How to write a compact DTO projection query with JPA

Last modified:

Imagine having a tool that can automatically detect if you are using JPA and Hibernate properly. Hypersistence Optimizer is that tool!

Introduction

In this article, we are going to see how we can write the best DTO projection JPQL query by omitting the package name when using JPA, Hibernate, and Spring.

As I already explained, DTO projections are the most efficient way of fetching data when using JPA and Hibernate.

How to write a compact DTO projection query with JPA @vlad_mihalcea https://t.co/EDgXuEbsFX pic.twitter.com/rAf1VZgtAA
— Java (@java) September 13, 2019

Domain Model

Let’s consider the following Post entity and its associated PostDTO Value Object.

We can see that the Post entity has seven attributes, while PostDTO has only two of those. If our business use case requires just the two attributes contained in PostDTO, then it’s going to be more efficient to fetch a PostDTO projection rather than a list of Post entities.

DTO projection query with JPA

The JPA specification defines the constructor expression for passing a fully-qualified DTO class name to be used as a placeholder for the selected entity attributes:

List<PostDTO> postDTOs = entityManager.createQuery("""
    select new com.vladmihalcea.book.hpjp.hibernate.forum.dto.PostDTO(
        p.id,
        p.title
    )
    from Post p
    where p.createdOn > :fromTimestamp
    """, PostDTO.class)
.setParameter(
    "fromTimestamp",
    LocalDate.of(2016, 1, 1).atStartOfDay()
)
.getResultList();

According to the JPA standard, the DTO constructor expression must take the fully-qualified name of the Java class representing the DTO object we want to hold the selected entity attributes.

But this is not nice at all!

I’d rather want to use the simple class name or, at least, a short folder name if there are two DTOs with the same name, but with different structures.

A simpler DTO projection query with JPA and Hibernate

So, basically, this is how I want to write DTO projections:

List<PostDTO> postDTOs = entityManager.createQuery("""
    select new PostDTO(
        p.id,
        p.title
    )
    from Post p
    where p.createdOn > :fromTimestamp
    """, PostDTO.class)
.setParameter(
    "fromTimestamp",
    LocalDate.of(2016, 1, 1).atStartOfDay()
)
.getResultList();

Basically, I want to be able to use the simple Java class name by default, instead of having to supply the fully-qualified class name for every JPA constructor expression.

In order to be able to use the simple Java class name, we need to use the ClassImportIntegrator utility provided by the Hibernate Types project:

Declarative configuration

If you are using a declarative configuration, then you first need to create a class implementing the Hibernate IntegratorProvider, and which returns the configured ClassImportIntegrator instance:

public class ClassImportIntegratorIntegratorProvider 
        implements IntegratorProvider {

    @Override
    public List<Integrator> getIntegrators() {
        return List.of(
            new ClassImportIntegrator(
                List.of(
                    PostDTO.class
                )
            )
        );
    }
}

Afterward, you need to set the hibernate.integrator_provider configuration property to the fully-qualified name of the ClassImportIntegratorIntegratorProvider.

If you’re using Spring Boot, you can declare the hibernate.integrator_provider property in the application.properties configuration file, like this:

spring.jpa.properties.hibernate.integrator_provider=com.vladmihalcea.book.hpjp.ClassImportIntegratorIntegratorProvider

If you’re Java EE, you can set the hibernate.integrator_provider property in the persistence.xml JPA configuration file, like this:

That’s it!

Programmatic configuration

You can also configure the hibernate.integrator_provider property programmatically, using a Spring Java-based configuration, either via the JPA or the native Hibernate API bootstrapping strategies.

Spring and JPA

To bootstrap JPA with Spring, you need to use the LocalContainerEntityManagerFactoryBean:

@Bean
public LocalContainerEntityManagerFactoryBean entityManagerFactory() {

    LocalContainerEntityManagerFactoryBean emf = 
       new LocalContainerEntityManagerFactoryBean();

    emf.setPersistenceUnitName(getClass().getSimpleName());
    emf.setPersistenceProvider(new HibernatePersistenceProvider());
    emf.setDataSource(dataSource());
    emf.setPackagesToScan(packagesToScan());

    emf.setJpaVendorAdapter(new HibernateJpaVendorAdapter());
    
    Properties properties = new Properties();
    
    properties.setProperty(
        "hibernate.dialect", 
        hibernateDialect
    );
    
    properties.put(
        "hibernate.integrator_provider",
        (IntegratorProvider) () -> Collections.singletonList(
            new ClassImportIntegrator(
                List.of(
                    PostDTO.class
                )
            )
        )
    );
    
    emf.setJpaProperties(properties);

    return emf;
}

Notice how we passed the hibernate.integrator_provider configuration property to the LocalContainerEntityManagerFactoryBean via its setJpaProperties method.

Spring and Hibernate

To bootstrap the native Hibernate with Spring, you need to use the SessionactoryBean:

@Bean
public LocalSessionFactoryBean sessionFactory() {

    LocalSessionFactoryBean sf = 
        new LocalSessionFactoryBean();
        
    sf.setDataSource(dataSource());
    sf.setPackagesToScan(packagesToScan());
    
    Properties properties = new Properties();
    
    properties.setProperty(
        "hibernate.dialect", 
        hibernateDialect
    );
    
    sf.setHibernateProperties(properties);
    
    sf.setHibernateIntegrators(
        new ClassImportIntegrator(
            List.of(
                PostDTO.class
            )
        )
    );
    
    return sf;
}

Using relative package names

By default, the ClassImportIntegrator will register the provided DTOs using their simple class name. However, if you have multiple DTOs with the same name located in different packages, you need to register the relative package name to differentiate between different DTO classes.

The fully-qualified name of the PostDTO class is com.vladmihalcea.book.hpjp.hibernate.forum.dto.PostDTO. Therefore, we can configure the ClassImportIntegrator to exclude the com.vladmihalcea.book.hpjp.hibernate path, so we can reference the PostDTO using the remaining relative path, forum.dto.PostDTO.

To exclude a package prefix, you need to call the excludePath method, as follows:

List<PostDTO> postDTOs = entityManager.createQuery("""
    select new forum.dto.PostDTO(
        p.id,
        p.title
    )
    from Post p
    where p.createdOn > :fromTimestamp
    """, PostDTO.class)
.setParameter(
    "fromTimestamp",
    LocalDate.of(2016, 1, 1).atStartOfDay()
)
.getResultList();

Cool, right?

Online Workshops

If you enjoyed this article, I bet you are going to love my upcoming Online Workshops!

Caching Best Practices with JPA and Hibernate (2.5 hours) on the 30th of September

High-Performance SQL (4 hours) on the 6th of October in collaboration with Voxxed Days Ticino

High-Performance SQL (12 hours) starting on the 28th of October in collaboration with Bouvet

Conclusion

Omitting the DTO package name in a JPA query is definitely the type of enhancement most Java developers wanted to have for a long time, as demonstrated by the positive reactions I got on this tweet.

Next week, I'm going to show you how you can omit the package name when doing DTO projections with @Java Persistence JPQL.

Who's interested in this awesome tip? pic.twitter.com/B249xV47nZ
— Vlad Mihalcea (@vlad_mihalcea) September 7, 2019