Posted on May 23, 2020

Springboot + JPA + Kotlin

Today you’ll learn how to develop REST API with Spring Boot 2 and Kotlin. Kotlin is 100% interoperable with Java. Which means you can call Java code from Kotlin and Kotlin code can be used from Java seamlessly. If you want to learn more about Kotlin, refer to Kotlin reference doc.

In this tutorial we'll use an in-memory database (H2) with Spring Data to persist and retrieve data via rest API.

Creating a New Project

Visit Spring Initializr and select Kotlin language. In this tutorial we'll use Maven, select Maven project. But you can also use Gradle.

Add the following dependencies:

Spring Data JPA
Spring Web
H2 Database
Spring Boot DevTools

Click Generate to downlaod .zip file and upack it into your working directory.

Understanding Application entry point

src/main/com/example/demo/DemoApplication.kt

package com.example.demo import org.springframework.boot.autoconfigure.SpringBootApplication import org.springframework.boot.runApplication @SpringBootApplication class DemoApplication fun main(args: Array<String>) { runApplication<DemoApplication>(*args) }

Main fun is the entry point for Springboot application. runApplication<DemoApplication>(*args) is Kotlin idiomatic alternative to SpringApplication.run(DemoApplication::class.java, *args) that you see mostly in Java.

Create Project structure

Create the following packages in src/main/com/example/demo directory:

config
controller
domain
repository
service

Create Entity Class

Create Product.kt in domain package.

package com.example.demo.domain import javax.persistence.Column import javax.persistence.Entity import javax.persistence.Id import javax.persistence.Table @Entity @Table(name = "product") class Product( @Id @GeneratedValue(strategy = GenerationType.AUTO var id: Long, @Column var name: String, @Column var price: Float) { override fun toString(): String { return "Product(name='$name', price=$price)" } }

Compared to Java, you can notice that we defined primary constructor as part of the class header. Kotlin will generate getter, setter, hashcode, and equals methods for us, which you can also override.

Create Repository Interface

Create ProductRepository.kt in domain package.

package com.example.demo.repository import com.example.demo.domain.Product import org.springframework.data.repository.CrudRepository import org.springframework.stereotype.Repository @Repository interface ProductRepository: CrudRepository<Product, Long> { fun findProductsByName(name: String): List<Product> }

Compared to Java, you can notice the use of : to implement CrudRepository

Service Interface and Implementation

Create ProductService.kt and ProductServiceImpl.kt in service package.

package com.example.demo.service import com.example.demo.domain.Product interface ProductService { fun createProduct(product: Product) fun findProductsByName(name: String): List<Product> }

package com.example.demo.service import com.example.demo.domain.Product import com.example.demo.repository.ProductRepository import org.springframework.beans.factory.annotation.Autowired import org.springframework.stereotype.Service @Service class ProductServiceImpl(@Autowired val productRepository: ProductRepository): ProductService { override fun createProduct(product: Product) { productRepository.save(product) } override fun findProductsByName(name: String): List<Product> { return productRepository.findProductsByName(name) } }

Notice how we used constructor injection as part of class header to inject ProductRepository.

Controller Class

Create ProductController.kt in controller package

package com.example.demo.controller import com.example.demo.domain.Product import com.example.demo.service.ProductService import org.springframework.beans.factory.annotation.Autowired import org.springframework.web.bind.annotation.* @RestController @RequestMapping("api/v1/products") class ProductController(@Autowired val productService: ProductService) { @PostMapping fun createProduct(@RequestBody product: Product) { productService.createProduct(product) } @GetMapping("{name}") fun getProductsByName(@PathVariable("name") name: String): List<Product> { return productService.findProductsByName(name) } }

Config Swagger UI

To configure Swagger UI add the following dependencies to pom.xml:

<dependency> <groupId>io.springfox</groupId> <artifactId>springfox-swagger2</artifactId> <version>2.9.2</version> </dependency> <dependency> <groupId>io.springfox</groupId> <artifactId>springfox-swagger-ui</artifactId> <version>2.9.2</version> </dependency>

Create SwaggerConfig.kt in config package.

package com.example.demo.config import org.springframework.context.annotation.Bean import org.springframework.context.annotation.Configuration import springfox.documentation.builders.PathSelectors import springfox.documentation.builders.RequestHandlerSelectors import springfox.documentation.spi.DocumentationType import springfox.documentation.spring.web.plugins.Docket import springfox.documentation.swagger2.annotations.EnableSwagger2 @EnableSwagger2 @Configuration class SwaggerConfig { @Bean fun api(): Docket = Docket(DocumentationType.SWAGGER_2) .select() .apis(RequestHandlerSelectors.any()) .paths(PathSelectors.any()) .build() }