線程池創建的四種

透過Executors線程池創建的四種方法分別爲：

newCachedThreadPool：創建一個可快取線程池，如果線程池長度超過處理需要，可靈活回收空閒線程，若無可回收，則新建線程。

newFixedThreadPool：創建一個定長線程池，可控制線程最大併發數，超出的線程會在隊列中等待。

newScheduledThreadPool：創建一個定長線程池，支援定時及週期性任務執行。

newSingleThreadExecutor：創建一個單線程化的線程池，它只會用唯一的工作線程來執行任務，保證所有任務按照指定順序(FIFO, LIFO, 優先級)執行。

public class ThreadPoolExecutor extends AbstractExecutorService{//第一個構造方法public ThreadPoolExecutor(int corePoolSize,                               int maximumPoolSize,                               long keepAliveTime,                               TimeUnit unit,                               BlockingQueue<Runnable> workQueue) {         this(corePoolSize, maximumPoolSize, keepAliveTime, unit, workQueue,              Executors.defaultThreadFactory(), defaultHandler);     }//第二個構造方法public ThreadPoolExecutor(int corePoolSize,                               int maximumPoolSize,                               long keepAliveTime,                               TimeUnit unit,                               BlockingQueue<Runnable> workQueue,                               ThreadFactory threadFactory) {         this(corePoolSize, maximumPoolSize, keepAliveTime, unit, workQueue,              threadFactory, defaultHandler);     }//第三個構造方法public ThreadPoolExecutor(int corePoolSize,                               int maximumPoolSize,                               long keepAliveTime,                               TimeUnit unit,                               BlockingQueue<Runnable> workQueue,                               RejectedExecutionHandler handler) {         this(corePoolSize, maximumPoolSize, keepAliveTime, unit, workQueue,              Executors.defaultThreadFactory(), handler);     }//第四個也是真正的初始化構造函數public ThreadPoolExecutor(int corePoolSize,                               int maximumPoolSize,                               long keepAliveTime,                               TimeUnit unit,                               BlockingQueue<Runnable> 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;     }}