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| 1 : | dl | 1.1 | <!DOCTYPE HTML PUBLIC "-//IETF//DTD HTML//EN"> |
| 2 : | <html> <head> | ||
| 3 : | <title>Concurrency Utilities</title> | ||
| 4 : | </head> | ||
| 5 : | |||
| 6 : | <body> | ||
| 7 : | |||
| 8 : | dl | 1.3 | <p> Utility classes commonly useful in concurrent programming. This |
| 9 : | package includes a few small standardized extensible frameworks, as | ||
| 10 : | well as some classes that provide useful functionality and are | ||
| 11 : | otherwise tedious or difficult to implement. Here are brief | ||
| 12 : | descriptions of the main components. See also the <tt>locks</tt> and | ||
| 13 : | <tt>atomic</tt> packages. | ||
| 14 : | dl | 1.1 | |
| 15 : | <h2>Executors</h2> | ||
| 16 : | |||
| 17 : | dl | 1.12 | <b>Interfaces.</b> {@link java.util.concurrent.Executor} is a simple |
| 18 : | standardized interface for defining custom thread-like subsystems, | ||
| 19 : | including thread pools, asynchronous IO, and lightweight task | ||
| 20 : | frameworks. Depending on which concrete Executor class is being used, | ||
| 21 : | tasks may execute in a newly created thread, an existing | ||
| 22 : | task-execution thread, or the thread calling <tt>execute()</tt>, and | ||
| 23 : | may execute sequentially or concurrently. {@link | ||
| 24 : | java.util.concurrent.ExecutorService} provides a more complete | ||
| 25 : | asynchronous task execution framework. An ExecutorService manages | ||
| 26 : | dl | 1.14 | queuing and scheduling of tasks, and allows controlled shutdown. The |
| 27 : | dl | 1.12 | {@link java.util.concurrent.ScheduledExecutorService} subinterface |
| 28 : | dl | 1.19 | and associated interfaces add support for delayed and periodic task execution. |
| 29 : | dl | 1.12 | ExecutorServices provide methods arranging asynchronous execution of |
| 30 : | any function expressed as {@link java.util.concurrent.Callable}, the | ||
| 31 : | result-bearing analog of {@link java.lang.Runnable}. A {@link | ||
| 32 : | java.util.concurrent.Future} returns the results of a function, allows | ||
| 33 : | dl | 1.16 | determination of whether execution has completed, and provides a means to |
| 34 : | jsr166 | 1.20 | cancel execution. A {@link java.util.concurrent.RunnableFuture} is |
| 35 : | dl | 1.19 | a Future that possesses a <tt>run</tt> method that upon execution, |
| 36 : | sets its results. | ||
| 37 : | dl | 1.12 | |
| 38 : | <p> | ||
| 39 : | |||
| 40 : | dl | 1.16 | <b>Implementations.</b> Classes {@link |
| 41 : | dl | 1.12 | java.util.concurrent.ThreadPoolExecutor} and {@link |
| 42 : | dl | 1.16 | java.util.concurrent.ScheduledThreadPoolExecutor} provide tunable, |
| 43 : | flexible thread pools. The {@link java.util.concurrent.Executors} | ||
| 44 : | class provides factory methods for the most common kinds and | ||
| 45 : | configurations of Executors, as well as a few utility methods for | ||
| 46 : | using them. Other utilities based on Executors include the concrete | ||
| 47 : | class {@link java.util.concurrent.FutureTask} providing a common | ||
| 48 : | extensible implementation of Futures, and {@link | ||
| 49 : | dl | 1.12 | java.util.concurrent.ExecutorCompletionService}, that assists in |
| 50 : | coordinating the processing of groups of asynchronous tasks. | ||
| 51 : | dl | 1.1 | |
| 52 : | <h2>Queues</h2> | ||
| 53 : | |||
| 54 : | dl | 1.6 | The java.util.concurrent {@link |
| 55 : | dl | 1.1 | java.util.concurrent.ConcurrentLinkedQueue} class supplies an |
| 56 : | dl | 1.6 | efficient scalable thread-safe non-blocking FIFO queue. Five |
| 57 : | implementations in java.util.concurrent support the extended {@link | ||
| 58 : | java.util.concurrent.BlockingQueue} interface, that defines blocking | ||
| 59 : | versions of put and take: {@link | ||
| 60 : | dl | 1.1 | java.util.concurrent.LinkedBlockingQueue}, {@link |
| 61 : | java.util.concurrent.ArrayBlockingQueue}, {@link | ||
| 62 : | java.util.concurrent.SynchronousQueue}, {@link | ||
| 63 : | java.util.concurrent.PriorityBlockingQueue}, and {@link | ||
| 64 : | dl | 1.2 | java.util.concurrent.DelayQueue}. The different classes cover the most |
| 65 : | common usage contexts for producer-consumer, messaging, parallel | ||
| 66 : | dl | 1.17 | tasking, and related concurrent designs. The {@link |
| 67 : | java.util.concurrent.BlockingDeque} interface extends | ||
| 68 : | <tt>BlockingQueue</tt> to support both FIFO and LIFO (stack-based) | ||
| 69 : | operations. Class {@link java.util.concurrent.LinkedBlockingDeque} | ||
| 70 : | provides an implementation. | ||
| 71 : | dl | 1.1 | |
| 72 : | |||
| 73 : | <h2>Timing</h2> | ||
| 74 : | |||
| 75 : | The {@link java.util.concurrent.TimeUnit} class provides multiple | ||
| 76 : | dl | 1.2 | granularities (including nanoseconds) for specifying and controlling |
| 77 : | dl | 1.9 | time-out based operations. Most classes in the package contain |
| 78 : | operations based on time-outs in addition to indefinite waits. In all | ||
| 79 : | cases that time-outs are used, the time-out specifies the minimum time | ||
| 80 : | that the method should wait before indicating that it | ||
| 81 : | timed-out. Implementations make a "best effort" to detect | ||
| 82 : | time-outs as soon as possible after they occur. However, an indefinite | ||
| 83 : | amount of time may elapse between a time-out being detected and a | ||
| 84 : | dl | 1.18 | thread actually executing again after that time-out. All methods |
| 85 : | that accept timeout parameters treat values less than or equal to | ||
| 86 : | zero to mean not to wait at all. To wait "forever", you can use | ||
| 87 : | a value of <tt>Long.MAX_VALUE</tt>. | ||
| 88 : | dholmes | 1.4 | |
| 89 : | dl | 1.1 | <h2>Synchronizers</h2> |
| 90 : | |||
| 91 : | dl | 1.13 | Four classes aid common special-purpose synchronization idioms. |
| 92 : | dl | 1.10 | {@link java.util.concurrent.Semaphore} is a classic concurrency tool. |
| 93 : | dl | 1.16 | {@link java.util.concurrent.CountDownLatch} is a very simple yet very |
| 94 : | dl | 1.11 | common utility for blocking until a given number of signals, events, |
| 95 : | or conditions hold. A {@link java.util.concurrent.CyclicBarrier} is a | ||
| 96 : | dl | 1.13 | resettable multiway synchronization point useful in some styles of |
| 97 : | dl | 1.11 | parallel programming. An {@link java.util.concurrent.Exchanger} allows |
| 98 : | dl | 1.13 | two threads to exchange objects at a rendezvous point, and is useful |
| 99 : | in several pipeline designs. | ||
| 100 : | dl | 1.1 | |
| 101 : | <h2>Concurrent Collections</h2> | ||
| 102 : | |||
| 103 : | dl | 1.17 | Besides Queues, this package supplies Collection implementations |
| 104 : | designed for use in multithreaded contexts: | ||
| 105 : | {@link java.util.concurrent.ConcurrentHashMap}, | ||
| 106 : | {@link java.util.concurrent.ConcurrentSkipListMap}, | ||
| 107 : | {@link java.util.concurrent.ConcurrentSkipListSet}, | ||
| 108 : | {@link java.util.concurrent.CopyOnWriteArrayList}, and | ||
| 109 : | {@link java.util.concurrent.CopyOnWriteArraySet}. | ||
| 110 : | When many threads are expected to access a given collection, | ||
| 111 : | a <tt>ConcurrentHashMap</tt> is normally preferable to | ||
| 112 : | a synchronized <tt>HashMap</tt>, and a | ||
| 113 : | <tt>ConcurrentSkipListMap</tt> is normally preferable | ||
| 114 : | to a synchronized <tt>TreeMap</tt>. A | ||
| 115 : | <tt>CopyOnWriteArrayList</tt> is preferable to | ||
| 116 : | a synchronized <tt>ArrayList</tt> when the expected number of reads | ||
| 117 : | and traversals greatly outnumber the number of updates to a list. | ||
| 118 : | dl | 1.1 | |
| 119 : | dl | 1.6 | <p>The "Concurrent" prefix used with some classes in this package is a |
| 120 : | shorthand indicating several differences from similar "synchronized" | ||
| 121 : | dl | 1.1 | classes. For example <tt>java.util.Hashtable</tt> and |
| 122 : | <tt>Collections.synchronizedMap(new HashMap())</tt> are | ||
| 123 : | dl | 1.6 | synchronized. But {@link java.util.concurrent.ConcurrentHashMap} is |
| 124 : | "concurrent". A concurrent collection is thread-safe, but not | ||
| 125 : | governed by a single exclusion lock. In the particular case of | ||
| 126 : | ConcurrentHashMap, it safely permits any number of concurrent reads as | ||
| 127 : | dl | 1.9 | well as a tunable number of concurrent writes. "Synchronized" classes |
| 128 : | can be useful when you need to prevent all access to a collection via | ||
| 129 : | a single lock, at the expense of poorer scalability. In other cases in | ||
| 130 : | which multiple threads are expected to access a common collection, | ||
| 131 : | "concurrent" versions are normally preferable. And unsynchronized | ||
| 132 : | collections are preferable when either collections are unshared, or | ||
| 133 : | are accessible only when holding other locks. | ||
| 134 : | dl | 1.1 | |
| 135 : | <p> Most concurrent Collection implementations (including most Queues) | ||
| 136 : | also differ from the usual java.util conventions in that their Iterators | ||
| 137 : | provide <em>weakly consistent</em> rather than fast-fail traversal. A | ||
| 138 : | weakly consistent iterator is thread-safe, but does not necessarily | ||
| 139 : | freeze the collection while iterating, so it may (or may not) reflect | ||
| 140 : | any updates since the iterator was created. | ||
| 141 : | |||
| 142 : | dl | 1.21 | <h2> Memory Consistency Properties </h2> |
| 143 : | |||
| 144 : | <a href="http://java.sun.com/docs/books/jls/third_edition/html/memory.html"> | ||
| 145 : | Chapter 17 of the Java Language Specification</a> defines the | ||
| 146 : | <i>happens-before</i> relation on memory operations such as reads and | ||
| 147 : | writes of shared variables. The results of a write by one thread are | ||
| 148 : | guaranteed to be visible to a read by another thread only if the write | ||
| 149 : | operation <i>happens-before</i> the read operation. The | ||
| 150 : | <tt>synchronized</tt> and <tt>volatile</tt> constructs, as well as the | ||
| 151 : | <tt>Thread.start()</tt> and <tt>Thread.join()</tt> methods, can form | ||
| 152 : | <i>happens-before</i> relationships. In particular: | ||
| 153 : | |||
| 154 : | <ul> | ||
| 155 : | <li>Each action in a thread happens before every action in that | ||
| 156 : | thread that comes later in the program's order. | ||
| 157 : | |||
| 158 : | <li>An unlock (<tt>synchronized</tt> block or method exit) on a | ||
| 159 : | monitor happens before every subsequent lock (<tt>synchronized</tt> | ||
| 160 : | dl | 1.23 | block or method entry) of that same monitor. And because |
| 161 : | the <i>happens-before</i> relation is transitive, all actions | ||
| 162 : | of a thread prior to unlocking happen before all actions | ||
| 163 : | subsequent to any thread locking that monitor. | ||
| 164 : | dl | 1.21 | |
| 165 : | <li>A write to a <tt>volatile</tt> field happens before every | ||
| 166 : | dl | 1.23 | subsequent read of that same field. Writes and reads of |
| 167 : | <tt>volatile</tt> fields have similar memory consistency effects | ||
| 168 : | as entering and exiting monitors, but do <em>not</em> entail | ||
| 169 : | mutual exclusion locking. | ||
| 170 : | dl | 1.21 | |
| 171 : | jsr166 | 1.22 | <li>A call to <tt>start</tt> on a thread happens before any action in the |
| 172 : | dl | 1.21 | started thread. |
| 173 : | |||
| 174 : | <li>All actions in a thread happen before any other thread | ||
| 175 : | successfully returns from a <tt>join</tt> on that thread. | ||
| 176 : | |||
| 177 : | </ul> | ||
| 178 : | |||
| 179 : | dl | 1.23 | |
| 180 : | dl | 1.21 | The methods of all classes in <tt>java.util.concurrent</tt> and its |
| 181 : | subpackages extend these guarantees to higher-level | ||
| 182 : | synchronization. In particular: | ||
| 183 : | |||
| 184 : | <ul> | ||
| 185 : | |||
| 186 : | <li>State changes to any object made prior to placement into any | ||
| 187 : | concurrent collection happen before this element is accessed via or | ||
| 188 : | removed from that collection. | ||
| 189 : | |||
| 190 : | <li>State changes to a <tt>Runnable</tt> object made prior to | ||
| 191 : | submission to an <tt>Executor</tt> happen before its execution. And | ||
| 192 : | similarly for a <tt>Callable</tt> object submitted to an | ||
| 193 : | <tt>ExecutorService</tt>. | ||
| 194 : | |||
| 195 : | <li>State changes to a <tt>Future</tt> made prior to it becoming | ||
| 196 : | available happen before access via <tt>Future.get()</tt>. | ||
| 197 : | |||
| 198 : | <li>Actions prior to "releasing" synchronizer methods such as | ||
| 199 : | <tt>Lock.unlock</tt>, <tt>Semaphore.release</tt>, | ||
| 200 : | <tt>CountDownLatch.countDown<tt> and <tt>Condition.signal</tt> | ||
| 201 : | happen before actions subsequent to "acquiring" methods such as | ||
| 202 : | <tt>Lock.lock</tt>, <tt>Semaphore.acquire</tt>, and | ||
| 203 : | <tt>CountDownLatch.await</tt> on the same synchronizer object. | ||
| 204 : | |||
| 205 : | <li>Actions prior to symmetric synchronizer methods such as | ||
| 206 : | <tt>CyclicBarrier.await</tt> and <tt>Exchanger.exchange</tt> happen | ||
| 207 : | before those subsequent to the matching actions in other threads. | ||
| 208 : | |||
| 209 : | </ul> | ||
| 210 : | |||
| 211 : | dl | 1.15 | @since 1.5 |
| 212 : | |||
| 213 : | dl | 1.1 | </body> </html> |
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