SpringDataJPAfindOne()更改为可选如何使用此？

我正在用 Java8学习 SpringBoot2.0。

我还参考了一些博客制作教程的例子。

教程的源代码是:

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

但是这个代码抛出了这个错误:

FindOne (id)

我正在考虑将 JPAfindOne()方法更改为 Optional< S >

怎么解决呢？

更多信息:

这是 typeyService 方法:

public Category findOne(Long id) {
return categoryRepository.findOne(id);
}
198135 次浏览
小开

Indeed, in the latest version of Spring Data, findOne returns an optional. If you want to retrieve the object from the Optional, you can simply use get() on the Optional. First of all though, a repository should return the optional to a service, which then handles the case in which the optional is empty. afterwards, the service should return the object to the controller.

小开

Optional api provides methods for getting the values. You can check isPresent() for the presence of the value and then make a call to get() or you can make a call to get() chained with orElse() and provide a default value.

The last thing you can try doing is using @Query() over a custom method.

小开
最佳答案

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);

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);

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.

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);

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

So, the actual method to invoke is now Optional<T> findById(ID id).

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

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.

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).

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

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

Here are three classical usage examples.

  1. Suppose that if the entity is found you want to get it otherwise you want to get a default value.

You could write :

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

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. Suppose that if the entity is found you want to return it, else you want to throw an exception.

You could write :

return repository.findById(id)
.orElseThrow(() -> new EntityNotFoundException(id));
  1. Suppose you want to apply a different processing according to if the entity is found or not (without necessarily throwing an exception).

You could write :

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

The method has been renamed to findById(…) returning an Optional so that you have to handle absence yourself:

Optional<Foo> result = repository.findById(…);


result.ifPresent(it -> …); // do something with the value if present
result.map(it -> …); // map the value if present
Foo foo = result.orElse(null); // if you want to continue just like before
小开

I always write a default method "findByIdOrError" in widely used CrudRepository repos/interfaces.

@Repository
public interface RequestRepository extends CrudRepository<Request, Integer> {


default Request findByIdOrError(Integer id) {
return findById(id).orElseThrow(EntityNotFoundException::new);
}
}
小开

Consider an User entity and UserRepository. In service package code like below.

Optional<User> resultUser = UserRepository.findById(userId);  //return Optional


User createdUser = resultUser.get();  //return User

Now you can access all the User entity attributes using getter.

createdUser.getId();
createdUser.getName();

like that.

小开

The findOne method of the CrudRepository interface has been replaced by findById since version 2.0 of Spring Data Commons. you replace findOne(id) by:

findById(id).orElse(null)

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