QueryDSL

What is QueryDSL?

Querydsl is an extensive Java framework, which allows for the generation of type-safe queries in a syntax similar to SQL

Why QueryDSL?

1. Code Completion
2. Type Safety
3. No Strings Involved
4. Adapts to Refactoring

Example QueryDSLs

query.from(employee).where(employee.name.eq("auhuman"));

query.from(employee).where(employee.age.gt(35)).and(employee.gender.eq(“Male”);

Maven Dependencies

<dependency>

    <groupId>com.mysema.querydsl</groupId>

    <artifactId>querydsl-apt</artifactId>

    <version>3.7.4</version>

</dependency>

<dependency>

    <groupId>com.mysema.querydsl</groupId>

    <artifactId>querydsl-jpa</artifactId>

    <version>3.7.4</version>

</dependency>

Maven Build Plugin

<plugin>

    <groupId>com.mysema.maven</groupId>

    <artifactId>apt-maven-plugin</artifactId>

    <version>1.1.3</version>

    <executions>

        <execution>

            <goals>

                <goal>process</goal>

            </goals>

            <configuration>

                <outputDirectory>target/generated-sources/java</outputDirectory>

                <processor>com.mysema.query.apt.jpa.JPAAnnotationProcessor</processor>

            </configuration>

        </execution>

    </executions>

</plugin>

Entity

@Entity
public class Employee {

    @Id
    @GeneratedValue
    private Long id;

    private String firstName, lastName, description;

    private Long count;

    public Long getId() {
        return id;
    }

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

    public String getFirstName() {
        return firstName;
    }

    public void setFirstName(String firstName) {
        this.firstName = firstName;
    }

    public String getLastName() {
        return lastName;
    }

    public void setLastName(String lastName) {
        this.lastName = lastName;
    }

    public String getDescription() {
        return description;
    }

    public void setDescription(String description) {
        this.description = description;
    }

    public Long getCount() {
        return count;
    }

    public void setCount(Long count) {
        this.count = count;
    }
}

Sample Code

@RequestMapping(value = "/", method = RequestMethod.GET)

@ResponseBodyList<Company> queryDSL(@RequestParam(value = "name", required = false) String name) {

    JPAQuery query = new JPAQuery(entityManager);    

    QCompany company = QCompany.company;    

   if (name == null) {

        return query.from(company).list(company);    

   } else {

        return query.from(company).where(company.name.eq(name)).list(company);    

   }

}

@RequestMapping(value = "/queryDSLGroupByCount", method = RequestMethod.GET)
@ResponseBody
List<Employee> queryDSLGroupByCount() {
    JPAQuery jpaQuery = new JPAQuery(entityManager);
    QEmployee employee = QEmployee.employee;
    return jpaQuery.from(employee).groupBy(employee.lastName).list(Projections.constructor(
            Employee.class,
            employee.lastName,
            employee.count()
    ));
}