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About Michal Jastak

Michał is a Chief Technology Officer in Java Division of AIS.PL, company developing mostly Web Applications of different kind, usually e-Government related.

JPA – Querydsl Projections

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 Querydsl version 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 @QueryProjection annotation 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 map method 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

 

Reference: JPA – Querydsl Projections from our JCG partner Michal Jastak at the Warlock’s Thoughts blog.

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One comment

  1. Hi,

    I’m trying to solve the nasty LazyInitializationException using Projections and Querydsl as you mention here, and it works perfect! I can load without problems a datatable with search results, but how do you solve the lazy problem when you want to edit an instance? I mean, when I click on a row, I execute a findById and fill a form with the data but, obviously, the session is gone when the view tries to render a select element. I’m a bit lost… what is the correct way to solve this?

    Thanks, really like your articles.

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