How to return a custom object from a Spring Data JPA GROUP BY query

java

dto

lombok

spring-data

byNikita Barsukov·Jan 21, 2025

You can utilize a constructor expression in your JPQL @Query to map the grouped results to your custom Data Transfer Object (DTO) in Spring Data JPA:

// Your friendly neighbourhood DTO
public class Summary {
    // The category we're concerned with
    private String category; 
    // Count, not the vampire
    private Long count; 

    // DTO Constructor aka "The Gatherer"
    public Summary(String category, Long count) {
        this.category = category;
        this.count = count;
    }
    // Getters and setters code...
}

// Repository aka "The Oracle"
public interface Repo extends JpaRepository<Entity, Long> {
    // The puzzle master at work 
    @Query("SELECT new package.path.to.Summary(e.category, COUNT(e)) FROM Entity e GROUP BY e.category")
    List<Summary> groupByCategory();
}

So, remember to craft your Summary DTO with a constructor that mirrors the Group By fields you want, then lob it inside a @Query in your repo to dish out a list of Summary munchies complete with the grouped data.

DTO crafting and interface utilization

Custom object creation - easy as pie

Ensure that your DTO class is properly constructed. Double-check that the constructor and properties are defined for each grouping column present in your JPA query.

The 'new' JPQL keyword - your magic wand

When you're knee-deep in crafting your JPQL queries, remember the 'new' keyword as this allows you to create an instance of your DTO class directly in the query. A worthwhile and nifty trick to keep up your sleeve.

Handling native queries - interface to the rescue

Venturing into the land of native queries? Ensure to adopt interface-based projections. This nifty feature allows a type-safe way to retrieve a subset of the data in your entity.

Mapping the puzzle - A to Z

When your query is done, you'd typically get a List<Object[]>. It's now up to you to translate this into a list of DTOs, ensuring the points meet in terms of constructor order and type.

Caveat for Null instances - don’t step on the Null

Before you dive headfirst into processing your results, do a null check. This will keep your code robust and ready to take on the world.

Code optimization with Lombok and static references - be lazy, the smart way

For trimming your DTOs, Lombok is your best ally — less boilerplate, more stuff that matters. Furthermore, static final strings for property names can be a guiding star for code maintenance.