Mercurial > hg > blitz_stable
annotate src/EDU/oswego/cs/dl/util/concurrent/WriterPreferenceReadWriteLock.java @ 32:243c74d599bf
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author | Dan Creswell <dan.creswell@gmail.com> |
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date | Sat, 12 Jun 2010 10:41:47 +0100 |
parents | 3dc0c5604566 |
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0
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1 /* |
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2 File: WriterPreferenceReadWriteLock.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 25aug1998 dl record writer thread |
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14 3May1999 dl add notifications on interrupt/timeout |
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15 |
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16 */ |
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17 |
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18 package EDU.oswego.cs.dl.util.concurrent; |
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19 |
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20 /** |
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21 * A ReadWriteLock that prefers waiting writers over |
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22 * waiting readers when there is contention. This class |
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23 * is adapted from the versions described in CPJ, improving |
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24 * on the ones there a bit by segregating reader and writer |
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25 * wait queues, which is typically more efficient. |
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26 * <p> |
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27 * The locks are <em>NOT</em> reentrant. In particular, |
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28 * even though it may appear to usually work OK, |
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29 * a thread holding a read lock should not attempt to |
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30 * re-acquire it. Doing so risks lockouts when there are |
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31 * also waiting writers. |
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32 * <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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33 **/ |
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34 |
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35 public class WriterPreferenceReadWriteLock implements ReadWriteLock { |
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36 |
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37 protected long activeReaders_ = 0; |
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38 protected Thread activeWriter_ = null; |
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39 protected long waitingReaders_ = 0; |
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40 protected long waitingWriters_ = 0; |
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41 |
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42 |
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43 protected final ReaderLock readerLock_ = new ReaderLock(); |
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44 protected final WriterLock writerLock_ = new WriterLock(); |
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45 |
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46 public Sync writeLock() { return writerLock_; } |
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47 public Sync readLock() { return readerLock_; } |
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48 |
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49 /* |
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50 A bunch of small synchronized methods are needed |
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51 to allow communication from the Lock objects |
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52 back to this object, that serves as controller |
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53 */ |
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54 |
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55 |
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56 protected synchronized void cancelledWaitingReader() { --waitingReaders_; } |
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57 protected synchronized void cancelledWaitingWriter() { --waitingWriters_; } |
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58 |
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59 |
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60 /** Override this method to change to reader preference **/ |
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61 protected boolean allowReader() { |
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62 return activeWriter_ == null && waitingWriters_ == 0; |
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63 } |
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64 |
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65 |
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66 protected synchronized boolean startRead() { |
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67 boolean allowRead = allowReader(); |
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68 if (allowRead) ++activeReaders_; |
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69 return allowRead; |
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70 } |
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71 |
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72 protected synchronized boolean startWrite() { |
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73 |
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74 // The allowWrite expression cannot be modified without |
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75 // also changing startWrite, so is hard-wired |
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76 |
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77 boolean allowWrite = (activeWriter_ == null && activeReaders_ == 0); |
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78 if (allowWrite) activeWriter_ = Thread.currentThread(); |
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79 return allowWrite; |
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80 } |
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81 |
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82 |
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83 /* |
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84 Each of these variants is needed to maintain atomicity |
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85 of wait counts during wait loops. They could be |
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86 made faster by manually inlining each other. We hope that |
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87 compilers do this for us though. |
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88 */ |
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89 |
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90 protected synchronized boolean startReadFromNewReader() { |
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91 boolean pass = startRead(); |
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92 if (!pass) ++waitingReaders_; |
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93 return pass; |
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94 } |
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95 |
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96 protected synchronized boolean startWriteFromNewWriter() { |
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97 boolean pass = startWrite(); |
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98 if (!pass) ++waitingWriters_; |
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99 return pass; |
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100 } |
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101 |
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102 protected synchronized boolean startReadFromWaitingReader() { |
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103 boolean pass = startRead(); |
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104 if (pass) --waitingReaders_; |
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105 return pass; |
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106 } |
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107 |
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108 protected synchronized boolean startWriteFromWaitingWriter() { |
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109 boolean pass = startWrite(); |
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110 if (pass) --waitingWriters_; |
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111 return pass; |
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112 } |
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113 |
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114 /** |
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115 * Called upon termination of a read. |
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116 * Returns the object to signal to wake up a waiter, or null if no such |
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117 **/ |
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118 protected synchronized Signaller endRead() { |
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119 if (--activeReaders_ == 0 && waitingWriters_ > 0) |
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120 return writerLock_; |
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121 else |
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122 return null; |
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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 * Called upon termination of a write. |
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128 * Returns the object to signal to wake up a waiter, or null if no such |
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129 **/ |
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130 protected synchronized Signaller endWrite() { |
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131 activeWriter_ = null; |
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132 if (waitingReaders_ > 0 && allowReader()) |
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133 return readerLock_; |
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134 else if (waitingWriters_ > 0) |
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135 return writerLock_; |
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136 else |
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137 return null; |
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138 } |
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139 |
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140 |
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141 /** |
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142 * Reader and Writer requests are maintained in two different |
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143 * wait sets, by two different objects. These objects do not |
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144 * know whether the wait sets need notification since they |
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145 * don't know preference rules. So, each supports a |
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146 * method that can be selected by main controlling object |
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147 * to perform the notifications. This base class simplifies mechanics. |
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148 **/ |
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149 |
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150 protected abstract class Signaller { // base for ReaderLock and WriterLock |
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151 abstract void signalWaiters(); |
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152 } |
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153 |
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154 protected class ReaderLock extends Signaller implements Sync { |
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155 |
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156 public void acquire() throws InterruptedException { |
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157 if (Thread.interrupted()) throw new InterruptedException(); |
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158 InterruptedException ie = null; |
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159 synchronized(this) { |
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160 if (!startReadFromNewReader()) { |
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161 for (;;) { |
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162 try { |
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163 ReaderLock.this.wait(); |
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164 if (startReadFromWaitingReader()) |
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165 return; |
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166 } |
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167 catch(InterruptedException ex){ |
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168 cancelledWaitingReader(); |
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169 ie = ex; |
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170 break; |
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171 } |
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172 } |
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173 } |
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174 } |
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175 if (ie != null) { |
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176 // fall through outside synch on interrupt. |
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177 // This notification is not really needed here, |
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178 // but may be in plausible subclasses |
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179 writerLock_.signalWaiters(); |
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180 throw ie; |
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181 } |
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182 } |
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183 |
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184 |
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185 public void release() { |
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186 Signaller s = endRead(); |
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187 if (s != null) s.signalWaiters(); |
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188 } |
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189 |
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190 |
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191 synchronized void signalWaiters() { ReaderLock.this.notifyAll(); } |
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192 |
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193 public boolean attempt(long msecs) throws InterruptedException { |
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194 if (Thread.interrupted()) throw new InterruptedException(); |
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195 InterruptedException ie = null; |
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196 synchronized(this) { |
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197 if (msecs <= 0) |
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198 return startRead(); |
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199 else if (startReadFromNewReader()) |
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200 return true; |
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201 else { |
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202 long waitTime = msecs; |
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203 long start = System.currentTimeMillis(); |
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204 for (;;) { |
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205 try { ReaderLock.this.wait(waitTime); } |
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206 catch(InterruptedException ex){ |
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207 cancelledWaitingReader(); |
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208 ie = ex; |
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209 break; |
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210 } |
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211 if (startReadFromWaitingReader()) |
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212 return true; |
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213 else { |
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214 waitTime = msecs - (System.currentTimeMillis() - start); |
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215 if (waitTime <= 0) { |
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216 cancelledWaitingReader(); |
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217 break; |
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218 } |
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219 } |
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220 } |
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221 } |
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222 } |
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223 // safeguard on interrupt or timeout: |
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224 writerLock_.signalWaiters(); |
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225 if (ie != null) throw ie; |
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226 else return false; // timed out |
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227 } |
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228 |
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229 } |
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230 |
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231 protected class WriterLock extends Signaller implements Sync { |
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232 |
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233 public void acquire() throws InterruptedException { |
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234 if (Thread.interrupted()) throw new InterruptedException(); |
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235 InterruptedException ie = null; |
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236 synchronized(this) { |
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237 if (!startWriteFromNewWriter()) { |
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238 for (;;) { |
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239 try { |
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240 WriterLock.this.wait(); |
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241 if (startWriteFromWaitingWriter()) |
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242 return; |
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243 } |
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244 catch(InterruptedException ex){ |
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245 cancelledWaitingWriter(); |
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246 WriterLock.this.notify(); |
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247 ie = ex; |
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248 break; |
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249 } |
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250 } |
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251 } |
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252 } |
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253 if (ie != null) { |
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254 // Fall through outside synch on interrupt. |
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255 // On exception, we may need to signal readers. |
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256 // It is not worth checking here whether it is strictly necessary. |
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257 readerLock_.signalWaiters(); |
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258 throw ie; |
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259 } |
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260 } |
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261 |
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262 public void release(){ |
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263 Signaller s = endWrite(); |
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264 if (s != null) s.signalWaiters(); |
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265 } |
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266 |
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267 synchronized void signalWaiters() { WriterLock.this.notify(); } |
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268 |
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269 public boolean attempt(long msecs) throws InterruptedException { |
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270 if (Thread.interrupted()) throw new InterruptedException(); |
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271 InterruptedException ie = null; |
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272 synchronized(this) { |
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273 if (msecs <= 0) |
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274 return startWrite(); |
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275 else if (startWriteFromNewWriter()) |
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276 return true; |
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277 else { |
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278 long waitTime = msecs; |
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279 long start = System.currentTimeMillis(); |
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280 for (;;) { |
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281 try { WriterLock.this.wait(waitTime); } |
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282 catch(InterruptedException ex){ |
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283 cancelledWaitingWriter(); |
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284 WriterLock.this.notify(); |
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285 ie = ex; |
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286 break; |
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287 } |
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288 if (startWriteFromWaitingWriter()) |
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289 return true; |
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290 else { |
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291 waitTime = msecs - (System.currentTimeMillis() - start); |
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292 if (waitTime <= 0) { |
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293 cancelledWaitingWriter(); |
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294 WriterLock.this.notify(); |
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295 break; |
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296 } |
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297 } |
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298 } |
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299 } |
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300 } |
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301 |
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302 readerLock_.signalWaiters(); |
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303 if (ie != null) throw ie; |
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304 else return false; // timed out |
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305 } |
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306 |
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307 } |
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308 |
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309 |
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310 |
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311 } |
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312 |