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31 * Written by Doug Lea with assistance from members of JCP JSR-166
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36 package java.util.concurrent;
39 * A service that decouples the production of new asynchronous tasks
40 * from the consumption of the results of completed tasks. Producers
41 * <tt>submit</tt> tasks for execution. Consumers <tt>take</tt>
42 * completed tasks and process their results in the order they
43 * complete. A <tt>CompletionService</tt> can for example be used to
44 * manage asynchronous IO, in which tasks that perform reads are
45 * submitted in one part of a program or system, and then acted upon
46 * in a different part of the program when the reads complete,
47 * possibly in a different order than they were requested.
49 * <p>Typically, a <tt>CompletionService</tt> relies on a separate
50 * {@link Executor} to actually execute the tasks, in which case the
51 * <tt>CompletionService</tt> only manages an internal completion
52 * queue. The {@link ExecutorCompletionService} class provides an
53 * implementation of this approach.
55 * <p>Memory consistency effects: Actions in a thread prior to
56 * submitting a task to a {@code CompletionService}
57 * <a href="package-summary.html#MemoryVisibility"><i>happen-before</i></a>
58 * actions taken by that task, which in turn <i>happen-before</i>
59 * actions following a successful return from the corresponding {@code take()}.
62 public interface CompletionService<V> {
64 * Submits a value-returning task for execution and returns a Future
65 * representing the pending results of the task. Upon completion,
66 * this task may be taken or polled.
68 * @param task the task to submit
69 * @return a Future representing pending completion of the task
70 * @throws RejectedExecutionException if the task cannot be
71 * scheduled for execution
72 * @throws NullPointerException if the task is null
74 Future<V> submit(Callable<V> task);
77 * Submits a Runnable task for execution and returns a Future
78 * representing that task. Upon completion, this task may be
81 * @param task the task to submit
82 * @param result the result to return upon successful completion
83 * @return a Future representing pending completion of the task,
84 * and whose <tt>get()</tt> method will return the given
85 * result value upon completion
86 * @throws RejectedExecutionException if the task cannot be
87 * scheduled for execution
88 * @throws NullPointerException if the task is null
90 Future<V> submit(Runnable task, V result);
93 * Retrieves and removes the Future representing the next
94 * completed task, waiting if none are yet present.
96 * @return the Future representing the next completed task
97 * @throws InterruptedException if interrupted while waiting
99 Future<V> take() throws InterruptedException;
103 * Retrieves and removes the Future representing the next
104 * completed task or <tt>null</tt> if none are present.
106 * @return the Future representing the next completed task, or
107 * <tt>null</tt> if none are present
112 * Retrieves and removes the Future representing the next
113 * completed task, waiting if necessary up to the specified wait
114 * time if none are yet present.
116 * @param timeout how long to wait before giving up, in units of
118 * @param unit a <tt>TimeUnit</tt> determining how to interpret the
119 * <tt>timeout</tt> parameter
120 * @return the Future representing the next completed task or
121 * <tt>null</tt> if the specified waiting time elapses
122 * before one is present
123 * @throws InterruptedException if interrupted while waiting
125 Future<V> poll(long timeout, TimeUnit unit) throws InterruptedException;