如何使用Hibernate和Spring Boot正确处理空结果集 [英] How to properly handle empty resultset with Hibernate and Spring Boot

问题描述

我有一个使用Hibernate和Spring Data JPA的CrudRepository的Spring应用程序.如果查询的数据存在于数据库中，那么一切似乎都可以正常工作.但是，如果存在不返回任何结果的查询，则CrudRepository返回null，我得到一个NullPointerException.因此，例如http://localhost:8080/api/id=3，如果数据库中存在ID为3的行，则可以正常工作.如果没有ID为3的行，它将失败并显示:

I have a Spring app that is using Hibernate and the Spring Data JPA's CrudRepository. Everything seems to work properly if the data that was queried for exists in the database. However, if there is a query that returns no result, the CrudRepository returns null and I get a NullPointerException. So for example http://localhost:8080/api/id=3 if there is a row with id 3 in the database it works fine. If there isn't a row with id of 3 it fails with a:

发生意外错误(类型=内部服务器错误， 状态= 500)

there was an unexpected error (type=Internal Server Error, status=500)

在客户端，在服务器上是NullPointerException.

On the client side and a NullPointerException on the server side.

处理无结果"查询的简单情况的正确方法是什么?

What is the proper way of dealing with a simple case of a "No Results" query?

推荐答案

检查返回值，如果它不是null，则以200 OK响应的形式返回它的某种表示形式.否则，返回404 Not Found.最后，您将拥有一个像这样的控制器:

Inspect the return value, if it's not null, return some representation of it as a 200 OK response. Otherwise, return a 404 Not Found. In the end, you would have a controller like:

@RequestMapping(...)
public ResponseEntity<?> getOne(...) {
    Something something = repository.findOne(...);
    if (something == null)
        return ResponseEntity.notFound().build();

    return ResponseEntity.ok(something);
}

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您可以重构前面的代码以合并Java 8的Optional<T>，如 JB Nizet . "https://stackoverflow.com/questions/37757048/how-to-properly-handle-empty-resultset-with-hibernate-and-spring-boot#comment62984085_37757236>评论.基本上，Optional<T>只是可能包含或可能不包含类型T的值的容器.您可以将此类型用作Spring Data JPA方法的返回类型，如下所示:

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You can refactor the preceding code to incorporate Java 8's Optional<T>, as JB Nizet mentioned in the comments. Basically, Optional<T> is just a container that may or may not hold a value of type T. You can use this type as the return type of Spring Data JPA methods, something like the following:

public interface SomethingRepository extends CrudRepository<Something, Long> {
    Optional<Something> findById(Long id);
}

然后为404 Not Found定义一个例外:

@ResponseStatus(HttpStatus.NOT_FOUND)
public class NotFoundException extends RuntimeException {}

如果在控制器中引发类型为NotFoundException的异常，Spring MVC的异常解析器将捕获该异常并将其转换为404 Not Found HTTP响应.

If you throw an exception of type NotFoundException in your controllers, Spring MVC's exception resolver would catch that exception and convert it to a 404 Not Found HTTP response.

最后，您的控制器将是:

Finally your controller would be like:

@RequestMapping(...)
public Something getOne(...) {
    return repository.findById(id).orElseThrow(NotFoundException::new);
}

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有关以下内容的详细讨论:

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For more detailed discussions on:

• Spring Data JPA的Optional<T>支持，在此处阅读.
• Java 8的Optional<T>， REST在实践中书.

• Spring Data JPA's Optional<T> support, read here.
• Spring MVC's exception resolvers, read here.
• Java 8's Optional<T>, read here.
• REST best practices, you can check out the REST in Practice book.

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