Mercurial > hg > blitz_condensed
annotate src/EDU/oswego/cs/dl/util/concurrent/WaitFreeQueue.java @ 34:6f68e94c1fb8 default tip
Add CondensedStats monitoring utility, equivalent to vmstat
author | Dominic Cleal <dominic-cleal@cdo2.com> |
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date | Thu, 05 Aug 2010 11:07:25 +0100 |
parents | 3dc0c5604566 |
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rev | line source |
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0
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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 } |