ExecutorService接口定义： http://donald-draper.iteye.com/blog/2365738
Future接口定义： http://donald-draper.iteye.com/blog/2365798
FutureTask解析： http://donald-draper.iteye.com/blog/2365980
CompletionService接口定义： http://donald-draper.iteye.com/blog/2366239
package java.util.concurrent;
/**
* A {@link CompletionService} that uses a supplied {@link Executor}
* to execute tasks. This class arranges that submitted tasks are,
* upon completion, placed on a queue accessible using {@code take}.
* The class is lightweight enough to be suitable for transient use
* when processing groups of tasks.
*
ExecutorCompletionService是执行器的补充或者说是辅助，用于执行任务。
任务提交到ExecutorCompletionService执行后，如果任务执行完，则添加到完成
队列，我们可以通过take，取回完成任务的结果。ExecutorCompletionService
是处理集合任务的一个轻量级的实现。
* <p>
*
* <b>Usage Examples.</b>
*
实例：
* Suppose you have a set of solvers for a certain problem, each
* returning a value of some type {@code Result}, and would like to
* run them concurrently, processing the results of each of them that
* return a non-null value, in some method {@code use(Result r)}. You
* could write this as:
*
假设将一个确定的问题，分成n个部分，这n个部分可以并发执行，每个部分返回一个非null，结果
在其他一些方法中，使用这些结果。
* <pre> {@code
* void solve(Executor e,
* Collection<Callable<Result>> solvers)
* throws InterruptedException, ExecutionException {
* CompletionService<Result> ecs
* = new ExecutorCompletionService<Result>(e);
* for (Callable<Result> s : solvers)
* ecs.submit(s);
* int n = solvers.size();
* for (int i = 0; i < n; ++i) {
* Result r = ecs.take().get();
* if (r != null)
* use(r);
* }
* }}</pre>
*
* Suppose instead that you would like to use the first non-null result
* of the set of tasks, ignoring any that encounter exceptions,
* and cancelling all other tasks when the first one is ready:
*
假设我们仅想用任务集合中第一个完成任务的结果，并忽略其他任务遇到的异常，
当第一个完成任务的结果可用时，取消其他任务。
* <pre> {@code
* void solve(Executor e,
* Collection<Callable<Result>> solvers)
* throws InterruptedException {
* CompletionService<Result> ecs
* = new ExecutorCompletionService<Result>(e);
* int n = solvers.size();
* List<Future<Result>> futures
* = new ArrayList<Future<Result>>(n);
* Result result = null;
* try {
* for (Callable<Result> s : solvers)
* futures.add(ecs.submit(s));
* for (int i = 0; i < n; ++i) {
* try {
* Result r = ecs.take().get();
* if (r != null) {
* result = r;
* break;
* }
* } catch (ExecutionException ignore) {}
* }
* }
* finally {
* for (Future<Result> f : futures)
* f.cancel(true);
* }
*
* if (result != null)
* use(result);
* }}</pre>
*/
public class ExecutorCompletionService<V> implements CompletionService<V> {
private final Executor executor;//执行器
private final AbstractExecutorService aes;//执行器服务
private final BlockingQueue<Future<V>> completionQueue;//任务完成队列
/**
* FutureTask extension to enqueue upon completion
扩展FutureTask的队列完成任务
*/
private class QueueingFuture extends FutureTask<Void> {
QueueingFuture(RunnableFuture<V> task) {
super(task, null);
this.task = task;
}
//这个是关键，在FutureTask那篇文章中，我们有讲，及当任务完成时调用done方法，
//done方法为抽象方法，待子类扩展
protected void done() {
//当任务任务执行结束时，添加到完成队列
completionQueue.add(task);
}
private final Future<V> task;
}
//根据Callable，创建
private RunnableFuture<V> newTaskFor(Callable<V> task) {
if (aes == null)
return new FutureTask<V>(task);
else
return aes.newTaskFor(task);
}
private RunnableFuture<V> newTaskFor(Runnable task, V result) {
if (aes == null)
return new FutureTask<V>(task, result);
else
return aes.newTaskFor(task, result);
}
/**
* Creates an ExecutorCompletionService using the supplied
* executor for base task execution and a
* {@link LinkedBlockingQueue} as a completion queue.
*
根据执行器构建ExecutorCompletionService，完成队列默认为LinkedBlockingQueue
* @param executor the executor to use
* @throws NullPointerException if executor is {@code null}
*/
public ExecutorCompletionService(Executor executor) {
if (executor == null)
throw new NullPointerException();
this.executor = executor;
this.aes = (executor instanceof AbstractExecutorService) ?
(AbstractExecutorService) executor : null;
//将完成任务放在LinkedBlockingQueue中
this.completionQueue = new LinkedBlockingQueue<Future<V>>();
}
/**
* Creates an ExecutorCompletionService using the supplied
* executor for base task execution and the supplied queue as its
* completion queue.
*
根据执行器和完成队列构建ExecutorCompletionService
* @param executor the executor to use
* @param completionQueue the queue to use as the completion queue
* normally one dedicated for use by this service. This
* queue is treated as unbounded -- failed attempted
* {@code Queue.add} operations for completed taskes cause
* them not to be retrievable.
* @throws NullPointerException if executor or completionQueue are {@code null}
*/
public ExecutorCompletionService(Executor executor,
BlockingQueue<Future<V>> completionQueue) {
if (executor == null || completionQueue == null)
throw new NullPointerException();
this.executor = executor;
this.aes = (executor instanceof AbstractExecutorService) ?
(AbstractExecutorService) executor : null;
this.completionQueue = completionQueue;
}
//提交执行Callable任务
public Future<V> submit(Callable<V> task) {
if (task == null) throw new NullPointerException();
RunnableFuture<V> f = newTaskFor(task);
executor.execute(new QueueingFuture(f));
return f;
}
//提交执行Runnable任务
public Future<V> submit(Runnable task, V result) {
if (task == null) throw new NullPointerException();
RunnableFuture<V> f = newTaskFor(task, result);
executor.execute(new QueueingFuture(f));
return f;
}
//从完成队列take完成任务的结果，没有则阻塞，直到有任务完成
public Future<V> take() throws InterruptedException {
return completionQueue.take();
}
//从完成队列获取完成任务的结果，没有则返回null
public Future<V> poll() {
return completionQueue.poll();
}
//从完成队列获取完成任务的结果，没有则超时等待，如果超时等待期间还没有完成任务，返回为null
public Future<V> poll(long timeout, TimeUnit unit)
throws InterruptedException {
return completionQueue.poll(timeout, unit);
}
}
总结：
ExecutorCompletionService内部关联一个执行器AbstractExecutorService和
一个阻塞的任务完成队列，默认为LinkedBlockingQueue。当提交任务，则包装成QueueingFuture，QueueingFuture
扩展了FutureTask，重写done方法，即在任务执行结束时，添加任务执行结果到完成队列。
而take，poll，超时poll直接委托给完成队列。