详解Java8中新引入的异步编程API之CompletableFuture

最新推荐文章于 2024-11-29 14:20:32 发布

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最新推荐文章于 2024-11-29 14:20:32 发布 · 1k 阅读

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#java #开发语言

本文介绍了Java 8引入的CompletableFuture，它是Java Future和CompletionStage API的扩展，用于异步编程。文中阐述了其概念、用途，详细讲解了创建异步任务、异步回调、组合处理等方法的使用，还提醒使用时要注意线程池选择、异常处理和结果获取等问题。

Java8中引入了不少新的特性，如：

lambda表达式：Lambda表达式允许在代码中直接定义匿名函数，使得Java对函数式编程的支持更加完善。它极大地简化了代码，使得开发者能够编写出更简洁、更灵活的代码。
Stream API：Stream API为处理数据提供了一种高效且易于使用的方式。通过使用Stream API，开发者可以编写出更简洁、更易于阅读的代码来处理集合数据。它提供了诸如filter、map、sorted等常用的方法，使得数据操作更加直观和方便。
Date/Time API：Java 8引入了新的日期和时间API，替代了之前复杂且易出错的java.util.Date和java.util.Calendar类。新的API提供了更加直观和灵活的方式来处理日期和时间。
接口的默认方法和静态方法：在Java 8中，接口可以包含默认方法和静态方法。默认方法允许在接口中定义方法的具体实现，而静态方法与类中的静态方法类似，可以在接口中使用static关键字定义。
函数式接口：Java 8引入了函数式接口的概念，这是一个只包含一个抽象方法的接口。Lambda表达式可以很容易地与函数式接口结合使用，从而支持函数式编程。
方法引用：方法引用是Lambda表达式的一个简化形式，它允许我们直接引用已存在的方法或构造器。
Optional容器：Optional是一个可以为null的容器对象。它的引入旨在减少空指针异常的发生，使得代码更加健壮。
CompletableFuture

今天，我们就来看看这个CompletableFuture，它是什么、怎么用、以及使用过程中需要注意什么。

是什么？(CompletableFuture,Future,CompletionStage,Function,Consumer,Supplier...)

CompletableFuture是Java 8中引入的一个功能强大的类，它位于包JUC(java.util.concurrent)下，是Java Future和CompletionStage API的扩展，专为异步编程而设计。它允许我们将任务运行在与主线程分离的其他线程中，并通过回调在主线程中得到异步任务执行的状态，包括是否完成、是否异常等信息。这种设计使得主线程不会阻塞或等待任务的完成，从而可以并行执行其他任务，极大地提高了程序的性能。

CompletableFuture的主要用途在于异步编程，特别是在需要执行耗时的操作（如I/O操作）时，可以将其放在后台线程中执行，同时主线程可以继续执行其他任务。一旦后台任务完成，主线程可以通过回调机制获取结果或处理异常情况（基于观察者设计模式）。

/**
 * A {@link Future} that may be explicitly completed (setting its
 * value and status), and may be used as a {@link CompletionStage},
 * supporting dependent functions and actions that trigger upon its
 * completion.
 *
 * <p>When two or more threads attempt to
 * {@link #complete complete},
 * {@link #completeExceptionally completeExceptionally}, or
 * {@link #cancel cancel}
 * a CompletableFuture, only one of them succeeds.
 *
 * <p>In addition to these and related methods for directly
 * manipulating status and results, CompletableFuture implements
 * interface {@link CompletionStage} with the following policies: <ul>
 *
 * <li>Actions supplied for dependent completions of
 * <em>non-async</em> methods may be performed by the thread that
 * completes the current CompletableFuture, or by any other caller of
 * a completion method.</li>
 *
 * <li>All <em>async</em> methods without an explicit Executor
 * argument are performed using the {@link ForkJoinPool#commonPool()}
 * (unless it does not support a parallelism level of at least two, in
 * which case, a new Thread is created to run each task).  To simplify
 * monitoring, debugging, and tracking, all generated asynchronous
 * tasks are instances of the marker interface {@link
 * AsynchronousCompletionTask}. </li>
 *
 * <li>All CompletionStage methods are implemented independently of
 * other public methods, so the behavior of one method is not impacted
 * by overrides of others in subclasses.  </li> </ul>
 *
 * <p>CompletableFuture also implements {@link Future} with the following
 * policies: <ul>
 *
 * <li>Since (unlike {@link FutureTask}) this class has no direct
 * control over the computation that causes it to be completed,
 * cancellation is treated as just another form of exceptional
 * completion.  Method {@link #cancel cancel} has the same effect as
 * {@code completeExceptionally(new CancellationException())}. Method
 * {@link #isCompletedExceptionally} can be used to determine if a
 * CompletableFuture completed in any exceptional fashion.</li>
 *
 * <li>In case of exceptional completion with a CompletionException,
 * methods {@link #get()} and {@link #get(long, TimeUnit)} throw an
 * {@link ExecutionException} with the same cause as held in the
 * corresponding CompletionException.  To simplify usage in most
 * contexts, this class also defines methods {@link #join()} and
 * {@link #getNow} that instead throw the CompletionException directly
 * in these cases.</li> </ul>
 *
 * @author Doug Lea
 * @since 1.8
 */
public class CompletableFuture<T> implements Future<T>, CompletionStage<T>