深入Callable及Runnable两个接口 获取线程返回结果

今天碰到一个需要获取线程返回结果的业务场景，所以了解到了Callable接口。

先来看下下面这个例子：

public class ThreadTest {

    public static void main(String[] args) throws Exception {
        ExecutorService exc = Executors.newCachedThreadPool();
        try {
            String result = null;
            FutureTask<String> task = (FutureTask<String>) exc.submit(new Runnable() {
                @Override
                public void run() {
                    for (int i = 0; i <; i++) {
                        try {
                            Thread.sleep(100L);
                        } catch (InterruptedException e) {
                            e.printStackTrace();
                        }
                        System.out.println(this.getClass() + "::线程执行中.." + i);
                    }
                }
            }, result);

            System.out.println("task return value:" + task.get());

            FutureTask<String> callableTask = (FutureTask<String>) exc.submit(new Callable<String>() {

                @Override
                public String call() throws InterruptedException {
                    for (int i = 0; i <; i++) {
                        Thread.sleep(100L);
                        System.out.println(this.getClass() + "::线程执行中.." + i);
                    }
                    return "success";
                }

            });

            System.out.println("提前出结果了 task return value:" + task.get());

            System.out.println("callableTask return value:" + callableTask.get());

        } finally {
            exc.shutdown();
        }

    }

}

运行结果如下：

class thread.ThreadTest$1::线程执行中..0
class thread.ThreadTest$1::线程执行中..1
class thread.ThreadTest$1::线程执行中..2
class thread.ThreadTest$1::线程执行中..3
class thread.ThreadTest$1::线程执行中..4
class thread.ThreadTest$1::线程执行中..5
class thread.ThreadTest$1::线程执行中..6
class thread.ThreadTest$1::线程执行中..7
class thread.ThreadTest$1::线程执行中..8
class thread.ThreadTest$1::线程执行中..9
task return value:null
提前出结果了 task return value:null
class thread.ThreadTest$2::线程执行中..0
class thread.ThreadTest$2::线程执行中..1
class thread.ThreadTest$2::线程执行中..2
class thread.ThreadTest$2::线程执行中..3
class thread.ThreadTest$2::线程执行中..4
class thread.ThreadTest$2::线程执行中..5
class thread.ThreadTest$2::线程执行中..6
class thread.ThreadTest$2::线程执行中..7
class thread.ThreadTest$2::线程执行中..8
class thread.ThreadTest$2::线程执行中..9
callableTask return value:success

可以得到以下几点：

1 Runnable，Callable两个接口方法体不一样，前者为run，后者为call，且返回值也不一样；

2 Runnable接口由于run方法返回void所以无法解决线程成功后返回相应结果的问题；但是实现Callable接口的线程类可以，因为Callable的执行方法体call方法

　可以返回对象。

3 由于runnable接口没有返回值，所以FutureTask为了解决此问题将runnable线程类通过支配器转换为callable线程。

4 当通过task对象调用get方法时，已经执行完成的现成可以立刻得到返回结果，但是还没执行完的线程一直在等待。

下面进入源码看看：

线程池执行submit方法时进入AbstractExecutorService类中的submit

  public <T> Future<T> submit(Callable<T> task) {
        if (task == null) throw new NullPointerException();
        RunnableFuture<T> ftask = newTaskFor(task);
        execute(ftask);
        return ftask;
    }

这里好理解，将线程放入任务，由线程池的execute方法去执行。

执行完成后，当调用get方法时，会进入FutureTask的get方法：

public V get() throws InterruptedException, ExecutionException {
        int s = state;
　　　　　//当线程状态为新建活着执行中时一直调用awaitDone方法
        if (s <= COMPLETING)
　　　　　　  //循环判断线程状态是否已经执行成功，如果执行成功返回线程状态；其中还包括线程取消，中断等情况的判断。可参见下方源码。
　　　　　　  //所以这里便是上面例子中为什么线程执行成功后即可立即得到结果，如果还没有执行成功
            s = awaitDone(false, 0L);

　　　　　　　//线程状态正常返回结果
            return report(s); 
    }

awaitDone源码

  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 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);
        }
    }

 @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);
    }

然后我们来看看FutureTask是如何对runnable线程进行转换的。代码也很简单：

public FutureTask(Runnable runnable, V result) {
        this.callable = Executors.callable(runnable, result);
        this.state = NEW;       // ensure visibility of callable
    }

public static <T> Callable<T> callable(Runnable task, T result) {
        if (task == null)
            throw new NullPointerException();
        return new RunnableAdapter<T>(task, result);
    }

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;
        }
    }