year in milliseconds; convenient as a time-out value **/
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329 // Not that it matters, but there is some variation across
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330 // standard sources about value at msec precision.
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331 // The value used is the same as in java.util.GregorianCalendar
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332 public static final long ONE_YEAR = (long)(365.2425 * ONE_DAY);
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333
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334 /** One century in milliseconds; convenient as a time-out value **/
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335 public static final long ONE_CENTURY = 100 * ONE_YEAR;
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336
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337
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338 }
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339
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340