[JAVA] Introduction to Spring Boot x OpenAPI ~ OpenAPI made with Generation gap pattern ~


Participated in the Java community event "JJUG CCC 2019 Fall". The story of creating a super-simple OpenAPI inspired by the content of the "API server development using OpenAPI Specification 3.0 that is not Swagger" that I heard there. (Shallow)

This goal

Write OpenAPI using Spring Boot

Development flow using OpenAPI (roughly)


Rule: ** Generation gap pattern ** → Do not edit the automatically generated class file.

1. Prepare API definition file

Use the sample in OAS (OpenAPI Specification)


2. Automatic code generation from definition

Use Gradle plugin [openapi-generator-gradle-plugin]. Gradle plugin that automatically generates code from API definition file.


plugins {
	id "org.openapi.generator" version "4.2.1"
Point 1: Compile settings

Set to include automatically generated class files at compile time


compileJava.dependsOn tasks.openApiGenerate
sourceSets.main.java.srcDir "${openApiGenerate.outputDir.get()}/src/main/java"
Point 2: Openapi Generator settings

The execution example of openapi-generator-gradle-plugin can be almost copied. Just manipulate the config file to create an interface.



openApiGenerate {
    generatorName = "spring"
    //Config settings
    configFile = "$rootDir/specs/config.json".toString()
    //Sample API specification
    inputSpec = "$rootDir/specs/petstore-v3.0.yaml".toString()
    outputDir = "$buildDir/generated".toString()
    apiPackage = "org.openapi.example.api"
    invokerPackage = "org.openapi.example.invoker"
    modelPackage = "org.openapi.example.model"
    configOptions = [
        dateLibrary: "java8"
    systemProperties = [
        modelDocs: 'false'


  "interfaceOnly": true

Automatic class generation is OK with Gradle task [openApiGenerate].

スクリーンショット 2019-11-24 9.40.36.png

When the task is executed, the following Java files are automatically generated


3. API implementation

Generate a controller by implementing from the automatically generated interface


public class PetsApiController implements PetsApi{
	public ResponseEntity<List<Pet>> listPets(@Valid Integer limit) {
		System.out.println("Here list pet");
		return new ResponseEntity<>(HttpStatus.OK);

Complete! !!

Completed code


Bonus: I tried using the OpenAPI documentation tool

Documents are automatically generated using the OpenAPI Document Tool (redoc).

docker run -it --rm -p 80:80 \
  -v $(pwd)/specs/petstore-v3.0.yaml:/usr/share/nginx/html/swagger.yaml \


Output image: Output API parameters and return values on an easy-to-read screen based on the definition file


Reference material

API server development with OpenAPI Specification 3.0, not Swagger https://www.slideshare.net/techblogyahoo/swagger-openapi-specification-30-api

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