> For the complete documentation index, see [llms.txt](https://jupiter-1992.gitbook.io/jupiter-note/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://jupiter-1992.gitbook.io/jupiter-note/hou-duan/bing-fa-bian-cheng/07-completablefuture.md).

# 07 CompletableFuture

> 本文为个人学习摘要笔记。\
> 原文地址：[廖雪峰 Java 教程 - 使用 CompletableFuture](https://www.liaoxuefeng.com/wiki/1252599548343744/1306581182447650)

Java 8 开始引入了 `CompletableFuture`，它针对 `Future` 做了改进，可以传入回调对象，当异步任务完成或者发生异常时，自动调用回调对象的回调方法。

`CompletableFuture` 的优点是：

* 异步任务结束时，会自动回调某个对象的方法；
* 异步任务出错时，会自动回调某个对象的方法；
* 主线程设置好回调后，不再关心异步任务的执行。

```java
public class T {

    public static void main(String[] args) throws Exception {
        // 创建异步执行任务:
        CompletableFuture<Double> cf = CompletableFuture.supplyAsync(T::fetchPrice);
        // 如果执行成功:
        cf.thenAccept((result) -> {
            System.out.println("price: " + result);
        });
        // 如果执行异常:
        cf.exceptionally((e) -> {
            e.printStackTrace();
            return null;
        });
        // 主线程不要立刻结束，否则CompletableFuture默认使用的线程池会立刻关闭:
        Thread.sleep(2000);
    }

    static Double fetchPrice() {
        try {
            Thread.sleep(1000);
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
        if (Math.random() < 0.3) {
            throw new RuntimeException("fetch price failed!");
        }
        return 5 + Math.random() * 20;
    }
}
```

除了实现了异步回调机制外，多个 `CompletableFuture` 可以串行执行。

```java
public class T {

    public static void main(String[] args) throws Exception {
        // 第一个任务:
        CompletableFuture<String> cfQuery = CompletableFuture.supplyAsync(() -> {
            return queryCode("中国石油");
        });
        // cfQuery成功后继续执行下一个任务:
        CompletableFuture<Double> cfFetch = cfQuery.thenApplyAsync(T::fetchPrice);
        // cfFetch成功后打印结果:
        cfFetch.thenAccept((result) -> {
            System.out.println("price: " + result);
        });
        // 主线程不要立刻结束，否则CompletableFuture默认使用的线程池会立刻关闭:
        Thread.sleep(2000);
    }

    static String queryCode(String name) {
        try {
            Thread.sleep(500);
        } catch (InterruptedException e) {
        }
        return "601857";
    }

    static Double fetchPrice(String code) {
        try {
            Thread.sleep(500);
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
        return 5 + Math.random() * 20;
    }
}
```

除了串行执行外，多个 `CompletableFuture` 还可以并行执行。

```java
public class T {

    public static void main(String[] args) throws Exception {
        // 两个CompletableFuture执行异步查询:
        CompletableFuture<String> cfQueryFromSina = CompletableFuture.supplyAsync(() -> {
            return queryCode("中国石油", "https://finance.sina.com.cn/code/");
        });
        CompletableFuture<String> cfQueryFrom163 = CompletableFuture.supplyAsync(() -> {
            return queryCode("中国石油", "https://money.163.com/code/");
        });

        // 用anyOf合并为一个新的CompletableFuture:
        CompletableFuture<Object> cfQuery = CompletableFuture.anyOf(cfQueryFromSina, cfQueryFrom163);

        // 两个CompletableFuture执行异步查询:
        CompletableFuture<Double> cfFetchFromSina = cfQuery.thenApplyAsync((code) -> {
            return fetchPrice((String) code, "https://finance.sina.com.cn/price/");
        });
        CompletableFuture<Double> cfFetchFrom163 = cfQuery.thenApplyAsync((code) -> {
            return fetchPrice((String) code, "https://money.163.com/price/");
        });

        // 用anyOf合并为一个新的CompletableFuture:
        CompletableFuture<Object> cfFetch = CompletableFuture.anyOf(cfFetchFromSina, cfFetchFrom163);

        // 最终结果:
        cfFetch.thenAccept((result) -> {
            System.out.println("price: " + result);
        });
        // 主线程不要立刻结束，否则CompletableFuture默认使用的线程池会立刻关闭:
        Thread.sleep(2000);
    }

    static String queryCode(String name, String url) {
        System.out.println("query code from " + url + "...");
        try {
            Thread.sleep((long) (Math.random() * 1000));
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
        return "601857";
    }

    static Double fetchPrice(String code, String url) {
        System.out.println("query price from " + url + "...");
        try {
            Thread.sleep((long) (Math.random() * 1000));
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
        return 5 + Math.random() * 20;
    }

}
```


---

# Agent Instructions
This documentation is published with GitBook. GitBook is the documentation platform designed so that both humans and AI agents can read, navigate, and reason over technical content effectively. Learn more at gitbook.com.

## Querying This Documentation
If you need additional information that is not directly available in this page, you can query the documentation dynamically by asking a question.

Perform an HTTP GET request on the current page URL with the `ask` query parameter:

```
GET https://jupiter-1992.gitbook.io/jupiter-note/hou-duan/bing-fa-bian-cheng/07-completablefuture.md?ask=<question>
```

The question should be specific, self-contained, and written in natural language.
The response will contain a direct answer to the question and relevant excerpts and sources from the documentation.

Use this mechanism when the answer is not explicitly present in the current page, you need clarification or additional context, or you want to retrieve related documentation sections.