Creating a Starter Project

Initializing the application

If you want to set up the application environment on your system then follow along with the upcoming instructions.

The easiest way to get started is using the Spring Initializr website. We’ll create a Spring Boot RESTful web service that stores data in the in-memory H2 database.

Once you visit the Spring Initializr website, choose either Maven Project or Gradle Project. For language, select Kotlin, of course. Then choose the desired version of Spring Boot—for the examples in this chapter, we use version 2.1.2. For the Group text box, type a desired top-level package name—for example, com.agiledeveloper. For the Artifact, type todo. In the text box next to Search for Dependencies, type “Web” and select it from the drop-down list that appears. Also, type “H2” and select it from the drop-down. As a final dependency, type “JPA” and select it from the drop-down. Finally, click the Generate Project button and save the generated zip file to your system.

Unzip the todo.zip file on your system and, using the command-line tool, cd to the todo directory.

Examine the pom.xml file if you selected a Maven project, or the build.gradle file if you selected Gradle instead. Take note of the dependencies. You’ll see dependencies on the H2 library, the Jackson-kotlin library, which will be used to create a JSON response, the JPA library, and the Kotlin-stdlib library compatible with JDK 8.

Spring Boot uses a special Kotlin-Spring compiler plugin to deal with a few things that conflict with the default Kotlin way and the Spring way. In Kotlin, classes are final by default. But Spring expects classes to be open. Without the Kotlin-Spring compiler plugin integration, each class written using Kotlin, like a controller, for example, will have to be explicitly marked open. Thanks to the plugin, we can write the classes without the open keyword. The plugin will inspect a class to see if it has some Spring-related meta-annotations, like @Component, @Async, @Transactional, and so on, and if it does, it’ll automatically open those classes during compilation. Since these are meta-annotations, in addition to opening classes that are decorated with these annotations, the plugin also opens classes that are decorated with derived annotations like @Component.

Kotlin vs. Java

The entry point for a Spring Boot application is the class containing the main() method. If we were to create the Spring Boot application using Java, we’d have something like this: