In this article, we will discuss typesafe queries. We are aware that Spring Data can derive a query based on Bean properties, but this convention has certain shortcomings.

Shortcomings

• When we need to filter a query based on a lot of properties, the method name will look ugly and pretty big. Like findPersonByNameAndHobbyAndCountry().
• If we want to filter a query based on runtime strategy, say country and hobby, country and name, hobby, and country etc. We can’t do it through Query Derivation technique.

It will be good if we have a method that takes a Predicate/Strategy and we build the predicate runtime and pass it to the same method — so a single method acts as the runtime query generator.

One can relate the same techniques with the JPA Criteria API. Yes, it is just like Criteria API.

The good part is that Spring Data incorporates the querydsl project to support this technique.

Querydsl tries provide a fluent API by which clients can build a query based on Bean properties. The API is derived from the persistence store or Object model, but it is store- and model-agnostic at the same time, so it allows the client to create and use the query with ease.

This API supports a variety of stores like JPA, Hibernate, JDO, native JDBC, Lucene, Hibernate Search, and MongoDB.

Due to this vast support, the Spring Data project integrates well with querydsl.

How Querydsl Works

Querydsl generates metamodel objects from Entity classes. A client/developer uses this metamodel Object to do typesafe queries. Querydsl generates this metamodel via APT processor, which uses the Java 6 Annotation Processing Tool (APT). This tool hooks with the compiler and, based on the store-specific processor, such as JPA and JpaAnnotationProcessor, it finds the annotation for JPA @Entity and @Embeddable and generates the metamodel sources in the target destination. It retains the same package structure.

Steps

1. In POM.xml, we need to add the querydsl-apt and querydsl-jpa dependency.

=<dependency>
    <groupId>com.querydsl</groupId>
    <artifactId>querydsl-apt</artifactId>
    <version>4.1.3</version>
</dependency>
<dependency> 
    <groupId>com.querydsl</groupId>
    <artifactId>querydsl-jpa</artifactId> 
    <version>4.1.3</version> 
</dependency>

2. Now, in the plugin section, we need to add JpaAnnotationProcessor.

<plugin>
    <groupId>com.mysema.maven</groupId>
    <artifactId>apt-maven-plugin</artifactId>
    <version>1.1.3</version>
    <executions>
        <execution>
            <phase>generate-sources</phase>
            <goals>
                <goal>process</goal>
            </goals>
            <configuration>
                <outputDirectory>target/generated-sources</outputDirectory>
                <processor>com.querydsl.apt.jpa.JPAAnnotationProcessor</processor>
            </configuration>
        </execution>
    </executions>
    <dependencies>
        <dependency>
            <groupId>com.querydsl</groupId>
            <artifactId>querydsl-apt</artifactId>
            <version>4.1.3</version>
        </dependency>
    </dependencies>
</plugin>

3. We define the output directory as target/generated-sources where metamodel classes are generated. Bind this plugin to the generate-sources phase.

4. Now execute the command 

mvn -X generate-sources

 5. It will create metamodel classes in the target/generated-sources directory.

6. As the package structure is the same, copy the metamodel classes under the src directory maintaining same package structure.

7.  Extend the QueryDslPredicateExecutor interface in the Custom repository.

8.  Write a query based on the metamodel classes.

Example

1. POM.xml

<?xml version="1.0" encoding="UTF-8"?>
<project xmlns="http://maven.apache.org/POM/4.0.0" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd">
    <modelVersion>4.0.0</modelVersion>

    <groupId>com.example</groupId>
    <artifactId>cabBook</artifactId>
    <version>0.0.1-SNAPSHOT</version>
    <packaging>jar</packaging>

    <name>CabBookingSystem</name>
    <description>CabBookingSystem Using Spring Boot</description>

    <parent>
        <groupId>org.springframework.boot</groupId>
        <artifactId>spring-boot-starter-parent</artifactId>
        <version>1.4.1.RELEASE</version>
        <relativePath/> <!-- lookup parent from repository -->
    </parent>

    <properties>
        <project.build.sourceEncoding>UTF-8</project.build.sourceEncoding>
        <project.reporting.outputEncoding>UTF-8</project.reporting.outputEncoding>
        <java.version>1.7</java.version>
    </properties>

    <dependencies>
        <dependency>
            <groupId>org.springframework.boot</groupId>
            <artifactId>spring-boot-starter-actuator</artifactId>
        </dependency>

        <dependency>
            <groupId>org.springframework.boot</groupId>
            <artifactId>spring-boot-starter-data-jpa</artifactId>
        </dependency>

        <dependency>
            <groupId>org.springframework.boot</groupId>
            <artifactId>spring-boot-starter-web</artifactId>
        </dependency>

        <dependency>
            <groupId>mysql</groupId>
            <artifactId>mysql-connector-java</artifactId>
            <scope>runtime</scope>
        </dependency>

        <dependency>
            <groupId>org.springframework.boot</groupId>
            <artifactId>spring-boot-starter-test</artifactId>
            <scope>test</scope>
        </dependency>

        <dependency>
            <groupId>com.querydsl</groupId>
            <artifactId>querydsl-apt</artifactId>
            <version>4.1.3</version>
        </dependency>

        <dependency> 
            <groupId>com.querydsl</groupId>
            <artifactId>querydsl-jpa</artifactId> 
            <version>4.1.3</version> 
        </dependency>
    </dependencies>

    <build>
        <plugins>
            <plugin>
                <groupId>org.springframework.boot</groupId>
                <artifactId>spring-boot-maven-plugin</artifactId>
            </plugin>
            <plugin>
                <groupId>com.mysema.maven</groupId>
                <artifactId>apt-maven-plugin</artifactId>
                <version>1.1.3</version>
            <executions>
                <execution>
                    <phase>generate-sources</phase>
                    <goals>
                        <goal>process</goal>
                    </goals>
                    <configuration>
                        <outputDirectory>target/generated-sources</outputDirectory>
                        <processor>com.querydsl.apt.jpa.JPAAnnotationProcessor</processor>
                    </configuration>
                </execution>
            </executions>
                <dependencies>
                    <dependency>
                        <groupId>com.querydsl</groupId>
                        <artifactId>querydsl-apt</artifactId>
                        <version>4.1.3</version>
                    </dependency>
                </dependencies>
            </plugin>
        </plugins>
    </build>
</project>

2. Execute mvn -X generate-sources. The following metamodel Objects are created:

package com.example.person;

import static com.querydsl.core.types.PathMetadataFactory.*;

import com.querydsl.core.types.dsl.*;

import com.querydsl.core.types.PathMetadata;
import javax.annotation.Generated;
import com.querydsl.core.types.Path;
import com.querydsl.core.types.dsl.PathInits;


/**
 * QPerson is a Querydsl query type for Person
 */
@Generated("com.querydsl.codegen.EntitySerializer")
public class QPerson extends EntityPathBase<Person> {

    private static final long serialVersionUID = 955983357L;

    public static final QPerson person = new QPerson("person");

    public final StringPath country = createString("country");

    public final StringPath gender = createString("gender");

    public final ListPath<Hobby, QHobby> hobby = this.<Hobby, QHobby>createList("hobby", Hobby.class, QHobby.class, PathInits.DIRECT2);

    public final NumberPath<Long> id = createNumber("id", Long.class);

    public final StringPath name = createString("name");

    public QPerson(String variable) {
        super(Person.class, forVariable(variable));
    }

    public QPerson(Path<? extends Person> path) {
        super(path.getType(), path.getMetadata());
    }

    public QPerson(PathMetadata metadata) {
        super(Person.class, metadata);
    }

}


package com.example.person;

import static com.querydsl.core.types.PathMetadataFactory.*;

import com.querydsl.core.types.dsl.*;

import com.querydsl.core.types.PathMetadata;
import javax.annotation.Generated;
import com.querydsl.core.types.Path;
import com.querydsl.core.types.dsl.PathInits;


/**
 * QHobby is a Querydsl query type for Hobby
 */
@Generated("com.querydsl.codegen.EntitySerializer")
public class QHobby extends EntityPathBase<Hobby> {

    private static final long serialVersionUID = -253362646L;

    private static final PathInits INITS = PathInits.DIRECT2;

    public static final QHobby hobby = new QHobby("hobby");

    public final NumberPath<Long> id = createNumber("id", Long.class);

    public final StringPath name = createString("name");

    public final QPerson person;

    public QHobby(String variable) {
        this(Hobby.class, forVariable(variable), INITS);
    }

    public QHobby(Path<? extends Hobby> path) {
        this(path.getType(), path.getMetadata(), PathInits.getFor(path.getMetadata(), INITS));
    }

    public QHobby(PathMetadata metadata) {
        this(metadata, PathInits.getFor(metadata, INITS));
    }

    public QHobby(PathMetadata metadata, PathInits inits) {
        this(Hobby.class, metadata, inits);
    }

    public QHobby(Class<? extends Hobby> type, PathMetadata metadata, PathInits inits) {
        super(type, metadata, inits);
        this.person = inits.isInitialized("person") ? new QPerson(forProperty("person")) : null;
    }

}

3. Entity classes are as follows

package com.example.person;

import java.util.ArrayList;
import java.util.List;

import javax.persistence.CascadeType;
import javax.persistence.Entity;
import javax.persistence.FetchType;
import javax.persistence.GeneratedValue;
import javax.persistence.GenerationType;
import javax.persistence.Id;
import javax.persistence.OneToMany;

@Entity
public class Person {

    @Id
    @GeneratedValue(strategy=GenerationType.AUTO)
    private Long id;
    private String name;
    private String country;
    private String gender;

    @OneToMany(mappedBy="person",targetEntity=Hobby.class,
        fetch=FetchType.EAGER,cascade=CascadeType.ALL)
        List<Hobby> hobby;

    public String getName() {
        return name;
    }
    public void setName(String name) {
        this.name = name;
    }
    public String getCountry() {
        return country;
    }
    public void setCountry(String country) {
        this.country = country;
    }
    public String getGender() {
        return gender;
    }
    public void setGender(String gender) {
        this.gender = gender;
    }

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

    public List<Hobby> getHobby() {
        return hobby;
    }
    public void setHobby(List<Hobby> hobby) {
        this.hobby = hobby;
    }

    public void addHobby(Hobby ihobby)
    {
        if(hobby == null)
        {
            hobby = new ArrayList<Hobby>();
        }
        hobby.add(ihobby);
    }
    @Override
    public String toString() {
        return "Person [id=" + id + ", name=" + name + ", country=" + country + ", gender=" + gender + "]";
        }
    }

    package com.example.person;

    import javax.persistence.Entity;
    import javax.persistence.GeneratedValue;
    import javax.persistence.GenerationType;
    import javax.persistence.Id;
    import javax.persistence.JoinColumn;
    import javax.persistence.ManyToOne;

    @Entity
    public class Hobby {
      @Id
      @GeneratedValue(strategy=GenerationType.AUTO)
      private Long id;

     @ManyToOne
     @JoinColumn(name="person_id")
    private Person person;
    private String name;
    public Long getId() {
        return id;
    }
    public void setId(Long id) {
        this.id = id;
    }
    public Person getPerson() {
        return person;
    }
    public void setPerson(Person person) {
        this.person = person;
    }
    public String getName() {
        return name;
    }
    public void setName(String name) {
        this.name = name;
    }
}

4. Extending QueryDslPredicateExecutor<T> in Custom Repository. By doing this, we can pass the Predicate into the CRUD methods. Now, the findAll method looks like:

Iterable<T> findAll(Predicate predicate);

package com.example.repo;

import java.util.List;

import org.springframework.data.jpa.repository.Query;
import org.springframework.data.querydsl.QueryDslPredicateExecutor;
import org.springframework.data.repository.CrudRepository;
import org.springframework.stereotype.Repository;

import com.example.person.Person;

@Repository
public interface PersonRepositary extends CrudRepository<Person, Long>,QueryDslPredicateExecutor<Person> {

    @Query("select p from Person p where p.country like ?1")
    List<Person> findByCountryContains(String country);

    List<Person> findPersonByHobbyName(String name);
}

package com.example.repo;
import org.springframework.data.querydsl.QueryDslPredicateExecutor;
import org.springframework.data.repository.CrudRepository;
import com.example.person.Hobby;

public interface HobbyRepository extends CrudRepository<Hobby,Long>,QueryDslPredicateExecutor<Hobby>  {
}

5. Create a method called findPersonByNameQueryDsl, build a Predicate based on the person name property:

BooleanExpression nameExpr = QPerson.person.name.contains("Shamik");

Pass this Predicate in the findAll(..) method:

package com.example.person;

import java.util.List;

import org.slf4j.Logger;
import org.slf4j.LoggerFactory;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.boot.CommandLineRunner;
import org.springframework.boot.SpringApplication;
import org.springframework.boot.autoconfigure.SpringBootApplication;
import org.springframework.context.annotation.Bean;
import org.springframework.data.jpa.repository.config.EnableJpaRepositories;

import com.example.repo.HobbyRepository;
import com.example.repo.PersonRepositary;
import com.querydsl.core.types.dsl.BooleanExpression;


@SpringBootApplication
@EnableJpaRepositories("com.example.repo")
public class PersonApplication {

    @Autowired
    HobbyRepository hRepo;
    private static final Logger log = LoggerFactory.getLogger(PersonApplication.class);

    @Bean
    public CommandLineRunner demo(PersonRepositary repository) {

        //insertPerson(repository);
        //findPersonByHobbyname(repository,"Photography");
        findPersonByNameQueryDsl(repository,"Photography");
        /*List<Person> pesonList = repository.findByCountryContains("India");
        for(Person p : pesonList)
        log.info("Person " + p);*/
        return null;
    }

    private void findPersonByNameQueryDsl(PersonRepositary repository,String hobby)
    {
        BooleanExpression nameExpr = QPerson.person.name.contains("Shamik");

        Iterable<Person> pList = repository.findAll(nameExpr);
        for(Person p : pList)
        log.info("Person " + p);
    }

    public static void main(String[] args) {
        SpringApplication.run(PersonApplication.class, args);
    }
}

Output

Hibernate: 
    select
        person0_.id as id1_1_,
        person0_.country as country2_1_,
        person0_.gender as gender3_1_,
        person0_.name as name4_1_ 
    from
        person person0_ 
    where
        person0_.name like ? escape '!'
Hibernate: 
    select
        hobby0_.person_id as person_i3_0_0_,
        hobby0_.id as id1_0_0_,
        hobby0_.id as id1_0_1_,
        hobby0_.name as name2_0_1_,
        hobby0_.person_id as person_i3_0_1_ 
    from
        hobby hobby0_ 
    where
        hobby0_.person_id=?
2016-10-04 18:34:18.520  INFO 7344 --- [main] com.example.person.PersonApplication     : Person Person [id=14, name=Shamik mitra, country=India, gender=male]