摘要：實現初始化線程池數量策略并執行線程。二簡述下從定義線程池，到其底層執行的簡單流程簡單定義了如上兩句代碼，自動創建個固定大小的線程的線程池，實現了接口的對象以后，自動啟動線程并執行對象的方法。關鍵的調用類方法，管理線程池并執行任務。

一.類圖說明

Executor接口類，執行Runnable接口execute。

ExecutorService接口類繼承Executor接口，包含提交執行Runnable和Callable接口submit以及shutdown，invokeAll接口。

ScheduledExecutorService接口類繼承ExecutorService接口，主要包含計劃執行接口schedule，scheduleAtFixedRate以及scheduleWithFixedDelay。

虛類AbstractExecutorService，實現ExecutorService接口。實現ExecutorService相關接口。

ThreadPoolExecutor類，繼承虛類AbstractExecutorService。實現初始化線程池數量、策略并執行線程。

ScheduledThreadPoolExecutor類，繼承ThreadPoolExecutor類并實現ScheduledExecutorService接口。實現計劃執行相關接口。

執行類，定義ThreadPoolExecutor和ScheduledThreadPoolExecutor類，并使用相關concurrent類方法。

二.簡述下從定義線程池，到其底層執行的簡單流程

ExecutorService executorService = Executors.newFixedThreadPool(3);
executorService.submit(new NewTask());

簡單定義了如上兩句代碼，JDK自動創建3個固定大小的線程的線程池，submit實現了Runnable接口的NewTask對象以后，JDK自動啟動線程并執行NewTask對象的run方法。流程是如何的呢？

1.Executors的newFixedThreadPool方法new了一個ThreadPoolExecutor對象，且new了一個LinkedBlockingQueue對象。

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

2.調用ThreadPoolExecutor的構造函數，其中ThreadFactory默認使用defaultThreadFactory，defaultHandler為AbortPolicy。

public ThreadPoolExecutor(int corePoolSize,
                              int maximumPoolSize,
                              long keepAliveTime,
                              TimeUnit unit,
                              BlockingQueue workQueue) {
        this(corePoolSize, maximumPoolSize, keepAliveTime, unit, workQueue,
             Executors.defaultThreadFactory(), defaultHandler);
    }

public ThreadPoolExecutor(int corePoolSize,
                              int maximumPoolSize,
                              long keepAliveTime,
                              TimeUnit unit,
                              BlockingQueue workQueue,
                              ThreadFactory threadFactory,
                              RejectedExecutionHandler handler) {
        if (corePoolSize < 0 ||
            maximumPoolSize <= 0 ||
            maximumPoolSize < corePoolSize ||
            keepAliveTime < 0)
            throw new IllegalArgumentException();
        if (workQueue == null || threadFactory == null || handler == null)
            throw new NullPointerException();
        this.corePoolSize = corePoolSize;
        this.maximumPoolSize = maximumPoolSize;
        this.workQueue = workQueue;
        this.keepAliveTime = unit.toNanos(keepAliveTime);
        this.threadFactory = threadFactory;
        this.handler = handler;
    }
    

3.初始化以后，調用AbstractExecutorService類的submit方法，執行Runnable對象。

public Future submit(Runnable task) {
        if (task == null) throw new NullPointerException();
        RunnableFuture ftask = newTaskFor(task, null);
        execute(ftask);
        return ftask;
    }
    

4.RunnableAdapter實現了Callable接口，FutureTask類包含callable類成員對象。
FutureTask實現RunnableFuture接口，RunnableFuture繼承Runnable接口。所以newTaskFor(task, null)方法返回一個FutureTask對象。

public class FutureTask implements RunnableFuture

public interface RunnableFuture extends Runnable, Future

RunnableFuture ftask = newTaskFor(task, null);

protected  RunnableFuture newTaskFor(Runnable runnable, T value) {
    return new FutureTask(runnable, value);
}

public FutureTask(Runnable runnable, V result) {
    this.callable = Executors.callable(runnable, result);
    this.state = NEW;       // ensure visibility of callable
}

public static  Callable callable(Runnable task, T result) {
    if (task == null)
        throw new NullPointerException();
    return new RunnableAdapter(task, result);
}

static final class RunnableAdapter implements Callable {
    final Runnable task;
    final T result;
    RunnableAdapter(Runnable task, T result) {
        this.task = task;
        this.result = result;
    }
    public T call() {
        task.run();
        return result;
    }
}

5.關鍵的調用ThreadPoolExecutor類execute方法，管理線程池并執行Runnable任務。主要邏輯如下：
1）如果線程數量小于corePoolSize，新來一個任務，創建一個新線程。
2）如果線程數量等于corePoolSize，則將任務緩存到workQueue中。
3）如果線程數量等于corePoolSize并且workQueue隊列已滿，則繼續創建線程直到等于maximumPoolSize。
4）如果線程數量等于maximumPoolSize，且workQueue隊列已滿，則根據defaultHandler策略執行相應措施。默認是AbortPolicy，拋出一個運行時異常RejectedExecutionException。另外還有3種策略：CallerRunsPolicy->如果線程池沒有shutdown，則直接調用Runnable的run方法執行。如果線程池shutdown，則直接丟棄；DiscardPolicy->直接丟棄，沒有任何異常；DiscardOldestPolicy->丟棄最老的任務，并調用線程池的execute方法執行，如果線程池沒有shutdown。

public void execute(Runnable command) {
        if (command == null)
            throw new NullPointerException();
        int c = ctl.get();
        if (workerCountOf(c) < corePoolSize) {
            if (addWorker(command, true))
                return;
            c = ctl.get();
        }
        if (isRunning(c) && workQueue.offer(command)) {
            int recheck = ctl.get();
            if (! isRunning(recheck) && remove(command))
                reject(command);
            else if (workerCountOf(recheck) == 0)
                addWorker(null, false);
        }
        else if (!addWorker(command, false))
            reject(command);
    }