How to fetch FetchType.LAZY associations with JPA and Hibernate in a Spring Controller

How to Fetch FetchType.LAZY Associations with JPA and Hibernate in a Spring Controller

Are you having trouble fetching FetchType.LAZY associations in your Spring Controller? Don't worry, you're not alone. Many developers struggle with this issue, but we're here to help!

In this blog post, we'll address the common problem of triggering lazy data loading and provide you with easy solutions to overcome it. So, let's dive in and understand the problem first.

The Problem

Let's consider the code snippet below:

@Entity
public class Person {

    @Id
    @GeneratedValue
    private Long id;

    @ManyToMany(fetch = FetchType.LAZY)
    private List<Role> roles;
    // ...
}

Here, we have a Person class with a many-to-many association that is marked as FetchType.LAZY. In our Spring Controller, we have the following code:

@Controller
@RequestMapping("/person")
public class PersonController {

    @Autowired
    PersonRepository personRepository;

    @RequestMapping("/get")
    public @ResponseBody Person getPerson() {
        Person person = personRepository.findOne(1L);
        return person;
    }
}

The issue arises when we try to access the lazy-loaded data. We receive an exception like:

failed to lazily initialize a collection of role: no.dusken.momus.model.Person.roles, could not initialize proxy - no Session

The Solution

To trigger the loading of lazy associations, we need to ensure that the session is still open when accessing the associations. There are a few ways to achieve this.

Solution 1: Eager Fetching

You can change the FetchType to EAGER in your Person class, like this:

@ManyToMany(fetch = FetchType.EAGER)
private List<Role> roles;

However, eager fetching can negatively impact performance, as it loads all associated data immediately, even if you don't need it.

Solution 2: Open Session in View

Another approach is to use the Open Session in View pattern. This pattern keeps the Hibernate session open until the view rendering is complete. To enable this pattern in Spring, follow these steps:

1. Add the necessary dependencies to your project. For example, if you're using Maven, add the following to your pom.xml:

<dependency>
    <groupId>org.springframework.boot</groupId>
    <artifactId>spring-boot-starter-data-jpa</artifactId>
</dependency>
<dependency>
    <groupId>org.springframework.boot</groupId>
    <artifactId>spring-boot-starter-web</artifactId>
</dependency>

1. In your application.properties or application.yml file, configure Hibernate to enable the Open Session in View pattern:

spring.jpa.open-in-view=true
spring:
  jpa:
    open-in-view: true

1. With this configuration, Spring will automatically open and close the session for each request, allowing you to fetch lazy associations in your controller without exceptions.

Solution 3: Fetch Associations Manually

If you prefer a more explicit approach, you can manually fetch the associations in your controller. Here's an example:

@RequestMapping("/get")
public @ResponseBody Person getPerson() {
    Person person = personRepository.findOne(1L);
    Hibernate.initialize(person.getRoles());
    return person;
}

By calling Hibernate.initialize(person.getRoles()), we force the lazy associations to be loaded before returning the response.

Conclusion

Now you know how to fetch FetchType.LAZY associations in a Spring Controller using JPA and Hibernate. The solutions discussed in this post give you different approaches to overcome the common problem of lazy data loading. Choose the method that best suits your needs and architectural preferences.

Don't let lazy associations stop you from building great applications! Implement these solutions and enjoy hassle-free lazy loading.

If you found this article helpful, share it with your fellow developers and engage with us in the comments below. We'd love to hear your thoughts and experiences with lazy loading in JPA and Hibernate.

Happy coding! 💻🚀