Java并发类库提供的线程池有哪几种？分别有什么特点？
问题：
Java并发库提供的线程池有哪几种？分别有什么特点？
知识点补充
Executor框架
ThreadPoolExecutor 线程池类
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| public ThreadPoolExecutor(int corePoolSize, int maximumPoolSize, long keepAliveTime, TimeUnit unit, BlockingQueue<Runnable> workQueue) { this(corePoolSize, maximumPoolSize, keepAliveTime, unit, workQueue, Executors.defaultThreadFactory(), defaultHandler); }
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参数说明：
corePoolSize：核心线程数。
maximumPoolSize：最大线程数。
keepAliveTime：线程存活时间。当线程数大于core数，那么超过该时间的线程将会被终结。
unit：keepAliveTime的单位。java.util.concurrent.TimeUnit类存在静态静态属性： NANOSECONDS、MICROSECONDS、MILLISECONDS、SECONDS
workQueue：Runnable的阻塞队列。若线程池已经被占满，则该队列用于存放无法再放入线程池中的Runnable。
回答问题：
通常开发者都是利用Executors提供的通用线程池创建方法，去创建不同配置的线程池，主要区别在于不同的ExecutorService类型或者不同的初始参数。
Executors目前提供了5种不同的线程池创建配置：
- newCachedThreadPool()，它是一种用来处理大量短时间工作任务的线程池；它会试图缓存线程并且重用，当无缓存线程可用时，就会创建新的工作线程；如果线程闲置的时间超过60s，则被终止并移除缓存；长时间闲置时，这种线程池，不会消耗什么资源。其内部使用SynchronousQueue作为工作队列。
构造方法（看英文注释是最好的理解哈）：
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| /** * Creates a thread pool that creates new threads as needed, but * will reuse previously constructed threads when they are * available. These pools will typically improve the performance * of programs that execute many short-lived asynchronous tasks. * Calls to {@code execute} will reuse previously constructed * threads if available. If no existing thread is available, a new * thread will be created and added to the pool. Threads that have * not been used for sixty seconds are terminated and removed from * the cache. Thus, a pool that remains idle for long enough will * not consume any resources. Note that pools with similar * properties but different details (for example, timeout parameters) * may be created using {@link ThreadPoolExecutor} constructors. * * @return the newly created thread pool */ public static ExecutorService newCachedThreadPool() { return new ThreadPoolExecutor(0, Integer.MAX_VALUE, 60L, TimeUnit.SECONDS, new SynchronousQueue<Runnable>()); }
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- newFixedThreadPool(int nThreadPool)，重要指定数目（nThreads)的线程，其背后使用的无界的工作队列，任何时候最多有nThreads个工作线程是活动的。这意味着，如果数量超过了活动队列数目，将在工作队列中等待空闲线程出现；如果有工作线程退出，将会有新的工作线程被创建，以补足指定的数目nThreads。
构造方法（看英文注释是最好的理解哈）：
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| /** * Creates a thread pool that reuses a fixed number of threads * operating off a shared unbounded queue. At any point, at most * {@code nThreads} threads will be active processing tasks. * If additional tasks are submitted when all threads are active, * they will wait in the queue until a thread is available. * If any thread terminates due to a failure during execution * prior to shutdown, a new one will take its place if needed to * execute subsequent tasks. The threads in the pool will exist * until it is explicitly {@link ExecutorService#shutdown shutdown}. * * @param nThreads the number of threads in the pool * @return the newly created thread pool * @throws IllegalArgumentException if {@code nThreads <= 0} */ public static ExecutorService newFixedThreadPool(int nThreads) { return new ThreadPoolExecutor(nThreads, nThreads, 0L, TimeUnit.MILLISECONDS, new LinkedBlockingQueue<Runnable>()); }
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- newSingleThreadExcutor()，它的特点在于工作线程数目被限制为1，操作了一个无界的工作队列，所以它保证了所有任务都是被顺序执行的。最多会有一个任务处于活动状态，并且不允许使用者改动线程池实例，因此可以避免其改变线程数目。
构造方法（看英文注释是最好的理解哈）：
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| /** * Creates an Executor that uses a single worker thread operating * off an unbounded queue. (Note however that if this single * thread terminates due to a failure during execution prior to * shutdown, a new one will take its place if needed to execute * subsequent tasks.) Tasks are guaranteed to execute * sequentially, and no more than one task will be active at any * given time. Unlike the otherwise equivalent * {@code newFixedThreadPool(1)} the returned executor is * guaranteed not to be reconfigurable to use additional threads. * * @return the newly created single-threaded Executor */ public static ExecutorService newSingleThreadExecutor() { return new FinalizableDelegatedExecutorService (new ThreadPoolExecutor(1, 1, 0L, TimeUnit.MILLISECONDS, new LinkedBlockingQueue<Runnable>())); }
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- newSingleThreadScheduledExcutor()和newScheduledThreadPool(int corePoolSize)，创建的是个ScheduledExecutorService，可以进行定时或周期性的工作调度，区别在于单一工作线程还是多个工作线程。
构造方法（注释是最好的解释哈）
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| /** * Creates a thread pool that can schedule commands to run after a * given delay, or to execute periodically. * @param corePoolSize the number of threads to keep in the pool, * even if they are idle * @return a newly created scheduled thread pool * @throws IllegalArgumentException if {@code corePoolSize < 0} */ public static ScheduledExecutorService newScheduledThreadPool(int corePoolSize) { return new ScheduledThreadPoolExecutor(corePoolSize); }
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- newWorkStealingPool(int parallelism)这是一个经常被人忽略的线程池，它是Java 8 才加入这个创建方法，其内部会创建ForkJoinPool，利用Work-Stealing算法，并行的处理任务，不保证处理顺序。
构造方法：
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| /** * Creates a thread pool that maintains enough threads to support * the given parallelism level, and may use multiple queues to * reduce contention. The parallelism level corresponds to the * maximum number of threads actively engaged in, or available to * engage in, task processing. The actual number of threads may * grow and shrink dynamically. A work-stealing pool makes no * guarantees about the order in which submitted tasks are * executed. * * @param parallelism the targeted parallelism level * @return the newly created thread pool * @throws IllegalArgumentException if {@code parallelism <= 0} * @since 1.8 */ public static ExecutorService newWorkStealingPool(int parallelism) { return new ForkJoinPool (parallelism, ForkJoinPool.defaultForkJoinWorkerThreadFactory, null, true); }
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参考：
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