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| 1 : | jsr166 | 1.1 | <!DOCTYPE HTML PUBLIC "-//IETF//DTD HTML//EN"> |
| 2 : | <html> | ||
| 3 : | <head> | ||
| 4 : | <title>JSR 166 Snapshot Introduction.</title> | ||
| 5 : | </head> | ||
| 6 : | |||
| 7 : | <body bgcolor="#ffffee" vlink="#0000aa" link="#cc0000"> | ||
| 8 : | <h1>JSR 166 Snapshot Introduction.</h1> | ||
| 9 : | |||
| 10 : | by <a href="http://gee.cs.oswego.edu/dl">Doug Lea</a> | ||
| 11 : | <p> | ||
| 12 : | |||
| 13 : | To join a mailing list discussing this JSR, go to: | ||
| 14 : | <A HREF="http://altair.cs.oswego.edu/mailman/listinfo/concurrency-interest"> http://altair.cs.oswego.edu/mailman/listinfo/concurrency-interest</A> . | ||
| 15 : | |||
| 16 : | <p> | ||
| 17 : | Package java.util.concurrent contains utility classes that are | ||
| 18 : | commonly useful in concurrent programming. Like package java.util, it | ||
| 19 : | includes a few small standardized extensible frameworks, as well as | ||
| 20 : | some classes that provide useful functionality and are otherwise | ||
| 21 : | tedious or difficult to implement. In this JSR, we have been | ||
| 22 : | conservative in selecting only those APIs and implementations that are | ||
| 23 : | useful enough to encourage nearly all concurrent programmers to use | ||
| 24 : | routinely. JSR 166 also includes a few changes and additions in | ||
| 25 : | packages outside of java.util.concurrent: java.lang, to address timing | ||
| 26 : | and uncaught exceptions, and java.util to better integrate queues, and | ||
| 27 : | to make Timers conform to new frameworks. The API covers: | ||
| 28 : | |||
| 29 : | <ul> | ||
| 30 : | <li> Queues | ||
| 31 : | <li> Executors | ||
| 32 : | <li> Locks | ||
| 33 : | <li> Condition variables | ||
| 34 : | <li> Atomic variables | ||
| 35 : | <li> Timing | ||
| 36 : | <li> Barriers | ||
| 37 : | <li> Concurrent Collections | ||
| 38 : | <li> Uncaught Exception Handlers | ||
| 39 : | </ul> | ||
| 40 : | |||
| 41 : | |||
| 42 : | The main rationale for JSR 166 is that threading primitives, such as | ||
| 43 : | synchronized blocks, Object.wait and Object.notify, are insufficient | ||
| 44 : | for many programming tasks. Currently, developers can use only the | ||
| 45 : | concurrency control constructs provided in the Java language | ||
| 46 : | itself. These are too low level for some applications, and are | ||
| 47 : | incomplete for others. As a result, application programmers are often | ||
| 48 : | forced to implement their own concurrency facilities, resulting in | ||
| 49 : | enormous duplication of effort creating facilities that are | ||
| 50 : | notoriously hard to get right and even harder to optimize. Offering a | ||
| 51 : | standard set of concurrency utilities will ease the task of writing a | ||
| 52 : | wide variety of multithreaded applications and generally improve the | ||
| 53 : | quality of the applications that use them. | ||
| 54 : | |||
| 55 : | <p> | ||
| 56 : | Here are brief descriptions and rationales of the main components. | ||
| 57 : | For details see the javadocs at <a | ||
| 58 : | href="http://gee.cs.oswego.edu/dl/concurrent/index.html">http://gee.cs.oswego.edu/dl/concurrent/index.html</a> | ||
| 59 : | |||
| 60 : | |||
| 61 : | <h2>Queues</h2> | ||
| 62 : | |||
| 63 : | A basic (nonblocking) Queue interface that is compatatible with | ||
| 64 : | java.util.Collections will be introduced into java.util. Also, | ||
| 65 : | although it is at the borders of being in scope of JSR-166, | ||
| 66 : | java.util.LinkedList will be adapted to support Queue, and | ||
| 67 : | a new non-thread-safe java.util.HeapPriorityQueue will be added. | ||
| 68 : | |||
| 69 : | <p> | ||
| 70 : | Four implementations in java.util.concurrent support the extended | ||
| 71 : | BlockingQueue interface, that defines blocking versions of put and | ||
| 72 : | take: LinkedBlockingQueue, ArrayBlockingQueue, SynchronousQueue, and | ||
| 73 : | PriorityBlockingQueue. Additionally, java.util.concurrent.LinkedQueue | ||
| 74 : | supplies an efficient thread-safe non-blocking queue. | ||
| 75 : | <p> | ||
| 76 : | Since the target release is JDK1.5, and generics are slated to be in | ||
| 77 : | 1.5, Queues should be parametrized on element type. (Also some others | ||
| 78 : | below.) We are ignoring this for now. | ||
| 79 : | |||
| 80 : | |||
| 81 : | <h2>Executors</h2> | ||
| 82 : | |||
| 83 : | Executors provide a simple standardized interface for defining custom | ||
| 84 : | thread-like subsystems, including thread pools, asynch-IO, and | ||
| 85 : | lightweight task frameworks. Executors also standardize ways of | ||
| 86 : | calling threads that compute functions returning results, via | ||
| 87 : | Futures. This is supported in part by defining java.lang.Callable, the | ||
| 88 : | argument/result analog of Runnable. | ||
| 89 : | |||
| 90 : | <p> While the Executor framework is intended to be extensible (so | ||
| 91 : | includes for example, AbstractExecutor that simplifies construction of | ||
| 92 : | new implementations), the most commonly used Executor will be | ||
| 93 : | ThreadExecutor, which can be configured to act as all sorts of thread | ||
| 94 : | pools, background threads, etc. The class is designed to be general | ||
| 95 : | enough to suffice for the vast majority of usages, even sophisticated | ||
| 96 : | ones, yet also includes methods and functionality that simplify | ||
| 97 : | routine usage. | ||
| 98 : | |||
| 99 : | <p> | ||
| 100 : | A few methods will also be added to the java.util.Timer to support | ||
| 101 : | Futures, and address other requests for enhancement. | ||
| 102 : | |||
| 103 : | <h2>Locks</h2> | ||
| 104 : | |||
| 105 : | The Lock interface supports locking disciplines that differ in | ||
| 106 : | semantics (reentrant, semaphore-based, etc), and that can be used in | ||
| 107 : | non-block-structured contexts including hand-over-hand and lock | ||
| 108 : | reordering algorithms. This flexibility comes at the price of more | ||
| 109 : | awkward syntax. Implementations include Semaphore, ReentrantMutex | ||
| 110 : | FIFOSemaphore, and CountDownLatch. | ||
| 111 : | |||
| 112 : | <p> | ||
| 113 : | The Locks class additionally supports trylock-designs using builtin | ||
| 114 : | locks without needing to use Lock classes. This requires adding new | ||
| 115 : | capabilities to builtin locks inside JVMs. | ||
| 116 : | |||
| 117 : | <p> | ||
| 118 : | A ReadWriteLock interface similarly defines locks that may be shared | ||
| 119 : | among readers but are exclusive to writers. For this release, only a | ||
| 120 : | single implementation, ReentrantReadWriteLock, is planned, since it | ||
| 121 : | covers all standard usage contexts. But programmers may create their | ||
| 122 : | own implementations to cover nonstandard requirements. | ||
| 123 : | |||
| 124 : | <h2>Conditions</h2> | ||
| 125 : | |||
| 126 : | A Condition class provides the kinds of condition variables associated | ||
| 127 : | with monitors in other cocurrent languages, as well as pthreads | ||
| 128 : | condvars. Their support reduces the need for tricky and/or | ||
| 129 : | inefficient solutions to many classic concurrent problems. Conditions | ||
| 130 : | also address the annoying problem that Object.wait(msecs) does not | ||
| 131 : | return an indication of whether the wait timed out. This leads to | ||
| 132 : | error-prone code. Since this method is in class Object, the problem is | ||
| 133 : | basically unfixable. | ||
| 134 : | <p> | ||
| 135 : | To avoid compatibility problems, the names of Condition methods need | ||
| 136 : | to be different than Object versions. The downside of this is that | ||
| 137 : | people can make the mistake of calling cond.notify instead of | ||
| 138 : | cond.signal. However, they will get IllegalMonitorState exceptions if | ||
| 139 : | they do, so they can detect the error if they ever run the code. | ||
| 140 : | <p> | ||
| 141 : | The implementation requires VM magic to atomically suspend and release | ||
| 142 : | lock. But it is unlikely to be very challenging for JVM providers, | ||
| 143 : | since most layer Java monitors on top of posix condvars or similar | ||
| 144 : | low-level functionality anyway. | ||
| 145 : | |||
| 146 : | <h2>Atomic variables</h2> | ||
| 147 : | |||
| 148 : | Classes AtomicInteger, AtomicLong, AtomicDouble, AtomicFloat, and | ||
| 149 : | AtomicReference provide simple scalar variables supporting | ||
| 150 : | compareAndSwap (CAS) and related atomic operations. These are | ||
| 151 : | desparately needed by those performing low-level concurrent system | ||
| 152 : | programming, but much less commonly useful in higher-level frameworks. | ||
| 153 : | |||
| 154 : | |||
| 155 : | <h2>Timing</h2> | ||
| 156 : | |||
| 157 : | Java has always supported sub-millisecond versions of several native | ||
| 158 : | time-out-based methods (such as Object.wait), but not methods to | ||
| 159 : | actually perform timing in finer-grained units. We address this by | ||
| 160 : | introducing java.lang.Clock, which provides multiple granularities for | ||
| 161 : | both accessing time and performing time-out based operations. | ||
| 162 : | |||
| 163 : | |||
| 164 : | <h2>Barriers</h2> | ||
| 165 : | |||
| 166 : | Barriers (multiway synchronization points) are very common in some | ||
| 167 : | styles of parallel programming, yet tricky to get right. The two most | ||
| 168 : | useful flavors (CyclicBarriers and Exchangers) don't have much of an | ||
| 169 : | interface in common, and only have one standard implementation each, | ||
| 170 : | so these are simply defined as public classes rather than interfaces | ||
| 171 : | and implementations. | ||
| 172 : | |||
| 173 : | |||
| 174 : | <h2>Concurrent Collections</h2> | ||
| 175 : | |||
| 176 : | There are no new interfaces, but JSR 166 will supply a few Collection | ||
| 177 : | implementations designed for use in multithreaded contexts: | ||
| 178 : | ConcurrentHashTable, CopyOnWriteArrayList, and CopyOnWriteArraySet. | ||
| 179 : | |||
| 180 : | <h2>Uncaught Exception Handlers</h2> | ||
| 181 : | |||
| 182 : | The java.lang.Thread class will be modified to allow per-thread | ||
| 183 : | installation of handlers for uncaught exceptions. Ths optionally | ||
| 184 : | disassociates these handlers from ThreadGroups, which has proven to be | ||
| 185 : | too inflexible in many multithreaded programs. (Note that the combination | ||
| 186 : | of features in JSR 166 make ThreadGroups even less likely to | ||
| 187 : | be used in most programs. Perhaps they will eventually be deprecated.) | ||
| 188 : | <p> | ||
| 189 : | Additionally, Threads and ThreadLocals will now support a means to | ||
| 190 : | clear and remove ThreadLocals, which is needed in some thread-pool and | ||
| 191 : | worker-thread designs. | ||
| 192 : | |||
| 193 : | <hr> | ||
| 194 : | <address><A HREF="http://gee.cs.oswego.edu/dl">Doug Lea</A></address> | ||
| 195 : | </body> | ||
| 196 : | </html> |
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