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Concurrent-Executor Framework

 
阅读更多

Executor Framework

 

 

 

Callable  VS  Runnable

1.代表任务,会被线程执行。

不同

1. Callable型的任务能够返回结果或者抛出异常,而Runnable型任务不能

 

//java.util.concurrent 
//Interface Callable<V>
V call() throws Exception

//java.lang 
//Interface Runnable
void run()
 

2. 

 

两个问题

问题1.Thread的target是Runnable类型的,不能直接将Callable型对象作为Thread 的target。

如何将Callable型对象交给Thread?

 写道
Executor接口中的execute(Runnable command)方法的参数也是Runnable型,我们其实也想让Callable型
任务也能够被执行。根据这个需求注意在Executor的子接口ExecutorService中就增加了相关方法submit(Runnable/Callable),submit方法在子类AbstractExecutorService中给出了实现。实现里通过将Runnable/Callable型任务包装成RunnableFuture(继承了Runnable)从而可以交给execute(Runnable command)执行。

问题3:为什么不让FutureTask直接实现Runnable和Future两个接口,而是增加RunnableFuture接口
 

 

问题2.call()方法有返回值——但是call方法并不是直接被调用,而是作为线程执行体被调用。

如何获取call()的返回值?

 

为了解决问题2,JDK1.5中提供了Future接口来代表call()方法的返回值,并提供了一个实现类FutureTask。可以通过

get方法来获得最后执行的结果。

为了解决问题1,让FutureTask实现了Runnable接口,这样可以将 FutureTask交给Thread执行,间接将Callable交给了

Thread。


   

 

 


 

 

FutureTask<V>

 

A cancellable asynchronous computation.

可取消的异步计算 

This class provides a base implementation of Future, with methods to start and cancel a computation, query to see if the computation is complete, and retrieve the result of the computation. 

 

The result can only be retrieved when the computation has completed; the get method will block if the computation has not yet completed. Once the computation has completed, the computation cannot be restarted or cancelled. 

 

A FutureTask can be used to wrap a Callable or Runnable object. Because FutureTask implements Runnable, a FutureTask can be submitted to an Executor for execution. 

 

In addition to serving as a standalone class, this class provides protected functionality that may be useful when creating customized task classes.

Executor接口

 

 

ExecutorService接口

 

 

AbstractExecutorService抽象类

 

 

 

 public Future<?> submit(Runnable task) {
        if (task == null) throw new NullPointerException();
        RunnableFuture<Object> ftask = newTaskFor(task, null);
        execute(ftask);//ftask是Runnable型
        return ftask;//ftask是Future型
    }

    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;
    }

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

protected <T> RunnableFuture<T> newTaskFor(Runnable runnable, T value) {
        return new FutureTask<T>(runnable, value);
    }

    protected <T> RunnableFuture<T> newTaskFor(Callable<T> callable) {
        return new FutureTask<T>(callable);
    }

为什么不让FutureTask直接实现Runnable和Future两个接口,而是增加RunnableFuture接口?

注意submit方法有返回值Future,通过这个Future就可以对。。

 

ThreadPoolExecutor类

 

 

 

 

 

ScheduledExecutorService接口

 

 

ScheduledThreadPoolExecutor类

 

 

 

 

 

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