DZone
Thanks for visiting DZone today,
Edit Profile
  • Manage Email Subscriptions
  • How to Post to DZone
  • Article Submission Guidelines
Sign Out View Profile
  • Post an Article
  • Manage My Drafts
Over 2 million developers have joined DZone.
Log In / Join
Refcards Trend Reports
Events Video Library
Refcards
Trend Reports

Events

View Events Video Library

Zones

Culture and Methodologies Agile Career Development Methodologies Team Management
Data Engineering AI/ML Big Data Data Databases IoT
Software Design and Architecture Cloud Architecture Containers Integration Microservices Performance Security
Coding Frameworks Java JavaScript Languages Tools
Testing, Deployment, and Maintenance Deployment DevOps and CI/CD Maintenance Monitoring and Observability Testing, Tools, and Frameworks
Culture and Methodologies
Agile Career Development Methodologies Team Management
Data Engineering
AI/ML Big Data Data Databases IoT
Software Design and Architecture
Cloud Architecture Containers Integration Microservices Performance Security
Coding
Frameworks Java JavaScript Languages Tools
Testing, Deployment, and Maintenance
Deployment DevOps and CI/CD Maintenance Monitoring and Observability Testing, Tools, and Frameworks

Generative AI has transformed nearly every industry. How can you leverage GenAI to improve your productivity and efficiency?

SBOMs are essential to circumventing software supply chain attacks, and they provide visibility into various software components.

Related

  • Decoding Database Speed: Essential Server Resources and Their Impact
  • Understanding Time Series Databases
  • Exploring Data Redaction Enhancements in Oracle Database 23ai
  • The Rise of the Intelligent AI Agent: Revolutionizing Database Management With Agentic DBA

Trending

  • The Architecture That Keeps Netflix and Slack Always Online
  • Why Traditional CI/CD Falls Short for Cloud Infrastructure
  • Understanding Time Series Databases
  • MCP Client Agent: Architecture and Implementation
  1. DZone
  2. Data Engineering
  3. Databases
  4. JPA - Querydsl Projections

JPA - Querydsl Projections

By 
Michal Jastak user avatar
Michal Jastak
·
May. 15, 13 · Interview
Likes (3)
Comment
Save
Tweet
Share
38.9K Views

Join the DZone community and get the full member experience.

Join For Free

In my last post: JPA - Basic Projections - I've mentioned about two basic possibilities of building JPA Projections. This post brings you more examples, this time based on Querydsl framework. Note, that I'm referring Querydslversion 3.1.1 here.

Reinvented constructor expressions


Take a look at the following code:...

import static com.blogspot.vardlokkur.domain.QEmployee.employee;
 
import javax.persistence.EntityManager;
import javax.persistence.PersistenceContext;
 
import org.springframework.beans.factory.annotation.Autowired;
 
import com.blogspot.vardlokkur.domain.EmployeeNameProjection;
 
import com.mysema.query.jpa.JPQLTemplates;
import com.mysema.query.jpa.impl.JPAQuery;
import com.mysema.query.types.ConstructorExpression;
...
 
public class ConstructorExpressionExample {
    
    ...
    @PersistenceContext
    private EntityManager entityManager;
 
    @Autowired
    private JPQLTemplates jpqlTemplates;
    
    public void someMethod() {
        ...
        final List<EmployeeNameProjection> projections = new JPAQuery(entityManager, jpqlTemplates)
                        .from(employee)
                        .orderBy(employee.name.asc())
                        .list(ConstructorExpression.create(EmployeeNameProjection.class, employee.employeeId,
                                        employee.name));
        ...                                
    }
    ...
}

The above Querydsl construction means: create new JPQL query [1][2], using employee as the data source, order the data using employee name [3], and return the list of EmployeeNameProjection, built using the 2-arg constructor called with employee ID and name [4].  This is very similar to the constructor expressions example from my previous post (JPA - Basic Projections), and leads to the following SQL query:

select EMPLOYEE_ID, EMPLOYEE_NAME from EMPLOYEE order by EMPLOYEE_NAME asc
As you see above, the main advantage comparing to the JPA constructor expressions is using Java class, instead of its name hard-coded in JPQL query.

Even more reinvented constructor expressions


Querydsl documentation [4] describes another way of using constructor expressions, requiring @QueryProjectionannotation and Query Type [1] usage for projection, see example below. Let's start with the projection class modification - note that I added @QueryProjection annotation on the class constructor.
package com.blogspot.vardlokkur.domain;
 
import java.io.Serializable;
 
import javax.annotation.concurrent.Immutable;
 
import com.mysema.query.annotations.QueryProjection;
 
@Immutable
public class EmployeeNameProjection implements Serializable {
 
    private final Long employeeId;
 
    private final String name;
 
    @QueryProjection
    public EmployeeNameProjection(Long employeeId, String name) {
        super();
        this.employeeId = employeeId;
        this.name = name;
    }
 
    public Long getEmployeeId() {
        return employeeId;
    }
 
    public String getName() {
        return name;
    }
 
}
Now we may use modified projection class (and corresponding Query Type [1] ) in following way:
...
import static com.blogspot.vardlokkur.domain.QEmployee.employee;
 
import javax.persistence.EntityManager;
import javax.persistence.PersistenceContext;
 
import org.springframework.beans.factory.annotation.Autowired;
 
import com.blogspot.vardlokkur.domain.EmployeeNameProjection;
import com.blogspot.vardlokkur.domain.QEmployeeNameProjection;
 
import com.mysema.query.jpa.JPQLTemplates;
import com.mysema.query.jpa.impl.JPAQuery;
 
...
 
public class ConstructorExpressionExample {
    ...
    @PersistenceContext
    private EntityManager entityManager;
 
    @Autowired
    private JPQLTemplates jpqlTemplates;
 
    public void someMethod() {
        ...
        final List<EmployeeNameProjection> projections = new JPAQuery(entityManager, jpqlTemplates)
            .from(employee)
            .orderBy(employee.name.asc())
            .list(new QEmployeeNameProjection(employee.employeeId, employee.name));
        ...
    }
    ...
}
Which leads to SQL query:

select EMPLOYEE_ID, EMPLOYEE_NAME from EMPLOYEE order by EMPLOYEE_NAME asc
In fact, when you take a closer look at the Query Type [1] generated for EmployeeNameProjection(QEmployeeNameProjection), you will see it is some kind of "shortcut" for creating constructor expression the way described in first section of this post.

Mapping projection

Querydsl provides another way of building projections, using factories based on MappingProjection.
package com.blogspot.vardlokkur.domain;
 
import static com.blogspot.vardlokkur.domain.QEmployee.employee;
 
import com.mysema.query.Tuple;
import com.mysema.query.types.MappingProjection;
 
public class EmployeeNameProjectionFactory extends MappingProjection<EmployeeNameProjection> {
 
    public EmployeeNameProjectionFactory() {
        super(EmployeeNameProjection.class, employee.employeeId, employee.name);
    }
 
    @Override
    protected EmployeeNameProjection map(Tuple row) {
        return new EmployeeNameProjection(row.get(employee.employeeId), row.get(employee.name));
    }
 
}
The above class is a simple factory creating EmployeeNameProjection instances using employee ID and name. Note that the factory constructor defines which employee properties will be used for building the projection, and mapmethod defines how the instances will be created.

Below you may find an example of using the factory:
...
import static com.blogspot.vardlokkur.domain.QEmployee.employee;
 
import javax.persistence.EntityManager;
import javax.persistence.PersistenceContext;
 
import org.springframework.beans.factory.annotation.Autowired;
 
import com.blogspot.vardlokkur.domain.EmployeeNameProjection;
import com.blogspot.vardlokkur.domain.EmployeeNameProjectionFactory
 
import com.mysema.query.jpa.JPQLTemplates;
import com.mysema.query.jpa.impl.JPAQuery;
...
 
public class MappingProjectionExample {
    
    ...
    @PersistenceContext
    private EntityManager entityManager;
 
    @Autowired
    private JPQLTemplates jpqlTemplates;
 
    public void someMethod() {
        ...
        final List<EmployeeNameProjection> projections = new JPAQuery(entityManager, jpqlTemplates)
                            .from(employee)
                            .orderBy(employee.name.asc())
                            .list(new EmployeeNameProjectionFactory());
        ....
    }
    ...
}
As you see, the one and only difference here, comparing to constructor expression examples, is the list method call.

Above example leads again to the very simple SQL query:

select EMPLOYEE_ID, EMPLOYEE_NAME from EMPLOYEE order by EMPLOYEE_NAME asc

Building projections this way is much more powerful, and doesn't require existence of n-arg projection constructor.

QBean based projection (JavaBeans strike again)


There is at least one more possibility of creating projection with Querydsl - QBean based - in this case we build the result list using:

... .list(Projections.bean(EmployeeNameProjection.class, employee.employeeId, employee.name))

This way requires EmployeeNameProjection class to follow JavaBean conventions, which is not always desired in application. Use it if you want, but you have been warned ;)

Few links for the dessert

  1. Using Query Types
  2. Querying
  3. Ordering
  4. Constructor projections


Database

Published at DZone with permission of Michal Jastak, DZone MVB. See the original article here.

Opinions expressed by DZone contributors are their own.

Related

  • Decoding Database Speed: Essential Server Resources and Their Impact
  • Understanding Time Series Databases
  • Exploring Data Redaction Enhancements in Oracle Database 23ai
  • The Rise of the Intelligent AI Agent: Revolutionizing Database Management With Agentic DBA

Partner Resources

×

Comments

The likes didn't load as expected. Please refresh the page and try again.

ABOUT US

  • About DZone
  • Support and feedback
  • Community research
  • Sitemap

ADVERTISE

  • Advertise with DZone

CONTRIBUTE ON DZONE

  • Article Submission Guidelines
  • Become a Contributor
  • Core Program
  • Visit the Writers' Zone

LEGAL

  • Terms of Service
  • Privacy Policy

CONTACT US

  • 3343 Perimeter Hill Drive
  • Suite 100
  • Nashville, TN 37211
  • [email protected]

Let's be friends: