首页 > 代码库 > 多线程（三）
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_1Start to execute the task task_5Start to execute the task task_3Start to execute the task task_4Start to execute the task task_2task_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_1Start to execute the task task_5Start to execute the task task_3Start to execute the task task_4Start to execute the task task_2nullnullnullnullnull
多线程（三）