Kotlin CRUD Rest Api using Spring Boot, Hibernate, Postgres, Docker and Docker Compose

Let's create a CRUD Rest API in Kotlin, using:

• Spring boot
• Gradle
• Hibernate
• Postgres
• Docker
• Docker Compose

If you prefer a video version:

All the code is available in the GitHub repository (link in the video description): https://youtube.com/live/BbT1PCAOS2s

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🏁 Intro

Here is a schema of the architecture of the application we are going to create:

We will create 5 endpoints for basic CRUD operations:

• Create
• Read all
• Read one
• Update
• Delete

Here are the steps we are going through:

1. Create a Spring Boot project using Spring Initializr
2. Configure the database connection
3. Create User.kt, UserRepository.kt and UserService.kt
4. Dockerize the application
5. Create docker-compose.yml to run the database and the application
6. Test the application with Postman and Tableplus

We will go with a step-by-step guide, so you can follow along.

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Requirements:

• Kotlin installed
• Docker installed and running
• (Optional): Postman and Tableplus to follow along, but any testing tool will work

Optional: VS Code with the following extensions:

• Java Extension Pack
• Spring Boot Extension Pack

---

🚀 Create a new Kotlin project

There are many ways to create a new Kotlin project, but I will use the Spring Initializr in VS Code.

To do this, you need to have the Java Extension Pack and the Spring Boot Extension Pack installed.

Open VS Code and click on the "Create Java Project" button:

This will open a prompt at the top of the screen. Click on the following in order:

• Spring boot
• Gradle
• 3.0.5 (it might change in the future)
• Kotlin
• com.example (just click enter)
• demo (just click enter)
• Jar
• Java 17

This will open another prompt. Click on the following in order:

• Spring Web (dependency to create a Rest API)
• Spring Data JPA (dependency to use Hibernate)
• PostgreSQL Driver (dependency to connect to Postgres)

Then you should select the folder where you want to create the project.

select a folder and clock on "Generate into this folder".

Now click the button at the bottom right of the screen to open the project in a new window.

We are done with the creation of the project.

Now we can start coding the application.

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👩‍💻 Code the application

There are two steps to code the application:

• Configure the database connection
• Create the User entity, the UserRepository and the UserService

🔗 Configure the database connection

Open the application.properties file in the src/main/resources folder (it should be empty).

Add the following content:

spring.datasource.url=${DB_URL}
spring.datasource.username=${PG_USER}
spring.datasource.password=${PG_PASSWORD}

spring.jpa.hibernate.ddl-auto=update
spring.jpa.properties.hibernate.dialect=org.hibernate.dialect.PostgreSQLDialect

Explanation:

• spring.datasource.url: the url of the database.

• spring.datasource.username: the username of the database.

• spring.datasource.password: the password of the database.

• spring.jpa.hibernate.ddl-auto: the way we want to update the database. We will use update to create the tables if they don't exist, and update them if they do.

• spring.jpa.properties.hibernate.dialect: the dialect of the database. We will use PostgreSQL.

We will use the environment variables later (and it will be a bit tricky).

📁 Create the resource structure

Create a new folder called users in the demo folder (or whatever you named your project).

Create three files in this folder:

• User.kt
• UserRepository.kt
• UserController.kt

Your folder shold look like this:

Now let's populate the files.

User.kt

The User.kt file will contain the entity of the user.

Open the file and add the following content (change the package name to match your project):

package com.example.demo.users

import jakarta.persistence.*

@Entity
@Table(name = "users")
data class User(
    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    val id: Long,
    val name: String,
    val email: String
)

Explanation:

• @Entity: decorator to tell Hibernate that this class is an entity.

• @Table: decorator to tell Hibernate the name of the table in the database ("users in this case").

• @Id: decorator to tell Hibernate that this field is the primary key.

• @GeneratedValue: decorator to auto-increment the id whenever we create a new user.

An user will have three fields: id, name and email.

UserRepository.kt

The UserRepository.kt file will contain the interface to interact with the database.

Open the file UserRepository.kt and add the following content (change the package name if you used a different one):

package com.example.demo.users

import org.springframework.data.repository.CrudRepository

interface UserRepository : CrudRepository<User, Long>

Explanation:

• interface UserRepository: the interface that will contain the methods to interact with the database. It will be of type CrudRepository. This is a generic interface that contains the basic methods to interact with the database. It will have a type User and an Int (the type of the primary key).

UserController.kt

The UserController.kt file will contain the Rest API.

Open the file UserController.kt and add the following content (change the package name if you used a different one):

package com.example.demo.users


import org.springframework.beans.factory.annotation.Autowired
import org.springframework.http.HttpStatus
import org.springframework.http.ResponseEntity
import org.springframework.web.bind.annotation.*

@RestController
@RequestMapping("/api/users")
class UserController(@Autowired private val userRepository: UserRepository) {

    @GetMapping("")
    fun getAllUsers(): List<User> =
        userRepository.findAll().toList()

    @PostMapping("")
    fun createUser(@RequestBody user: User): ResponseEntity<User> {
        val createdUser = userRepository.save(user)
        return ResponseEntity(createdUser, HttpStatus.CREATED)
    }

    @GetMapping("/{id}")
    fun getUserById(@PathVariable("id") userId: Int): ResponseEntity<User> {
        val user = userRepository.findById(userId).orElse(null)
        return if (user != null) ResponseEntity(user, HttpStatus.OK)
               else ResponseEntity(HttpStatus.NOT_FOUND)
    }

    @PutMapping("/{id}")
    fun updateUserById(@PathVariable("id") userId: Int, @RequestBody user: User): ResponseEntity<User> {

        val existingUser = userRepository.findById(userId).orElse(null)

        if (existingUser == null) {
            return ResponseEntity(HttpStatus.NOT_FOUND)
        }

        val updatedUser = existingUser.copy(name = user.name, email = user.email)
        userRepository.save(updatedUser)
        return ResponseEntity(updatedUser, HttpStatus.OK)
    }

    @DeleteMapping("/{id}")
    fun deleteUserById(@PathVariable("id") userId: Int): ResponseEntity<User> {
        if (!userRepository.existsById(userId)) {
            return ResponseEntity(HttpStatus.NOT_FOUND)
        }
        userRepository.deleteById(userId)
        return ResponseEntity(HttpStatus.NO_CONTENT)
    }
}

Explanation:

• @RestController: decorator for Spring.

• @RequestMapping: to tell Spring the base url of the Rest API. In this case, it will be /api/users.

• @Autowired: to tell Spring to inject the UserRepository.

Then we have the five methods to interact with the database:

• getAllUsers: to get all the users.
• createUser: to create a new user.
• getUserById: to get a user by id.
• updateUserById: to update a user by id.
• deleteUserById: to delete a user by id.

Our Rest API is ready to get Dockerized.

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🐳 Dockerization

Now the fun part: Dockerization.

In this project, I decided to build the Kotlin project directly inside the Docker image.

Another option would be to build the project locally and then copy the jar file to the Docker image.

🐋 Dockerfile

Create a new file called Dockerfile in the root of the project.

Add the following content (explanation is in the comments):

# Start with a base image containing Java runtime
FROM amazoncorretto:17-alpine-jdk

# Create a directory
WORKDIR /app

# Copy all the files from the current directory to the image
COPY . .

# build the project avoiding tests
RUN ./gradlew clean build -x test

# Expose port 8080
EXPOSE 8080

# Run the jar file
CMD ["java", "-jar", "./build/libs/demo-0.0.1-SNAPSHOT.jar"]

⚠️ The unusual part here are the ARG lines. They are used to pass arguments to the Docker image. They are defined in the docker-compose.yml file.

🐙 docker-compose.yml

Let's create the docker-compose.yml file at the root of the project.

Add the following content (explanation is in the comments):

version: '3.9'

services: 
  kotlinapp: 
    container_name: kotlinapp
    build: # this is the build context: .  
      context: .
      dockerfile: Dockerfile
      args: # these are the arguments that are passed to the dockerfile
        DB_URL: ${DB_URL} 
        PG_USER: ${PG_USER}
        PG_PASSWORD: ${PG_PASSWORD}
    ports: # port exposed to the host machine
      - "8080:8080"
    environment: # these are the environment variables that are passed to the dockerfile
      DB_URL: jdbc:postgresql://db:5432/postgres
      PG_USER: postgres
      PG_PASSWORD: postgres
    depends_on: # this is the dependency on the db service
      - db
  db:
    container_name: db
    image: postgres:12
    environment: # environment variables for the Postgres container
      POSTGRES_USER: postgres
      POSTGRES_PASSWORD: postgres
      POSTGRES_DB: postgres
    ports: # port exposed to the host machine
      - "5432:5432"
    volumes: # volume used to persist data
      - pgdata:/var/lib/postgresql/data

volumes: # volume creation
  pgdata: {}

---

Build and run the project

Now we can build and run the project.

💽 Run the Postgres database

First, we need to run the Postgres database.

docker compose up -d db

To check if it's running, you can use the following command:

docker compose logs

and the

docker ps -a

If the output is like the following one, you are good to go:

You should see something like that, you are good to go.

As additional test, you can connect to the database using TablePlus (or any other database client).

You can create a new connection using the following parameters:

• Host: localhost
• Port: 5432
• Database: postgres
• User: postgres
• Password: postgres

Then click on the Test Connection button. The database is connected but emptt for now.

🏗️ Build the project

Let's build the project inside the Docker image.

docker compose build

And the output should be something like that:

🏃‍♂️ Run the project

Now we can run the project.

docker compose up kotlinapp

And this should be the output:

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🧪 Test the project

Now we can test the project. We will use Postman, but you can use any other tool.

📝 Create a user

To create a new user, make a POST request to localhost:8080/api/users.

The body of the request should be like that:

{
    "name": "aaa",
    "email": "aaa@mail"
}

The output should be something like that:

Let's create two more users, make a POST request to localhost:8080/api/users.

{
    "name": "bbb",
    "email": "bbb@mail"
}

{
    "name": "ccc",
    "email": "ccc@mail"
}

📝 Get all users

To get all users, make a GET request to localhost:8000/api/users.

The output should be something like that:

📝 Get a user

To get a user, make a GET request to localhost:8000/api/users/{id}.

For example GET request to localhost:8000/api/users/1.

The output should be something like that:

📝 Update a user

To update a user, make a PUT request to localhost:8000/api/users/{id}.

For example PUT request to localhost:8000/api/users/2.

The body of the request should be like that:

{
    "name": "Francesco",
    "email": "francesco@mail"
}

The output should be something like that:

📝 Delete a user

To delete a user, make a DELETE request to localhost:8000/api/users/{id}.

For example DELETE request to localhost:8000/api/users/1.

On Postman you should see something like that:

Final test

As a final test, we can check the database using TablePlus.

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🏁Conclusion

We made it! We have built a CRUD rest API in Kotlin, using:

• Spring boot
• Gradle
• Hibernate
• Postgres
• Docker
• Docker Compose

If you prefer a video version:

All the code is available in the GitHub repository (link in the video description): https://youtube.com/live/BbT1PCAOS2s

That's all.

If you have any question, drop a comment below.

Francesco