API documentation with OpenAPI

MicroProfile OpenAPI is a MicroProfile community programming model for the OpenAPI specification. MicroProfile OpenAPI helps you to generate structured documentation from your JAX-RS applications.

Structured documentation helps other microservices and developers to understand and communicate with your application. Other developers need relevant information to build an application that communicates with the REST API of your application. MicroProfile OpenAPI facilitates this communication by generating human and machine-readable documentation on a simple interface that doesn’t require access to the source code. You can implement MicroProfile OpenAPI for Open Liberty by enabling the MicroProfile OpenAPI feature.

The two main approaches to generate an OpenAPI document are the code-first approach and the design-first approach. In the code-first approach, a developer can generate documentation of the REST API from the source code. The reference document that is produced lists all the API endpoints with descriptions of how to use them. Alternatively, in the design-first approach, you can include a pre-generated OpenAPI document that was written separately from the code. The JAX-RS framework handles basic API generation for JAX-RS annotations and generates a skeleton OpenAPI treemap. You can use this treemap as a starting point and augment it with annotations and code to produce a complete OpenAPI document. Additionally, you can use this manually created documentation to generate stubs, or testable versions of code modules, such as client libraries for the API.

The code-first approach

In the code-first approach, you can initially generate basic API documentation of the REST API from annotations that are specified in the source code. Then, you can augment the existing annotations with OpenAPI annotations, which are processed to generate the documentation. Adding annotations takes less work than manually defining the OpenAPI document, and gives a useful explanation of the different parts of the API.

In the following example, the OpenAPI annotations @APIResponses, @Operation, and @Parameter are added to the purchaseCar() JAX-RS method.

         value = {
                 responseCode = "404",
                 description = "The requested car could not be found at the dealership, and could not be purchased.",
                 content = @Content(mediaType = "text/plain")),
                 responseCode = "200",
                 description = "The requested car was successfully purchased.",
                 content = @Content(mediaType = "application/json"))})
         summary = "Purchases the specified car from the dealership, and adds the car to your garage.",
         description = "Retrieves the car with the specified registration from the dealership if it exists, and
                        adds this to the caller's garage. The boolean response represents the state of the internal operation.")
 public Response purchaseCar(
              description = "The registration of the car to be added to the inventory.",
              required = true,
              example = "NX15 9012",
              schema = @Schema(type = SchemaType.STRING))
    @PathParam("registration") String registration) {
  boolean success = manager.purchaseCar(registration);
     if (!success) {
         return Response.status(Response.Status.NOT_FOUND)
                        .entity("{ \"error\" : "
                                + "\"The car with registration " + registration
                                + " could not be added to the inventory\" }")
     return Response.ok(success).build();

The default format of the generated document is yaml, but documents can also be provided in JSON format. The following OpenAPI document is generated after the OpenAPI annotations are added from the example in a yaml format:

     summary: "Purchases the specified car from the dealership, and adds the car\
       \ to your garage."
     description: "Retrieves the car with the specified registration from the dealership\
       \ if it exists, and adds this to the caller's garage. The boolean response\
       \ represents the state of the internal operation."
     - name: registration
       in: path
       description: The registration of the car to be added to the inventory.
       required: true
         type: string
       example: NX15 9012
         description: "The requested car could not be found at the dealership, and\
           \ could not be purchased."
           text/plain: {}
         description: The requested car was successfully purchased.
           application/json: {}

The information that is provided through the OpenAPI annotations augments the basic API documentation that is generated by the JAX-RS framework.

For more information, see the MicroProfile OpenAPI Javadoc for the annotations that are available.

The design-first approach

An alternative approach is to design the REST API in an editor, such as the Swagger editor, before you write any code. With this approach, you can spot and rectify any issues in the design before it is implemented. In large companies, subject matter experts review the API to ensure it’s consistent and usable. This API design then forms a contract and must be implemented as agreed.

You can write this API design in yaml or JSON format and place it in the META-INF directory of your application. Optionally, you can create stubs for the API code. The code first and design-first approaches are not mutually exclusive. You can augment manually created API documents by adding annotations to the code as you would in a code-first approach.

For more information, see Using pregenerated OpenAPI documents.

Filter components in and out of API documentation

If you want to update or remove certain elements and fields of the OpenAPI document you can use a filter, which is called one time after all other documentation mechanisms are finished. Filters give the users the flexibility to modify the document. With filters, you can implement an interface in your application. When a component of the documentation is processed, it calls your implementation and you can either add or remove content. For more information, see Filtering the OpenAPI tree elements.

View API documentation

The raw OpenAPI document can be retrieved by using the /openapi endpoint. This UI is built from the Open Source Swagger UI and renders the generated /openapi document in a user-friendly browser interface.

Some deprecated annotations were removed between Microprofile OpenAPI versions 1.1 and 2.0, which might result in breaking changes between those versions. For more information, see the Release Notes for Microprofile OpenAPI 2.0.