Easy OpenAPI 3 specification for your Spring Boot REST API

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The springdoc-openapi library allows us to automatically generate an OpenAPI specification for our rest API built with Spring Boot. This specification is also useful when we need a Swagger documentation or we want to automate client code generation.

You can find an example project with springdoc-openapi in the spring-boot-angular-scaffolding repository.

Add Maven dependencies

First, we are going to add the Maven dependencies for the springdoc-openapi-ui and springdoc-openapi-data-rest libraries to our pom.xml:

As a result, we now have an automatically generated OpenAPI specification for our Spring Boot project and all its REST endpoints. Just start the app and visit http://localhost:8080/v3/api-docs:

OpenAPI specification

Documentation via Swagger UI

All you need to do is visit the http://localhost:8080/swagger-ui.html url and verify the generated documentation:

Spring Boot OpenAPI 3 documentation before applying custom configuration

As we can see, the generated specification and documentation contain only basic information and use the default naming convention. We’re going to customize it with some additional configuration.

Add custom configuration

The code used below is available in the spring-boot-angular-scaffolding repository.

Customize specification with annotations

If we want to avoid implementation details leaking through names like HomePageDto or home-page-controller, we can customize those names with annotations. We’re going to use @Schema on our HomePageDto object:

Next, we’re going to change the controller name and add a description for our example endpoint:

As a result, we will see a more friendly version of our docs when running the app again:

Swagger ui after adding annotations to code

Add project info

Furthermore, we can add some project details to the generated specification. Fortunately, we have project name, description and version already defined in our pom.xml file:

Thanks to the Automatic Property Expansion feature we can use the existing build configuration. Let’s create custom properties in our application.properties file and assign their values:

Following the Spring Boot documentation on Externalized Configuration, specifically the Type-safe Configuration Properties chapter, we’re going to create the OpenApiProperties class:

Finally, we can use those properties in our OpenAPI configuration class. Additionally, we can specify the license of the project as well:

Consequently, the swagger-ui page will include the specified project details:

Project details in swagger ui

Obviously, we will see all those changes in the specification as well:

OpenAPI spec after configuration

Springdoc-openapi modules

It’s important to realize that in this example we added only one springdoc-openapi module to our project – Spring Data Rest. However, Springdoc comes with multiple modules, offering support for Spring Security, Hateoas, Kotlin etc.

Specification in the yaml format

Another springdoc-openapi feature worth mentioning is the ability to generate the API specification as a yaml file. We can download the api-docs.yaml file under the http://localhost:8080/v3/api-docs.yaml url:

Openapi specification for the Spring Boot API in the yaml format

Learn more on documenting Spring Boot API

Photo by Taryn Elliott from Pexels

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