Use Spring Data CrudRepository for database access

In this part, you will migrate the service layer to use the Spring Data CrudRepository instead of JdbcTemplate for database access. CrudRepository is a Spring Data interface for generic CRUD operations on a repository of a specific type. It provides several methods out of the box for interacting with a database.

Update your application

First, you need to adjust the Message class for work with the CrudRepository API:

Add the @Table annotation to the Message class to declare mapping to a database table.
Add the @Id annotation before the id field.
These annotations also require additional imports.
// Message.kt package com.example.demo import org.springframework.data.annotation.Id import org.springframework.data.relational.core.mapping.Table @Table("MESSAGES") data class Message(@Id val id: String?, val text: String)
In addition, to make the use of the Message class more idiomatic, you can set the default value for id property to null and flip the order of the data class properties:
@Table("MESSAGES") data class Message(val text: String, @Id val id: String? = null)
Now if you need to create a new instance of the Message class, you can only specify the text property as a parameter:
val message = Message("Hello") // id is null
Declare an interface for the CrudRepository that will work with the Message data class. Create the MessageRepository.kt file and add the following code to it:
// MessageRepository.kt package com.example.demo import org.springframework.data.repository.CrudRepository interface MessageRepository : CrudRepository<Message, String>
Update the MessageService class. It will now use the MessageRepository instead of executing SQL queries:
// MessageService.kt package com.example.demo import org.springframework.data.repository.findByIdOrNull import org.springframework.stereotype.Service @Service class MessageService(private val db: MessageRepository) { fun findMessages(): List<Message> = db.findAll().toList() fun findMessageById(id: String): Message? = db.findByIdOrNull(id) fun save(message: Message): Message = db.save(message) }
Extension functions
The findByIdOrNull() function is an extension function for CrudRepository interface in Spring Data JDBC.
CrudRepository save() function
This function works with an assumption that the new object doesn't have an id in the database. Hence, the id should be null for insertion.
If the id isn't null, CrudRepository assumes that the object already exists in the database and this is an update operation as opposed to an insert operation. After the insert operation, the id will be generated by the data store and assigned back to the Message instance. This is why the id property should be declared using the var keyword.
Update the messages table definition to generate the ids for the inserted objects. Since id is a string, you can use the RANDOM_UUID() function to generate the id value by default:
-- schema.sql CREATE TABLE IF NOT EXISTS messages ( id VARCHAR(60) DEFAULT RANDOM_UUID() PRIMARY KEY, text VARCHAR NOT NULL );
Update the name of the database in the application.properties file located in the src/main/resources folder:
spring.application.name=demo spring.datasource.driver-class-name=org.h2.Driver spring.datasource.url=jdbc:h2:file:./data/testdb2 spring.datasource.username=name spring.datasource.password=password spring.sql.init.schema-locations=classpath:schema.sql spring.sql.init.mode=always

Here is the complete code of the application:

// 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) }

// Message.kt package com.example.demo import org.springframework.data.annotation.Id import org.springframework.data.relational.core.mapping.Table @Table("MESSAGES") data class Message(val text: String, @Id val id: String? = null)

// MessageRepository.kt package com.example.demo import org.springframework.data.repository.CrudRepository interface MessageRepository : CrudRepository<Message, String>

// MessageService.kt package com.example.demo import org.springframework.data.repository.findByIdOrNull import org.springframework.stereotype.Service @Service class MessageService(private val db: MessageRepository) { fun findMessages(): List<Message> = db.findAll().toList() fun findMessageById(id: String): Message? = db.findByIdOrNull(id) fun save(message: Message): Message = db.save(message) }

// MessageController.kt package com.example.demo import org.springframework.http.ResponseEntity import org.springframework.web.bind.annotation.GetMapping import org.springframework.web.bind.annotation.PathVariable import org.springframework.web.bind.annotation.PostMapping import org.springframework.web.bind.annotation.RequestBody import org.springframework.web.bind.annotation.RequestMapping import org.springframework.web.bind.annotation.RestController import java.net.URI @RestController @RequestMapping("/") class MessageController(private val service: MessageService) { @GetMapping fun listMessages() = ResponseEntity.ok(service.findMessages()) @PostMapping fun post(@RequestBody message: Message): ResponseEntity<Message> { val savedMessage = service.save(message) return ResponseEntity.created(URI("/${savedMessage.id}")).body(savedMessage) } @GetMapping("/{id}") fun getMessage(@PathVariable id: String): ResponseEntity<Message> = service.findMessageById(id).toResponseEntity() private fun Message?.toResponseEntity(): ResponseEntity<Message> = // If the message is null (not found), set response code to 404 this?.let { ResponseEntity.ok(it) } ?: ResponseEntity.notFound().build() }

Run the application

Congratulations! The application is ready to run again. After replacing JdbcTemplate with CrudRepository, the functionality remains the same, so the application works just as before.

You can now run the POST and GET HTTP requests from the requests.http file and get the same results.

What's next

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31 October 2025