Documenting Your Data’s Reach: Generating API Docs for Spring Data REST

Spring Data REST is a fantastic tool for rapidly exposing your JPA entities as hypermedia-driven REST APIs. However, even the most intuitive APIs benefit from clear and comprehensive documentation. While HATEOAS provides discoverability at runtime, static documentation offers a bird’s-eye view, making it easier for developers to understand the API’s structure, available resources, and supported operations.

This article will explore how to generate documentation for your Spring Data REST APIs, focusing on popular tools and approaches within the Spring ecosystem.

Why Document Your Spring Data REST API?

• Onboarding New Developers: Clear documentation significantly speeds up the learning curve for developers integrating with your API.
• Improved Collaboration: Provides a shared understanding of the API’s capabilities for both internal and external teams.
• Reduced Ambiguity: Clarifies endpoints, request/response formats, and available actions.
• Client Library Generation: Well-structured documentation can be used to automatically generate client libraries in various languages.
• Long-Term Maintainability: Serves as a reference point as your API evolves.

Popular Tools and Approaches

Here are some common ways to generate documentation for your Spring Data REST APIs:

1. Spring REST Docs (Recommended for Accuracy and Maintainability)

Spring REST Docs is a powerful tool specifically designed for documenting RESTful APIs built with Spring. It works by writing tests that interact with your API and, as a byproduct, generates accurate and up-to-date documentation snippets.

How it Works with Spring Data REST:

• Write Integration Tests: You write tests that exercise your Spring Data REST endpoints (e.g., creating, retrieving, updating, deleting entities).
• Document API Interactions: Within your tests, you use Spring REST Docs’ DSL to document the request and response details, including:
  • Path parameters
  • Query parameters
  • Request body fields
  • Response body fields (including HATEOAS links)
  • HTTP status codes
  • Headers
• Generate Documentation: Spring REST Docs uses the information captured during the tests to generate Asciidoctor or Markdown documentation.

Example (Gradle setup and a simplified test snippet):

build.gradle:

plugins {
    id 'org.springframework.boot' version '...'
    id 'java'
    id 'org.asciidoctor.jvm.convert' version '3.3.2'
}

// ... other dependencies

dependencies {
    implementation 'org.springframework.boot:spring-boot-starter-data-rest'
    implementation 'org.springframework.boot:spring-boot-starter-web'
    testImplementation 'org.springframework.boot:spring-boot-starter-test'
    testImplementation 'org.springframework.restdocs:spring-restdocs-mockmvc'
}

asciidoctor {
    inputs.dir snippetsDir
    outputDir file('build/asciidoc')
}

task copyDocument(type: Copy) {
    dependsOn asciidoctor
    from file('build/asciidoc/html5')
    into file('build/resources/static/docs') // Or your preferred output directory
}

bootJar {
    dependsOn copyDocument
}

Simplified Integration Test:

import org.junit.jupiter.api.Test;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.boot.test.autoconfigure.restdocs.AutoConfigureRestDocs;
import org.springframework.boot.test.autoconfigure.web.servlet.AutoConfigureMockMvc;
import org.springframework.boot.test.context.SpringBootTest;
import org.springframework.http.MediaType;
import org.springframework.test.web.servlet.MockMvc;

import static org.springframework.restdocs.mockmvc.MockMvcRestDocumentation.document;
import static org.springframework.restdocs.payload.PayloadDocumentation.*;
import static org.springframework.test.web.servlet.request.MockMvcRequestBuilders.get;
import static org.springframework.test.web.servlet.result.MockMvcResultMatchers.status;

@SpringBootTest
@AutoConfigureMockMvc
@AutoConfigureRestDocs(outputDir = "build/generated-snippets")
public class ProductDocumentationIntegrationTest {

    @Autowired
    private MockMvc mockMvc;

    @Test
    void getProduct() throws Exception {
        this.mockMvc.perform(get("/products/1"))
                .andExpect(status().isOk())
                .andDo(document("product-get",
                        responseFields(
                                fieldWithPath("id").description("The product ID"),
                                fieldWithPath("name").description("The product name"),
                                fieldWithPath("price").description("The product price"),
                                fieldWithPath("_links.self.href").description("Link to the product itself"),
                                fieldWithPath("_links.category.href").description("Link to the product's category")
                                // Add documentation for other fields and links
                        )));
    }
}

Pros:

• Accurate: Documentation is generated from actual API interactions, reducing the risk of outdated information.
• Maintainable: Changes to the API will likely break tests, forcing you to update the documentation.
• Comprehensive: Allows detailed documentation of requests, responses, headers, and HATEOAS links.
• Integrates Well with Spring: Seamlessly works with Spring’s testing framework.

Cons:

• Requires Writing Tests: Adds the overhead of writing detailed integration tests.
• Steeper Learning Curve: Might have a slightly higher initial learning curve compared to purely annotation-based approaches.

2. OpenAPI (Swagger) Specification (Annotation-Based)

OpenAPI (formerly Swagger) is a widely adopted specification for describing RESTful APIs. You can generate OpenAPI documentation for your Spring Data REST API using libraries like springdoc-openapi.

How it Works with Spring Data REST:

• Add Dependency: Include the springdoc-openapi-starter-data-rest dependency in your project.

  Maven:

  <dependency>
      <groupId>org.springdoc</groupId>
      <artifactId>springdoc-openapi-starter-data-rest</artifactId>
      <version>...</version>
  </dependency>
  

  Gradle:

  implementation 'org.springdoc:springdoc-openapi-starter-data-rest:...'
  

• Configuration (Optional): You can customize the generated OpenAPI specification using properties in your application.yml or @OpenAPIDefinition and @Operation annotations. However, for basic Spring Data REST exposure, minimal configuration is often needed.

• Access Documentation: Once your application is running, the OpenAPI specification will typically be available at endpoints like /v3/api-docs (in JSON format) and a UI (Swagger UI) at /swagger-ui.html.

Example (application.yml for basic info):

springdoc:
  api-docs:
    path: /v3/api-docs
  swagger-ui:
    path: /swagger-ui.html
    config:
      defaultModelsExpandDepth: -1 # Collapse models by default

Pros:

• Easy Setup: Adding the dependency and accessing the UI is straightforward.
• Interactive UI (Swagger UI): Provides a user-friendly interface to explore and test your API.
• Widely Adopted Standard: OpenAPI specifications can be used by various tools for client generation, testing, and more.
• Annotation-Driven (with customizations): You can add annotations to your entities, repositories, and custom controllers for more detailed documentation.

Cons:

• Limited Automatic Documentation of HATEOAS: While springdoc-openapi can document the basic structure and fields of your entities, fully capturing the dynamic nature of HATEOAS links might require more manual configuration or customizers.
• Potential for Outdated Documentation: If not carefully maintained with annotations, the documentation might drift from the actual API behavior.

3. Manual Documentation (Not Recommended for Evolving APIs)

You could manually write documentation using tools like Markdown, Confluence, or dedicated API documentation platforms.

Pros:

• Full Control: You have complete control over the content and format.

Cons:

• Time-Consuming: Requires significant manual effort to create and maintain.
• Prone to Errors: Easily becomes outdated as the API evolves.
• Difficult to Keep Synchronized: Ensuring accuracy with the codebase is challenging.

Best Practices for Documenting Spring Data REST APIs:

• Choose the Right Tool: For Spring-based APIs, Spring REST Docs offers the highest accuracy and maintainability. OpenAPI is a good choice for broader adoption and interactive exploration. Avoid relying solely on manual documentation for evolving APIs.
• Focus on Key Information: Document endpoints, HTTP methods, request/response formats (including data types), authentication requirements, and any specific business logic.
• Document HATEOAS Links: Clearly explain the purpose and structure of the _links section and the meaning of different rel values. Provide examples of how clients can use these links to navigate the API.
• Keep Documentation Up-to-Date: Integrate documentation generation into your build process to ensure it stays synchronized with your codebase. Tests in Spring REST Docs serve this purpose well.
• Provide Examples: Include clear and concise examples of requests and responses.
• Consider Different Formats: Generate documentation in formats that are easily consumable by developers (e.g., HTML, Markdown, OpenAPI JSON/YAML).
• Host Your Documentation: Make your API documentation easily accessible to your consumers (e.g., host it on a dedicated documentation website).

Conclusion:

Documenting your Spring Data REST API is essential for its successful adoption and long-term maintainability. While Spring Data REST simplifies API creation, investing in proper documentation ensures that developers can effectively understand and integrate with your data resources. By leveraging tools like Spring REST Docs or OpenAPI, you can generate accurate, discoverable, and up-to-date documentation that empowers your API consumers. Choose the approach that best fits your project’s needs and prioritize keeping your documentation in sync with your evolving API.