java 41 lines · 7 steps

Projection interfaces in Spring Data JPA

A repository maps a grouped revenue query straight onto a typed interface, computed fields and all.

Explained by highlit

Spring Intermediate projections jpql aggregation repository pattern interface default methods

1package com.example.orders.repository;
2 
3import com.example.orders.domain.Order;
4import org.springframework.data.jpa.repository.JpaRepository;
5import org.springframework.data.jpa.repository.Query;
6import org.springframework.data.repository.query.Param;
7 
8import java.math.BigDecimal;
9import java.time.Instant;
10import java.util.List;
11 
12public interface OrderRepository extends JpaRepository<Order, Long> {
13 
14    interface CustomerRevenue {
15        Long getCustomerId();
16        String getCustomerName();
17        long getOrderCount();
18        BigDecimal getTotalRevenue();
19 
20        default boolean isHighValue() {
21            return getTotalRevenue().compareTo(new BigDecimal("1000")) >= 0;
22        }
23    }
24 
25    @Query("""
26            select c.id as customerId,
27                   c.name as customerName,
28                   count(o) as orderCount,
29                   coalesce(sum(o.total), 0) as totalRevenue
30            from Order o
31            join o.customer c
32            where o.placedAt >= :since
33              and o.status = com.example.orders.domain.OrderStatus.COMPLETED
34            group by c.id, c.name
35            having sum(o.total) > :threshold
36            order by sum(o.total) desc
37            """)
38    List<CustomerRevenue> findRevenueByCustomerSince(
39            @Param("since") Instant since,
40            @Param("threshold") BigDecimal threshold);
41}

01 / 01

STEP 01

Walkthrough

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Three takeaways

1Interface projections let Spring Data return exactly the columns a query selects, without a dedicated entity or DTO class.
2JPQL aliases must match projection getter names so Spring can bind each selected value to the right accessor.
3Default methods on a projection interface add derived logic that lives with the data rather than in calling code.

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