【源码解析】聊聊线程池实现原理与源码深度解析（二）

AbstractExecutorService

上一篇文章中，主要介绍了AbstractExecutorService的线程执行的核心流程，execute() 这个方法显然是没有返回执行任务的结果，如果我们需要获取任务执行的结果，怎么办？

Callable 就是一个可以获取线程执行的结果。

public abstract class AbstractExecutorService implements ExecutorService {

  	/*
     * 将任务包装成FutureTask任务。带返回值参数的
     */
    protected <T> RunnableFuture<T> newTaskFor(Runnable runnable, T value) {
        return new FutureTask<T>(runnable, value);
    }

    /**
    ** 不带返回值的
    **/
    protected <T> RunnableFuture<T> newTaskFor(Callable<T> callable) {
        return new FutureTask<T>(callable);
    }

    /**
     * @throws RejectedExecutionException {@inheritDoc}
     * @throws NullPointerException       {@inheritDoc}
     */
    public Future<?> submit(Runnable task) {
        if (task == null) throw new NullPointerException();
        //1.将任务包装成RunableFuture对象，由于RunnableFuture是实现Runable类，所以execute的参数是一个可拓展的类型
        RunnableFuture<Void> ftask = newTaskFor(task, null);
        //2，交给具体的执行器进行实现
        execute(ftask);
        return ftask;
    }

    /**
     * @throws RejectedExecutionException {@inheritDoc}
     * @throws NullPointerException       {@inheritDoc}
     */
    public <T> Future<T> submit(Runnable task, T result) {
        if (task == null) throw new NullPointerException();
        RunnableFuture<T> ftask = newTaskFor(task, result);
        execute(ftask);
        return ftask;
    }

    /**
     * @throws RejectedExecutionException {@inheritDoc}
     * @throws NullPointerException       {@inheritDoc}
     */
    public <T> Future<T> submit(Callable<T> task) {
        if (task == null) throw new NullPointerException();
        //将任务装成成一个FutureTask任务
        RunnableFuture<T> ftask = newTaskFor(task);
        //执行任务
        execute(ftask);
        return ftask;
    }
  }

submit其实是一个重载的方法，分别是一个task，以及可以传递获取结果的任务，以及使用callable。

demo

从源码上看三个方法其实都是将任务进行了封装，然后调用线程池执行的核心方法

    public static void main(String[] args) throws ExecutionException, InterruptedException {
        Callable<Integer> resultCallable = new Callable<Integer>() {
            @Override
            public Integer call() throws Exception {
                return 1 + 1;
            }
        };

        ExecutorService threadPool = Executors.newFixedThreadPool(1);
        Future<Integer> resultTask = threadPool.submit(resultCallable);
        System.out.println(resultTask.get());
        threadPool.shutdown();
    }

FutureTask

public class FutureTask<V> implements RunnableFuture<V> {
     
    /* NEW -> COMPLETING -> NORMAL
     * NEW -> COMPLETING -> EXCEPTIONAL
     * NEW -> CANCELLED
     * NEW -> INTERRUPTING -> INTERRUPTED
     */
    private volatile int state;
    private static final int NEW          = 0; // 初始化状态
    private static final int COMPLETING   = 1; // 结果计算完成或响应中断到赋值给返回值的状态
    private static final int NORMAL       = 2; // 任务正常完成，结果被set
    private static final int EXCEPTIONAL  = 3; // 任务抛出异常
    private static final int CANCELLED    = 4; // 任务被取消
    private static final int INTERRUPTING = 5; // 线程中断状态被设置为true 线程未响应中断
    private static final int INTERRUPTED  = 6; // 线程已被中断

    /** The underlying callable; nulled out after running */
    private Callable<V> callable; // 需要执行的任务
    /** The result to return or exception to throw from get() */
    // 执行callable的线程，调用FutureTask.run()方法通过CAS设置
    private Object outcome; // non-volatile, protected by state reads/writes
    /** The thread running the callable; CASed during run() */
    // 执行callable的线程，调用FutureTask.run()方法通过CAS设置
    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; // 初始化状态是new      // ensure visibility of callable
    }
}

 /* 继承了Runnable ，因为线程池中执行的也是Runnbale的任务
 */
public interface RunnableFuture<V> extends Runnable, Future<V> {
    /**
     * Sets this Future to the result of its computation
     * unless it has been cancelled.
     */
    void run();
}

FutureTask 实现RunnableFuture，也间接实现了run方法。

重点

我们知道 execute(ftask); 本质就是利用线程池进行执行，而线程执行的时候，其实就是启动对应任务的run方法。

task.run();

		// 这里是什么时候调用的，其实是
    // execute(ftask)传入的任务 task.run()
    public void run() {
        //不是新建状态 直接中止
        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 {
                    //核心，执行任务的call方法，你看就是调用普通的方法一样。
                    result = c.call();
                    //同步调用获取结果值
                    ran = true;
                } catch (Throwable ex) {
                    result = null;
                    ran = false;
                    setException(ex);
                }
                if (ran)
                    //设置结果值
                    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;
            //响应中断
            if (s >= INTERRUPTING)
                handlePossibleCancellationInterrupt(s);
        }
    }

判断当前任务状态，非NEW直接返回
执行对应c.call() 其实就是执行callable中的call方法。
将返回值set进去

    protected void set(V v) {
        //CAS 去设置当前任务执行状态 new-completing
        if (UNSAFE.compareAndSwapInt(this, stateOffset, NEW, COMPLETING)) {
            //返回结果outcome
            outcome = v;
            UNSAFE.putOrderedInt(this, stateOffset, NORMAL); // final state
            finishCompletion();
        }
    }

get

    public V get() throws InterruptedException, ExecutionException {
        int s = state;
        //如果是在执行中，则等待一会
        if (s <= COMPLETING)
            s = awaitDone(false, 0L);
        //返回结果
        return report(s);
    }

    /**
     * @throws CancellationException {@inheritDoc}
     */
    public V get(long timeout, TimeUnit unit)
        throws InterruptedException, ExecutionException, TimeoutException {
        if (unit == null)
            throw new NullPointerException();
        //设置了超时时间，则等待一定的时间，如果还没有获取到返回异常
        int s = state;
        if (s <= COMPLETING &&
            (s = awaitDone(true, unit.toNanos(timeout))) <= COMPLETING)
            throw new TimeoutException();
        return report(s);
    }

    private V report(int s) throws ExecutionException {
        Object x = outcome;
        //执行完成 返回x结果
        if (s == NORMAL)
            return (V)x;
        //如果任务取消，返回异常
        if (s >= CANCELLED)
            throw new CancellationException();
        throw new ExecutionException((Throwable)x);
    }

awaitDone

    private int awaitDone(boolean timed, long nanos)
        throws InterruptedException {
        final long deadline = timed ? System.nanoTime() + nanos : 0L;
        WaitNode q = null;
        boolean queued = false;
        for (;;) {
            //如果线程执行interrupted，直接抛出异常，并且将任务移除
            if (Thread.interrupted()) {
                removeWaiter(q);
                throw new InterruptedException();
            }

            int s = state;
            //状态大于COMPLETING 说明完成了
            if (s > COMPLETING) {
                if (q != null)
                    q.thread = null;
                return s;
            }
            //
            else if (s == COMPLETING) // cannot time out yet
                Thread.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);
            }
            else
                LockSupport.park(this);
        }
    }