关于Spring项目中JpaRepository.getOne方法的使用困惑咨询

Hey there! Let's walk through the getOne() method you're using in your ExampleService—it's got some nuanced behavior that's easy to miss, and I'll break down what's going on, potential issues, and better alternatives for your use case.

What getOne() Actually Does #

First off, getOne() is built on top of JPA's EntityManager.getReference(). Here's the key thing to remember:

• It doesn't immediately hit the database to fetch the entity. Instead, it returns a lazy-loaded proxy object.
• The proxy only triggers a database query when you access a property of the entity that's not its ID.
• If the entity with the given ID doesn't exist, it won't throw an error right away—you'll get an EntityNotFoundException only when you try to access the proxy's non-ID properties.

The Problem in Your Current Code #

In your fetchAndSet method, you're setting this proxy object to exampleParentEntity.setChildEntity(...). This can cause issues later:

• If you try to access any properties of childEntity outside of an active transaction (like after the service method completes, in a controller or view layer), you'll hit a LazyInitializationException. That's because the EntityManager that created the proxy is no longer open, so it can't fetch the real entity data from the database.

Better Alternatives for Your Use Case #

Most of the time, when you need to fetch an entity to associate it with another, you'll want to use a method that returns the actual entity immediately. Here are your best options:

1. findById() (Returns Optional) #

This method immediately executes a SQL query to fetch the entity. It returns an Optional, so you can handle cases where the ID doesn't exist gracefully:

@Component
public class ExampleService{
    @Autowired
    ChildOfJpaRepository childOfJpaRepository;

    public void fetchAndSet(ExampleParentEntity exampleParentEntity, Long id){
        childOfJpaRepository.findById(id)
            .ifPresent(childEntity -> exampleParentEntity.setChildEntity(childEntity));
        // Or handle the empty case if needed:
        // .orElseThrow(() -> new EntityNotFoundException("Child entity not found with id: " + id));
    }
}

2. findById(id).orElseThrow(...) (For Required Entities) #

If you're certain the entity with the given ID must exist (and want to throw an exception if it doesn't), use this approach. It avoids dealing with Optional and makes your intent clear:

public void fetchAndSet(ExampleParentEntity exampleParentEntity, Long id){
    ChildEntity childEntity = childOfJpaRepository.findById(id)
        .orElseThrow(() -> new EntityNotFoundException("Child entity not found with id: " + id));
    exampleParentEntity.setChildEntity(childEntity);
}

When Should You Use getOne()? #

getOne() isn't bad—it's just meant for specific scenarios:

• You only need the entity's ID to establish a relationship (and never access its other properties).
• You're sure you'll access the entity's properties within the same active transaction (so the proxy can fetch the data when needed).
• You want to avoid an unnecessary database query for performance reasons, and the above conditions are met.

内容的提问来源于stack exchange，提问作者pokemzok