ExecutorService

 对于多线程有了一点了解之后，那么来看看java.lang.concurrent包下面的一些东西。在此之前，我们运行一个线程都是显式调用了Thread的start()方法。我们用concurrent下面的类来实现一下线程的运行，而且这将成为以后常用的方法或者实现思路。 


        看一个简单的例子： 

public class CacheThreadPool {      public static void main(String[] args) {          ExecutorService exec=Executors.newCachedThreadPool();          for(int i=0;i<5;i++)              exec.execute(new LiftOff());          exec.shutdown();//并不是终止线程的运行，而是禁止在这个Executor中添加新的任务      }  }  

        这个例子其实很容易看懂，ExecutorService中有一个execute方法，这个方法的参数是Runnable类型。也就是说，将一个实现了Runnable类型的类的实例作为参数传入execute方法并执行，那么线程就相应的执行了。 


        一、ExecutorService 
        先看看ExecutorService，这是一个接口，简单的列一下这个接口： 

public interface ExecutorService extends Executor {        void shutdown();        List<Runnable> shutdownNow();        boolean isShutdown();        boolean isTerminated();        boolean awaitTermination(long timeout, TimeUnit unit)        <T> Future<T> submit(Callable<T> task);        <T> Future<T> submit(Runnable task, T result);        Future<?> submit(Runnable task);        <T> List<Future<T>> invokeAll(Collection<? extends Callable<T>> tasks)        <T> List<Future<T>> invokeAll(Collection<? extends Callable<T>> tasks, long timeout, TimeUnit unit)        <T> T invokeAny(Collection<? extends Callable<T>> tasks)        <T> T invokeAny(Collection<? extends Callable<T>> tasks,                      long timeout, TimeUnit unit)  }          ExecuteService继承了Executor，Executor也是一个接口，里面只有一个方法： void execute(Runnable command)  

        二、Executors 
        Executors是一个类，直接援引JDK文档的说明来说一下这个类的作用： 
       

        Factory and utility methods for Executor, ExecutorService, ScheduledExecutorService, ThreadFactory, and Callable classes defined in this package. This class supports the following kinds of methods:         Methods that create and return an ExecutorService set up with commonly useful configuration settings.       Methods that create and return a ScheduledExecutorService set up with commonly useful configuration settings.       Methods that create and return a "wrapped" ExecutorService, that disables reconfiguration by making implementation-specific methods inaccessible.       Methods that create and return a ThreadFactory that sets newly created threads to a known state.       Methods that create and return a Callable out of other closure-like forms, so they can be used in execution methods requiring Callable.       

        在上面的例子中，我们用到了newCachedThreadPool()方法。看一下这个方法： 

public static ExecutorService newCachedThreadPool() {          return new ThreadPoolExecutor(0, Integer.MAX_VALUE,                                        60L, TimeUnit.SECONDS,                                        new SynchronousQueue<Runnable>());      }  

        在源码中我们可以知道两点，1、这个方法返回类型是ExecutorService；2、此方法返回值实际是另一个类的实例。看一下这个类的信息： 

public class ThreadPoolExecutor extends AbstractExecutorService {      ..........      private final BlockingQueue<Runnable> workQueue;//这个变量在下面会提到      ..........  }  

        ThreadPoolExecutor继承了AbstractExecutorService,而AbstractExecutorService又实现了ExecutorService接口。所以，根据多态，ThreadPoolExecutor可以看作是ExecutorService类型。 


        线程执行的最关键的一步是执行了executor方法，根据java的动态绑定，实际执行的是ThreadPoolExecutor所实现的executor方法。看看源码： 

public class ThreadPoolExecutor extends AbstractExecutorService {      ..........      public void execute(Runnable command) {          if (command == null)              throw new NullPointerException();          if (poolSize >= corePoolSize || !addIfUnderCorePoolSize(command)) {              if (runState == RUNNING && workQueue.offer(command)) {                  if (runState != RUNNING || poolSize == 0)                      ensureQueuedTaskHandled(command);              }              else if (!addIfUnderMaximumPoolSize(command))                  reject(command); // is shutdown or saturated          }      }      ..........  }  

        根据程序正常执行的路线来看，这个方法中比较重要的两个地方分别是： 
        1、workQueue.offer(command) 
        workQueue在上面提到过，是BlockingQueue<Runnable>类型的变量，这条语句就是将Runnable类型的实例加入到队列中。 
        2、ensureQueuedTaskHandled(command) 
        这个是线程执行的关键语句。看看它的源码： 

public class ThreadPoolExecutor extends AbstractExecutorService {      ..........      private void ensureQueuedTaskHandled(Runnable command) {          final ReentrantLock mainLock = this.mainLock;          mainLock.lock();          boolean reject = false;          Thread t = null;          try {              int state = runState;              if (state != RUNNING && workQueue.remove(command))                  reject = true;              else if (state < STOP &&                       poolSize < Math.max(corePoolSize, 1) &&                       !workQueue.isEmpty())                  t = addThread(null);          } finally {              mainLock.unlock();          }          if (reject)              reject(command);          else if (t != null)              t.start();      }      ..........  }  

        在这里我们就可以看到最终执行了t.start()方法来运行线程。在这之前的重点是t=addThread(null)方法，看看addThread方法的源码： 

public class ThreadPoolExecutor extends AbstractExecutorService {      ..........      private Thread addThread(Runnable firstTask) {          Worker w = new Worker(firstTask);          Thread t = threadFactory.newThread(w);          if (t != null) {              w.thread = t;              workers.add(w);              int nt = ++poolSize;              if (nt > largestPoolSize)                  largestPoolSize = nt;          }          return t;      }      ..........  }  

        这里两个重点，很明显： 
        1、Worker w = new Worker(firstTask) 
        2、Thread t = threadFactory.newThread(w) 
        先看Worker是个什么结构： 

public class ThreadPoolExecutor extends AbstractExecutorService {      ..........      private final class Worker implements Runnable {          ..........          Worker(Runnable firstTask) {              this.firstTask = firstTask;          }            private Runnable firstTask;          ..........            public void run() {              try {                  Runnable task = firstTask;                  firstTask = null;                  while (task != null || (task = getTask()) != null) {                      runTask(task);                      task = null;                  }              } finally {                  workerDone(this);              }          }      }        Runnable getTask() {          for (;;) {              try {                  int state = runState;                  if (state > SHUTDOWN)                      return null;                  Runnable r;                  if (state == SHUTDOWN)  // Help drain queue                      r = workQueue.poll();                  else if (poolSize > corePoolSize || allowCoreThreadTimeOut)                      r = workQueue.poll(keepAliveTime, TimeUnit.NANOSECONDS);                  else                      r = workQueue.take();                  if (r != null)                      return r;                  if (workerCanExit()) {                      if (runState >= SHUTDOWN) // Wake up others                          interruptIdleWorkers();                      return null;                  }                  // Else retry              } catch (InterruptedException ie) {                  // On interruption, re-check runState              }          }      }      }      ..........  }  

        Worker是一个内部类。根据之前可以知道，传入addThread的参数是null，也就是说Work中firstTask为null。
        在看看newThread是一个什么方法： 

public class Executors {      ..........      static class DefaultThreadFactory implements ThreadFactory {          ..........          public Thread newThread(Runnable r) {              Thread t = new Thread(group, r,                                    namePrefix + threadNumber.getAndIncrement(),                                    0);              if (t.isDaemon())                  t.setDaemon(false);              if (t.getPriority() != Thread.NORM_PRIORITY)                  t.setPriority(Thread.NORM_PRIORITY);              return t;          }          ..........      }      ..........  }  

        通过源码可以得知threadFactory的实际类型是DefaultThreadFactory，而DefaultThreadFactory是Executors的一个嵌套内部类。 


        之前我们提到了t.start()这个方法执行了线程。那么现在从头顺一下，看看到底是执行了谁的run方法。首先知道，t=addThread(null)，而addThread内部执行了下面三步，Worker w = new Worker(null);Thread t = threadFactory.newThread(w);return t;这里两个t是一致的。 
        从这里可以看出，t.start()实际上执行的是Worker内部的run方法。run()内部会在if条件里面使用“短路”：判断firstTask是否为null，若不是null则直接执行firstTask的run方法；如果是null，则调用getTask()方法来获取Runnable类型实例。从哪里获取呢？workQueue！在execute方法中，执行ensureQueuedTaskHandled(command)之前就已经把Runnable类型实例放入到workQueue中了，所以这里可以从workQueue中获取到。