Mercurial > hg > blitz_stable
annotate src/EDU/oswego/cs/dl/util/concurrent/FIFOReadWriteLock.java @ 8:fa7203ea1622
Remove some dead code (Dave Brosius)
author | Dan Creswell <dan.creswell@gmail.com> |
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date | Sat, 23 May 2009 11:27:41 +0100 |
parents | 3dc0c5604566 |
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1 /* |
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2 File: FIFOReadWriteLock.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 23nov2001 dl Replace main algorithm with fairer |
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13 version based on one by Alexander Terekhov |
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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 /** |
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20 * This class implements a policy for reader/writer locks in which |
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21 * threads contend in a First-in/First-out manner for access (modulo |
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22 * the limitations of FIFOSemaphore, which is used for queuing). This |
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23 * policy does not particularly favor readers or writers. As a |
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24 * byproduct of the FIFO policy, the <tt>attempt</tt> methods may |
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25 * return <tt>false</tt> even when the lock might logically be |
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26 * available, but, due to contention, cannot be accessed within the |
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27 * given time bound. <p> |
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28 * |
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29 * This lock is <em>NOT</em> reentrant. Current readers and |
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30 * writers should not try to re-obtain locks while holding them. |
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31 * <p> |
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32 * |
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33 * [<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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34 * |
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35 * @see FIFOSemaphore |
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36 **/ |
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37 |
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38 public class FIFOReadWriteLock implements ReadWriteLock { |
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39 |
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40 /** |
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41 * Fair Semaphore serving as a kind of mutual exclusion lock. |
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42 * Writers acquire on entry, and hold until rwlock exit. |
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43 * Readers acquire and release only during entry (but are |
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44 * blocked from doing so if there is an active writer). |
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45 **/ |
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46 protected final FIFOSemaphore entryLock = new FIFOSemaphore(1); |
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47 |
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48 /** |
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49 * Number of threads that have entered read lock. Note that this is |
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50 * never reset to zero. Incremented only during acquisition of read |
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51 * lock while the "entryLock" is held, but read elsewhere, so is |
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52 * declared volatile. |
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53 **/ |
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54 protected volatile int readers; |
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55 |
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56 /** |
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57 * Number of threads that have exited read lock. Note that this is |
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58 * never reset to zero. Accessed only in code protected by |
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59 * synchronized(this). When exreaders != readers, the rwlock is |
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60 * being used for reading. Else if the entry lock is held, it is |
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61 * being used for writing (or in transition). Else it is free. |
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62 * Note: To distinguish these states, we assume that fewer than 2^32 |
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63 * reader threads can simultaneously execute. |
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64 **/ |
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65 protected int exreaders; |
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66 |
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67 protected void acquireRead() throws InterruptedException { |
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68 entryLock.acquire(); |
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69 ++readers; |
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70 entryLock.release(); |
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71 } |
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72 |
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73 protected synchronized void releaseRead() { |
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74 /* |
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75 If this is the last reader, notify a possibly waiting writer. |
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76 Because waits occur only when entry lock is held, at most one |
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77 writer can be waiting for this notification. Because increments |
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78 to "readers" aren't protected by "this" lock, the notification |
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79 may be spurious (when an incoming reader in in the process of |
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80 updating the field), but at the point tested in acquiring write |
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81 lock, both locks will be held, thus avoiding false alarms. And |
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82 we will never miss an opportunity to send a notification when it |
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83 is actually needed. |
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84 */ |
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85 |
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86 if (++exreaders == readers) |
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87 notify(); |
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88 } |
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89 |
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90 protected void acquireWrite() throws InterruptedException { |
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91 // Acquiring entryLock first forces subsequent entering readers |
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92 // (as well as writers) to block. |
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93 entryLock.acquire(); |
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94 |
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95 // Only read "readers" once now before loop. We know it won't |
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96 // change because we hold the entry lock needed to update it. |
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97 int r = readers; |
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98 |
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99 try { |
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100 synchronized(this) { |
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101 while (exreaders != r) |
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102 wait(); |
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103 } |
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104 } |
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105 catch (InterruptedException ie) { |
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106 entryLock.release(); |
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107 throw ie; |
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108 } |
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109 } |
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110 |
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111 protected void releaseWrite() { |
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112 entryLock.release(); |
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113 } |
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114 |
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115 protected boolean attemptRead(long msecs) throws InterruptedException { |
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116 if (!entryLock.attempt(msecs)) |
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117 return false; |
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118 |
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119 ++readers; |
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120 entryLock.release(); |
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121 return true; |
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122 } |
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123 |
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124 protected boolean attemptWrite(long msecs) throws InterruptedException { |
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125 long startTime = (msecs <= 0)? 0 : System.currentTimeMillis(); |
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126 |
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127 if (!entryLock.attempt(msecs)) |
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128 return false; |
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129 |
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130 int r = readers; |
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131 |
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132 try { |
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133 synchronized(this) { |
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134 while (exreaders != r) { |
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135 long timeLeft = (msecs <= 0)? 0: |
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136 msecs - (System.currentTimeMillis() - startTime); |
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137 |
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138 if (timeLeft <= 0) { |
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139 entryLock.release(); |
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140 return false; |
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141 } |
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142 |
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143 wait(timeLeft); |
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144 } |
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145 return true; |
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146 } |
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147 } |
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148 catch (InterruptedException ie) { |
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149 entryLock.release(); |
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150 throw ie; |
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151 } |
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152 } |
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153 |
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154 // support for ReadWriteLock interface |
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155 |
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156 protected class ReaderSync implements Sync { |
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157 public void acquire() throws InterruptedException { |
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158 acquireRead(); |
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159 } |
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160 public void release() { |
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161 releaseRead(); |
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162 } |
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163 public boolean attempt(long msecs) throws InterruptedException { |
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164 return attemptRead(msecs); |
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165 } |
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166 } |
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167 |
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168 protected class WriterSync implements Sync { |
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169 public void acquire() throws InterruptedException { |
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170 acquireWrite(); |
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171 } |
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172 public void release() { |
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173 releaseWrite(); |
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174 } |
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175 public boolean attempt(long msecs) throws InterruptedException { |
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176 return attemptWrite(msecs); |
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177 } |
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178 } |
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179 |
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180 protected final Sync readerSync = new ReaderSync(); |
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181 protected final Sync writerSync = new WriterSync(); |
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182 |
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183 public Sync writeLock() { return writerSync; } |
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184 public Sync readLock() { return readerSync; } |
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185 |
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186 } |