CompletionService讲解

秒速五厘米 2022-09-09 09:22 109阅读 0赞

CompletionService的源码注释

package java.util.concurrent;
 
 /**
 * A service that decouples the production of new asynchronous tasks
 * from the consumption of the results of completed tasks. Producers
 * {@code submit} tasks for execution. Consumers {@code take}
 * completed tasks and process their results in the order they
 * complete. A {@code CompletionService} can for example be used to
 * manage asynchronous I/O, in which tasks that perform reads are
 * submitted in one part of a program or system, and then acted upon
 * in a different part of the program when the reads complete,
 * possibly in a different order than they were requested.
 *
 * Typically, a {@code CompletionService} relies on a separate
 * {@link Executor} to actually execute the tasks, in which case the
 * {@code CompletionService} only manages an internal completion
 * queue. The {@link ExecutorCompletionService} class provides an
 * implementation of this approach.
 *
 * Memory consistency effects: Actions in a thread prior to
 * submitting a task to a {@code CompletionService}
 * <a href="package-summary.html#MemoryVisibility">happen-before</a>
 * actions taken by that task, which in turn happen-before
 * actions following a successful return from the corresponding {@code take()}.
 */
 public interface CompletionService<V> {
 /**
 * Submits a value-returning task for execution and returns a Future
 * representing the pending results of the task. Upon completion,
 * this task may be taken or polled.
 *
 * @param task the task to submit
 * @return a Future representing pending completion of the task
 * @throws RejectedExecutionException if the task cannot be
 * scheduled for execution
 * @throws NullPointerException if the task is null
 */
 Future<V> submit(Callable<V> task);
 
 /**
 * Submits a Runnable task for execution and returns a Future
 * representing that task. Upon completion, this task may be
 * taken or polled.
 *
 * @param task the task to submit
 * @param result the result to return upon successful completion
 * @return a Future representing pending completion of the task,
 * and whose {@code get()} method will return the given
 * result value upon completion
 * @throws RejectedExecutionException if the task cannot be
 * scheduled for execution
 * @throws NullPointerException if the task is null
 */
 Future<V> submit(Runnable task, V result);
 
 /**
 * Retrieves and removes the Future representing the next
 * completed task, waiting if none are yet present.
 *
 * @return the Future representing the next completed task
 * @throws InterruptedException if interrupted while waiting
 */
 Future<V> take() throws InterruptedException;
 
 /**
 * Retrieves and removes the Future representing the next
 * completed task, or {@code null} if none are present.
 *
 * @return the Future representing the next completed task, or
 * {@code null} if none are present
 */
 Future<V> poll();
 
 /**
 * Retrieves and removes the Future representing the next
 * completed task, waiting if necessary up to the specified wait
 * time if none are yet present.
 *
 * @param timeout how long to wait before giving up, in units of
 * {@code unit}
 * @param unit a {@code TimeUnit} determining how to interpret the
 * {@code timeout} parameter
 * @return the Future representing the next completed task or
 * {@code null} if the specified waiting time elapses
 * before one is present
 * @throws InterruptedException if interrupted while waiting
 */
 Future<V> poll(long timeout, TimeUnit unit) throws InterruptedException;
 }

主要目的，任务的生产和消费解耦。

eg:

public class Test {
 
 public static void main(String[] args) throws InterruptedException, ExecutionException {
 ExecutorService executorService = Executors.newFixedThreadPool(10);
 
 CompletionService<String> completionService = new ExecutorCompletionService<>(executorService);
 
 for (int i = 0; i< 5; i++){
 int finalI = i;
 completionService.submit(() -> {
 TimeUnit.SECONDS.sleep(Math.abs(3 - finalI));
 return "Hello" + finalI;
 });
 }
 
 for (int i = 0; i< 5; i++){
 System.out.println(completionService.take().get());
 }
 }
 }