Runnable Callable Future FutureTask

 (一)runnable 与Callable的区别

@FunctionalInterface
public interface Runnable {
   public abstract void run();
}
 
@FunctionalInterface
public interface Callable<V> {
   V call() throws Exception;
}

• call()方法可以有返回值，返回类型为泛型V，代表着支持所有类型的返回值。

• call()方法定义时声明了可以抛出异常：throws Exception，而run()则不行。

public interface Future<V> {
   /**
    尝试取消执行此任务。如果任务已完成、已取消或由于其他原因无法取消，则此尝试将失败
    */
   boolean cancel(boolean mayInterruptIfRunning);
 
   /**
    如果此任务在完成之前被取消，则返回True
    */
   boolean isCancelled();
 
   /**
    如果此任务已完成，则返回｛true｝。完成可能是由于正常终止、异常或取消——在所有这些情况下，此方法都将返回｛ true｝
    */
   boolean isDone();
 
   /**
    如果需要，等待计算完成，然后检索其结果,如果计算被取消，则返回计算结果@throws CancellationException
    如果计算引发异常，则返回@throws ExecutionException
    如果当前线程在等待时中断，则返回异常@throws InterruptedException
    */
   V get() throws InterruptedException, ExecutionException;
 
   /**
    如果需要，最多等待给定的时间以完成计算，然后检索其结果（如果可用）
    @param timeout等待的最长时间
    @param unit超时参数的时间单位
    */
   V get(long timeout, TimeUnit unit)
       throws InterruptedException, ExecutionException, TimeoutException;
}

（二）Future与FutureTask

FutureTask实现了RunnableFuture接口，而RunnableFuture除开继承了Runnable外，还继承了Future接口.Future接口中主要提供了两类方法：获取任务执行结果的get方法以及取消任务的cancel方法.

在FutureTask出现之前，Java中的多线程编程执行任务后是不能获取执行结果的。而FutureTask整合了Runnable、Callable、Future三个接口，使得我们的多线程任务执行后可以异步获取到多线程的执行结果。FutureTask会将执行结束后的结果保存在成员变量：outcome中，等待获取执行结果的线程则读取outcome成员值即可。

public class FutureTask<V> implements RunnableFuture<V> {
    /**
     * 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;
    private Callable<V> callable;
    private Object outcome; // non-volatile, protected by state reads/writes
    private volatile Thread runner;
    private volatile WaitNode waiters;

   
    @SuppressWarnings("unchecked")
    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);
    }

    public FutureTask(Callable<V> callable) {
        if (callable == null)
            throw new NullPointerException();
        this.callable = callable;
        this.state = NEW;       // ensure visibility of callable
    }

   ...

    public V get(long timeout, TimeUnit unit)
        throws InterruptedException, ExecutionException, TimeoutException {
        if (unit == null)
            throw new NullPointerException();
        int s = state;
        //如果状态小于COMPLETING，表示FutureTask任务还没完成，则调用awaitDone让当前线程等待
        if (s <= COMPLETING &&
            (s = awaitDone(true, unit.toNanos(timeout))) <= COMPLETING)
            throw new TimeoutException();
        return report(s);
    }

    public void run() {
        //r如果状态值state不是NEW,捉着设置runner值失败，直接返回
        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 {
                    //调用callable方法，获取结果
                    result = c.call();
                    //运行成功
                    ran = true;
                } catch (Throwable ex) {
                    result = null;
                    ran = false;
                    //设置异常
                    setException(ex);
                }
                if (ran)
                //设置返回结果
                    set(result);
            }
        } finally {
        
            runner = null;
            int s = state;
            if (s >= INTERRUPTING)
                handlePossibleCancellationInterrupt(s);
        }
    }

    protected void set(V v) {
        if (UNSAFE.compareAndSwapInt(this, stateOffset, NEW, COMPLETING)) {
            //将结果赋值给outcome
            outcome = v;
            UNSAFE.putOrderedInt(this, stateOffset, NORMAL); // final state
            finishCompletion();
        }
    }

    
    private int awaitDone(boolean timed, long nanos)
        throws InterruptedException {
        final long deadline = timed ? System.nanoTime() + nanos : 0L;
        WaitNode q = null;
        boolean queued = false;
        for (;;) {
            //如果当前线程设置了中断标记
            if (Thread.interrupted()) {
                //从列表中移除节点q，并抛出中断异常
                removeWaiter(q);
                throw new InterruptedException();
            }

            int s = state;
            //如果状态已经完成，表示FutureTask任务已结束
            if (s > COMPLETING) {
                if (q != null)
                    q.thread = null;
                return s;
            }
            //还有一些后续操作没有完成，当前线程让出执行权
            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,
            //timed为true表示需要设置超时                                         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);
        }
    }

}

•  submit与execute是否都可以接受异常，是否都有返回值

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 {@code Executor} 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);
}

execute方法是在executor接口中定义的，执行execute方法时，如果任务不可被接受，则抛出RejectedExecutionException。如果command是null，则抛出NullPointerException。但是执行runnable接口时，其run方法是不抛出异常的。

public interface ExecutorService extends Executor {
     <T> Future<T> submit(Callable<T> task);
     <T> Future<T> submit(Runnable task, T result);
     Future<?> submit(Runnable task);
}

submit方法是在executorService接口中定义的，如果任务不可被接受，则抛出RejectedExecutionException。如果command是null，则抛出NullPointerException。submit执行时，会将Callable或者runnable接口封装成futureTask，然后调用futuretask的run方法，最后实际调用了futuretask内部的Callable成员变量的call方法。对于runnable接口封装成的futureTask，最后实际调用的是runnable的run方法。而对于Callable接口封装成的futureTask，最后实际调用的是Callable的call方法.

总结：execute方法接受的是runnable接口，是不会往外抛异常的。而submit方法，即可接受runnable接口，也可接受Callable接口。抛不抛异常看传入的接口类型。