Concurrency Utilities

From 65bf3316cf384588453604be6b4f0ed3751a8b0f Mon Sep 17 00:00:00 2001 From: tromey Date: Tue, 9 Jan 2007 19:58:05 +0000 Subject: Merged gcj-eclipse branch to trunk. git-svn-id: svn+ssh://gcc.gnu.org/svn/gcc/trunk@120621 138bc75d-0d04-0410-961f-82ee72b054a4 --- .../jsr166/java/util/concurrent/package.html | 222 +++++++++++++++++++++ 1 file changed, 222 insertions(+) create mode 100644 libjava/classpath/external/jsr166/java/util/concurrent/package.html (limited to 'libjava/classpath/external/jsr166/java/util/concurrent/package.html') diff --git a/libjava/classpath/external/jsr166/java/util/concurrent/package.html b/libjava/classpath/external/jsr166/java/util/concurrent/package.html new file mode 100644 index 00000000000..20227e1fece --- /dev/null +++ b/libjava/classpath/external/jsr166/java/util/concurrent/package.html @@ -0,0 +1,222 @@ + + +Concurrency Utilities + + + + +

Utility classes commonly useful in concurrent programming. This +package includes a few small standardized extensible frameworks, as +well as some classes that provide useful functionality and are +otherwise tedious or difficult to implement. Here are brief +descriptions of the main components. See also the locks and +atomic packages. + +

Executors

+ +Interfaces. {@link java.util.concurrent.Executor} is a simple +standardized interface for defining custom thread-like subsystems, +including thread pools, asynchronous IO, and lightweight task +frameworks. Depending on which concrete Executor class is being used, +tasks may execute in a newly created thread, an existing +task-execution thread, or the thread calling execute(), and +may execute sequentially or concurrently. {@link +java.util.concurrent.ExecutorService} provides a more complete +asynchronous task execution framework. An ExecutorService manages +queuing and scheduling of tasks, and allows controlled shutdown. The +{@link java.util.concurrent.ScheduledExecutorService} subinterface +and associated interfaces add support for delayed and periodic task execution. +ExecutorServices provide methods arranging asynchronous execution of +any function expressed as {@link java.util.concurrent.Callable}, the +result-bearing analog of {@link java.lang.Runnable}. A {@link +java.util.concurrent.Future} returns the results of a function, allows +determination of whether execution has completed, and provides a means to +cancel execution. A {@link java.util.concurrent.RunnableFuture} is +a Future that possesses a run method that upon execution, +sets its results. + +

+ +Implementations. Classes {@link +java.util.concurrent.ThreadPoolExecutor} and {@link +java.util.concurrent.ScheduledThreadPoolExecutor} provide tunable, +flexible thread pools. The {@link java.util.concurrent.Executors} +class provides factory methods for the most common kinds and +configurations of Executors, as well as a few utility methods for +using them. Other utilities based on Executors include the concrete +class {@link java.util.concurrent.FutureTask} providing a common +extensible implementation of Futures, and {@link +java.util.concurrent.ExecutorCompletionService}, that assists in +coordinating the processing of groups of asynchronous tasks. + +

Queues

+ +The java.util.concurrent {@link +java.util.concurrent.ConcurrentLinkedQueue} class supplies an +efficient scalable thread-safe non-blocking FIFO queue. Five +implementations in java.util.concurrent support the extended {@link +java.util.concurrent.BlockingQueue} interface, that defines blocking +versions of put and take: {@link +java.util.concurrent.LinkedBlockingQueue}, {@link +java.util.concurrent.ArrayBlockingQueue}, {@link +java.util.concurrent.SynchronousQueue}, {@link +java.util.concurrent.PriorityBlockingQueue}, and {@link +java.util.concurrent.DelayQueue}. The different classes cover the most +common usage contexts for producer-consumer, messaging, parallel +tasking, and related concurrent designs. The {@link +java.util.concurrent.BlockingDeque} interface extends +BlockingQueue to support both FIFO and LIFO (stack-based) +operations. Class {@link java.util.concurrent.LinkedBlockingDeque} +provides an implementation. + + +

Timing

+ +The {@link java.util.concurrent.TimeUnit} class provides multiple +granularities (including nanoseconds) for specifying and controlling +time-out based operations. Most classes in the package contain +operations based on time-outs in addition to indefinite waits. In all +cases that time-outs are used, the time-out specifies the minimum time +that the method should wait before indicating that it +timed-out. Implementations make a "best effort" to detect +time-outs as soon as possible after they occur. However, an indefinite +amount of time may elapse between a time-out being detected and a +thread actually executing again after that time-out. All methods +that accept timeout parameters treat values less than or equal to +zero to mean not to wait at all. To wait "forever", you can use +a value of Long.MAX_VALUE. + +

Synchronizers

+ +Four classes aid common special-purpose synchronization idioms. +{@link java.util.concurrent.Semaphore} is a classic concurrency tool. +{@link java.util.concurrent.CountDownLatch} is a very simple yet very +common utility for blocking until a given number of signals, events, +or conditions hold. A {@link java.util.concurrent.CyclicBarrier} is a +resettable multiway synchronization point useful in some styles of +parallel programming. An {@link java.util.concurrent.Exchanger} allows +two threads to exchange objects at a rendezvous point, and is useful +in several pipeline designs. + +

Concurrent Collections

+ +Besides Queues, this package supplies Collection implementations +designed for use in multithreaded contexts: +{@link java.util.concurrent.ConcurrentHashMap}, +{@link java.util.concurrent.ConcurrentSkipListMap}, +{@link java.util.concurrent.ConcurrentSkipListSet}, +{@link java.util.concurrent.CopyOnWriteArrayList}, and +{@link java.util.concurrent.CopyOnWriteArraySet}. +When many threads are expected to access a given collection, +a ConcurrentHashMap is normally preferable to +a synchronized HashMap, and a +ConcurrentSkipListMap is normally preferable +to a synchronized TreeMap. A +CopyOnWriteArrayList is preferable to +a synchronized ArrayList when the expected number of reads +and traversals greatly outnumber the number of updates to a list. + +

The "Concurrent" prefix used with some classes in this package is a +shorthand indicating several differences from similar "synchronized" +classes. For example java.util.Hashtable and +Collections.synchronizedMap(new HashMap()) are +synchronized. But {@link java.util.concurrent.ConcurrentHashMap} is +"concurrent". A concurrent collection is thread-safe, but not +governed by a single exclusion lock. In the particular case of +ConcurrentHashMap, it safely permits any number of concurrent reads as +well as a tunable number of concurrent writes. "Synchronized" classes +can be useful when you need to prevent all access to a collection via +a single lock, at the expense of poorer scalability. In other cases in +which multiple threads are expected to access a common collection, +"concurrent" versions are normally preferable. And unsynchronized +collections are preferable when either collections are unshared, or +are accessible only when holding other locks. + +

Most concurrent Collection implementations (including most Queues) +also differ from the usual java.util conventions in that their Iterators +provide weakly consistent rather than fast-fail traversal. A +weakly consistent iterator is thread-safe, but does not necessarily +freeze the collection while iterating, so it may (or may not) reflect +any updates since the iterator was created. + + +

Memory Consistency Properties

+ + +Chapter 17 of the Java Language Specification defines the +happens-before relation on memory operations such as reads and +writes of shared variables. The results of a write by one thread are +guaranteed to be visible to a read by another thread only if the write +operation happens-before the read operation. The +{@code synchronized} and {@code volatile} constructs, as well as the +{@code Thread.start()} and {@code Thread.join()} methods, can form +happens-before relationships. In particular: + +

Each action in a thread happens-before every action in that + thread that comes later in the program's order. + +
An unlock ({@code synchronized} block or method exit) of a + monitor happens-before every subsequent lock ({@code synchronized} + block or method entry) of that same monitor. And because + the happens-before relation is transitive, all actions + of a thread prior to unlocking happen-before all actions + subsequent to any thread locking that monitor. + +
A write to a {@code volatile} field happens-before every + subsequent read of that same field. Writes and reads of + {@code volatile} fields have similar memory consistency effects + as entering and exiting monitors, but do not entail + mutual exclusion locking. + +
A call to {@code start} on a thread happens-before any action in the + started thread. + +
All actions in a thread happen-before any other thread + successfully returns from a {@code join} on that thread. + +

+ + +The methods of all classes in {@code java.util.concurrent} and its +subpackages extend these guarantees to higher-level +synchronization. In particular: + +

Actions in a thread prior to placing an object into any concurrent + collection happen-before actions subsequent to the access or + removal of that element from the collection in another thread. + +
Actions in a thread prior to the submission of a {@code Runnable} + to an {@code Executor} happen-before its execution begins. + Similarly for {@code Callables} submitted to an {@code ExecutorService}. + +
Actions taken by the asynchronous computation represented by a + {@code Future} happen-before actions subsequent to the + retrieval of the result via {@code Future.get()} in another thread. + +
Actions prior to "releasing" synchronizer methods such as + {@code Lock.unlock}, {@code Semaphore.release}, and + {@code CountDownLatch.countDown} happen-before actions + subsequent to a successful "acquiring" method such as + {@code Lock.lock}, {@code Semaphore.acquire}, + {@code Condition.await}, and {@code CountDownLatch.await} on the + same synchronizer object in another thread. + +
For each pair of threads that successfully exchange objects via + an {@code Exchanger}, actions prior to the {@code exchange()} + in each thread happen-before those subsequent to the + corresponding {@code exchange()} in another thread. + +
Actions prior to calling {@code CyclicBarrier.await} + happen-before actions performed by the barrier action, and + actions performed by the barrier action happen-before actions + subsequent to a successful return from the corresponding {@code await} + in other threads. + +

+ +@since 1.5 + + -- cgit v1.2.3