Groovy Kotlin Example

Introduction

The point of this example is to show how various JVM languages can coexist under a single gradle project. In this case we are using Groovy as a baseline language and Kotlin as a newer language, but the same approach could be used to transition between any two JVM based languages.

Ideally, projects wouldn't mingle languages as there is some complexity, but in the real world, many projects use unsafe, risky languages such as Gradle. Organizations would like to move to newer, nicer, safer languages such as Kotlin, but it isn't realistic to rewrite projects in one fell swoop.

The safety and correctness of the legacy features must be preserved. At the same time, progress must be made in transitioning to the newer language.

How to get value from this example?

Rather than focusing on the technology or design here (which are pedestrian), instead focus on the way that changes are introduced. I.e. look at the project change log and see how changes are introduced. I hope that you will derive value from that. To that end, you may want to skip to the Change Log

Project Implementation

This is a Spring Boot project implementing CRUD REST web services which users a SQLite backend. SQLite was chosen because it is real persistence but doesn't require any configuration or set-up beyond what is in this project. The database DDL is generated dynamically by Hibernate when the application boots.

Other than the use of SQLite, this is a fairly standard architecture with the following layers:

1. Controller (web service endpoint)
2. Service (business logic)
3. Repository
4. Database

Change Log

COMMIT #1 Initial Groovy code. These changes are intended to represent a legacy application written in Groovy.

COMMIT #2 Initial Kotlin code along with Ktlint and Detekt configuration. Note that this is mostly independent functionality. There is only one dependency. The application class ExampleApplication which is written in Groovy depends on the Kotlin code.

COMMIT #3 Added Fruit to Pie. This necessitates refactoring Fruit to make it Kotlin so that we can reference it from the Kotlin Pie.

COMMIT #4 Add FruitServiceInterface as a Java interface so that we can reference a Groovy service from a Kotlin service.

COMMIT #5 Simplify example using Kotlin interface to reference from Groovy rather than Java interface.

Building and running

Build

./gradlew build

Run

./gradlew bootRun

Use

Run

curl --location --request POST 'localhost:50005/gkexample/fruit/apple' \
--header 'Content-Type: application/json' \
--data-raw '{
"variety": "black diamond",
"flavor": "sweet",
"color": "purple"
}'

Result

{
    "id": 1,
    "variety": "black diamond",
    "flavor": "sweet",
    "color": "purple"
}

Run

curl --location --request POST 'localhost:50005/gkexample/fruit/orange' \
--header 'Content-Type: application/json' \
--data-raw '{
"variety": "Cam Sanh",
"flavor": "sweet",
"color": "green"
}'

Result

{
    "id": 3,
    "variety": "Cam Sanh",
    "flavor": "sweet",
    "color": "green"
}

Run

curl --location --request GET 'localhost:50005/gkexample/fruit/all'

Result

[
    {
        "id": 1,
        "variety": "black diamond",
        "flavor": "sweet",
        "color": "purple"
    },
    {
        "id": 3,
        "variety": "Cam Sanh",
        "flavor": "sweet",
        "color": "green"
    }
]