What Is QueryDSL?

QueryDSL (Query Domain Specific Language) is a Java-based framework. It is a type-safe, fluent API for building SQL-like queries in a programmatic way.

Official Documentation for the latest version of QueryDSL 

Key Features and Concepts of QueryDSL

• Type Safety: 
• Code Generation
• JPA Integration
• Support for Collections
• DSL for Expressing Predicates
• Support for Joins
• Extensibility

Process of Query Optimization Using QueryDSL

To optimize query using QueryDSL in spring boot application, We have to follow below points.

1) Add dependency for QueryDSL in your application:

<dependency>
     <groupId>com.querydsl</groupId>
     <artifactId>querydsl-apt</artifactId>
     <version>5.0.0</version>
 </dependency>
 <dependency>
     <groupId>com.querydsl</groupId>
     <artifactId>querydsl-jpa</artifactId>
     <version>5.0.0</version>
 </dependency>

2) Create An entity Class Testing with Primary key and two other column:

@Data
@Builder
@AllArgsConstructor
@NoArgsConstructor
@Entity
public class Testing {
    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private Long id;
   
    private String firstName;
   
    private String lastName;
}

3) Create an Repository class for custom functions:

import org.springframework.beans.factory.annotation.Autowired;

import com.querydsl.jpa.impl.JPAQueryFactory;

public class TestingRepository {
    @Autowired
    private EntityManager entityManager;

    public List<Testing> findPeopleByLastName(String lastName) {
        JPAQueryFactory queryFactory = new JPAQueryFactory(entityManager);
        QTesting person = QTesting.testing;
        return queryFactory.selectFrom(person)
                .where(person.lastName.eq(lastName))
                .fetch();
    }
}

4) Create Database table

CREATE TABLE `testing` (
  `id` bigint NOT NULL AUTO_INCREMENT,
  `first_name` varchar(45) DEFAULT NULL,
  `last_name` varchar(45) DEFAULT NULL,
  PRIMARY KEY (`id`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_0900_ai_ci;

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