Mercurial > hg > blitz_stable
annotate src/EDU/oswego/cs/dl/util/concurrent/ReentrantWriterPreferenceReadWriteLock.java @ 32:243c74d599bf
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author | Dan Creswell <dan.creswell@gmail.com> |
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date | Sat, 12 Jun 2010 10:41:47 +0100 |
parents | 3dc0c5604566 |
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0
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1 /* |
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2 File: ReentrantWriterPreferenceReadWriteLock.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 26aug1998 dl Create public version |
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12 7sep2000 dl Readers are now also reentrant |
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13 19jan2001 dl Allow read->write upgrades if the only reader |
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14 10dec2002 dl Throw IllegalStateException on extra release |
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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 import java.util.*; |
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19 |
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20 /** |
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21 * A writer-preference ReadWriteLock that allows both readers and |
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22 * writers to reacquire |
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23 * read or write locks in the style of a ReentrantLock. |
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24 * Readers are not allowed until all write locks held by |
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25 * the writing thread have been released. |
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26 * Among other applications, reentrancy can be useful when |
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27 * write locks are held during calls or callbacks to methods that perform |
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28 * reads under read locks. |
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29 * <p> |
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30 * <b>Sample usage</b>. Here is a code sketch showing how to exploit |
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31 * reentrancy to perform lock downgrading after updating a cache: |
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32 * <pre> |
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33 * class CachedData { |
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34 * Object data; |
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35 * volatile boolean cacheValid; |
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36 * ReentrantWriterPreferenceReadWriteLock rwl = ... |
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37 * |
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38 * void processCachedData() { |
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39 * rwl.readLock().acquire(); |
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40 * if (!cacheValid) { |
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41 * |
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42 * // upgrade lock: |
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43 * rwl.readLock().release(); // must release first to obtain writelock |
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44 * rwl.writeLock().acquire(); |
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45 * if (!cacheValid) { // recheck |
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46 * data = ... |
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47 * cacheValid = true; |
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48 * } |
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49 * // downgrade lock |
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50 * rwl.readLock().acquire(); // reacquire read without giving up lock |
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51 * rwl.writeLock().release(); // release write, still hold read |
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52 * } |
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53 * |
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54 * use(data); |
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55 * rwl.readLock().release(); |
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56 * } |
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57 * } |
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58 * </pre> |
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59 * |
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60 * |
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61 * <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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62 * @see ReentrantLock |
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63 **/ |
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64 |
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65 public class ReentrantWriterPreferenceReadWriteLock extends WriterPreferenceReadWriteLock { |
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66 |
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67 /** Number of acquires on write lock by activeWriter_ thread **/ |
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68 protected long writeHolds_ = 0; |
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69 |
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70 /** Number of acquires on read lock by any reader thread **/ |
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71 protected HashMap readers_ = new HashMap(); |
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72 |
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73 /** cache/reuse the special Integer value one to speed up readlocks **/ |
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74 protected static final Integer IONE = new Integer(1); |
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75 |
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76 |
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77 protected boolean allowReader() { |
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78 return (activeWriter_ == null && waitingWriters_ == 0) || |
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79 activeWriter_ == Thread.currentThread(); |
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80 } |
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81 |
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82 protected synchronized boolean startRead() { |
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83 Thread t = Thread.currentThread(); |
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84 Object c = readers_.get(t); |
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85 if (c != null) { // already held -- just increment hold count |
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86 readers_.put(t, new Integer(((Integer)(c)).intValue()+1)); |
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87 ++activeReaders_; |
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88 return true; |
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89 } |
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90 else if (allowReader()) { |
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91 readers_.put(t, IONE); |
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92 ++activeReaders_; |
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93 return true; |
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94 } |
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95 else |
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96 return false; |
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97 } |
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98 |
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99 protected synchronized boolean startWrite() { |
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100 if (activeWriter_ == Thread.currentThread()) { // already held; re-acquire |
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101 ++writeHolds_; |
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102 return true; |
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103 } |
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104 else if (writeHolds_ == 0) { |
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105 if (activeReaders_ == 0 || |
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106 (readers_.size() == 1 && |
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107 readers_.get(Thread.currentThread()) != null)) { |
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108 activeWriter_ = Thread.currentThread(); |
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109 writeHolds_ = 1; |
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110 return true; |
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111 } |
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112 else |
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113 return false; |
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114 } |
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115 else |
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116 return false; |
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117 } |
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118 |
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119 |
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120 protected synchronized Signaller endRead() { |
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121 Thread t = Thread.currentThread(); |
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122 Object c = readers_.get(t); |
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123 if (c == null) |
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124 throw new IllegalStateException(); |
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125 --activeReaders_; |
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126 if (c != IONE) { // more than one hold; decrement count |
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127 int h = ((Integer)(c)).intValue()-1; |
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128 Integer ih = (h == 1)? IONE : new Integer(h); |
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129 readers_.put(t, ih); |
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130 return null; |
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131 } |
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132 else { |
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133 readers_.remove(t); |
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134 |
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135 if (writeHolds_ > 0) // a write lock is still held by current thread |
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136 return null; |
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137 else if (activeReaders_ == 0 && waitingWriters_ > 0) |
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138 return writerLock_; |
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139 else |
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140 return null; |
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141 } |
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142 } |
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143 |
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144 protected synchronized Signaller endWrite() { |
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145 --writeHolds_; |
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146 if (writeHolds_ > 0) // still being held |
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147 return null; |
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148 else { |
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149 activeWriter_ = null; |
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150 if (waitingReaders_ > 0 && allowReader()) |
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151 return readerLock_; |
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152 else if (waitingWriters_ > 0) |
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153 return writerLock_; |
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154 else |
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155 return null; |
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156 } |
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157 } |
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158 |
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159 } |
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160 |