# OpenAPI Generator

# Video

# Intro

# What is OpenAPI Generator?

An open source project used to generate applications based on OpenAPI v2.0/v3.0 documents. With support for over 40 different languages it can be used to create both the client and server applications based on the same OpenAPI Specification (OAS) file, helping to prevent any drift between your API specification and your code. OpenAPI Generator is even able to generate HTML documentation based on your OAS.

# What is Contract-First Development?

The practice of having a clearly defined API at the outset of a project. An important aspect of contract-first development is an agreement with your team that any updates to the contract happen before any updates to the code.

# Lab

# Setup

# Install OpenAPI Generator

Instructions for installation of OpenAPI Generator can be found here: https://openapi-generator.tech/docs/installation

# Download OpenAPI Specification File

You can get your OpenAPI Specification using one of the following options: a. Use your own OAS file. b. Download the demo application file here (opens new window). c. Retrieve your OAS file from Apicurio Studio (opens new window) created in the previous lab. See below.

If you completed the previous lab you should be able to follow the lab by downloading that OAS file. Here are the instructions for retrieving your OAS file off of Apicurio.

  1. Navigate to the APIs dashboard here (opens new window).
  2. Choose the API you want to download.
  3. Select "Download" in the dropdown menu. apicurio

# View OpenAPI Generator Documentation (Optional)

  1. Go to the OpenAPI Generator Webpage (opens new window).
  2. Click the "Generators" button.
  3. Take some time to view the different generators, following the links to get more information.

# Instructions

  1. Generate Spring Boot Application:

    • Run the command below, note replace todo.yaml with a path to your file.
    openapi-generator generate \
         -g spring \
         --library spring-boot \
         -i todo.yaml \
         -o ${PWD} \
         -p groupId=com.redhat \
         -p artifactId=todo \
         -p artifactVersion=1.0.0-SNAPSHOT \
         -p basePackage=com.redhat.todo \
         -p configPackage=com.redhat.todo.config \
         -p apiPackage=com.redhat.todo.api \
         -p modelPackage=com.redhat.todo.model \
         -p sourceFolder=src/main/gen \
         -p dateLibrary=java8 \
         -p java8=true

    Note: We are using the sourceFolder parameter to change where OpenAPI Generator places the generated Java code. By placing it in src/main/gen instead of src/main/java we are indicating that this code is created by the generator and should be treated as immutable.

    • Open the code in an IDE of your choice and take some time to look around the code. Take note that we are currently generating all the files related to the application inside of the src/main/gen folder.
  2. Add the src/main/gen to your source directories:

    • Add the following plugin to your pom.xml:
    • Validate you can run the application:
      • Run mvn spring-boot:run
      • Navigate to http://localhost:8080/swagger-ui.html
  3. View the code inside src/main/gen/com/redhat/todo:

    • View the api/TodosApi.java:
      • The interface created from the paths inside of your OAS file.
      • Contains all of your path's Swagger annotations.
      • There is also a basic implementation of each of your path methods returning a 503 Not Implemented. You may have noticed this if you attempted this in the previous step.
    • View the api/TodosApiController.java:
      • Currently there is very little besides the @RequestMapping annotation inside of this controller.
      • This will be where we implement the methods of the TodosApi interface.
    • View model/Todo.java:
      • The POJO object represented by the schema section of your OAS file.
  4. Implement the TodosApiController:

    • Create a src/main/java source folder – this is the folder we will be putting all the code we plan on modifying.
    • Move TodosApiController.java from the src/main/gen folder to the same package inside the src/main/java folder.
    • Stub the getTodo method by adding the following code:
      public ResponseEntity<Todo> getTodo(String todoId) {
          Todo response = new Todo();
          response.setName("Stubbed Todo Item");
          response.setDescription("Stubbed Todo Description");
          return ResponseEntity.ok(response);
      Note: we are returning a 200 response code. This is compliant with our OAS specification. Currently we don't have any validation on which response code we are returning but it is best practice to follow what is specified by your OpenAPI document. And in the future we will be looking at Schemathesis (opens new window) which will run test to validate that all expected response codes are returned.
    • Prevent the regeneration of the TodosApisController.java:
      • In order to prevent files that you add to your src/main/java from being recreated in src/main/gen they need to be added to your .openapi-generator-ignore file.
      • Add the following lines to prevent any *Controller.java files from being added, as well as preventing us from overriding our pom.xml:
    • Validate the endpoint:
      • Run mvn spring-boot:run
      • Navigate to http://localhost:8080/swagger-ui.html
      • Validate GET /todos/{todoId} returns 200 stubbed data.
    • (Optional) Stub out other endpoints – remember that the response codes should match your OAS document.
  5. Regenerate code on Maven builds:

    • Add the following Maven plugin to your pom.xml:


      Warning: If you did not add pom.xml to your .openapi-generator-ignore file the next build will override your pom.xml and you will need to add back the previous plugin.

      • Explore the new OAS file being pulled from GitHub:
        • Note the new user schema as well as the new paths associated with the new user object.
      • Validate that TodosApisController.java has been deleted from your src/main/gen folder and regenerate your files with mvn generate-sources.
      • Validate api/UsersApi.java and model/User.java are generated and the *Controller.java are not being generated.

# Wrap-up

At this point we have created a stubbed out application. You should be able to check in your application with the single TodosApisController.java source file and if another developer pulls it down, all of the other files needed to run the server will be generated automatically every time before the compile phase of the Maven lifecycle. As a bonus assignment you can modify the inputSpec parameter for your openapi-generator-maven-plugin to point back to your local file and see how changes in the OAS affect the code.

Hopefully, this showed how easy it is to go from one of these specification documents to an up and running application.

Last Updated: 12/18/2021, 5:53:27 PM