Mercurial > hg > blitz_stable

annotate src/EDU/oswego/cs/dl/util/concurrent/Channel.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: Channel.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 25aug1998 dl added peek
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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 buffers, queues, pipes, conduits, etc.
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19 * <p>
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20 * A Channel represents anything that you can put items
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21 * into and take them out of. As with the Sync
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22 * interface, both
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23 * blocking (put(x), take),
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24 * and timeouts (offer(x, msecs), poll(msecs)) policies
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25 * are provided. Using a
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26 * zero timeout for offer and poll results in a pure balking policy.
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27 * <p>
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28 * To aid in efforts to use Channels in a more typesafe manner,
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29 * this interface extends Puttable and Takable. You can restrict
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30 * arguments of instance variables to this type as a way of
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31 * guaranteeing that producers never try to take, or consumers put.
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32 * for example:
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33 * <pre>
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34 * class Producer implements Runnable {
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35 * final Puttable chan;
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36 * Producer(Puttable channel) { chan = channel; }
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37 * public void run() {
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38 * try {
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39 * for(;;) { chan.put(produce()); }
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40 * }
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41 * catch (InterruptedException ex) {}
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42 * }
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43 * Object produce() { ... }
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44 * }
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45 *
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46 *
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47 * class Consumer implements Runnable {
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48 * final Takable chan;
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49 * Consumer(Takable channel) { chan = channel; }
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50 * public void run() {
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51 * try {
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52 * for(;;) { consume(chan.take()); }
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53 * }
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54 * catch (InterruptedException ex) {}
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55 * }
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56 * void consume(Object x) { ... }
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57 * }
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58 *
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59 * class Setup {
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60 * void main() {
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61 * Channel chan = new SomeChannelImplementation();
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62 * Producer p = new Producer(chan);
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63 * Consumer c = new Consumer(chan);
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64 * new Thread(p).start();
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65 * new Thread(c).start();
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66 * }
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67 * }
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68 * </pre>
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69 * <p>
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70 * A given channel implementation might or might not have bounded
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71 * capacity or other insertion constraints, so in general, you cannot tell if
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72 * a given put will block. However,
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73 * Channels that are designed to
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74 * have an element capacity (and so always block when full)
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75 * should implement the
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76 * BoundedChannel
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77 * subinterface.
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78 * <p>
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79 * Channels may hold any kind of item. However,
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80 * insertion of null is not in general supported. Implementations
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81 * may (all currently do) throw IllegalArgumentExceptions upon attempts to
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82 * insert null.
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83 * <p>
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84 * By design, the Channel interface does not support any methods to determine
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85 * the current number of elements being held in the channel.
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86 * This decision reflects the fact that in
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87 * concurrent programming, such methods are so rarely useful
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88 * that including them invites misuse; at best they could
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89 * provide a snapshot of current
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90 * state, that could change immediately after being reported.
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91 * It is better practice to instead use poll and offer to try
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92 * to take and put elements without blocking. For example,
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93 * to empty out the current contents of a channel, you could write:
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94 * <pre>
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95 * try {
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96 * for (;;) {
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97 * Object item = channel.poll(0);
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98 * if (item != null)
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99 * process(item);
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100 * else
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101 * break;
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102 * }
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103 * }
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104 * catch(InterruptedException ex) { ... }
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105 * </pre>
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106 * <p>
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107 * However, it is possible to determine whether an item
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108 * exists in a Channel via <code>peek</code>, which returns
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109 * but does NOT remove the next item that can be taken (or null
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110 * if there is no such item). The peek operation has a limited
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111 * range of applicability, and must be used with care. Unless it
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112 * is known that a given thread is the only possible consumer
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113 * of a channel, and that no time-out-based <code>offer</code> operations
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114 * are ever invoked, there is no guarantee that the item returned
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115 * by peek will be available for a subsequent take.
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116 * <p>
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117 * When appropriate, you can define an isEmpty method to
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118 * return whether <code>peek</code> returns null.
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119 * <p>
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120 * Also, as a compromise, even though it does not appear in interface,
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121 * implementation classes that can readily compute the number
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122 * of elements support a <code>size()</code> method. This allows careful
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123 * use, for example in queue length monitors, appropriate to the
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124 * particular implementation constraints and properties.
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125 * <p>
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126 * All channels allow multiple producers and/or consumers.
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127 * They do not support any kind of <em>close</em> method
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128 * to shut down operation or indicate completion of particular
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129 * producer or consumer threads.
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130 * If you need to signal completion, one way to do it is to
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131 * create a class such as
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132 * <pre>
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133 * class EndOfStream {
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134 * // Application-dependent field/methods
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135 * }
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136 * </pre>
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137 * And to have producers put an instance of this class into
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138 * the channel when they are done. The consumer side can then
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139 * check this via
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140 * <pre>
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141 * Object x = aChannel.take();
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142 * if (x instanceof EndOfStream)
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143 * // special actions; perhaps terminate
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144 * else
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145 * // process normally
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146 * </pre>
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147 * <p>
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148 * In time-out based methods (poll(msecs) and offer(x, msecs),
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149 * time bounds are interpreted in
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150 * a coarse-grained, best-effort fashion. Since there is no
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151 * way in Java to escape out of a wait for a synchronized
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152 * method/block, time bounds can sometimes be exceeded when
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153 * there is a lot contention for the channel. Additionally,
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154 * some Channel semantics entail a ``point of
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155 * no return'' where, once some parts of the operation have completed,
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156 * others must follow, regardless of time bound.
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157 * <p>
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158 * Interruptions are in general handled as early as possible
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159 * in all methods. Normally, InterruptionExceptions are thrown
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160 * in put/take and offer(msec)/poll(msec) if interruption
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161 * is detected upon entry to the method, as well as in any
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162 * later context surrounding waits.
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163 * <p>
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164 * If a put returns normally, an offer
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165 * returns true, or a put or poll returns non-null, the operation
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166 * completed successfully.
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167 * In all other cases, the operation fails cleanly -- the
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168 * element is not put or taken.
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169 * <p>
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170 * As with Sync classes, spinloops are not directly supported,
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171 * are not particularly recommended for routine use, but are not hard
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172 * to construct. For example, here is an exponential backoff version:
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173 * <pre>
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174 * Object backOffTake(Channel q) throws InterruptedException {
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175 * long waitTime = 0;
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176 * for (;;) {
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177 * Object x = q.poll(0);
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178 * if (x != null)
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179 * return x;
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180 * else {
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181 * Thread.sleep(waitTime);
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182 * waitTime = 3 * waitTime / 2 + 1;
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183 * }
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184 * }
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185 * </pre>
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186 * <p>
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187 * <b>Sample Usage</b>. Here is a producer/consumer design
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188 * where the channel is used to hold Runnable commands representing
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189 * background tasks.
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190 * <pre>
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191 * class Service {
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192 * private final Channel channel = ... some Channel implementation;
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193 *
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194 * private void backgroundTask(int taskParam) { ... }
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195 *
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196 * public void action(final int arg) {
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197 * Runnable command =
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198 * new Runnable() {
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199 * public void run() { backgroundTask(arg); }
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200 * };
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201 * try { channel.put(command) }
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202 * catch (InterruptedException ex) {
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203 * Thread.currentThread().interrupt(); // ignore but propagate
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204 * }
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205 * }
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206 *
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207 * public Service() {
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208 * Runnable backgroundLoop =
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209 * new Runnable() {
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210 * public void run() {
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211 * for (;;) {
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212 * try {
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213 * Runnable task = (Runnable)(channel.take());
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214 * task.run();
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215 * }
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216 * catch (InterruptedException ex) { return; }
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217 * }
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218 * }
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219 * };
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220 * new Thread(backgroundLoop).start();
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221 * }
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222 * }
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223 *
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224 * </pre>
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225 * <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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226 * @see Sync
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227 * @see BoundedChannel
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228 **/
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229
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230 public interface Channel extends Puttable, Takable {
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231
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232 /**
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233 * Place item in the channel, possibly waiting indefinitely until
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234 * it can be accepted. Channels implementing the BoundedChannel
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235 * subinterface are generally guaranteed to block on puts upon
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236 * reaching capacity, but other implementations may or may not block.
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237 * @param item the element to be inserted. Should be non-null.
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238 * @exception InterruptedException if the current thread has
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239 * been interrupted at a point at which interruption
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240 * is detected, in which case the element is guaranteed not
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241 * to be inserted. Otherwise, on normal return, the element is guaranteed
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242 * to have been inserted.
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243 **/
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244 public void put(Object item) throws InterruptedException;
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245
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246 /**
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247 * Place item in channel only if it can be accepted within
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248 * msecs milliseconds. The time bound is interpreted in
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249 * a coarse-grained, best-effort fashion.
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250 * @param item the element to be inserted. Should be non-null.
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251 * @param msecs the number of milliseconds to wait. If less than
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252 * or equal to zero, the method does not perform any timed waits,
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253 * but might still require
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254 * access to a synchronization lock, which can impose unbounded
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255 * delay if there is a lot of contention for the channel.
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256 * @return true if accepted, else false
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257 * @exception InterruptedException if the current thread has
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258 * been interrupted at a point at which interruption
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259 * is detected, in which case the element is guaranteed not
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260 * to be inserted (i.e., is equivalent to a false return).
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261 **/
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262 public boolean offer(Object item, long msecs) throws InterruptedException;
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263
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264 /**
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265 * Return and remove an item from channel,
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266 * possibly waiting indefinitely until
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267 * such an item exists.
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268 * @return some item from the channel. Different implementations
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269 * may guarantee various properties (such as FIFO) about that item
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270 * @exception InterruptedException if the current thread has
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271 * been interrupted at a point at which interruption
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272 * is detected, in which case state of the channel is unchanged.
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273 *
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274 **/
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275 public Object take() throws InterruptedException;
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276
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277
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278 /**
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279 * Return and remove an item from channel only if one is available within
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280 * msecs milliseconds. The time bound is interpreted in a coarse
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281 * grained, best-effort fashion.
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282 * @param msecs the number of milliseconds to wait. If less than
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283 * or equal to zero, the operation does not perform any timed waits,
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284 * but might still require
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285 * access to a synchronization lock, which can impose unbounded
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286 * delay if there is a lot of contention for the channel.
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287 * @return some item, or null if the channel is empty.
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288 * @exception InterruptedException if the current thread has
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289 * been interrupted at a point at which interruption
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290 * is detected, in which case state of the channel is unchanged
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291 * (i.e., equivalent to a null return).
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292 **/
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293
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294 public Object poll(long msecs) throws InterruptedException;
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295
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296 /**
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297 * Return, but do not remove object at head of Channel,
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298 * or null if it is empty.
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299 **/
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300
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301 public Object peek();
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302
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303 }
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304