Mercurial > hg > blitz_condensed
annotate src/EDU/oswego/cs/dl/util/concurrent/SynchronizedRef.java @ 28:4b2f23b6feca
Add a schema compatibility check to indexer that parallels storage.
author | Dan Creswell <dan.creswell@gmail.com> |
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date | Sat, 12 Jun 2010 10:11:15 +0100 |
parents | 3dc0c5604566 |
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0
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1 /* |
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2 File: SynchronizedRef.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 */ |
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13 |
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14 package EDU.oswego.cs.dl.util.concurrent; |
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15 import java.io.*; |
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16 |
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17 /** |
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18 * A simple class maintaining a single reference variable that |
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19 * is always accessed and updated under synchronization. |
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20 * <p> |
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21 * <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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22 **/ |
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23 |
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24 public class SynchronizedRef extends SynchronizedVariable { |
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25 /** The maintained reference **/ |
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26 protected Object value_; |
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27 |
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28 /** |
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29 * Create a SynchronizedRef initially holding the given reference |
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30 * and using its own internal lock. |
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31 **/ |
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32 public SynchronizedRef(Object initialValue) { |
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33 super(); |
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34 value_ = initialValue; |
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35 } |
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36 |
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37 /** |
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38 * Make a new SynchronizedRef with the given initial value, |
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39 * and using the supplied lock. |
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40 **/ |
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41 public SynchronizedRef(Object initialValue, Object lock) { |
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42 super(lock); |
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43 value_ = initialValue; |
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44 } |
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45 |
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46 /** |
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47 * Return the current value |
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48 **/ |
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49 public final Object get() { synchronized(lock_) { return value_; } } |
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50 |
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51 /** |
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52 * Set to newValue. |
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53 * @return the old value |
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54 **/ |
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55 |
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56 public Object set(Object newValue) { |
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57 synchronized (lock_) { |
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58 Object old = value_; |
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59 value_ = newValue; |
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60 return old; |
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61 } |
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62 } |
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63 |
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64 /** |
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65 * Set value to newValue only if it is currently assumedValue. |
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66 * @return true if successful |
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67 **/ |
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68 public boolean commit(Object assumedValue, Object newValue) { |
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69 synchronized(lock_) { |
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70 boolean success = (assumedValue == value_); |
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71 if (success) value_ = newValue; |
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72 return success; |
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73 } |
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74 } |
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75 |
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76 |
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77 /** |
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78 * Atomically swap values with another SynchronizedRef. |
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79 * Uses identityHashCode to avoid deadlock when |
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80 * two SynchronizedRefs attempt to simultaneously swap with each other. |
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81 * (Note: Ordering via identyHashCode is not strictly guaranteed |
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82 * by the language specification to return unique, orderable |
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83 * values, but in practice JVMs rely on them being unique.) |
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84 * @return the new value |
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85 **/ |
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86 |
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87 public Object swap(SynchronizedRef other) { |
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88 if (other == this) return get(); |
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89 SynchronizedRef fst = this; |
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90 SynchronizedRef snd = other; |
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91 if (System.identityHashCode(fst) > System.identityHashCode(snd)) { |
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92 fst = other; |
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93 snd = this; |
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94 } |
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95 synchronized(fst.lock_) { |
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96 synchronized(snd.lock_) { |
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97 fst.set(snd.set(fst.get())); |
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98 return get(); |
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99 } |
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100 } |
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101 } |
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102 |
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103 |
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104 } |