Mercurial > hg > blitz_condensed
annotate src/EDU/oswego/cs/dl/util/concurrent/SynchronizedVariable.java @ 0:3dc0c5604566
Initial checkin of blitz 2.0 fcs - no installer yet.
author | Dan Creswell <dan.creswell@gmail.com> |
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date | Sat, 21 Mar 2009 11:00:06 +0000 |
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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 < 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 &= 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 } |