<p>上一篇分析了线程池的执行原理，主要关于线程池的生命周期和任务如何在池里创建、运行和终止。不过上次研究的是execute方法，执行的是Runnable任务，它不返回任何值。如果希望任务完成后返回结果，那么需要使用Callable接口，这也是本文要研究的主题。</p>    <pre>  <code class="language-java">ExecutorService es = Executors.newSingleThreadExecutor();  Future<?> task = es.submit(new MyThread());  try {      //限定时间获取结果      task.get(5, TimeUnit.SECONDS);  } catch (TimeoutException e) {      //超时触发线程中止      System.out.println("thread over time");  } catch (ExecutionException e) {     //抛出执行异常      throw e;  } finally {     //如果任务还在运行，执行中断      boolean mayInterruptIfRunning = true;      task.cancel(mayInterruptIfRunning);  }</code></pre>    <p>上面代码是Future的一个简单例子：MyThread实现Callable接口，执行时要求在限定时间内获取结果，超时执行会抛出TimeoutException，执行异常会抛出ExecutionException。最后在finally里，如果任务还在执行，就进行取消；如果任务已经执行完，取消操作也没有影响。</p>    <p style="text-align:center"><img src="https://simg.open-open.com/show/bc192907d3008855e6aa848394f550a7.png"></p>    <p>图1 FutureTask</p>    <p>Future接口代表一个异步任务的结果，提供了相应方法判断任务是否完成或者取消。从图1可知，RunnableFuture同时继承了Future和Runnable，是一个可运行、可知结果的任务，FutureTask是具体的实现类。</p>    <h3><strong>FutureTask的状态</strong></h3>    <pre>  <code class="language-java">private volatile int state;  private static final int NEW          = 0;  private static final int COMPLETING   = 1;  private static final int NORMAL       = 2;  private static final int EXCEPTIONAL  = 3;  private static final int CANCELLED    = 4;  private static final int INTERRUPTING = 5;  private static final int INTERRUPTED  = 6;</code></pre>    <p>FutureTask有7种状态，初始状态从NEW开始，状态转换路径可以归纳为图2所示。在后文的代码，会使用int的大小比较判断状态处于哪个范围，需要留意上面状态的排列顺序。</p>    <p style="text-align:center"><img src="https://simg.open-open.com/show/beda8415725d631b0f347860c37b7366.png"></p>    <p>图2 FutureTask状态路径</p>    <p>FutureTask的状态路径，取决于run和cancel的调用顺序，在后文分析时，对号入座这几条路径。</p>    <ol>     <li>NEW -> COMPLETING -> NORMAL 正常的流程</li>     <li>NEW -> COMPLETING -> EXCEPTIONAL 异常的流程</li>     <li>NEW -> CANCELLED 被取消流程</li>     <li>NEW -> INTERRUPTING -> INTERRUPTED 被中断流程</li>    </ol>    <h3><strong>FutureTask的变量</strong></h3>    <ul>     <li>int state</li>     <li>Thread runner</li>     <li>WaitNode waiters</li>     <li>Callable<V> callable</li>     <li>Object outcome</li>    </ul>    <p>state、runner、waiters三个变量没有使用原子类，而是使用Unsafe对象进行原子操作。代码中会见到很多形如compareAndSwap的方法，入门原理可以看我以前写的 认识非阻塞的同步机制CAS 。</p>    <p>callable是要执行的任务，runner是执行任务的线程，outcome是返回的结果（正常结果或Exception结果）</p>    <pre>  <code class="language-java">static final class WaitNode {      volatile Thread thread;      volatile WaitNode next;      WaitNode() { thread = Thread.currentThread(); }  }</code></pre>    <p>waiters的数据结构是WaitNode，保存了Thread和下个WaitNode的引用。waiters保存了等待结果的线程，每次操作只会增减头，所以是一个栈结构，详细见后文对get方法的分析。</p>    <h3><strong>FutureTask的创建</strong></h3>    <pre>  <code class="language-java">public FutureTask(Callable<V> callable) {      if (callable == null)          throw new NullPointerException();      this.callable = callable;      this.state = NEW;       // ensure visibility of callable  }    public FutureTask(Runnable runnable, V result) {      this.callable = Executors.callable(runnable, result);      this.state = NEW;       // ensure visibility of callable  }</code></pre>    <p>FutureTask可以接受Callable或者Runnable，state从NEW开始。如果是Runnable，需要调用Executors.callable转成Callable，返回的结果是预先传入的result。转换过程使用一个实现了Callable的RunnableAdapter包装Runnable和result，代码比较简单。</p>    <pre>  <code class="language-java">static final class RunnableAdapter<T> implements Callable<T> {      final Runnable task;      final T result;      RunnableAdapter(Runnable task, T result) {          this.task = task;          this.result = result;      }      public T call() {          task.run();          return result;      }  }</code></pre>    <p>提交FutureTask到线程池的submit定义在AbstractExecutorService，根据入参的不同，有三个submit方法。下面以提交Callable为例：</p>    <pre>  <code class="language-java">public <T> Future<T> submit(Callable<T> task) {     if (task == null) throw new NullPointerException();     RunnableFuture<T> ftask = newTaskFor(task);     execute(ftask);     return ftask;  }    protected <T> RunnableFuture<T> newTaskFor(Callable<T> callable) {         return new FutureTask<T>(callable);  }</code></pre>    <p>FutureTask在newTaskFor创建，然后调用线程池的execute执行，最后返回Future。获取Future后，就可以调用get获取结果，或者调用cancel取消任务。</p>    <h3><strong>FutureTask的运行</strong></h3>    <p>FutureTask实现了Runnable，在线程池里执行时调用的方法是run。</p>    <pre>  <code class="language-java">public void run() {      //1      if (state != NEW ||          !UNSAFE.compareAndSwapObject(this, runnerOffset,null, Thread.currentThread()))          return;      //2      try {          Callable<V> c = callable;          if (c != null && state == NEW) {              V result;              boolean ran;              try {                  result = c.call();                  ran = true;              } catch (Throwable ex) {                  result = null;                  ran = false;                  setException(ex);              }              if (ran)                  set(result);          }      } finally {         //3          runner = null;          int s = state;          if (s >= INTERRUPTING)              handlePossibleCancellationInterrupt(s);      }  }</code></pre>    <p>标记1处检查FutureTask的状态，如果不是处于NEW，说明状态已经进入四条路径之一，也就没有必要继续了。如果状态是NEW，则将执行任务的线程交给runner。</p>    <p>标记2处开始正式执行任务，调用call方法获取结果，没有异常就算成功，最后执行set方法；出现异常就调用setException方法。</p>    <p>标记3处，无论任务执行是否成功，都需要将runner重新置为空。</p>    <pre>  <code class="language-java">protected void set(V v) {      if (UNSAFE.compareAndSwapInt(this, stateOffset, NEW, COMPLETING)) {          outcome = v;          UNSAFE.putOrderedInt(this, stateOffset, NORMAL); // final state          finishCompletion();      }  }    protected void setException(Throwable t) {      if (UNSAFE.compareAndSwapInt(this, stateOffset, NEW, COMPLETING)) {          outcome = t;          UNSAFE.putOrderedInt(this, stateOffset, EXCEPTIONAL); // final state          finishCompletion();      }  }</code></pre>    <p>任务执行成功与失败，分别对应NEW -> COMPLETING -> NORMAL和NEW -> COMPLETING -> EXCEPTIONAL两条路径。这里先将状态修改为中间状态，再对结果赋值，最后再修改为最终状态。</p>    <pre>  <code class="language-java">private void finishCompletion() {      // assert state > COMPLETING;      for (WaitNode q; (q = waiters) != null;) {          if (UNSAFE.compareAndSwapObject(this, waitersOffset, q, null)) {              for (;;) {                  Thread t = q.thread;                  if (t != null) {                      q.thread = null;                      LockSupport.unpark(t);                  }                  WaitNode next = q.next;                  if (next == null)                      break;                  q.next = null; // unlink to help gc                  q = next;              }              break;          }      }      done();      callable = null;        // to reduce footprint  }</code></pre>    <p>最后调用finishCompletion执行任务完成，唤醒并删除所有在waiters中等待的线程。done方法是空的，供子类实现，最后callable也设置为空。</p>    <p>FutureTask还有个runAndReset，逻辑和run类似，但没有调用set方法来设置结果，执行完成后将任务重新初始化。</p>    <pre>  <code class="language-java">protected boolean runAndReset() {      if (state != NEW ||          !UNSAFE.compareAndSwapObject(this, runnerOffset,                                       null, Thread.currentThread()))          return false;      boolean ran = false;      int s = state;      try {          Callable<V> c = callable;          if (c != null && s == NEW) {              try {                  c.call(); // don't set result                  ran = true;              } catch (Throwable ex) {                  setException(ex);              }          }      } finally {          // runner must be non-null until state is settled to          // prevent concurrent calls to run()          runner = null;          // state must be re-read after nulling runner to prevent          // leaked interrupts          s = state;          if (s >= INTERRUPTING)              handlePossibleCancellationInterrupt(s);      }      return ran && s == NEW;  }</code></pre>    <h3><strong>FutureTask的取消</strong></h3>    <p>对于已经提交执行的任务，可以调用cancel执行取消。</p>    <pre>  <code class="language-java">public boolean cancel(boolean mayInterruptIfRunning) {     //1      if (!(state == NEW &&            UNSAFE.compareAndSwapInt(this, stateOffset, NEW,                mayInterruptIfRunning ? INTERRUPTING : CANCELLED)))          return false;      try {    // in case call to interrupt throws exception         //2          if (mayInterruptIfRunning) {              try {                  Thread t = runner;                  if (t != null)                      t.interrupt();              } finally { // final state                  UNSAFE.putOrderedInt(this, stateOffset, INTERRUPTED);              }          }      } finally {          finishCompletion();      }      return true;  }</code></pre>    <p>标记1处判断任务状态，为NEW才能被取消。如果mayInterruptIfRunning是true，代表任务需要被中断，走NEW -> INTERRUPTING -> INTERRUPTED流程。否则代表任务被取消，走NEW -> CANCELLED流程。</p>    <p>标记2处理任务被中断的情况，这里仅仅是对线程发出中断请求，不确保任务能检测并处理中断，详细原理去看Java的中断机制。</p>    <p>最后调用finishCompletion完成收尾工作。</p>    <pre>  <code class="language-java">public boolean isCancelled() {      return state >= CANCELLED;  }</code></pre>    <p>判断任务是否被取消，具体逻辑是判断state >= CANCELLED，包括了被中断一共两条路径的结果。</p>    <h3><strong>FutureTask获取结果</strong></h3>    <p>调用FutureTask的get方法获取任务的执行结果，可以阻塞直到获取结果，也可以限制范围时间内获取结果，否则抛出TimeoutException。</p>    <pre>  <code class="language-java">public V get() throws InterruptedException, ExecutionException {      int s = state;      if (s <= COMPLETING)          s = awaitDone(false, 0L);      return report(s);  }    public V get(long timeout, TimeUnit unit)      throws InterruptedException, ExecutionException, TimeoutException {      if (unit == null)          throw new NullPointerException();      int s = state;      if (s <= COMPLETING &&          (s = awaitDone(true, unit.toNanos(timeout))) <= COMPLETING)          throw new TimeoutException();      return report(s);  }</code></pre>    <p>get的核心实现调用了awaitDone，入参为是否开启时间限制和最大的等待时间。</p>    <pre>  <code class="language-java">private int awaitDone(boolean timed, long nanos)      throws InterruptedException {      final long deadline = timed ? System.nanoTime() + nanos : 0L;      WaitNode q = null;      boolean queued = false;      for (;;) {          if (Thread.interrupted()) {              removeWaiter(q);              throw new InterruptedException();          }            int s = state;          if (s > COMPLETING) {    //1              if (q != null)                  q.thread = null;              return s;          }          else if (s == COMPLETING) // cannot time out yet    //2              Thread.yield();          else if (q == null)     //3              q = new WaitNode();          else if (!queued)    //4              queued = UNSAFE.compareAndSwapObject(this, waitersOffset,                                                   q.next = waiters, q);          else if (timed) {    //5              nanos = deadline - System.nanoTime();              if (nanos <= 0L) {                  removeWaiter(q);                  return state;              }              LockSupport.parkNanos(this, nanos);          }          else     //6              LockSupport.park(this);      }  }</code></pre>    <p>awaitDone主要逻辑是一个无限循环，首先判断线程是否被中断，是的话移除waiter并抛出中断异常。接下来是一串if-else，一共六种情况。</p>    <ol>     <li>判断任务状态是否已经完成，是就直接返回；</li>     <li>任务状态是COMPLETING，代表在set结果时被阻塞了，这里先让出资源；</li>     <li>如果WaitNode为空，就为当前线程初始化一个WaitNode；</li>     <li>如果当前的WaitNode还没有加入waiters，就加入；</li>     <li>如果是限定时间执行，判断有无超时，超时就将waiter移出，并返回结果，否则阻塞一定时间；</li>     <li>如果没有限定时间，就一直阻塞到下次被唤醒。</li>    </ol>    <p>LockSupport是用来创建锁和其他同步类的基本线程阻塞原语。park和unpark的作用分别是阻塞线程和解除阻塞线程。</p>    <pre>  <code class="language-java">private V report(int s) throws ExecutionException {     Object x = outcome;     if (s == NORMAL)         return (V)x;     if (s >= CANCELLED)         throw new CancellationException();     throw new ExecutionException((Throwable)x);  }</code></pre>    <p>最后get调用report，使用outcome返回结果。</p>    <p style="text-align:center"><img src="https://simg.open-open.com/show/d7c532dd4fdbd4277271d131ef69e9a8.png"></p>    <p>图3</p>    <p>看图3，如果多个线程向同一个FutureTask实例get结果，但FutureTask又没有执行完毕，线程将会阻塞并保存在waiters中。待FutureTask获取结果后，唤醒waiters等待的线程，并返回同一个结果。</p>    <h3><strong>总结</strong></h3>    <p style="text-align:center"><img src="https://simg.open-open.com/show/2032bd1bf05efa607d34b723ce0597b4.png"></p>    <p>图4</p>    <p>图4归纳了FutureTask的作用，任务的调用线程Caller和线程池的工作线程通过FutureTask交互。对比线程池的执行原理，FutureTask是比较简单的。</p>    <p> </p>    <p>来自：http://www.jianshu.com/p/f624934b9a23</p>    <p> </p>