/* * Written by Doug Lea with assistance from members of JCP JSR-166 * Expert Group and released to the public domain, as explained at * http://creativecommons.org/publicdomain/zero/1.0/ */ package java.util.concurrent; /** * A <tt>Future</tt> represents the result of an asynchronous * computation. Methods are provided to check if the computation is * complete, to wait for its completion, and to retrieve the result of * the computation. The result can only be retrieved using method * <tt>get</tt> when the computation has completed, blocking if * necessary until it is ready. Cancellation is performed by the * <tt>cancel</tt> method. Additional methods are provided to * determine if the task completed normally or was cancelled. Once a * computation has completed, the computation cannot be cancelled. * If you would like to use a <tt>Future</tt> for the sake * of cancellability but not provide a usable result, you can * declare types of the form {@code Future<?>} and * return <tt>null</tt> as a result of the underlying task. * Future异步计算任务的结果。方法提供了检查计算任务是否完成,等待计算任务完成时, 获取计算任务结果。当计算任务完成时,调用get方法可以获取计算任务结果,如果需要 将阻塞至准备好。我们可以使用cancel取消线程的执行。另外提供了确定任务是否 正常结束或取消的方法。当任务已经完成时,任务不可被取消。 如果我们想取消任务,而不需要获取结果,则我们可以声明类型为Future<?>, 则任务完成返回为null。 * <p> 一些示例 * <b>Sample Usage</b> (Note that the following classes are all * made-up.) <p> * <pre> {@code * interface ArchiveSearcher { String search(String target); } * class App { * ExecutorService executor = ... * ArchiveSearcher searcher = ... * void showSearch(final String target) * throws InterruptedException { * Future<String> future * = executor.submit(new Callable<String>() { * public String call() { * return searcher.search(target); * }}); * displayOtherThings(); // do other things while searching * try { * displayText(future.get()); // use future * } catch (ExecutionException ex) { cleanup(); return; } * } * }}</pre> * * The {@link FutureTask} class is an implementation of <tt>Future</tt> that * implements <tt>Runnable</tt>, and so may be executed by an <tt>Executor</tt>. * For example, the above construction with <tt>submit</tt> could be replaced by: FutureTask提供了Future和Runnable的实现,所以也可以用执行器去执行,比如上面submit方法的 Callable构造,我们可以用FutureTask来代理 * <pre> {@code * FutureTask<String> future = * new FutureTask<String>(new Callable<String>() { * public String call() { * return searcher.search(target); * }}); * executor.execute(future);}</pre> * * <p>Memory consistency effects: Actions taken by the asynchronous computation * [url=package-summary.html#MemoryVisibility] <i>happen-before</i>[/url] * actions following the corresponding {@code Future.get()} in another thread. * 内存一直性:任务的执行发生在获取任务执行的结果之前 * @see FutureTask * @see Executor * @since 1.5 * @author Doug Lea * @param <V> The result type returned by this Future's <tt>get</tt> method */ public interface Future<V> { /** * Attempts to cancel execution of this task. This attempt will * fail if the task has already completed, has already been cancelled, * or could not be cancelled for some other reason. If successful, * and this task has not started when <tt>cancel</tt> is called, * this task should never run. If the task has already started, * then the <tt>mayInterruptIfRunning</tt> parameter determines * whether the thread executing this task should be interrupted in * an attempt to stop the task. * 尝试取消任务的执行。如果任务已经完成,或已经被取消,或者由于其他原因 不能被取消,则尝试失败。如果任务还没有开始,取消方法被调用,则尝试成功, 任务绝对不会执行。当任务已经被执行,mayInterruptIfRunning参数可以决定在尝试 停止任务时,执行中的任务是否可以被中断。 * <p>After this method returns, subsequent calls to {@link #isDone} will * always return <tt>true</tt>. Subsequent calls to {@link #isCancelled} * will always return <tt>true</tt> if this method returned <tt>true</tt>. * 当方法返回时,isDone方法将返回true。如果cancel返回true,则#isCancelled方法返回true * @param mayInterruptIfRunning <tt>true</tt> if the thread executing this * task should be interrupted; otherwise, in-progress tasks are allowed * to complete * @return <tt>false</tt> if the task could not be cancelled, * typically because it has already completed normally; * <tt>true</tt> otherwise */ boolean cancel(boolean mayInterruptIfRunning); /** * Returns <tt>true</tt> if this task was cancelled before it completed * normally. * 在任务正常完成前,如果任务被取消成功,则返回true * @return <tt>true</tt> if this task was cancelled before it completed */ boolean isCancelled(); /** * Returns <tt>true</tt> if this task completed. * 如果任务执行完,则返回true * Completion may be due to normal termination, an exception, or * cancellation -- in all of these cases, this method will return * <tt>true</tt>. * 由于任务正常结束,异常发生或任务被取消,方法返回true * @return <tt>true</tt> if this task completed */ boolean isDone(); /** * Waits if necessary for the computation to complete, and then * retrieves its result. 如果需要,等待计算任务完成,取回任务计算结果 * @return the computed result * @throws CancellationException if the computation was cancelled * @throws ExecutionException if the computation threw an * exception * @throws InterruptedException if the current thread was interrupted * while waiting */ V get() throws InterruptedException, ExecutionException; /** * Waits if necessary for at most the given time for the computation * to complete, and then retrieves its result, if available. * 如果需要,超时等待计算任务完成,如果结果可用,取回任务计算结果 * @param timeout the maximum time to wait * @param unit the time unit of the timeout argument * @return the computed result * @throws CancellationException if the computation was cancelled * @throws ExecutionException if the computation threw an * exception * @throws InterruptedException if the current thread was interrupted * while waiting * @throws TimeoutException if the wait timed out */ V get(long timeout, TimeUnit unit) throws InterruptedException, ExecutionException, TimeoutException; }
Future接口定义
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