Java Executor分析

Executor框架能够快速建立Java线程池。
Executors是Executor框架的基础。全部的Executor和ExecutorService都实现Executor接口，其中ExecutorService是比较重要的接口，Executors提供了不少静态方法能够实现不少ThreadPool。经过ExecutorService的submit能够分别将Callable和Runnable任务提交到线程中处理；经过shutdown()关闭全部线程，Callable返回的Future有结果，Runnable则没有。

Executors类：

public class Executors {
    public static ExecutorService newFixedThreadPool(int nThreads) {
        return new ThreadPoolExecutor(nThreads, nThreads, 0L, TimeUnit.MILLISECONDS, new LinkedBlockingQueue());
    }

    public static ExecutorService newFixedThreadPool(int nThreads, ThreadFactory threadFactory) {
        return new ThreadPoolExecutor(nThreads, nThreads, 0L, TimeUnit.MILLISECONDS, new LinkedBlockingQueue(), threadFactory);
    }

    public static ExecutorService newSingleThreadExecutor() {
        return new Executors.FinalizableDelegatedExecutorService(new ThreadPoolExecutor(1, 1, 0L, TimeUnit.MILLISECONDS, new LinkedBlockingQueue()));
    }

    public static ExecutorService newSingleThreadExecutor(ThreadFactory threadFactory) {
        return new Executors.FinalizableDelegatedExecutorService(new ThreadPoolExecutor(1, 1, 0L, TimeUnit.MILLISECONDS, new LinkedBlockingQueue(), threadFactory));
    }

    public static ExecutorService newCachedThreadPool() {
        return new ThreadPoolExecutor(0, 2147483647, 60L, TimeUnit.SECONDS, new SynchronousQueue());
    }

    public static ExecutorService newCachedThreadPool(ThreadFactory threadFactory) {
        return new ThreadPoolExecutor(0, 2147483647, 60L, TimeUnit.SECONDS, new SynchronousQueue(), threadFactory);
    }

    public static ScheduledExecutorService newSingleThreadScheduledExecutor() {
        return new Executors.DelegatedScheduledExecutorService(new ScheduledThreadPoolExecutor(1));
    }

    public static ScheduledExecutorService newSingleThreadScheduledExecutor(ThreadFactory threadFactory) {
        return new Executors.DelegatedScheduledExecutorService(new ScheduledThreadPoolExecutor(1, threadFactory));
    }

    public static ScheduledExecutorService newScheduledThreadPool(int corePoolSize) {
        return new ScheduledThreadPoolExecutor(corePoolSize);
    }

    public static ScheduledExecutorService newScheduledThreadPool(int corePoolSize, ThreadFactory threadFactory) {
        return new ScheduledThreadPoolExecutor(corePoolSize, threadFactory);
    }
}

主要方法：
1. newFixedThreadPool返回ThreadPoolExecutor，ThreadPoolExecutor建立固定数目的线程，当有任务submit时，若是没有线程可执行时，则加入任务队列。ThreadPoolExecutor刚开始不建立线程，当有任务提交时，若是poolSize<corePoolSize则建立线程，当poolSize==corePoolSize时，新增任务加入任务队列，若是此时任务队列满了而且poolSize<maximumPoolSize，则建立线程处理，不然就拒绝任务，拒绝策略取决于RejectedExecutionHandler，Worker线程则一直在等待队列getTask和run间循环；

2. newCachedThreadPool返回的也是ThreadPoolExecutor，区别是cache线程数大小没有限制，有60秒的IDLE，若是超过60秒没有任务则会被关闭；

3. newSingleThreadExecutor返回Executors.FinalizableDelegatedExecutorService，底层也是ThreadPoolExecutor，区别是线程是1~1，IDLE为0秒；

4. newScheduledThreadPool返回ScheduledExecutorService，ScheduledExecutorService继承ThreadPoolExecutor并实现了ScheduledExecutorService，ScheduledExecutorService封装一个DelayedWorkQueue，Worker执行一次任务后add进去，另外一个Worker触发take，take须要等待一个delay，重复循环下去。

5. newWorkStealingPool返回ForkJoinPool，ForkJoinPool分而治之，ForkJoinPool默认建立CPU数目建立并行等级，会动态的增长或关闭线程，最大线程数限制为32767，空闲线程会主动为别的线程分担任务，从而使CPU处于饱满状态，咱们要使用ForkJoin框架，必须首先建立一个ForkJoinTask。它提供在任务中执行fork( )和join( )操做的机制，一般状况下咱们不须要直接继承ForkJoinTask类，而只须要继承它的子类，Fork/Join框架提供了如下两个子类：RecursiveAction用于没有返回结果的任务，RecursiveTask 用于有返回结果的任务。ForkJoinTask须要经过ForkJoinPool来执行，任务分割出的子任务会添加到当前工做线程所维护的双端队列中，进入队列的头部。当一个工做线程的队列里暂时没有任务时，它会随机从其余工做线程的队列的尾部获取一个任务。