深入Callable及Runnable两个接口 获取线程返回结果

科技优家 2017-04-13 10:11

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 < 10; 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 < 10; 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..0class thread.ThreadTest$1::线程执行中..1class thread.ThreadTest$1::线程执行中..2class thread.ThreadTest$1::线程执行中..3class thread.ThreadTest$1::线程执行中..4class thread.ThreadTest$1::线程执行中..5class thread.ThreadTest$1::线程执行中..6class thread.ThreadTest$1::线程执行中..7class thread.ThreadTest$1::线程执行中..8class thread.ThreadTest$1::线程执行中..9task return value:null提前出结果了 task return value:nullclass thread.ThreadTest$2::线程执行中..0class thread.ThreadTest$2::线程执行中..1class thread.ThreadTest$2::线程执行中..2class thread.ThreadTest$2::线程执行中..3class thread.ThreadTest$2::线程执行中..4class thread.ThreadTest$2::线程执行中..5class thread.ThreadTest$2::线程执行中..6class thread.ThreadTest$2::线程执行中..7class thread.ThreadTest$2::线程执行中..8class thread.ThreadTest$2::线程执行中..9callableTask 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： 当通过task对象调用get方法时，已经执行完成的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;        }    }