并发系列（二）——FutureTask类源码简析

背景

　　本文基于JDK 11，主要介绍FutureTask类中的run()、get()和cancel() 方法，没有过多解析相应interface中的注释，但阅读源码时建议先阅读注释，明白方法的主要的功能，再去看源码会更快。

　　文中若有不正确的地方欢迎大伙留言指出，谢谢了！

1、FutureTask类图

　　1.1 FutureTask简介

　　FutureTask类图如下（使用IDEA生成）。如图所示，FutureTask实现了Future接口的所有方法，并且实现了Runnable接口，其中，Runnable接口的现实类用于被线程执行，而Future代表的是异步计算的结果。因此，FutureTask类可以理解为，执行run()（实现Runnable接口中的方法），通过Future的get()方法获取结果。

　　1.2 FutureTask的属性

 //任务线程总共有七中状态如下：
    * Possible state transitions:
     * NEW -> COMPLETING -> NORMAL
     * NEW -> COMPLETING -> EXCEPTIONAL
     * NEW -> CANCELLED
     * NEW -> INTERRUPTING -> INTERRUPTED
     */
    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 */
    //在run()方法中调用
    private Callable<V> callable;
    /** The result to return or exception to throw from get() */
    //任务执行结果，callable.call()正常执行的返回值
    private Object outcome; // non-volatile, protected by state reads/writes
    /** The thread running the callable; CASed during run() */
    //任务线程
    private volatile Thread runner;
    /** Treiber stack of waiting threads */
    //等待任务结果的线程组成的节点，放在链表对列中
    private volatile WaitNode waiters;

 2、源码解析

　　2.1 run()方法

public void run() {
        //1、若是任务的状态不是NEW，且使用CAS将runner置为当前线程则直接返回
        if (state != NEW ||
            !RUNNER.compareAndSet(this, null, Thread.currentThread()))
            return;
        try {
            Callable<V> c = callable;
            //2、任务不为null，且state的状态为NEW的情况下才执行任务
            if (c != null && state == NEW) {
                V result;
                boolean ran;
                try {
                    //执行任务并接收执行结果
                    result = c.call();
                    //正常执行结果则将标识置为true
                    ran = true;
                } catch (Throwable ex) {
                    //3、任务发生异常，执行或cancel()，则结果置为null，并记录异常信息
                    result = null;
                    ran = false;
                    setException(ex);
                }
                //4、任务正常结束，则设置返回结果
                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 interrupts
            int s = state;
            //5、若是异常导致，走另一个流程
            if (s >= INTERRUPTING)
                handlePossibleCancellationInterrupt(s);
        }
    }

　　1）若任务的状态不是NEW，或者使用CAS将runner置为当前线程失败，则直接返回的原因是防止多线程调用；

　　2）再度确认任务执行的前置条件；

　　3）任务执行异常，将result置为null，并记录异常，setException()源码如下：

protected void setException(Throwable t) {
        //使用CAS将状态置为中间态COMPLETING
        if (STATE.compareAndSet(this, NEW, COMPLETING)) {
            outcome = t;
            STATE.setRelease(this, EXCEPTIONAL); // final state
            //任务处于结束态时，遍历唤醒等待result的线程
            finishCompletion();
        }
    }

　　任务的状态变化为NEW  - >  COMPLETING  ->  EXCEPTIONAL

　　4）任务正常结果则会设置result之后，唤醒waitNode的链表对列中等待任务结果的线程；

　　5）异常后的调用逻辑如下：

 //保证调用cancel在run方法返回之前中断执行任务
    private void handlePossibleCancellationInterrupt(int s) {
        // It is possible for our interrupter to stall before getting a
        // chance to interrupt us.  Let's spin-wait patiently.
        if (s == INTERRUPTING)
            //自旋等待
            while (state == INTERRUPTING)
            //当前线程让出CPU执行权
                Thread.yield(); // wait out pending interrupt
    }

 　　2.2  get()方法

　　源码分析如下：

public V get() throws InterruptedException, ExecutionException {
        int s = state;
        if (s <= COMPLETING)
            //等待任务完成
            s = awaitDone(false, 0L);
        //返回结果
        return report(s);
    }

　　其中，等待过程分析如下：

private int awaitDone(boolean timed, long nanos)
        throws InterruptedException {
        // The code below is very delicate, to achieve these goals:
        // - call nanoTime exactly once for each call to park
        // - if nanos <= 0L, return promptly without allocation or nanoTime
        // - if nanos == Long.MIN_VALUE, don't underflow
        // - if nanos == Long.MAX_VALUE, and nanoTime is non-monotonic
        //   and we suffer a spurious wakeup, we will do no worse than
        //   to park-spin for a while
        long startTime = 0L;    // Special value 0L means not yet parked
        WaitNode q = null;
        boolean queued = false;
        for (;;) {
            int s = state;
            //1、任务的状态已经处于最终的状态，则将任务线程的引用置为null，直接返回状态
            if (s > COMPLETING) {
                if (q != null)
                    q.thread = null;
                return s;
            }
            //2、任务的状态为COMPLETING说明任务已经接近完成，则当前线程让出CPU权限以便任务执行线程获取到CPU执行权
            else if (s == COMPLETING)
                // We may have already promised (via isDone) that we are done
                // so never return empty-handed or throw InterruptedException
                Thread.yield();
            //3、当前线程被中断，则将当前线程从等待任务结果的对列中移除，并抛出异常
            else if (Thread.interrupted()) {
                removeWaiter(q);
                throw new InterruptedException();
            }
            //4、任务线程的状态小于COMPLETING，则将当前调用get()方法的线程新建一个Node
            else if (q == null) {
                if (timed && nanos <= 0L)
                    return s;
                q = new WaitNode();
            }
            //5、若由当前线程构成的Node未加入链表中，则加入
            else if (!queued)
                queued = WAITERS.weakCompareAndSet(this, q.next = waiters, q);
            //6、是否开启了超时获取结果
            else if (timed) {
                final long parkNanos;
                if (startTime == 0L) { // first time
                    startTime = System.nanoTime();
                    if (startTime == 0L)
                        startTime = 1L;
                    parkNanos = nanos;
                } else {
                    long elapsed = System.nanoTime() - startTime;
                    //7、超时则从栈中移除当前线程
                    if (elapsed >= nanos) {
                        removeWaiter(q);
                        return state;
                    }
                    parkNanos = nanos - elapsed;
                }
                // nanoTime may be slow; recheck before parking
                //当前线程挂起
                if (state < COMPLETING)
                    LockSupport.parkNanos(this, parkNanos);
            }
            else
                LockSupport.park(this);
        }
    }

　　获取到返回的状态值后，根据其状态值判断是返回结果还是抛出异常。

　　2.2 cancel()方法

public boolean cancel(boolean mayInterruptIfRunning) {
        //1、若任务线程的状态为NEW，则将其状态从NEW置为INTERRUPTING、CANCELLED
        if (!(state == NEW && STATE.compareAndSet
              (this, NEW, mayInterruptIfRunning ? INTERRUPTING : CANCELLED)))
            //CAS改变任务线程的状态失败，则直接返回false，表示cancel失败
            return false;
        try {    // in case call to interrupt throws exception
            //2、改变任务线程的状态成功后，根据是否中断running的任务线程的标识位，决定是否中断正在运行的任务线程
            if (mayInterruptIfRunning) {
                try {
                    Thread t = runner;
                    //任务线程不为null，则使用interrupt()中断
                    if (t != null)
                        t.interrupt();
                } finally { // final state
                    //设置状态
                    STATE.setRelease(this, INTERRUPTED);
                }
            }
        } finally {
            //3、清理等待任务结果的等待线程
            finishCompletion();
        }
        return true;
    }

 3、总结

　　1）执行run()方法，是在调用在Callable的call()方法，其实在初始化时被指定；

　　2）调用get()方法，若是任务线程还在执行，则会把调用get的线程封装成waitNode塞入到FutureTask类内部的阻塞链表对列中，可以有多个线程同时调用get()方法；

　　3）cancel()方法是通过对任务线程调用interrupt()实现；