详解Java CompletableFuture使用方法以及与FutureTask的区别

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我是靠谱客的博主谦让棉花糖，最近开发中收集的这篇文章主要介绍详解Java CompletableFuture使用方法以及与FutureTask的区别，觉得挺不错的，现在分享给大家，希望可以做个参考。

概述

futureTask 创建异步任务

        FutureTask<String> stringFutureTask = new FutureTask<>(() -> {
            return "aa";
        });
        executorService.execute(stringFutureTask);
        System.out.println(stringFutureTask.get());
 
        CompletableFuture<String> future1 = CompletableFuture.supplyAsync(() -> {
            return "aa";
        }, executorService); // 不用手动提交了
        System.out.println(future1.get());

还有很多异步回调, 组合处理

创建任务

1. .supplyAsync

创建一个带返回值的任务

2. .runAsync

创建一个不带返回值的任务

        ExecutorService executorService = Executors.newFixedThreadPool(1);
 
        // 带返回值
        CompletableFuture future = CompletableFuture.supplyAsync(() -> {
            try {
                System.out.println("future　" + new Date());
                Thread.sleep(2000);
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
            return "aaa";
        }, executorService); // 推荐使用

以上两个方法都有两个构造方法, 默认不指定自定义线程池, 他会指定默认的提交任务的方法

    // 查看cpu的核数是否大于1核
    private static final boolean useCommonPool =
        (ForkJoinPool.getCommonPoolParallelism() > 1);
 
    // 如果大于1核 则调用execute方法, 每次创建一个线程
    private static final Executor asyncPool = useCommonPool ?
        ForkJoinPool.commonPool() : new ThreadPerTaskExecutor();
 
    static final class ThreadPerTaskExecutor implements Executor {
        public void execute(Runnable r) { new Thread(r).start(); }
    }

所以推荐自定义线程池的方式

异步回调

指的是异步任务结束后调用的任务

1. .thenApply

带返回值的异步调用函数, 有入参, 有出参

2. .thenAccept

不带返回值的异步回调函数, 有入参

 
        CompletableFuture future = CompletableFuture.supplyAsync(() -> {
            try {
                System.out.println("future　" + new Date());
                Thread.sleep(2000);
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
            return "aaa";
        }, executorService);
 
        // future执行完之后执行的异步任务
        CompletableFuture<String> thenApply = future.thenApply((result) -> {
            System.out.println("future2　" +new Date());
            System.out.println(result);
            return "bbb" + result;
        });

3. .exceptionally

异步任务出现异常调用的回调方法

        CompletableFuture future = CompletableFuture.supplyAsync(() -> {
            try {
                System.out.println("future　" + new Date());
                Thread.sleep(2000);
                int a = 1 / 0;
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
            return "aaa";
        }, executorService);
 
        CompletableFuture<String> exceptionally = future.exceptionally((result) -> {
            System.out.println("future3 " + result);
            return "bbb" + result;
        });
        
        // 出现异常则返回异常, 没异常则返回future的返回值
        System.out.println(exceptionally.get());

去掉异常

4. .whenComplete

当主任务出现异常时, 会终止任务,get的时候会抛出主任务的异常, 入参值为null, 否则正常运行

        CompletableFuture future = CompletableFuture.supplyAsync(() -> {
            try {
                System.out.println("future　" + new Date());
                Thread.sleep(2000);
                int a = 1/0;
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
            return "aaa";
        }, executorService);
 
        CompletableFuture<String> exceptionally = future.whenComplete((result, error) -> {
            System.out.println("future3 " + result);
            System.out.println("future3 " + error);
        });
        System.out.println(exceptionally.get());