J.U.C之Executor框架入門指引

一、Executor接口

This interface provides a way of decoupling task submission from the mechanics of how each task will be run, including details of thread use, scheduling, etc.  An {@code Executor} is normally used instead of explicitly creating threads. 
For example, rather than invoking {@code new Thread(new(RunnableTask())).start()} for each of a set of tasks

executor框架是jdk1.5時引入的一個接口，主要目的是解耦任務的建立和任務的執行，在jdk1.5以前，咱們用線程建立一個任務時，一般是這樣 new Thread(new(RunnableTask())).start() ，當引入executor後咱們這樣來建立執行任務:

Executor executor = anExecutor;
executor.execute(new RunnableTask1());
executor.execute(new RunnableTask2());

但因爲executor接口只定義了方法void execute(Runnable command) 而沒有定義具體的實現，於是對於executor的不一樣實現，execute多是建立一個新的線程並當即啓動，有多是使用已有的工做線程運行，或者可能將任務放入等待隊列等待可用的工做線程。好比:

• 同步執行

  class DirectExecutor implements Executor {
     public void execute(Runnable r) {
       r.run();
     }
   }}

• 異步執行

  class ThreadPerTaskExecutor implements Executor {
     public void execute(Runnable r) {
       new Thread(r).start();
     }
   }}

• 排隊執行

  class SerialExecutor implements Executor {
     final Queue<Runnable> tasks = new ArrayDeque<Runnable>();
     final Executor executor;
     Runnable active;
  
     SerialExecutor(Executor executor) {
       this.executor = executor;
     }
  
     public synchronized void execute(final Runnable r) {
       tasks.offer(new Runnable() {
         public void run() {
           try {
             r.run();
           } finally {
             scheduleNext();
           }
         }
       });
       if (active == null) {
         scheduleNext();
       }
     }
  
     protected synchronized void scheduleNext() {
       if ((active = tasks.poll()) != null) {
         executor.execute(active);
       }
     }
   }}

二、ExecutorService接口

除了繼承Executor接口的功能外，還提供了關閉執行器的方法，更加通用的submit方法(除了能夠接收runnable接口任務還能夠接收callable接口任務,使用callable接口任務一般是須要獲取執行結果的任務，它經過返回的Future來獲取callable任務的執行結果)和批量運行Callable接口任務。

三、ScheduledExecutorService接口

除了繼承ExecutorService接口功能外，還提供了延時執行和間隔執行的功能(scheduleWithFixedDelay,scheduleAtFixedRate)

class BeeperControl {
   private final ScheduledExecutorService scheduler =
     Executors.newScheduledThreadPool(1);

   public void beepForAnHour() {
     final Runnable beeper = new Runnable() {
       public void run() { System.out.println("beep"); }
     };
     final ScheduledFuture<?> beeperHandle =
       scheduler.scheduleAtFixedRate(beeper, 10, 10, SECONDS);
     scheduler.schedule(new Runnable() {
       public void run() { beeperHandle.cancel(true); }
     }, 60 * 60, SECONDS);
   }
 }}

四、Executors 工廠

對於上述3個接口,jdk1.5 都提供了默認的實現，可是若是用戶本身去建立這些個默認實現的實例，就必需要了解這些默認實例的實現細節，而Executors 至關於就是一個簡單工廠，經過提供一些簡單的參數就能夠建立出來咱們想要的執行器。Executors爲咱們提供了五類執行器的建立:

• 建立固定線程數的Executor,返回ThreadPoolExecutor類型實例

  public static ExecutorService newFixedThreadPool(int nThreads)
  public static ExecutorService newFixedThreadPool(int nThreads, ThreadFactory threadFactory)

• 單個線程的Executor,返回FinalizableDelegatedExecutorService或DelegatedScheduledExecutorService類型實例

  public static ExecutorService newSingleThreadExecutor() 
  public static ExecutorService newSingleThreadExecutor(ThreadFactory threadFactory)
  public static ScheduledExecutorService newSingleThreadScheduledExecutor()
  public static ScheduledExecutorService newSingleThreadScheduledExecutor(ThreadFactory threadFactory)

• 可緩存的Executor

  public static ExecutorService newCachedThreadPool()
  public static ExecutorService newCachedThreadPool(ThreadFactory threadFactory)

• 延時、週期性的Executor

  public static ScheduledExecutorService newScheduledThreadPool(int corePoolSize)
  public static ScheduledExecutorService newScheduledThreadPool(int corePoolSize, ThreadFactory threadFactory)

• fork/join Executor,返回ForkJoinPool類實例

  public static ExecutorService newWorkStealingPool(int parallelism)//並行級別
  public static ExecutorService newWorkStealingPool()