Day 3 - Spring in Kotlin

Challenge - create a SpringBoot app

• Create a SpringBoot application from scratch
  • Use Spring Initializr
  • Add “Spring Web” / “Spring Data JDBC” / “H2 Database”

Observe the project

• Observe the pom.xml
  • What is different from what you are used to ?
• Analyze code structure
  • How does it differ from what you are used to ?
• Open the DemoApplication.kt file
  • What do you notice ?

Create a RestController

1) Return a hardcoded list of Message on GET

• A message is composed by an id (String nullable) and a text

2) Add the missing Layers

• Create a Service that will contain 2 functions : findMessages / addMessage

3) What else do we need to do ?

4) Add the Database / repository

• Create the Repository needed by the Service to retrieve the data

5) Plug everything together

• Use our repository in our Service
• Finalize our Service implementation by saving the Message
• Use our Service in our Controller
• Add the POST mapping to be able to create new messages

Configure the database

• Add an application.properties file

  spring.datasource.driver-class-name=org.h2.Driver
  spring.datasource.url=jdbc:h2:file:./data/testdb
  spring.datasource.username=sa
  spring.datasource.password=password
  spring.datasource.schema=classpath:sql/schema.sql
  spring.datasource.initialization-mode=always
  

Add the init script

• In the application.properties we set the schema to sql/schema.sql
  • Create this file that initializes the db schema

    CREATE TABLE IF NOT EXISTS messages
    (
    id   VARCHAR(60) DEFAULT RANDOM_UUID() PRIMARY KEY,
    text VARCHAR NOT NULL
    );
    

Use our favorite mapper (MapStruct)

• In real life we would not expose our Entities directly from our Controller
  • Let’s add a decoupling layer
  • Change the public contract :
    • Pass a AddMessage command in the POST and return a MessageDto in the GET
• Create our MessageMapper to map :
  • AddMessage -> Message
  • Message -> MessageDto
• Use it in our Service
• Refactor calls to the Mapper to use extension functions to hide mapper usage to the outside world

Final version available in day 4

---

Table of contents

• Step-by-step guide

---