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annotate src/EDU/oswego/cs/dl/util/concurrent/SynchronizedVariable.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: SynchronizedVariable.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 30Jun1998 dl Create public version
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12 */
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13
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14 package EDU.oswego.cs.dl.util.concurrent;
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15
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16 /**
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17 * Base class for simple, small classes
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18 * maintaining single values that are always accessed
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19 * and updated under synchronization. Since defining them for only
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20 * some types seemed too arbitrary, they exist for all basic types,
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21 * although it is hard to imagine uses for some.
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22 * <p>
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23 * These classes mainly exist so that you do not have to go to the
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24 * trouble of writing your own miscellaneous classes and methods
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25 * in situations including:
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26 * <ul>
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27 * <li> When you need or want to offload an instance
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28 * variable to use its own synchronization lock.
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29 * When these objects are used to replace instance variables, they
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30 * should almost always be declared as <code>final</code>. This
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31 * helps avoid the need to synchronize just to obtain the reference
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32 * to the synchronized variable itself.
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33 *
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34 * <li> When you need methods such as set, commit, or swap.
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35 * Note however that
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36 * the synchronization for these variables is <em>independent</em>
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37 * of any other synchronization perfromed using other locks.
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38 * So, they are not
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39 * normally useful when accesses and updates among
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40 * variables must be coordinated.
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41 * For example, it would normally be a bad idea to make
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42 * a Point class out of two SynchronizedInts, even those
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43 * sharing a lock.
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44 *
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45 * <li> When defining <code>static</code> variables. It almost
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46 * always works out better to rely on synchronization internal
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47 * to these objects, rather than class locks.
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48 * </ul>
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49 * <p>
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50 * While they cannot, by nature, share much code,
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51 * all of these classes work in the same way.
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52 * <p>
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53 * <b>Construction</b> <br>
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54 * Synchronized variables are always constructed holding an
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55 * initial value of the associated type. Constructors also
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56 * establish the lock to use for all methods:
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57 * <ul>
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58 * <li> By default, each variable uses itself as the
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59 * synchronization lock. This is the most common
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60 * choice in the most common usage contexts in which
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61 * SynchronizedVariables are used to split off
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62 * synchronization locks for independent attributes
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63 * of a class.
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64 * <li> You can specify any other Object to use as the
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65 * synchronization lock. This allows you to
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66 * use various forms of `slave synchronization'. For
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67 * example, a variable that is always associated with a
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68 * particular object can use that object's lock.
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69 * </ul>
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70 * <p>
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71 * <b>Update methods</b><br>
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72 * Each class supports several kinds of update methods:
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73 * <ul>
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74 * <li> A <code>set</code> method that sets to a new value and returns
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75 * previous value. For example, for a SynchronizedBoolean b,
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76 * <code>boolean old = b.set(true)</code> performs a test-and-set.
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77 * <p>
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78 * <li> A <code>commit</code> method that sets to new value only
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79 * if currently holding a given value.
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80 *
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81 * For example, here is a class that uses an optimistic update
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82 * loop to recompute a count variable represented as a
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83 * SynchronizedInt.
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84 * <pre>
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85 * class X {
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86 * private final SynchronizedInt count = new SynchronizedInt(0);
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87 *
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88 * static final int MAX_RETRIES = 1000;
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89 *
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90 * public boolean recomputeCount() throws InterruptedException {
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91 * for (int i = 0; i &lt; MAX_RETRIES; ++i) {
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92 * int current = count.get();
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93 * int next = compute(current);
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94 * if (count.commit(current, next))
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95 * return true;
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96 * else if (Thread.interrupted())
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97 * throw new InterruptedException();
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98 * }
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99 * return false;
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100 * }
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101 * int compute(int l) { ... some kind of computation ... }
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102 * }
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103 * </pre>
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104 * <p>
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105 * <li>A <code>swap</code> method that atomically swaps with another
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106 * object of the same class using a deadlock-avoidance strategy.
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107 * <p>
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108 * <li> Update-in-place methods appropriate to the type. All
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109 * numerical types support:
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110 * <ul>
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111 * <li> add(x) (equivalent to return value += x)
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112 * <li> subtract(x) (equivalent to return value -= x)
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113 * <li> multiply(x) (equivalent to return value *= x)
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114 * <li> divide(x) (equivalent to return value /= x)
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115 * </ul>
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116 * Integral types also support:
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117 * <ul>
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118 * <li> increment() (equivalent to return ++value)
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119 * <li> decrement() (equivalent to return --value)
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120 * </ul>
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121 * Boolean types support:
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122 * <ul>
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123 * <li> or(x) (equivalent to return value |= x)
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124 * <li> and(x) (equivalent to return value &amp;= x)
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125 * <li> xor(x) (equivalent to return value ^= x)
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126 * <li> complement() (equivalent to return x = !x)
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127 * </ul>
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128 * These cover most, but not all of the possible operators in Java.
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129 * You can add more compute-and-set methods in subclasses. This
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130 * is often a good way to avoid the need for ad-hoc synchronized
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131 * blocks surrounding expressions.
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132 * </ul>
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133 * <p>
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134 * <b>Guarded methods</b> <br>
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135 * All <code>Waitable</code> subclasses provide notifications on
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136 * every value update, and support guarded methods of the form
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137 * <code>when</code><em>predicate</em>, that wait until the
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138 * predicate hold, then optionally run any Runnable action
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139 * within the lock, and then return. All types support:
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140 * <ul>
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141 * <li> whenEqual(value, action)
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142 * <li> whenNotEqual(value, action)
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143 * </ul>
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144 * (If the action argument is null, these return immediately
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145 * after the predicate holds.)
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146 * Numerical types also support
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147 * <ul>
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148 * <li> whenLess(value, action)
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149 * <li> whenLessEqual(value, action)
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150 * <li> whenGreater(value, action)
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151 * <li> whenGreaterEqual(value, action)
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152 * </ul>
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153 * The Waitable classes are not always spectacularly efficient since they
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154 * provide notifications on all value changes. They are
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155 * designed for use in contexts where either performance is not an
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156 * overriding issue, or where nearly every update releases guarded
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157 * waits anyway.
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158 * <p>
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159 * <b>Other methods</b> <br>
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160 * This class implements Executor, and provides an <code>execute</code>
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161 * method that runs the runnable within the lock.
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162 * <p>
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163 * All classes except SynchronizedRef and WaitableRef implement
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164 * <code>Cloneable</code> and <code>Comparable</code>.
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165 * Implementations of the corresponding
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166 * methods either use default mechanics, or use methods that closely
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167 * correspond to their java.lang analogs. SynchronizedRef does not
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168 * implement any of these standard interfaces because there are
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169 * many cases where it would not make sense. However, you can
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170 * easily make simple subclasses that add the appropriate declarations.
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171 *
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172 * <p>
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173 *
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174 *
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175 *
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176 * <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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177 **/
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178
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179 public class SynchronizedVariable implements Executor {
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180
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181 protected final Object lock_;
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182
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183 /** Create a SynchronizedVariable using the supplied lock **/
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184 public SynchronizedVariable(Object lock) { lock_ = lock; }
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185
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186 /** Create a SynchronizedVariable using itself as the lock **/
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187 public SynchronizedVariable() { lock_ = this; }
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188
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189 /**
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190 * Return the lock used for all synchronization for this object
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191 **/
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192 public Object getLock() { return lock_; }
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193
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194 /**
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195 * If current thread is not interrupted, execute the given command
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196 * within this object's lock
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197 **/
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198
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199 public void execute(Runnable command) throws InterruptedException {
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200 if (Thread.interrupted()) throw new InterruptedException();
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201 synchronized (lock_) {
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202 command.run();
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203 }
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204 }
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205 }