Java Executor分析

Executor框架能夠快速建立Java線程池。
Executors是Executor框架的基礎。全部的Executor和ExecutorService都實現Executor接口，其中ExecutorService是比較重要的接口，Executors提供了不少靜態方法能夠實現不少ThreadPool。經過ExecutorService的submit能夠分別將Callable和Runnable任務提交到線程中處理；經過shutdown()關閉全部線程，Callable返回的Future有結果，Runnable則沒有。

Executors類：

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建立固定數目的線程，當有任務submit時，若是沒有線程可執行時，則加入任務隊列。ThreadPoolExecutor剛開始不建立線程，當有任務提交時，若是poolSize<corePoolSize則建立線程，當poolSize==corePoolSize時，新增任務加入任務隊列，若是此時任務隊列滿了而且poolSize<maximumPoolSize，則建立線程處理，不然就拒絕任務，拒絕策略取決於RejectedExecutionHandler，Worker線程則一直在等待隊列getTask和run間循環；

2. newCachedThreadPool返回的也是ThreadPoolExecutor，區別是cache線程數大小沒有限制，有60秒的IDLE，若是超過60秒沒有任務則會被關閉；

3. newSingleThreadExecutor返回Executors.FinalizableDelegatedExecutorService，底層也是ThreadPoolExecutor，區別是線程是1~1，IDLE爲0秒；

4. newScheduledThreadPool返回ScheduledExecutorService，ScheduledExecutorService繼承ThreadPoolExecutor並實現了ScheduledExecutorService，ScheduledExecutorService封裝一個DelayedWorkQueue，Worker執行一次任務後add進去，另外一個Worker觸發take，take須要等待一個delay，重複循環下去。

5. newWorkStealingPool返回ForkJoinPool，ForkJoinPool分而治之，ForkJoinPool默認建立CPU數目建立並行等級，會動態的增長或關閉線程，最大線程數限制爲32767，空閒線程會主動爲別的線程分擔任務，從而使CPU處於飽滿狀態，咱們要使用ForkJoin框架，必須首先建立一個ForkJoinTask。它提供在任務中執行fork( )和join( )操做的機制，一般狀況下咱們不須要直接繼承ForkJoinTask類，而只須要繼承它的子類，Fork/Join框架提供瞭如下兩個子類：RecursiveAction用於沒有返回結果的任務，RecursiveTask 用於有返回結果的任務。ForkJoinTask須要經過ForkJoinPool來執行，任務分割出的子任務會添加到當前工做線程所維護的雙端隊列中，進入隊列的頭部。當一個工做線程的隊列裏暫時沒有任務時，它會隨機從其餘工做線程的隊列的尾部獲取一個任務。