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.

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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.
note
These annotations also require additional imports.
```
// Message.kt
package 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 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 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
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

// DemoApplication.kt {...}

// Message.kt {...}

// MessageRepository.kt {...}

// MessageService.kt {...}

// MessageController.kt {...}

Use Spring Data CrudRepository for database access﻿

Update your application﻿

note

Run the application﻿

What's next﻿

Use Spring Data CrudRepository for database access

Update your application

Run the application

What's next