Generating an OpenAPI document and SDK from Spring Boot

You have a Spring Boot API and need to generate SDKs or API documentation for other teams. Rather than writing and maintaining separate OpenAPI documents, we will walk through how to generate them directly from your Spring Boot code and then use them to create and customize an SDK.

We’ll work with real code you can run locally, building a simple bookstore API to demonstrate how to properly document API structures, including inheritance between models, endpoint definitions, response types, and error handling. The examples illustrate how Spring Boot annotations map to OpenAPI concepts, so you can see how your code translates into API specifications.

Setting up a Spring Boot project

First, create a new Spring Boot project using Spring Initializr . Select the following options:

• Project: Maven
• Language: Java
• Spring Boot: 3.5.x (or the latest stable version)
• Project Metadata: Fill in as appropriate
• Dependencies: Spring Web

Download the project and extract it to your preferred directory.

Adding OpenAPI dependencies

Open the pom.xml file and add the following dependency:

Configuring application properties

Open the src/main/resources/application.properties file and add the following configuration:

These properties configure the application name that identifies your service, the endpoint where the OpenAPI document will be available (/api-docs), the version of the OpenAPI document to generate, and the URL path where you can access the Swagger UI documentation (/swagger-ui.html).

After starting your application, you can view the OpenAPI document at http://localhost:8080/api-docs and access the interactive Swagger UI at http://localhost:8080/swagger-ui.html.

Writing a Spring Boot application

You can find all the code for this step in the example application.

Open the src/main/java/com/bookstore/BookstoreApplication.java file in your text editor to see where to begin when adding OpenAPI annotations to your project.

Defining the main application configuration with annotations

The BookstoreApplication class is the entry point for the API, and it’s also where we define the main OpenAPI documentation properties:

The @OpenAPIDefinition annotation populates the OpenAPI document with essential context for anyone who wants to use the API. The title, version, and description fields describe what the API does, its current state, and how it can be used.

The @Server annotation defines available endpoints for the API. In the example, there are two options:

• A production server at https://api.bookstore.example.com that uses live data
• A localhost server at http://localhost:8080 for testing with sample data

Defining data models with annotations

Next, let’s look at how you can use OpenAPI annotations to describe API data structures in the Models.java file:

The @Schema annotation can be used at both the class and field levels:

• At the class level, @Schema describes what a Publication, Book, or Magazine represents in the API.
• At the field level, fields like id and author are documented with a description and example values.

The Publication class acts as the base schema in the OpenAPI specification. By using @JsonTypeInfo and @JsonSubTypes, we tell OpenAPI that a Publication can be either a Book or Magazine. This polymorphism is reflected in the OpenAPI document as a oneOf schema, allowing endpoints to accept or return either type. API clients will include a type field set to either BOOK or MAGAZINE to identify the publication type.

Here’s how we define an Order class that references the Publication schema:

The Order class uses the @Schema annotation to document the items field, which references the Publication schema. This tells OpenAPI that Orders can contain an array of either books or magazines, using the polymorphic structure defined earlier.

For the order status, we use an enumeration:

This appears in the OpenAPI document as a string field with a set of allowed values.

Finally, we define an error response model:

Defining API endpoints with annotations

Now, let’s define the API endpoints in the PublicationsController.java file:

The @Tag annotation groups operations under “Publications” in the OpenAPI document. Combined with @RequestMapping("/publications"), it tells API consumers that these endpoints handle publication-related operations.

For each endpoint method, we use annotations to document their purpose and responses:

The @Operation and @ApiResponses annotations document what the endpoint does and what responses to expect. For example, getPublication is annotated to show that it returns a publication successfully (200 status) or returns an error (404 status) when the publication isn’t found.

The @Parameter annotation describes the requirements for input parameters, such as the ID path parameter in this example.

Examining the generated OpenAPI document

Now that we’ve built the Spring Boot application, let’s generate and examine the OpenAPI document to understand how the Java code translates into API specifications.

First, install the necessary dependencies in the project and start the application with the following commands:

Download the OpenAPI document while running the application:

This command saves the OpenAPI document as openapi.yaml in your current directory.

Let’s explore the generated OpenAPI document to see how the Spring Boot annotations translate into an OpenAPI specification.

The OpenAPI Specification version information

The OpenAPI document begins with version information:

This version is determined by the configuration in our application.properties file. It tells API consumers which version of the OpenAPI Specification to expect.

API information

Next comes the info object, which is generated from the @OpenAPIDefinition annotation:

Notice how each field in the Java annotation maps directly to its counterpart in the OpenAPI document output. This one-to-one mapping makes it easy to understand how your code affects the final API documentation.

Server information

Server configurations defined with @Server annotations appear in the servers array:

Polymorphic models

One of the more complex aspects of the API is how polymorphic models are represented. The Publication class has been translated into a schema that supports polymorphism through a discriminator:

Key aspects to notice:

• The @Schema annotations provide descriptions and examples
• The @JsonTypeInfo annotation determines the discriminator property
• The @JsonSubTypes annotation defines the possible concrete implementations

API endpoints

Finally, let’s examine how controller methods translate into API endpoints. Here’s how the getPublication endpoint appears in the OpenAPI document:

The mapping is clear:

• The @Operation annotation provides the summary and description.
• Each @ApiResponse maps to an entry in the responses object.
• The @Parameter annotation documents the path parameter.

Creating an SDK from the OpenAPI document

Now that we have an OpenAPI document for the Spring Boot API, we can create an SDK using Speakeasy.

First, make sure you have Speakeasy installed:

Now, generate a TypeScript SDK using the following command:

Follow the onscreen prompts to provide the configuration details for the new SDK, such as the name, schema location, and output path. Enter openapi.yaml when prompted for the OpenAPI document location and select your preferred language (for example, TypeScript) when prompted for which language you would like to generate.

You’ll see the steps taken by Speakeasy to create the SDK in the terminal:

Speakeasy validates the OpenAPI document to check that it’s ready for code generation. Validation issues will be printed in the terminal. The generated SDK will be saved as a folder in your project.

If you get ESLint styling errors, run the speakeasy quickstart command from outside your project.

Speakeasy also suggests improvements for your SDK using Speakeasy Suggest, which is an AI-powered tool in Speakeasy Studio. You can see suggestions by opening the link to your Speakeasy Studio workspace in the terminal:

After running this command, you’ll find the generated SDK code in the specified output directory. This SDK can be used by clients to interact with your Spring Boot API in a type safe manner.

In the SDK README.md file, you’ll find documentation about your Speakeasy SDK. TypeScript SDKs generated with Speakeasy include an installable Model Context Protocol (MCP) server where the various SDK methods are exposed as tools that AI applications can invoke. Your SDK documentation includes instructions for installing the MCP server.

Note that the SDK is not ready for production use. To get it production-ready, follow the steps outlined in your Speakeasy workspace.

Customizing the SDK

The example app added retry logic to the SDK’s listPublications operation to handle network errors gracefully. This was done using one of Speakeasy’s OpenAPI extensions, x-speakeasy-retries.

Instead of modifying the OpenAPI document directly, this extension was added to the Spring Boot controller, and the OpenAPI document and SDK were regenerated.

These imports were added to src/main/java/com/bookstore/PublicationsController.java:

The listPublications operation was modified to include the retry configuration:

The OpenAPI document was then regenerated:

The OpenAPI document includes the retry configuration for the listPublications operation:

After recreating the SDK using Speakeasy:

The created SDK now includes retry logic for the listPublications operation, automatically handling network errors and 5XX responses.

Issues and feedback

Need some assistance or have a suggestion? Reach out to our team at support@speakeasy.com.

If you haven’t already, take a look at our blog to learn more about OpenAPI, SDK generation, and more, including:

• Native JSONL support in your SDKs
• Comprehensive SDK testing
• Model Context Protocol: TypeScript SDKs for the agentic AI ecosystem