1/* 2 * Written by Doug Lea with assistance from members of JCP JSR-166 3 * Expert Group and released to the public domain, as explained at 4 * http://creativecommons.org/licenses/publicdomain 5 */ 6 7/** 8 * 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 13 * {@link java.util.concurrent.locks} and 14 * {@link java.util.concurrent.atomic} packages. 15 * 16 * <h2>Executors</h2> 17 * 18 * <b>Interfaces.</b> 19 * 20 * {@link java.util.concurrent.Executor} is a simple standardized 21 * interface for defining custom thread-like subsystems, including 22 * thread pools, asynchronous IO, and lightweight task frameworks. 23 * Depending on which concrete Executor class is being used, tasks may 24 * execute in a newly created thread, an existing task-execution thread, 25 * or the thread calling {@link java.util.concurrent.Executor#execute 26 * execute}, and may execute sequentially or concurrently. 27 * 28 * {@link java.util.concurrent.ExecutorService} provides a more 29 * complete asynchronous task execution framework. An 30 * ExecutorService manages queuing and scheduling of tasks, 31 * and allows controlled shutdown. 32 * 33 * The {@link java.util.concurrent.ScheduledExecutorService} 34 * subinterface and associated interfaces add support for 35 * delayed and periodic task execution. ExecutorServices 36 * provide methods arranging asynchronous execution of any 37 * function expressed as {@link java.util.concurrent.Callable}, 38 * the result-bearing analog of {@link java.lang.Runnable}. 39 * 40 * A {@link java.util.concurrent.Future} returns the results of 41 * a function, allows determination of whether execution has 42 * completed, and provides a means to cancel execution. 43 * 44 * A {@link java.util.concurrent.RunnableFuture} is a {@code Future} 45 * that possesses a {@code run} method that upon execution, 46 * sets its results. 47 * 48 * <p> 49 * 50 * <b>Implementations.</b> 51 * 52 * Classes {@link java.util.concurrent.ThreadPoolExecutor} and 53 * {@link java.util.concurrent.ScheduledThreadPoolExecutor} 54 * provide tunable, flexible thread pools. 55 * 56 * The {@link java.util.concurrent.Executors} class provides 57 * factory methods for the most common kinds and configurations 58 * of Executors, as well as a few utility methods for using 59 * them. Other utilities based on {@code Executors} include the 60 * concrete class {@link java.util.concurrent.FutureTask} 61 * providing a common extensible implementation of Futures, and 62 * {@link java.util.concurrent.ExecutorCompletionService}, that 63 * assists in coordinating the processing of groups of 64 * asynchronous tasks. 65 * 66 * <h2>Queues</h2> 67 * 68 * The {@link java.util.concurrent.ConcurrentLinkedQueue} class 69 * supplies an efficient scalable thread-safe non-blocking FIFO 70 * queue. 71 * 72 * <p>Five implementations in {@code java.util.concurrent} support 73 * the extended {@link java.util.concurrent.BlockingQueue} 74 * interface, that defines blocking versions of put and take: 75 * {@link java.util.concurrent.LinkedBlockingQueue}, 76 * {@link java.util.concurrent.ArrayBlockingQueue}, 77 * {@link java.util.concurrent.SynchronousQueue}, 78 * {@link java.util.concurrent.PriorityBlockingQueue}, and 79 * {@link java.util.concurrent.DelayQueue}. 80 * The different classes cover the most common usage contexts 81 * for producer-consumer, messaging, parallel tasking, and 82 * related concurrent designs. 83 * 84 * <p>The {@link java.util.concurrent.BlockingDeque} interface 85 * extends {@code BlockingQueue} to support both FIFO and LIFO 86 * (stack-based) operations. 87 * Class {@link java.util.concurrent.LinkedBlockingDeque} 88 * provides an implementation. 89 * 90 * <h2>Timing</h2> 91 * 92 * The {@link java.util.concurrent.TimeUnit} class provides 93 * multiple granularities (including nanoseconds) for 94 * specifying and controlling time-out based operations. Most 95 * classes in the package contain operations based on time-outs 96 * in addition to indefinite waits. In all cases that 97 * time-outs are used, the time-out specifies the minimum time 98 * that the method should wait before indicating that it 99 * timed-out. Implementations make a "best effort" 100 * to detect time-outs as soon as possible after they occur. 101 * However, an indefinite amount of time may elapse between a 102 * time-out being detected and a thread actually executing 103 * again after that time-out. All methods that accept timeout 104 * parameters treat values less than or equal to zero to mean 105 * not to wait at all. To wait "forever", you can use a value 106 * of {@code Long.MAX_VALUE}. 107 * 108 * <h2>Synchronizers</h2> 109 * 110 * Five classes aid common special-purpose synchronization idioms. 111 * <ul> 112 * 113 * <li>{@link java.util.concurrent.Semaphore} is a classic concurrency tool. 114 * 115 * <li>{@link java.util.concurrent.CountDownLatch} is a very simple yet 116 * very common utility for blocking until a given number of signals, 117 * events, or conditions hold. 118 * 119 * <li>A {@link java.util.concurrent.CyclicBarrier} is a resettable 120 * multiway synchronization point useful in some styles of parallel 121 * programming. 122 * 123 * <li>An {@link java.util.concurrent.Exchanger} allows two threads to 124 * exchange objects at a rendezvous point, and is useful in several 125 * pipeline designs. 126 * 127 * </ul> 128 * 129 * <h2>Concurrent Collections</h2> 130 * 131 * Besides Queues, this package supplies Collection implementations 132 * designed for use in multithreaded contexts: 133 * {@link java.util.concurrent.ConcurrentHashMap}, 134 * {@link java.util.concurrent.ConcurrentSkipListMap}, 135 * {@link java.util.concurrent.ConcurrentSkipListSet}, 136 * {@link java.util.concurrent.CopyOnWriteArrayList}, and 137 * {@link java.util.concurrent.CopyOnWriteArraySet}. 138 * When many threads are expected to access a given collection, a 139 * {@code ConcurrentHashMap} is normally preferable to a synchronized 140 * {@code HashMap}, and a {@code ConcurrentSkipListMap} is normally 141 * preferable to a synchronized {@code TreeMap}. 142 * A {@code CopyOnWriteArrayList} is preferable to a synchronized 143 * {@code ArrayList} when the expected number of reads and traversals 144 * greatly outnumber the number of updates to a list. 145 146 * <p>The "Concurrent" prefix used with some classes in this package 147 * is a shorthand indicating several differences from similar 148 * "synchronized" classes. For example {@code java.util.Hashtable} and 149 * {@code Collections.synchronizedMap(new HashMap())} are 150 * synchronized. But {@link 151 * java.util.concurrent.ConcurrentHashMap} is "concurrent". A 152 * concurrent collection is thread-safe, but not governed by a 153 * single exclusion lock. In the particular case of 154 * ConcurrentHashMap, it safely permits any number of 155 * concurrent reads as well as a tunable number of concurrent 156 * writes. "Synchronized" classes can be useful when you need 157 * to prevent all access to a collection via a single lock, at 158 * the expense of poorer scalability. In other cases in which 159 * multiple threads are expected to access a common collection, 160 * "concurrent" versions are normally preferable. And 161 * unsynchronized collections are preferable when either 162 * collections are unshared, or are accessible only when 163 * holding other locks. 164 * 165 * <p>Most concurrent Collection implementations (including most 166 * Queues) also differ from the usual java.util conventions in that 167 * their Iterators provide <em>weakly consistent</em> rather than 168 * fast-fail traversal. A weakly consistent iterator is thread-safe, 169 * but does not necessarily freeze the collection while iterating, so 170 * it may (or may not) reflect any updates since the iterator was 171 * created. 172 * 173 * <h2><a name="MemoryVisibility">Memory Consistency Properties</a></h2> 174 * 175 * <a href="http://java.sun.com/docs/books/jls/third_edition/html/memory.html"> 176 * Chapter 17 of the Java Language Specification</a> defines the 177 * <i>happens-before</i> relation on memory operations such as reads and 178 * writes of shared variables. The results of a write by one thread are 179 * guaranteed to be visible to a read by another thread only if the write 180 * operation <i>happens-before</i> the read operation. The 181 * {@code synchronized} and {@code volatile} constructs, as well as the 182 * {@code Thread.start()} and {@code Thread.join()} methods, can form 183 * <i>happens-before</i> relationships. In particular: 184 * 185 * <ul> 186 * <li>Each action in a thread <i>happens-before</i> every action in that 187 * thread that comes later in the program's order. 188 * 189 * <li>An unlock ({@code synchronized} block or method exit) of a 190 * monitor <i>happens-before</i> every subsequent lock ({@code synchronized} 191 * block or method entry) of that same monitor. And because 192 * the <i>happens-before</i> relation is transitive, all actions 193 * of a thread prior to unlocking <i>happen-before</i> all actions 194 * subsequent to any thread locking that monitor. 195 * 196 * <li>A write to a {@code volatile} field <i>happens-before</i> every 197 * subsequent read of that same field. Writes and reads of 198 * {@code volatile} fields have similar memory consistency effects 199 * as entering and exiting monitors, but do <em>not</em> entail 200 * mutual exclusion locking. 201 * 202 * <li>A call to {@code start} on a thread <i>happens-before</i> any 203 * action in the started thread. 204 * 205 * <li>All actions in a thread <i>happen-before</i> any other thread 206 * successfully returns from a {@code join} on that thread. 207 * 208 * </ul> 209 * 210 * 211 * The methods of all classes in {@code java.util.concurrent} and its 212 * subpackages extend these guarantees to higher-level 213 * synchronization. In particular: 214 * 215 * <ul> 216 * 217 * <li>Actions in a thread prior to placing an object into any concurrent 218 * collection <i>happen-before</i> actions subsequent to the access or 219 * removal of that element from the collection in another thread. 220 * 221 * <li>Actions in a thread prior to the submission of a {@code Runnable} 222 * to an {@code Executor} <i>happen-before</i> its execution begins. 223 * Similarly for {@code Callables} submitted to an {@code ExecutorService}. 224 * 225 * <li>Actions taken by the asynchronous computation represented by a 226 * {@code Future} <i>happen-before</i> actions subsequent to the 227 * retrieval of the result via {@code Future.get()} in another thread. 228 * 229 * <li>Actions prior to "releasing" synchronizer methods such as 230 * {@code Lock.unlock}, {@code Semaphore.release}, and 231 * {@code CountDownLatch.countDown} <i>happen-before</i> actions 232 * subsequent to a successful "acquiring" method such as 233 * {@code Lock.lock}, {@code Semaphore.acquire}, 234 * {@code Condition.await}, and {@code CountDownLatch.await} on the 235 * same synchronizer object in another thread. 236 * 237 * <li>For each pair of threads that successfully exchange objects via 238 * an {@code Exchanger}, actions prior to the {@code exchange()} 239 * in each thread <i>happen-before</i> those subsequent to the 240 * corresponding {@code exchange()} in another thread. 241 * 242 * <li>Actions prior to calling {@code CyclicBarrier.await} 243 * <i>happen-before</i> actions performed by the barrier action, and 244 * actions performed by the barrier action <i>happen-before</i> actions 245 * subsequent to a successful return from the corresponding {@code await} 246 * in other threads. 247 * 248 * </ul> 249 * 250 * @since 1.5 251 */ 252package java.util.concurrent; 253