In this application, I am making a simple RestFul API of Expense Tracker with Spring Boot and Kotlin with the basic CRUD functionality. Here I am using MongoDB as a database. You can clone this project as Open in IntelliJ IDEA start the application, will have the backend up and running at port 8080 in no time. Kotlin makes it so easy to spring up a backend. Spring applications created using Kotlin, when compared to those created using Java, are concise and easier to maintain. Kotlin is a first-class language for programming Spring applications.
Here medium article which goes into much detail and explanation on the same. https://debanshudatta.medium.com/making-simple-spring-boot-application-with-kotlin-and-mongodb-66889a86b1b1