Spring Data JPA findOne() 更改为 Optional 如何使用? [英] Spring Data JPA findOne() change to Optional how to use this?

问题描述

我正在学习 SpringBoot2.0 和 Java8.

我遵循了一些博客制作教程示例.

And I followed some blog-making tutorial example.

教程源码为:

@GetMapping("/{id}/edit")
public String edit(@PathVariable Long id, Model model) {
  model.addAttribute("categoryDto", categoryService.findOne(id));
  return "category/edit";
}

但是这段代码抛出了这个错误:

But this code is throwing this error:

categoryService.findOne(id)

categoryService.findOne(id)

我正在考虑将 JPA findOne() 方法更改为 Optional<>

I'm thinking about changing the JPA findOne() method to Optional< S >

如何解决?

更多信息:

这是 categoryService 方法:

This is the categoryService method:

public Category findOne(Long id) {
  return categoryRepository.findOne(id);
}

推荐答案

至少从 2.0 版本开始，Spring-Data-Jpa 修改了 findOne().
现在，findOne() 既没有相同的签名，也没有相同的行为.
之前，它在 CrudRepository 接口中定义为:

From at least, the 2.0 version, Spring-Data-Jpa modified findOne().
Now, findOne() has neither the same signature nor the same behavior.
Previously, it was defined in the CrudRepository interface as:

T findOne(ID primaryKey);

现在，您将在 CrudRepository 中找到的单个 findOne() 方法是在 QueryByExampleExecutor 接口中定义的方法:

Now, the single findOne() method that you will find in CrudRepository is the one defined in the QueryByExampleExecutor interface as:

<S extends T> Optional<S> findOne(Example<S> example);

最后由SimpleJpaRepository实现，CrudRepository接口的默认实现.
此方法是按示例搜索查询，您不希望将其作为替代.

That is implemented finally by SimpleJpaRepository, the default implementation of the CrudRepository interface.
This method is a query by example search and you don't want that as a replacement.

其实在新的API中，行为相同的方法还是有的，只是方法名变了.
它在 CrudRepository 接口:

In fact, the method with the same behavior is still there in the new API, but the method name has changed.
It was renamed from findOne() to findById() in the CrudRepository interface :

Optional<T> findById(ID id); 

现在它返回一个Optional，这对于防止NullPointerException来说还不错.

Now it returns an Optional, which is not so bad to prevent NullPointerException.

所以，实际调用的方法现在是 Optional;findById(ID id).

如何使用?
学习可选用法.以下是有关其规范的重要信息:

How to use that?
Learning Optional usage. Here's important information about its specification:

可能包含也可能不包含非空值的容器对象.如果一个value 存在，isPresent() 将返回 true 并且 get() 将返回价值.

A container object which may or may not contain a non-null value. If a value is present, isPresent() will return true and get() will return the value.

依赖于存在或不存在的其他方法提供了包含的值，例如 orElse()(返回默认值如果值不存在)和 ifPresent()(如果值存在).

Additional methods that depend on the presence or absence of a contained value are provided, such as orElse() (return a default value if value not present) and ifPresent() (execute a block of code if the value is present).

---

关于如何使用 Optional 和 Optional 的一些提示；findById(ID id).

---

Some hints on how to use Optional with Optional<T> findById(ID id).

通常，当您通过 id 查找实体时，您希望将其返回或在未检索到的情况下进行特定处理.

Generally, as you look for an entity by id, you want to return it or make a particular processing if that is not retrieved.

以下是三个经典用法示例.

Here are three classical usage examples.

1. 假设如果找到实体，您想要获取它，否则您想要获取默认值.

你可以写:

Foo foo = repository.findById(id)
                    .orElse(new Foo());

或者在有意义的情况下获取 null 默认值(与 API 更改之前的行为相同):

or get a null default value if it makes sense (same behavior as before the API change) :

Foo foo = repository.findById(id)
                    .orElse(null);

1. 假设如果找到实体，您想返回它，否则您想抛出异常.

你可以写:

return repository.findById(id)
        .orElseThrow(() -> new EntityNotFoundException(id));

1. 假设您想根据是否找到实体来应用不同的处理(不必抛出异常).

你可以写:

Optional<Foo> fooOptional = fooRepository.findById(id);
if (fooOptional.isPresent()) {
    Foo foo = fooOptional.get();
    // processing with foo ...
} else {
    // alternative processing....
}

这篇关于Spring Data JPA findOne() 更改为 Optional 如何使用?的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！