Class - java.util.concurrent.ExecutorCompletionService
Created by : Mr Dk.
2021 / 02 / 16 🧧 21:41
Ningbo, Zhejiang, China
Definition
该类提供了 CompletionService 接口的轻量级实现。类内使用了一个 Executor 来执行任务，在任务执行完成后，将会保存到一个队列中，通过 take() 函数来进行访问。
/**
* 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.
*
* <p>
*
* <b>Usage Examples.</b>
*
* 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:
*
* <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:
*
* <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> {
}
Executor
类内维护了 Executor 对象以执行任务。特别地，如果 Executor 对象实现了 AbstractExecutorService 抽象类，则将使用 AbstractExecutorService 提供的函数：
private final Executor executor;
private final AbstractExecutorService aes;
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);
}
Queueing Future
类内定义了一个阻塞队列来维护所有已完成任务的 Future 对象。FutureTask 类被继承为 QueueingFuture 类，重写了任务完成后会被回调的 done() 函数 (FutureTask 中默认什么都不做)。在 done() 中，使已经完成的任务加入阻塞队列。
private final BlockingQueue<Future<V>> completionQueue;
/**
* 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;
}
Constructor
构造函数需要传入一个 Executor 对象。根据 Executor 是否继承自 AbstractExecutorService 决定是否启用成员变量 aes。阻塞队列默认会被初始化为 LinkedBlockingQueue，但用户也可以自行传入继承自 BlockingQueue 的阻塞队列。
/**
* 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;
}
Submit
将任务包装为可被执行的 QueueFuture 对象，然后使用 executor 执行任务。任务完成后，其 done() 会被调用使其进入完成队列。
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;
}
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;
}
Take / Poll
以阻塞 (take()) 或非阻塞超时 (poll()) 的方式从完成队列中取出任务的 FutureTask 对象。
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);
}