jdk8中java.util.concurrent包分析

?并发框架分类

1. Executor相关类

Interfaces.?Executor?is a simple standardized interface for defining custom thread-like subsystems,including thread pools,asynchronous I/O,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.?ExecutorService?provides a more complete asynchronous task execution framework. An ExecutorService manages queuing and scheduling of tasks,and allows controlled shutdown. The?ScheduledExecutorService?subinterface and associated interfaces add support for delayed and periodic task execution. ExecutorServices provide methods arranging asynchronous execution of any function expressed as?Callable,the result-bearing analog of?Runnable. A?Future?returns the results of a function,allows determination of whether execution has completed,and provides a means to cancel execution. A?RunnableFuture?is aFuture?that possesses a?run?method that upon execution,sets its results.

Implementations.?Classes?ThreadPoolExecutor?and?ScheduledThreadPoolExecutor?provide tunable,flexible thread pools. The?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?FutureTask?providing a common extensible implementation of Futures,and?ExecutorCompletionService,that assists in coordinating the processing of groups of asynchronous tasks.

Class?ForkJoinPool?provides an Executor primarily designed for processing instances of?ForkJoinTask?and its subclasses. These classes employ a work-stealing scheduler that attains high throughput for tasks conforming to restrictions that often hold in computation-intensive parallel processing.

2.future相关类

3.Queue相关类

The?ConcurrentLinkedQueue?class supplies an efficient scalable thread-safe non-blocking FIFO queue. The?ConcurrentLinkedDeque?class is similar,but additionally supports the?Deque?interface.

Five implementations in?java.util.concurrent?support the extended?BlockingQueue?interface,that defines blocking versions of put and take:?LinkedBlockingQueue,?ArrayBlockingQueue,?SynchronousQueue,?PriorityBlockingQueue,and?DelayQueue. The different classes cover the most common usage contexts for producer-consumer,messaging,parallel tasking,and related concurrent designs.

Extended interface?TransferQueue,and implementation?LinkedTransferQueue?introduce a synchronous?transfer?method (along with related features) in which a producer may optionally block awaiting its consumer.

The?BlockingDeque?interface extends?BlockingQueue?to support both FIFO and LIFO (stack-based) operations. Class?LinkedBlockingDeque?provides an implementation.

4.?atomic相关类

<div class="cnblogs_code">

  <span style="color: #0000ff;">private <span style="color: #0000ff;">static <span style="color: #0000ff;">final AtomicReferenceFieldUpdater<Node,Node> leftUpdater =<span style="color: #000000;">

AtomicReferenceFieldUpdater.newUpdater(Node.<span style="color: #0000ff;">class,Node.<span style="color: #0000ff;">class,"left"<span style="color: #000000;">);

<span style="color: #0000ff;">private <span style="color: #0000ff;">static AtomicReferenceFieldUpdater<Node,Node> rightUpdater =<span style="color: #000000;">

AtomicReferenceFieldUpdater.newUpdater(Node.<span style="color: #0000ff;">class,"right"<span style="color: #000000;">);

Node getLeft() { <span style="color: #0000ff;">return<span style="color: #000000;"> left;  }

<span style="color: #0000ff;">boolean<span style="color: #000000;"> compareAndSetLeft(Node expect,Node update) {

<span style="color: #0000ff;">return leftUpdater.compareAndSet(<span style="color: #0000ff;">this<span style="color: #000000;">,expect,update);

}

<span style="color: #008000;">//<span style="color: #008000;"> ... and so on

}

5. lock相关类

5.1 lock

Interfaces and classes providing a framework for locking and waiting for conditions that is distinct from built-in synchronization and monitors.

：Condition?factors out the?Object?monitor methods (wait,?notify?and?notifyAll) into distinct objects to give the effect of having multiple wait-sets per object,by combining them with the use of arbitrary?Lock?implementations.

As an example,suppose we have a bounded buffer which supports?put?and?take?methods. If a?take?is attempted on an empty buffer,then the thread will block until an item becomes available; if a?put?is attempted on a full buffer,then the thread will block until a space becomes available. We would like to keep waiting?put?threads and?takethreads in separate wait-sets so that we can use the optimization of only notifying a single thread at a time when items or spaces become available in the buffer. This can be achieved using two?Condition?instances.

(The?ArrayBlockingQueue?class provides this functionality,so there is no reason to implement this sample usage class.)

：Lock?implementations provide more extensive locking operations than can be obtained using?synchronized?methods and statements.

：A?ReadWriteLock?maintains a pair of associated?locks,one for read-only operations and one for writing.

?6.Timing

The?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 ofLong.MAX_VALUE.

7.Synchronizers

Five classes aid common special-purpose synchronization idioms.

• Semaphore?is a classic concurrency tool.
• CountDownLatch?is a very simple yet very common utility for blocking until a given number of signals,events,or conditions hold.
• A?CyclicBarrier?is a resettable multiway synchronization point useful in some styles of parallel programming.
• A?Phaser?provides a more flexible form of barrier that may be used to control phased computation among multiple threads.
• An?Exchanger?allows two threads to exchange objects at a rendezvous point,and is useful in several pipeline designs.

8.Concurrent Collections

Besides Queues,this package supplies Collection implementations designed for use in multithreaded contexts:?ConcurrentHashMap,?ConcurrentSkipListMap,?ConcurrentSkipListSet,?CopyOnWriteArrayList,and?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 synchronizedArrayList?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. ButConcurrentHashMap?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.

java.util?conventions in that their??and??provide?weakly consistent?rather than fast-fail traversal:

• they may proceed concurrently with other operations
• they will never throw?ConcurrentModificationException
• they are guaranteed to traverse elements as they existed upon construction exactly once,and may (but are not guaranteed to) reflect any modifications subsequent to construction.

9.Memory Consistency Properties

?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?synchronized?and?volatile?constructs,as well as the?Thread.start()?and?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 (synchronized?block or method exit) of a monitor?happens-before?every subsequent lock (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?volatile?field?happens-before?every subsequent read of that same field. Writes and reads of?volatile?fields have similar memory consistency effects as entering and exiting monitors,but do?not?entail mutual exclusion locking.
• A call to?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?join?on that thread.

The methods of all classes in?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?Runnable?to an?Executor?happen-before?its execution begins. Similarly for?Callables?submitted to an?ExecutorService.
• Actions taken by the asynchronous computation represented by a?Future?happen-before?actions subsequent to the retrieval of the result via?Future.get()?in another thread.
• Actions prior to "releasing" synchronizer methods such as?Lock.unlock,?Semaphore.release,and?CountDownLatch.countDown?happen-before?actions subsequent to a successful "acquiring" method such as?Lock.lock,?Semaphore.acquire,Condition.await,and?CountDownLatch.await?on the same synchronizer object in another thread.
• For each pair of threads that successfully exchange objects via an?Exchanger,actions prior to the?exchange()?in each thread?happen-before?those subsequent to the corresponding?exchange()?in another thread.
• Actions prior to calling?CyclicBarrier.await?and?Phaser.awaitAdvance?(as well as its variants)?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?await?in other threads.

参考文献：

【1】?https://docs.oracle.com/javase/8/docs/api/java/util/concurrent/package-summary.html

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