多线程（三）

接着说线程池，使用java自带的线程池，一般的情况下线程池对应的实现类是ThreadPoolExecutor，当然不排除自己来写一个线程池，扯远了，ThreadPoolExecutor 扩展自抽象类AbstractExecutorService，其中AbstractExecutorService 默认的实现了：

默认的实现的方法中首先我们看看，上面我们说到的submit（）方法：

源代码：

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

    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;
    }

可以看到的是：

针对submit（）接受的参数的类型是实现了Runable 或者Callable 接口的类，然后封装为 RunnableFuture<T> ftask = newTaskFor(task); 交给execute（ftask）执行，返回执行的结果。这个execute(Runnable)方法将在子类ThreadPoolExecutor中实现。

我们可以首先来看一下executor接口的说明：

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 <tt>Executor</tt> 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);
}

在将来某个时刻执行给定的任务command，说明这个方法是将来会去执行，在主线程(submit方法)中是不会阻塞的。另外，注意到如果提交的是Runnable接口，返回的future对象中其结果是在任务提交时候就指定了的，要么是null，要么是T result。上面的分析可以简单测试下：

//Callable 任务
class CallabelTask implements Callable<String>{
        String name;
        public CallabelTask(String name){
            this.name = name;
        }
        @Override
        public String call() throws Exception {
            System.out.println("Start to execute the task " + name);
            TimeUnit.SECONDS.sleep(5);
            return name + " is done!";
        }
        
    }

public static void submitCallableTask()throws Exception{
        ExecutorService executor = Executors.newCachedThreadPool();
        List<Future<String>> results = new ArrayList<Future<String>>(5);
        for(int i = 0; i < 5; ){
            results.add(executor.submit(new CallabelTask("task_"+(++i))));
        }
        System.out.println("All the tasks have been submited through invokeAll method!");
        executor.shutdown();
        for(Future<String> f : results)
            System.out.println(f.get());
    }

public static void main(String[] args) throws Exception { submitCallableTask();
}
//调用submitCallableTask的结果：
All the tasks have been submited through invokeAll method!
Start to execute the task task_1
Start to execute the task task_5
Start to execute the task task_3
Start to execute the task task_4
Start to execute the task task_2
task_1 is done!
task_2 is done!
task_3 is done!
task_4 is done!
task_5 is done!

从返回的结果看，主线程在执行完executor.submit(new CallabelTask("task_"+(++i))) 之后并没有阻塞，而是继续往下执行println语句打印出语句：

All the tasks have been submited through invokeAll method!

而提交的任务将由其他线程在“将来某个时刻“去执行。当然在主线程去获取执行结果f.get() 的时候肯定是要阻塞的，因为既然要获取结果了，当然要等到任务执行完毕返回才有啊！

如果把上面的CallabelTask改变成RunnableTask，则返回的结果将是null，原因是提交的时候在new TaskFor方法中就已经指定了返回结果为null,就是上面的submit源代码中的这句话：

RunnableFuture<Object> ftask = newTaskFor(task, null);

测试的代码是：

class RunnableTask implements Runnable{
        String name;
        public RunnableTask(String name){
            this.name = name;
        }
        @Override
        public void run() {
            System.out.println("Start to execute the task " + name);
            try {
                TimeUnit.SECONDS.sleep(5);
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
        }
    }
//RunnableTask 执行结果：
All the tasks have been submited through invokeAll method!
Start to execute the task task_1
Start to execute the task task_5
Start to execute the task task_3
Start to execute the task task_4
Start to execute the task task_2
null
null
null
null
null