Mercurial > hg > blitz_stable

annotate src/EDU/oswego/cs/dl/util/concurrent/WaitFreeQueue.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: WaitFreeQueue.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 16Jun1998 dl Create public version
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12 5Aug1998 dl replaced int counters with longs
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13 17nov2001 dl Simplify given Bill Pugh's observation
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14 that counted pointers are unnecessary.
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15 */
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16
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17 package EDU.oswego.cs.dl.util.concurrent;
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18
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19 /**
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20 * A wait-free linked list based queue implementation.
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21 * <p>
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22 *
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23 * While this class conforms to the full Channel interface, only the
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24 * <code>put</code> and <code>poll</code> methods are useful in most
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25 * applications. Because the queue does not support blocking
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26 * operations, <code>take</code> relies on spin-loops, which can be
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27 * extremely wasteful. <p>
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28 *
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29 * This class is adapted from the algorithm described in <a
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30 * href="http://www.cs.rochester.edu/u/michael/PODC96.html"> Simple,
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31 * Fast, and Practical Non-Blocking and Blocking Concurrent Queue
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32 * Algorithms</a> by Maged M. Michael and Michael L. Scott. This
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33 * implementation is not strictly wait-free since it relies on locking
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34 * for basic atomicity and visibility requirements. Locks can impose
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35 * unbounded waits, although this should not be a major practical
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36 * concern here since each lock is held for the duration of only a few
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37 * statements. (However, the overhead of using so many locks can make
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38 * it less attractive than other Channel implementations on JVMs where
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39 * locking operations are very slow.) <p>
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40 *
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41 * @see BoundedLinkedQueue
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42 * @see LinkedQueue
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43 *
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44 * <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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45
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46 **/
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47
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48 public class WaitFreeQueue implements Channel {
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49
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50 /*
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51 This is a straightforward adaptation of Michael & Scott
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52 algorithm, with CAS's simulated via per-field locks,
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53 and without version numbers for pointers since, under
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54 Java Garbage Collection, you can never see the "wrong"
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55 node with the same address as the one you think you have.
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56 */
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57
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58 /** List nodes for Queue **/
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59 protected final static class Node {
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60 protected final Object value;
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61 protected volatile Node next;
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62
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63 /** Make a new node with indicated item, and null link **/
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64 protected Node(Object x) { value = x; }
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65
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66 /** Simulate a CAS operation for 'next' field **/
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67 protected synchronized boolean CASNext(Node oldNext, Node newNext) {
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68 if (next == oldNext) {
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69 next = newNext;
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70 return true;
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71 }
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72 else
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73 return false;
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74 }
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75 }
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76
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77 /** Head of list is always a dummy node **/
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78 protected volatile Node head = new Node(null);
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79 /** Pointer to last node on list **/
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80 protected volatile Node tail = head;
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81
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82 /** Lock for simulating CAS for tail field **/
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83 protected final Object tailLock = new Object();
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84
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85 /** Simulate CAS for head field, using 'this' lock **/
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86 protected synchronized boolean CASHead(Node oldHead, Node newHead) {
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87 if (head == oldHead) {
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88 head = newHead;
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89 return true;
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90 }
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91 else
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92 return false;
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93 }
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94
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95 /** Simulate CAS for tail field **/
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96 protected boolean CASTail(Node oldTail, Node newTail) {
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97 synchronized(tailLock) {
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98 if (tail == oldTail) {
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99 tail = newTail;
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100 return true;
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101 }
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102 else
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103 return false;
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104 }
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105 }
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106
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107 public void put(Object x) throws InterruptedException {
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108 if (x == null) throw new IllegalArgumentException();
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109 if (Thread.interrupted()) throw new InterruptedException();
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110 Node n = new Node(x);
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111
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112 for(;;) {
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113 Node t = tail;
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114 // Try to link new node to end of list.
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115 if (t.CASNext(null, n)) {
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116 // Must now change tail field.
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117 // This CAS might fail, but if so, it will be fixed by others.
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118 CASTail(t, n);
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119 return;
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120 }
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121
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122 // If cannot link, help out a previous failed attempt to move tail
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123 CASTail(t, t.next);
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124 }
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125 }
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126
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127 public boolean offer(Object x, long msecs) throws InterruptedException {
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128 put(x);
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129 return true;
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130 }
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131
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132 /** Main dequeue algorithm, called by poll, take. **/
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133 protected Object extract() throws InterruptedException {
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134 for (;;) {
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135 Node h = head;
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136 Node first = h.next;
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137
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138 if (first == null)
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139 return null;
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140
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141 Object result = first.value;
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142 if (CASHead(h, first))
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143 return result;
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144 }
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145 }
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146
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147 public Object peek() {
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148 Node first = head.next;
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149
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150 if (first == null)
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151 return null;
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152
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153 // Note: This synch unnecessary after JSR-133.
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154 // It exists only to guarantee visibility of returned object,
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155 // No other synch is needed, but "old" memory model requires one.
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156 synchronized(this) {
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157 return first.value;
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158 }
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159 }
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160
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161 /**
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162 * Spin until poll returns a non-null value.
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163 * You probably don't want to call this method.
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164 * A Thread.sleep(0) is performed on each iteration
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165 * as a heuristic to reduce contention. If you would
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166 * rather use, for example, an exponential backoff,
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167 * you could manually set this up using poll.
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168 **/
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169 public Object take() throws InterruptedException {
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170 if (Thread.interrupted()) throw new InterruptedException();
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171 for(;;) {
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172 Object x = extract();
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173 if (x != null)
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174 return x;
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175 else
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176 Thread.sleep(0);
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177 }
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178 }
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179
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180 /**
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181 * Spin until poll returns a non-null value or time elapses.
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182 * if msecs is positive, a Thread.sleep(0) is performed on each iteration
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183 * as a heuristic to reduce contention.
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184 **/
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185 public Object poll(long msecs) throws InterruptedException {
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186 if (Thread.interrupted()) throw new InterruptedException();
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187 if (msecs <= 0)
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188 return extract();
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189
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190 long startTime = System.currentTimeMillis();
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191 for(;;) {
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192 Object x = extract();
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193 if (x != null)
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194 return x;
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195 else if (System.currentTimeMillis() - startTime >= msecs)
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196 return null;
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197 else
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198 Thread.sleep(0);
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199 }
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200
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201 }
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202 }