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Spring

Introduction

The Spring Framework provides first-class support for Kotlin and lets developers write Kotlin applications almost as if the Spring Framework was a native Kotlin framework. Most of the code samples of the reference documentation are provided in Kotlin in addition to Java.

Spring Framework - Kotlin

Spring Boot

Spring Boot is a framework that simplifies the creation of stand-alone, Spring based applications.

Create a new “JVM Application” with Amper

To enable Spring Boot support, add the following to the module.yaml file:

yaml
settings:
    springBoot: enabled

Mixed projects (containing Java and Kotlin sources simultaneously) are supported.

Examples of Spring Boot projects:

Amper - Spring

RESTful Web Service

You will build a service that will accept HTTP GET requests at http://localhost:8080/greeting.

It will respond with a JSON representation of a greeting, as the following listing shows:

json
{"id":1,"content":"Hello, World!"}

You can customize the greeting with an optional name parameter in the query string, as the following listing shows:

http://localhost:8080/greeting?name=User

The name parameter value overrides the default value of World and is reflected in the response, as the following listing shows:

json
{"id":1,"content":"Hello, User!"}

Create a Resource Representation Class

The service will handle GET requests for /greeting, optionally with a name parameter in the query string. The GET request should return a 200 OK response with JSON in the body that represents a greeting. It should resemble the following output:

json
{
    "id": 1,
    "content": "Hello, World!"
}

The id field is a unique identifier for the greeting, and content is the textual representation of the greeting.

To model the greeting representation, create a resource representation class. To do so, provide a data class for the id and content data, as the following listing shows:

java
data class Greeting(val id: Long, val content: String)
Note

This application uses the Jackson JSON library to automatically marshal instances of type Greeting into JSON. Jackson is included by default by the web starter.

Create a Resource Controller

In Spring’s approach to building RESTful web services, HTTP requests are handled by a controller. These components are identified by the @RestController annotation, and the GreetingController class shown in the following listing handles GET requests for /greeting by returning a new instance of the Greeting class:

java
private const val template = "Hello, %s!"

@RestController
class GreetingController {

  private val counter = AtomicLong(0)

  @GetMapping("/greeting")
  fun greeting(@RequestParam name: String = "World") =
    Greeting(counter.fetchAndAdd(1), template.format(name))

}

Building a RESTful Web Service

Test the Service

http://localhost:8080/greeting?name=david

Test

Spring Boot provides a @SpringBootTest annotation that works by creating the ApplicationContext used in your tests through SpringApplication. In addition to @SpringBootTest a number of other annotations are also provided for testing more specific slices of an application.

By default, @SpringBootTest will not start a server. You can use the webEnvironment attribute of @SpringBootTest to further refine how your tests run:

Pending