The best way to map a projection query to a DTO (Data Transfer Object) with JPA and Hibernate

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Introduction

While answering questions on the Hibernate forum, I stumbled on the following question. What was different than previous questions on the same topic was that the original poster realized the shortcoming of the JPA constructor expression which can be addressed elegantly by Hibernate.

Because this has been a recurring theme on StackOverflow and the Hibernate forum, I decided to dedicate an article to the best way of mapping DTO projections using JPA and Hibernate.

Domain Model

Considering we have the following Post entity:

Post entity for DTO projection

As previously explained, fetching entities only makes sense if you plan to modify them. If you are only interested in a DTO projection, it’s more efficient to select only the columns that are really needed by the caller.

Assuming we want to select just the id and the title from our Post entities, it would be a waste of resources to select an entire Post entity, so let’s see how you could achieve this goal using JPA and Hibernate.

DTO projections using JPA

When using JPA or Hibernate, you can execute both entity queries via JPQL or Criteria API or native SQL queries.

DTO projections using Tuple and JPQL

If you don’t want to supply a DTO class for your projection, you can use the JPA Tuple.

So, to use Tuple projection, your JPQL query looks as follows:

List<Tuple> postDTOs = entityManager
.createQuery(
    "select " +
    "       p.id as id, " +
    "       p.title as title " +
    "from Post p " +
    "where p.createdOn > :fromTimestamp", Tuple.class)
.setParameter( "fromTimestamp", Timestamp.from(
    LocalDateTime.of( 2016, 1, 1, 0, 0, 0 )
        .toInstant( ZoneOffset.UTC ) ))
.getResultList();

assertFalse( postDTOs.isEmpty() );

Tuple postDTO = postDTOs.get( 0 );
assertEquals( 
    1L, 
    postDTO.get( "id" ) 
);

assertEquals( 
    "High-Performance Java Persistence", 
    postDTO.get( "title" ) 
);

As you can see, the Tuple is a convenient way of fetching DTO projections as you don’t require to specify a DTO class for every type of projection that needs to be supported.

DTO projections using a Constructor Expression and JPQL

If you don’t want to use a Tuple because you want the DTO projection to use a specific class, you can use a Constructor Expression by specifying the NEW keyword along with the fully-qualified name of the class representing the DTO projection and the list of attributes that will be passed as constructor arguments.

The DTO class must provide a constructor that takes all the attributes fetched by the result set projection.

So, the DTO projection must look as follows:

public class PostDTO {

    private Long id;

    private String title;

    public PostDTO(Number id, String title) {
        this.id = id.longValue();
        this.title = title;
    }

    public Long getId() {
        return id;
    }

    public String getTitle() {
        return title;
    }
}

Therefore, the constructor expression JPQL query looks as follows:

List<PostDTO> postDTOs = entityManager
.createQuery(
    "select new " +
    "   com.vladmihalcea.book.hpjp.hibernate.query.dto.projection.jpa.PostDTO(" +
    "       p.id, " +
    "       p.title " +
    "   ) " +
    "from Post p " +
    "where p.createdOn > :fromTimestamp", PostDTO.class)
.setParameter( "fromTimestamp", Timestamp.from(
    LocalDateTime.of( 2016, 1, 1, 0, 0, 0 )
        .toInstant( ZoneOffset.UTC ) ))
.getResultList();

DTO projections using Tuple and native SQL queries

Starting from Hibernate ORM 5.2.11, because the HHH-11897 Jira issue got fixed, you can use Tuple for native SQL queries.

List<Tuple> postDTOs = entityManager
.createNativeQuery(
    "SELECT " +
    "       p.id AS id, " +
    "       p.title AS title " +
    "FROM Post p " +
    "WHERE p.created_on > :fromTimestamp", Tuple.class)
.setParameter( "fromTimestamp", Timestamp.from(
    LocalDateTime.of( 2016, 1, 1, 0, 0, 0 )
        .toInstant( ZoneOffset.UTC ) ))
.getResultList();

assertFalse( postDTOs.isEmpty() );

Tuple postDTO = postDTOs.get( 0 );
assertEquals( 
    1L, 
    ((Number) postDTO.get( "id" )).longValue() 
);

assertEquals( 
    "High-Performance Java Persistence", 
    postDTO.get( "title" ) 
);

DTO projections using a ConstructorResult

For native SQL queries, you can no longer use a Constructor Expression, so you need to use a named native query and configure a given SqlResultSetMapping so that you can populate the DTO class either via its constructor or its fields.

If we use the same PostDTO class type introduced previously, we have to provide the following SqlResultSetMapping:

@NamedNativeQuery(
    name = "PostDTO",
    query =
        "SELECT " +
        "       p.id AS id, " +
        "       p.title AS title " +
        "FROM Post p " +
        "WHERE p.created_on > :fromTimestamp",
    resultSetMapping = "PostDTO"
)
@SqlResultSetMapping(
    name = "PostDTO",
    classes = @ConstructorResult(
        targetClass = PostDTO.class,
        columns = {
            @ColumnResult(name = "id"),
            @ColumnResult(name = "title")
        }
    )
)

Now, the SQL projection named native query is executed as follows:

List<PostDTO> postDTOs = entityManager
.createNamedQuery("PostDTO")
.setParameter( "fromTimestamp", Timestamp.from(
    LocalDateTime.of( 2016, 1, 1, 0, 0, 0 )
        .toInstant( ZoneOffset.UTC ) ))
.getResultList();

DTO projections using Hibernate

While you can use all the JPA features with Hibernate, there are many more features Hibernate has to offer than the standard Java Persistence specification.

DTO projections using ResultTransformer and JPQL

As previously explained, the ResultTransformer allows you to customize the result set any way you like so you can use it to transform the typical Object[] array projection into a DTO result set.

This time, you don’t need to provide a constructor to match the entity attributes being selected by the query.

Although you don’t even have to provide setters in your DTO class, here, we need the setter because BigInteger might be returned for the id database column while we need it to be cast as a Long.

Hibernate can set the appropriate fields using Reflection, so it’s more flexible than the previous JPA Constructor Expression alternative.

Considering we have the following DTO class:

public class PostDTO {

    private Long id;

    private String title;

    public Long getId() {
        return id;
    }

    public void setId(Number id) {
        this.id = id.longValue();
    }

    public String getTitle() {
        return title;
    }

    public void setTitle(String title) {
        this.title = title;
    }
}

We can transform the result set using the setResultTransformer method of the Hibernate-specific org.hibernate.query.Query interface which you can unwrap from the JPA Query.

List<PostDTO> postDTOs = entityManager
.createQuery(
    "select " +
    "       p.id as id, " +
    "       p.title as title " +
    "from Post p " +
    "where p.createdOn > :fromTimestamp")
.setParameter( "fromTimestamp", Timestamp.from(
    LocalDateTime.of( 2016, 1, 1, 0, 0, 0 ).toInstant( ZoneOffset.UTC ) ))
.unwrap( org.hibernate.query.Query.class )
.setResultTransformer( Transformers.aliasToBean( PostDTO.class ) )
.getResultList();

DTO projections using ResultTransformer and a Native SQL query

If you want to use a native SQL query, you don’t need to go through all the trouble of declaring a SqlResultSetMapping since you can use the AliasToBeanResultTransformer just like it was the case for the aforementioned JPQL example.

List postDTOs = entityManager
.createNativeQuery(
    "select " +
    "       p.id as \"id\", " +
    "       p.title as \"title\" " +
    "from Post p " +
    "where p.created_on > :fromTimestamp")
.setParameter( "fromTimestamp", Timestamp.from(
    LocalDateTime.of( 2016, 1, 1, 0, 0, 0 ).toInstant( ZoneOffset.UTC ) ))
.unwrap( org.hibernate.query.NativeQuery.class )
.setResultTransformer( Transformers.aliasToBean( PostDTO.class ) )
.getResultList();

Cool, right?

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Conclusion

As you can see, there are multiple options to generate a DTO projection with JPA and Hibernate. Although the JPA specification offers both the Constructor Expression and the Tuple result, the ResultTransformer can be a much more flexible alternative.

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19 Comments on “The best way to map a projection query to a DTO (Data Transfer Object) with JPA and Hibernate

  1. Hi Vlad and thanks for your nice article.

    Hibernate caches entities. What about DTOs. If i want to access to a DTO in several views, What should i do?

    I use “Business Objects” for transferring data between the Presentation Layer and the Business Layer and “Entity Objects” for transferring data between the Business Layer and the Data Access Layer in this case, the Hibernate(separation of concerns). I’m Thinking if i want a BO for reading, will use DTO projection but if i want to write a BO to a DB, will write a mapping system to map the BO to the corresponding Entity. Is that right? does the Hibernate have a mapping system for that?

  2. Remove the LongDetails.class parameter in the createNativeQuery method. That’s causing the issue.

    • Each column in the executing SQL query is distinct from an alias perspective.

  3. Hi Vlad,

    Is it possible to map an object inside a DTO object using HQL query directly? I am constructing a DTO by having another object passed as a parameter.

    • It depends on what do you mean by “object inside DTO”. If it’s an entity, then mixing DTOs and entities does not sound like a great idea. If it’s another DTO, then sure it’s possible. You can even build hierarchical DTOs. Check out my High-Performance Java Persistence book for more details.

  4. Hello, thank you very much for this article, is’t very clear how to use DTO projections with your examples.

    I have a doubt about generating a list of related entities in our DTO.
    In my case, I have an entity (Post.java) that has a @OneToMany relationship with another entity (Comment.java) and I’m trying to generate a DTO that contains some values of Post.java and a list containing DTOs from the related Comment.java.

    I have your book High-Performance Java Persistence, but couldn’t find an example of such case (if it’s even possible), perhaps I missed it.

    If you could help me I would really apreciate it. Thank you in advance!

    • Sure, it is possible. You can find an example in the Fetching chapter where I show the difference between fetching all data and using a Recursive CTE to limit the result set on the database side. All the code is on GitHub. Check out this example for instance.

  5. Great Article, but it made me think…
    It states that it is more efficient to select only the needed columns for DTO projections, but what if performance isn’t an issue?
    Isn’t safety (next to efficiency) an other reason to select only the needed columns in DTO projections (while you can modify the data but cannot accidentally persist it to the database)?

  6. Hi Vlad.
    I am trying to use native query to fetch a projection interface which has String, Long and a JsonNode as methods.

    However, when i try to use the above approach ,i.e., using a scalar for each of the types, and by using your dialect, I am facing “No converter found capable of converting from type ObjectNode” error.

    When I use the same interface at repository using @Query annotation, I get No dialect mapping for JDBC type: 1111.

    How can I resolve this issue? Does hibernate support createNativeQuery for interfaces? I would like to send List which will have a tree structure.

  7. Hello,
    when using DTO projections using ResultTransformer and a Native SQL query we’ve encountered an Connection leak.
    I’ve created an example project for demonstration. If you trigger requests on the /fail endpoint and check your (postgres) DB you’ll find that connections are not getting closed.
    It is a Spring Boot project and it seems like an bug.

    Here is the code:
    https://github.com/fleske/connection_leak
    Can you please explain what i happening and if there is an solution for this?
    Thanks

  8. Hi Vlad thanks for the article. Just want to add
    To make it work with Spring Data repository -> from Spring data JPA 2.0.5 – we need to add @Query(nativeQuery=true) on top of function in repository and EntityName.FunctionName in NamedNativeQuery

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