J.U.C之Executor框架入門指引

這是個坑發表於2020-09-21
框架
1、Executor介面
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時引入的一個介面，主要目的是解耦任務的建立和任務的執行，在jdk1.5之前，我們用執行緒建立一個任務時，通常是這樣 new Thread(new(RunnableTask())).start() ，當引入executor後我們這樣來建立執行任務:

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

但由於executor介面只定義了方法void execute(Runnable command) 而沒有定義具體的實現，因而對於executor的不同實現，execute可能是建立一個新的執行緒並立即啟動，有可能是使用已有的工作執行緒執行，或者可能將任務放入等待佇列等待可用的工作執行緒。比如:

  • 同步執行

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

  • 非同步執行

    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);
     }
   }
 }}
2、ExecutorService介面

除了繼承Executor介面的功能外，還提供了關閉執行器的方法，更加通用的submit方法(除了可以接收runnable介面任務還可以接收callable介面任務,使用callable介面任務通常是需要獲取執行結果的任務，它通過返回的Future來獲取callable任務的執行結果)和批量執行Callable介面任務。

3、ScheduledExecutorService介面

除了繼承ExecutorService介面功能外，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);
   }
 }}
4、Executors 工廠

對於上述3個介面,jdk1.5 都提供了預設的實現，但是如果使用者自己去建立這些個預設實現的例項，就必須要了解這些預設例項的實現細節，而Executors 相當於就是一個簡單工廠，通過提供一些簡單的引數就可以建立出來我們想要的執行器。Executors為我們提供了五類執行器的建立:

  • 建立固定執行緒數的Executor,返回ThreadPoolExecutor型別例項

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

  • 單個執行緒的Executor,返回FinalizableDelegatedExecutorService或DelegatedScheduledExecutorService型別例項

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

  • 可快取的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類例項

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

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