API Documentation

API Documentation is a written guide that explains how to use an API.

It helps developers understand:

• What endpoints are available
• How to call them
• What data to send
• What response they will receive

In simple words: API documentation is a manual for using an API correctly.

What Is API Documentation?

API documentation tells developers:

• What the API does
• Which endpoints are available
• What HTTP methods to use (GET, POST, PUT, DELETE)
• What parameters to send
• What response to expect
• What errors can occur

Without documentation, even a good API becomes difficult to use.

Why API Documentation Is Important

Without Documentation

• Developers get confused
• APIs are hard to use
• Integration takes more time
• Frequent back-and-forth communication

With Good Documentation

• APIs are easy to understand
• Faster development
• Fewer mistakes
• Better developer experience
• Easier onboarding for new developers

Well-documented APIs are more trustworthy and easier to adopt, especially for public or partner APIs.

What Should Good API Documentation Include?

Good API documentation should be clear, complete, and easy to follow. Its main goal is to help developers understand and use the API correctly without confusion.

Here are the key sections every good API documentation must include:

1. API Overview

This section gives a high-level understanding of the API.

Include:

• Purpose of the API
• What problem it solves
• Base URL
• Authentication method

Example:

Base URL:<https://api.example.com>
Authentication:BearerToken

2. Authentication & Authorization

Explain how to access the API securely.

Include:

• Authentication type (API Key, JWT, OAuth, Basic Auth)
• Where credentials should be sent (Header, Query, Cookie)

Example:

Authorization: Bearer <token>

3. Endpoint Details

Explain each endpoint clearly.

Include:

• HTTP Method (GET, POST, PUT, DELETE)
• Endpoint URL
• What the endpoint does

Example:

GET /api/users

Description: Fetch all users

4. Request Parameters

Explain all inputs the API expects.

Include:

• Path parameters
• Query parameters
• Request body fields

Example:

GET /api/users/{id}

5. Request Examples

Show real API request examples.

Example:

GET /api/users/1

Examples help developers understand how to use the API quickly.

6. Response Structure

Explain what the API returns.

Include:

• Field names
• Data types
• Meaning of each field

Example:

{
"id":1,
"name":"John Doe"
}

7. Error Responses

Document all possible errors clearly.

Include:

• HTTP status codes
• Error messages
• Reason for the error

Example:

{
"error":"User not found",
"status":404
}

8. HTTP Status Codes

List commonly used status codes.

Examples:

• 200 – Success
• 201 – Created
• 400 – Bad Request
• 401 – Unauthorized
• 404 – Not Found
• 500 – Internal Server Error

9. API Versioning Information

Explain how API versions are handled.

Examples:

• /api/v1/users
• Header-based versioning

This helps clients handle changes safely.

10. Rate Limits & Constraints (If Any)

Explain any usage limits.

Include:

• Request limits
• Timeout rules
• Payload size limits

Example:

100 requestsperminuteperuser

11. Examples & Use Cases

Provide:

• Common use cases
• Sample workflows

This makes documentation practical and real-world friendly.

12. Changelog / Updates

Mention:

• Breaking changes
• New features
• Deprecated endpoints

This helps developers stay updated.

Types of API Documentation

API documentation can be created in two main ways. Each approach has its own use cases, advantages, and limitations.

1. Manual Documentation

Manual documentation is written and maintained by developers.

Characteristics:

• Created manually by the development team
• Written using Markdown, PDF, Wiki, or text files
• Offers full control over wording and structure

Drawbacks:

• Hard to maintain over time
• Can easily become outdated
• Requires extra effort to keep in sync with code

2. Auto-Generated Documentation

Auto-generated documentation is created automatically from the source code.

Characteristics:

• Generated directly from controllers and annotations
• Always stays in sync with the code
• Minimal manual effort required

Benefits:

• Always up to date
• Ideal for production systems
• Provides interactive API testing

Popular Tools:

• Swagger / OpenAPI
• Springdoc OpenAPI (Spring Boot)

Manual vs Auto-Generated API Documentation

Popular API Documentation Tools

API Documentation in Spring Boot

In Spring Boot, API documentation can be generated automatically, which saves time and prevents manual mistakes.

Instead of writing documentation by hand, Spring Boot can scan your code and create interactive API docs.

The most popular and recommended solution is:

👉 Swagger using springdoc-openapi

What Is OpenAPI?

OpenAPI is a standard specification used to describe REST APIs in a clear and structured way.

It defines:

• Available API endpoints
• HTTP methods (GET, POST, PUT, DELETE)
• Request parameters
• Request bodies
• Response formats
• Error codes

👉 Swagger UI is a visual interface built on top of OpenAPI that allows developers to:

• View complete API documentation
• Test APIs directly from the browser
• Understand request and response structures easily

Add Swagger (springdoc-openapi) Dependency

To enable Swagger UI in a Spring Boot 3+ application, add the following dependency.

Spring Boot 3+

<dependency>
<groupId>org.springdoc</groupId>
<artifactId>springdoc-openapi-starter-webmvc-ui</artifactId>
<version>2.5.0</version>
</dependency>

No extra configuration required Springdoc automatically scans your controllers and generates API documentation.

Swagger UI URLs

After starting your Spring Boot application, access the documentation at:

Swagger UI (Interactive Docs)

<http://localhost:8080/swagger-ui.html>

OpenAPI JSON (Specification)

<http://localhost:8080/v3/api-docs>

Basic API Example (Spring Boot + Swagger)

@RestController
@RequestMapping("/api/users")
public class UserController {

    @GetMapping("/{id}")
    public User getUser(@PathVariable Long id) {
        return new User(id, "John", "[email protected]");
    }
}

Swagger automatically documents:

Without writing any extra documentation, Swagger generates:

• Endpoint URL → /api/users/{id}
• HTTP method → GET
• Path Parameter → id
• Response Structure → User object
• Response Status Codes

Enhancing Documentation with Annotations

Swagger (springdoc-openapi) provides annotations that help make your API documentation clearer and more detailed.

@Operation – Describe an Endpoint

@Operation(
    summary = "Get user by ID",
    description = "Fetch a user using their unique ID"
)
@GetMapping("/{id}")
public User getUser(@PathVariable Long id) {
    return userService.findById(id);
}

Adds a summary and description for the endpoint in Swagger UI.

@Parameter – Describe Parameters

@GetMapping("/{id}")
public User getUser(
    @Parameter(description = "User ID", example = "1")
    @PathVariable Long id
) {
    return userService.findById(id);
}

Documents path, query, and header parameters with descriptions and examples.

@ApiResponse – Document Responses

@Operation(summary = "Get user by ID")
@ApiResponses({
    @ApiResponse(responseCode = "200", description = "User found"),
    @ApiResponse(responseCode = "404", description = "User not found")
})
@GetMapping("/{id}")
public User getUser(@PathVariable Long id) {
    return userService.findById(id);
}

Adds HTTP response codes and descriptions to Swagger UI.

Documenting Request Body

@Operation(summary = "Create new user")
@PostMapping
public User createUser(
    @RequestBody(description = "User details", required = true) User user
) {
    return userService.save(user);
}

Swagger shows:

• JSON structure
• Required fields
• Example values

Grouping APIs (For Large Projects)

@Bean
public GroupedOpenApi userApi() {
    return GroupedOpenApi.builder()
            .group("Users")
            .pathsToMatch("/api/users/**")
            .build();
}

Useful for:

• Large applications
• Microservices
• Organizing APIs by modules or functional areas

Securing Swagger UI (Optional)

While Swagger UI is great for development, you usually don’t want it publicly accessible in production.

• Restrict access

  Only allow authorized users (e.g., developers, admins) to view Swagger UI.

• Enable only in dev or test environments

  Disable Swagger for production environments to prevent exposing internal API details.

• Protect using Spring Security

  Use Spring Security to secure the /swagger-ui.html and /v3/api-docs endpoints.

Example: Enable Swagger Only in Dev Profile

@Configuration
@Profile("dev")
publicclassSwaggerConfig {
// Swagger configuration beans here
}

Swagger will be active only when the application runs in the dev profile.

Example: Secure Swagger with Spring Security

@Override
protectedvoidconfigure(HttpSecurity http)throws Exception {
    http
        .authorizeRequests()
        .requestMatchers("/swagger-ui.html","/v3/api-docs/**").hasRole("ADMIN")
        .anyRequest().authenticated()
        .and()
        .formLogin();
}

Only users with ADMIN role can access Swagger in production.

Best Practices for API Documentation

1. Use clear summaries and descriptions
2. Document all request parameters
3. Define response codes clearly
4. Add request and response examples
5. Keep documentation updated
6. Use consistent naming
7. Disable Swagger in production if required

API Documentation vs Code Comments

Benefits of Swagger API Documentation

• Auto-generated and always updated
• Interactive testing in browser
• Improves developer productivity
• Reduces bugs and misunderstandings
• Makes APIs easy to explore

Conclusion

• API Documentation explains how to use an API
• Good documentation saves time and reduces errors
• Swagger / OpenAPI is the best choice for Spring Boot
• Auto-generated docs are always accurate
• Clear documentation = Happy developers

If you are building REST APIs in Spring Boot, Swagger should be your default choice for API documentation.