The Java Persistence API (JPA) is a Java specification for storing, accessing, and managing data in a relational database. Spring Data JPA is a Spring framework implementation of JPA that makes it easier to work with JPA in a Spring application.
In this article, we'll cover some advanced topics in Spring Data JPA, including:
A projection is a way of specifying which fields of an entity you want to retrieve from the database. Spring Data JPA supports two types of projections:
With interface-based projections, you create an interface that defines the fields you want to retrieve, and Spring Data JPA generates an implementation of that interface at runtime. This is the recommended way of using projections, as it allows you to take advantage of the type safety and code completion features of your IDE.
To use interface-based projections, you first need to create an interface that defines the fields you want to retrieve:
public interface UserNameOnly {
String getUsername();
}
Then you can use that interface in your query:
List<UserNameOnly> users = userRepository.findAll(projection(UserNameOnly.class));
With dynamic projections, you can specify the fields you want to retrieve as a String. This is less type-safe than interface-based projections, but can be useful if you want to retrieve a field that isn't part of your entity model.
To use dynamic projections, you first need to create a class that defines the fields you want to retrieve:
public class UserNameOnly {
private String username;
public String getUsername() {
return username;
}
}
Then you can use that class in your query:
List<UserNameOnly> users = userRepository.findAll(projection(UserNameOnly.class, "username"));
Querydsl is a library that allows you to create type-safe queries in a Java-based domain-specific language. It can be used with JPA, JDO, and other data access frameworks.
To use Querydsl with Spring Data JPA, you need to add the Querydsl JPA dependency to your project:
<dependency>
<groupId>com.querydsl</groupId>
<artifactId>querydsl-jpa</artifactId>
<version>${querydsl.version}</version>
</dependency>
You also need to configure Querydsl to generate code for your entities:
<build>
<plugins>
...
<plugin>
<groupId>com.mysema.maven</groupId>
<artifactId>apt-maven-plugin</artifactId>
<version>1.1.3</version>
<configuration>
<outputDirectory>target/generated-sources/java</outputDirectory>
<processor>com.querydsl.apt.jpa.JPAAnnotationProcessor</processor>
</configuration>
<executions>
<execution>
<goals>
<goal>process</goal>
</goals>
</execution>
</executions>
</plugin>
...
</plugins>
</build>
Once Querydsl is configured, you can use it to create type-safe queries:
QUser user = QUser.user;
List<User> users = queryFactory
.selectFrom(user)
.where(user.username.eq("johndoe"))
.fetch();
The Criteria API is a Java API for building type-safe queries. It is part of the JPA specification and can be used with any JPA-compliant data access framework.
To use the Criteria API with Spring Data JPA, you need to create a CriteriaQuery:
CriteriaBuilder builder = entityManager.getCriteriaBuilder();
CriteriaQuery<User> criteria = builder.createQuery(User.class);
You can then use the CriteriaQuery to build your query:
Root<User> user = criteria.from(User.class);
criteria.select(user);
criteria.where(builder.equal(user.get("username"), "johndoe"));
Finally, you can execute the query:
List<User> users = entityManager.createQuery(criteria).getResultList();
In this article, we covered some advanced topics in Spring Data JPA, including projections, Querydsl, and the Criteria API. We also saw how to use each of these features in a Spring application.