分析 Java 线程池 Callable 任务执行原理
KariShaffer
7年前
<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>