CompletionService详解
我们知道，通过 Future 和 FutureTask 可以获得线程任务的执行结果，但它们有一定的缺陷：
- Future：多个线程任务的执行结果，我们可以通过轮询的方式去获取，但普通轮询会有被阻塞的可能，升级轮询会非常消耗cpu。
- FutureTask：虽然我们可以调用 done 方法，在线程任务执行结束后立即返回或做其他处理，但对批量线程任务结果的管理方面有所不足。
为了更好地应对大量线程任务结果处理的问题，JDK提供了功能强大的 CompletionService。CompletionService是一个接口，使用创建时提供的 Executor 对象（通常是线程池）来执行任务，并在内部维护了一个阻塞队列（QueueingFuture），当任务执行结束就把任务的执行结果的 Future 对象加入到阻塞队列中。
该接口只有一个实现类： ExecutorCompletionService。
1 CompletionService 解析
1.1 构造方法
首先看一下 ExecutorCompletionService 的构造函数：
/**
* Creates an ExecutorCompletionService using the supplied
* executor for base task execution and a
* {@link LinkedBlockingQueue} as a completion queue.
*
* @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;
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.
*
* @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 tasks 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;
}
这两个构造方法都需要传入一个线程池，如果不指定 completionQueue，那么默认会使用无界的 LinkedBlockingQueue。任务执行结果的 Future 对象就是加入到 completionQueue 中。
1.2 方法
CompletionService 接口提供的方法有 5 个：
public interface CompletionService<V> {
//提交线程任务
Future<V> submit(Callable<V> task);
//提交线程任务
Future<V> submit(Runnable task, V result);
//阻塞等待
Future<V> take() throws InterruptedException;
//非阻塞等待
Future<V> poll();
//带时间的非阻塞等待
Future<V> poll(long timeout, TimeUnit unit) throws InterruptedException;
}
方法简介如下：
- submit(Callable task)：提交线程任务，交由 Executor 对象去执行，并将结果放入阻塞队列；
- take()：在阻塞队列中获取并移除一个元素，该方法是阻塞的，即获取不到的话线程会一直阻塞；
- poll()：在阻塞队列中获取并移除一个元素，该方法是非阻塞的，获取不到即返回 null ；
- poll(long timeout, TimeUnit unit)：从阻塞队列中非阻塞地获取并移除一个元素，在设置的超时时间内获取不到即返回 null ；
接下来，我们重点看一下submit 的源码：
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;
}
从submit 方法的源码中可以确认两点：
- 线程任务确实是由 Executor 对象执行的；
- 提交某个任务时，该任务首先将被包装为一个QueueingFuture。
继续追查 QueueingFuture，可以发现：
该类重写了 FutureTask 的done方法，当计算完成时，把Executor执行的计算结果放入BlockingQueue中，而放入结果是按任务完成顺序来进行的，即先完成的任务先放入阻塞队列。
/**
* FutureTask extension to enqueue upon completion
*/
private class QueueingFuture extends FutureTask<Void> {
QueueingFuture(RunnableFuture<V> task) {
super(task, null);
this.task = task;
}
protected void done() { completionQueue.add(task); }
private final Future<V> task;
}
由此，CompletionService 实现了生产者提交任务和消费者获取结果的解耦，任务的完成顺序由 CompletionService 来保证，消费者一定是按照任务完成的先后顺序来获取执行结果。
2 CompletionService 使用示例
下面我们使用一个小例子，领略一下 CompletionService 的便利：
import java.util.Date;
import java.util.concurrent.*;
/**
* @author guozhengMu
* @version 1.0
* @date 2019/11/8 20:25
* @description
* @modify
*/
public class CompletionServiceTest {
public static void main(String[] args) {
ExecutorService executor = Executors.newCachedThreadPool();
CompletionService<String> cs = new ExecutorCompletionService<>(executor);
// 此线程池运行5个线程
for (int i = 0; i < 5; i++) {
final int index = i;
cs.submit(() -> {
String name = Thread.currentThread().getName();
System.out.println(name + " 启动：" + new Date());
TimeUnit.SECONDS.sleep(10 - index * 2);
return name;
});
}
executor.shutdown();
for (int i = 0; i < 5; i++) {
try {
System.out.println(cs.take().get() + " 结果：" + new Date());
} catch (Exception e) {
e.printStackTrace();
}
}
}
}
运行结果：
pool-1-thread-2 启动：Sun Nov 10 11:34:13 CST 2019
pool-1-thread-4 启动：Sun Nov 10 11:34:13 CST 2019
pool-1-thread-3 启动：Sun Nov 10 11:34:13 CST 2019
pool-1-thread-5 启动：Sun Nov 10 11:34:13 CST 2019
pool-1-thread-1 启动：Sun Nov 10 11:34:13 CST 2019
pool-1-thread-5 结果：Sun Nov 10 11:34:15 CST 2019
pool-1-thread-4 结果：Sun Nov 10 11:34:17 CST 2019
pool-1-thread-3 结果：Sun Nov 10 11:34:19 CST 2019
pool-1-thread-2 结果：Sun Nov 10 11:34:21 CST 2019
pool-1-thread-1 结果：Sun Nov 10 11:34:23 CST 2019
通过观察运行结果，可以看到，结果输出的顺序与线程任务执行完成的顺序一致。
3 完整源码
package java.util.concurrent;
public class ExecutorCompletionService<V> implements CompletionService<V> {
// 线程池
private final Executor executor;
private final AbstractExecutorService aes;
// 阻塞队列：存放线程执行结果
private final BlockingQueue<Future<V>> completionQueue;
//内部封装的一个用来执线程的FutureTask
private class QueueingFuture extends FutureTask<Void> {
QueueingFuture(RunnableFuture<V> task) {
super(task, null);
this.task = task;
}
//线程执行完成后调用此函数将结果放入阻塞队列
protected void done() {
completionQueue.add(task);
}
private final Future<V> task;
}
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);
}
//构造函数，这里一般传入一个线程池对象executor的实现类
public ExecutorCompletionService(Executor executor) {
if (executor == null)
throw new NullPointerException();
this.executor = executor;
this.aes = (executor instanceof AbstractExecutorService) ?
(AbstractExecutorService) executor : null;
this.completionQueue = new LinkedBlockingQueue<Future<V>>();//默认的是链表阻塞队列
}
//构造函数，可以自己设定阻塞队列
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;
}
//提交线程任务，其实最终还是executor去提交
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;
}
//提交线程任务，其实最终还是executor去提交
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;
}
public Future<V> take() throws InterruptedException {
return completionQueue.take();
}
public Future<V> poll() {
return completionQueue.poll();
}
public Future<V> poll(long timeout, TimeUnit unit) throws InterruptedException {
return completionQueue.poll(timeout, unit);
}
}