When building an API, it is essential to handle errors and exceptions properly. Not only does this help improve the user experience, but it also provides meaningful information to the API consumers. In this blog post, we will explore how to document API error handling and exception mapping using Java Spring REST Docs.

Why document API errors and exceptions?

Documenting API errors and exceptions is crucial because it helps API consumers understand how to handle and interpret error responses. By providing clear and consistent error messages, developers can diagnose and resolve issues more efficiently.

Additionally, documenting errors and exceptions helps maintain an updated and well-documented API. With proper documentation, developers can easily refer to the error structure and its mapping to understand the possible error scenarios or exceptions that can occur during API interaction.

Using Java Spring REST Docs

Java Spring REST Docs is a powerful tool that allows developers to generate documentation for their APIs. It integrates seamlessly with Spring Framework and provides a simple way to document HTTP endpoints, request, and response payloads.

Handling errors and exceptions

To document API errors and exceptions, we need to define a standardized structure for error responses. This structure should include relevant information such as error code, error message, and a detailed description of the error.

In Spring MVC, we can use the @ExceptionHandler annotation to handle exceptions globally or at the controller level. We can then map these exceptions to appropriate error response payloads using a custom exception handler.

Here’s an example of an exception handler that maps a NotFoundException to a 404 Not Found error response:

@ControllerAdvice
public class GlobalExceptionHandler {

    @ExceptionHandler(NotFoundException.class)
    @ResponseStatus(HttpStatus.NOT_FOUND)
    @ResponseBody
    public ErrorResponse handleNotFoundException(NotFoundException ex) {
        return new ErrorResponse(ex.getErrorCode(), ex.getMessage());
    }
}

Documenting error responses with Spring REST Docs

Once we have properly handled the errors and exceptions, we can use Spring REST Docs to document the error responses.

In the test class, we can use the andDo method from the MockMvcResultHandlers class to perform additional operations after the request has been executed. We can then use the andDo method to generate a documentation snippet for the error response using Spring REST Docs.

Here’s an example of how to document an error response using Spring REST Docs:

@Test
public void shouldReturnNotFoundWhenResourceIsNotFound() throws Exception {
    mockMvc.perform(get("/api/users/{id}", 123))
            .andExpect(status().isNotFound())
            .andDo(document("user-not-found",
                    responseFields(
                            fieldWithPath("errorCode").description("The error code"),
                            fieldWithPath("message").description("The error message")
                    )
            ));
}

In the above example, we perform a GET request to the /api/users/{id} endpoint, expecting a 404 Not Found error response. We then use the andDo method to generate a documentation snippet that describes the fields of the error response.

Generating API documentation

To generate the API documentation, we can execute the tests and run the Spring REST Docs build plugin. The plugin will process the documentation snippets and generate API documentation in various formats, such as HTML or Markdown.

By including the generated documentation in our API project, we can provide an up-to-date and easily accessible resource for API consumers to understand the error handling and exception mapping.

Conclusion

Properly documenting API errors and exceptions is essential for creating robust and user-friendly APIs. With Java Spring REST Docs, we can easily document error handling and exception mapping, providing API consumers with clear and consistent error responses.

By adopting a standardized structure for error responses and leveraging Spring REST Docs, we can generate comprehensive API documentation that helps developers understand and handle errors effectively.

#API #documentation