1 系列目录

• 线程池接口分析以及FutureTask设计实现
• 线程池源码分析-ThreadPoolExecutor

该系列打算从一个最简单的Executor执行器开始一步一步扩展到ThreadPoolExecutor，希望能粗略的描述出线程池的各个实现细节。针对JDK1.7中的线程池

2 Executor接口说明

Executor执行器，就是执行一个Runnable任务，可同步可异步，接口定义如下：

public interface Executor {/*** Executes the given command at some time in the future.  The command* may execute in a new thread, in a pooled thread, or in the calling* thread, at the discretion of the <tt>Executor</tt> implementation.** @param command the runnable task* @throws RejectedExecutionException if this task cannot be* accepted for execution.* @throws NullPointerException if command is null*/void execute(Runnable command);
}

ExecutorService则继承了Executor，描述了线程池应该具有的功能特性，来详细看下接口，这些接口都有详细的文档可以阅读，这里就不再列出来了，目前只说明我们重点关注的接口。

<T> Future<T> submit(Callable<T> task);

可以提交一个Callable，并且返回一个Future用于追踪提交的任务。如何追踪一个任务的状态和返回数据呢？那就需要将提交的任务进行封装，对任务的执行、执行过程中的异常、中断、返回结果进行统一的监控处理。下面就来看看AbstractExecutorService对上述submit的实现

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);
}

从上面看到就是对Callable封装成一个新的任务，即FutureTask，调用Executor的原始接口execute方法来执行FutureTask，并且返回给用户FutureTask对象，用于追踪任务的状态和数据，下面就需要我们来详细看看FutureTask如何对任务进行封装的

3 FutureTask的实现细节

3.1 FutureTask的属性和构造函数

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;/** The underlying callable; nulled out after running */
private Callable<V> callable;
/** The result to return or exception to throw from get() */
private Object outcome; // non-volatile, protected by state reads/writes
/** The thread running the callable; CASed during run() */
private volatile Thread runner;
/** Treiber stack of waiting threads */
private volatile WaitNode waiters;public FutureTask(Callable<V> callable) {if (callable == null)throw new NullPointerException();this.callable = callable;this.state = NEW;       // ensure visibility of callable
}

有一个状态变量state,一个Callable callable即原始任务，Object outcome存放原始任务的输出结果或者异常，Thread runner运行该任务的线程，WaitNode waiters等待获取任务结果的等待者

3.2 FutureTask的get方法实现

使用FutureTask阻塞式等待任务执行结果，一种是永远阻塞另一种就是阻塞一定时间否则报超时异常，如下2个方法

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);
}

阻塞式等待的核心逻辑就在上述awaitDone方法中，来详细看看

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) {if (q != null)q.thread = null;return s;}else if (s == COMPLETING) // cannot time out yetThread.yield();else if (q == null)q = new WaitNode();else if (!queued)queued = UNSAFE.compareAndSwapObject(this, waitersOffset,q.next = waiters, q);else if (timed) {nanos = deadline - System.nanoTime();if (nanos <= 0L) {removeWaiter(q);return state;}LockSupport.parkNanos(this, nanos);}elseLockSupport.park(this);}
}

可以看到有一个for循环不断处理着各种情况：

1 从最开始的WaitNode q = null，构建了一个WaitNode，即代表着当前线程作为一个等待者，WaitNode就是一个简单的链表，如下

static final class WaitNode {volatile Thread thread;volatile WaitNode next;WaitNode() { thread = Thread.currentThread(); }
}

2 构建好WaitNode之后就要将该WaitNode放入链表中，这时候就会涉及多线程问题，使用UNSAFE的CAS来解决，这种方式也是AtomicLong等众多原子类的底层实现方式

3 成功放入WaitNode链表之后，采用LockSupport的park阻塞当前线程，要么只阻塞一定时间要么一直阻塞，直到被LockSupport的unpark唤醒。LockSupport在锁的底层实现AQS中也非常常见，使用了LockSupport就可以不用在for循环里不断判断当前任务状态而浪费CPU，只需要当前任务完成之后，使用LockSupport对等待线程进行unpark，就可以使等待的线程退出等待继续往下执行

4 如果LockSupport阻塞时间到了，还未收到unpark，则需要从等待者链表中删除当前线程代表的等待者

3.3 FutureTask的任务执行过程

public void run() {if (state != NEW ||!UNSAFE.compareAndSwapObject(this, runnerOffset,null, Thread.currentThread()))return;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 {// 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 interruptsint s = state;if (s >= INTERRUPTING)handlePossibleCancellationInterrupt(s);}
}

1 一旦FutureTask任务开始执行了，就需要将当前执行线程设置到FutureTask的volatile Thread runner属性中

2 执行原始任务Callable的call方法，可能成功也可能失败也可能被中断被取消

文档中有如下状态的迁移过程：

Possible state transitions:* NEW -> COMPLETING -> NORMAL* NEW -> COMPLETING -> EXCEPTIONAL* NEW -> CANCELLED* NEW -> INTERRUPTING -> INTERRUPTED

来看下成功和失败方法

protected void set(V v) {if (UNSAFE.compareAndSwapInt(this, stateOffset, NEW, COMPLETING)) {outcome = v;UNSAFE.putOrderedInt(this, stateOffset, NORMAL); // final statefinishCompletion();}
}protected void setException(Throwable t) {if (UNSAFE.compareAndSwapInt(this, stateOffset, NEW, COMPLETING)) {outcome = t;UNSAFE.putOrderedInt(this, stateOffset, EXCEPTIONAL); // final statefinishCompletion();}
}

都是首先将状态变成COMPLETING正在结束中，然后设置outcome，成功则设置正常的返回值，失败则设置成异常，然后根据划定最终的状态结果，成功就是NORMAL，失败就是EXCEPTIONAL，最后呢调用finishCompletion，去unpark之前说的WaitNode中对应的线程们

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 gcq = next;}break;}}done();callable = null;        // to reduce footprint
}

这里就是遍历WaitNode链表，对每一个WaitNode对应的线程依次进行LockSupport.unpark(t)，使其结束阻塞。WaitNode通知完毕后，调用一个done方法，目前该方法是空的实现，所以你如果想在任务完成后执行一些动作的时候就可以重写该方法

有一个问题就是：为什么一定要加入COMPLETING状态呢？能不能直接过度到NORMAL或者EXCEPTIONAL？

目前我的理解是：NORMAL或者EXCEPTIONAL是一种最终状态，所以在出现该状态前，outcome必须已经被设置了，即有如下代码：

protected void set(V v) {outcome = v;UNSAFE.compareAndSwapInt(this, stateOffset, NEW, NORMAL)finishCompletion();
}

但是因为存在外部直接取消该任务，所以结果状态的设置和outcome必须是同步的，且outcome在前，为了保证代码的同步可以使用锁

protected void set(V v) {synchronized(){outcome = v;UNSAFE.compareAndSwapInt(this, stateOffset, NEW, NORMAL)finishCompletion();}
}

为了减少锁带来的开支，就可以引入一个中间状态COMPLETING，通过CAS来间接实现锁的竞争，同时又保证outcome在最终状态NORMAL或者EXCEPTIONAL之前被设置

3.4 FutureTask任务的取消

public boolean cancel(boolean mayInterruptIfRunning) {if (state != NEW)return false;if (mayInterruptIfRunning) {if (!UNSAFE.compareAndSwapInt(this, stateOffset, NEW, INTERRUPTING))return false;Thread t = runner;if (t != null)t.interrupt();UNSAFE.putOrderedInt(this, stateOffset, INTERRUPTED); // final state}else if (!UNSAFE.compareAndSwapInt(this, stateOffset, NEW, CANCELLED))return false;finishCompletion();return true;
}

取消任务，有2种情况，一种该任务正在运行，一种就是非运行状态，所以需要用户给出明示是否中断正在运行的任务，即需要一个参数mayInterruptIfRunning

中断任务就是通过中断运行该任务的线程，即直接调用该线程的interrupt()方法

4 结束语

FutureTask大部分就简单分析完了，其他的自己去看下就行了。至此我们了解了一个任务被提交经过了封装，变成了一个新的任务FutureTask,同时FutureTask也明确了该任务的整个执行过程，只留出核心execute(futureTask)方法需要被子类来实现，下一篇文章就重点介绍下ThreadPoolExecutor对该核心方法的实现