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
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
, and may execute sequentially or concurrently.
provides a more complete asynchronous task execution framework. An ExecutorService manages queuing and scheduling of tasks, and allows controlled shutdown. The
subinterface and associated interfaces add support for delayed and periodic task execution. ExecutorServices provide methods arranging asynchronous execution of any function expressed as
, the result-bearing analog of
returns the results of a function, allows determination of whether execution has completed, and provides a means to cancel execution. A
that possesses a
method that upon execution, sets its results.
class supplies an efficient scalable thread-safe non-blocking FIFO queue. The
class is similar, but additionally supports the
DelayQueue 。 不同的类涵盖了生产者 - 消费者，消息传递，并行任务和相关并发设计的最常见使用情境。
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
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.
CyclicBarrier is a resettable multiway synchronization point useful in some styles of parallel programming.
Phaser provides a more flexible form of barrier that may be used to control phased computation among multiple threads.
Exchanger allows two threads to exchange objects at a rendezvous point, and is useful in several pipeline designs.
Besides Queues, this package supplies Collection implementations designed for use in multithreaded contexts:
. When many threads are expected to access a given collection, a
is normally preferable to a synchronized
, and a
is normally preferable to a synchronized
is preferable to a synchronized
when the expected number of reads and traversals greatly outnumber the number of updates to a list.
Collections.synchronizedMap(new HashMap())是同步的。 但是
ConcurrentHashMap是“并发”的。 并发集合是线程安全的，但不受单个排除锁定的控制。 在ConcurrentHashMap的特殊情况下，它安全地允许任意数量的并发读取以及可调数量的并发写入。 当需要通过单个锁来阻止对集合的所有访问时，“同步”类可能非常有用，但代价是可扩展性较差。 在其他情况下，多个线程需要访问一个通用集合，“并发”版本通常更可取。 如果任何一个集合是非共享的，或者只有在持有其他锁时才可访问，则非同步集合更可取。
java.util约定不同，因为它们的 Iterators和 Spliterators提供 弱一致性而不是快速失败遍历：
- they may proceed concurrently with other operations
- they will never throw
- 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.
Memory Consistency Properties
Chapter 17 of The Java™ Language Specification
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
the read operation. The
constructs, as well as the
methods, can form
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
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
- 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
CountDownLatch.countDown happen-before actions subsequent to a successful "acquiring" method such as
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
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.