@RestControllerAdvice Causes /v3/api-docs 500 Error In SpringDoc OpenAPI UI - How To Fix

Introduction

In the realm of Spring Boot applications, SpringDoc OpenAPI is a powerful tool for generating API documentation. However, developers sometimes encounter issues when integrating @RestControllerAdvice with SpringDoc, particularly when dealing with global exception handling. This article delves into a specific bug where using @RestControllerAdvice can lead to a 500 Internal Server Error when accessing the /v3/api-docs endpoint, effectively breaking the Swagger UI. We'll explore the root cause, steps to reproduce the issue, expected behavior, and a practical workaround. Additionally, we'll discuss potential improvements to SpringDoc to prevent this problem in the future.

Understanding the Bug: @RestControllerAdvice and SpringDoc OpenAPI

The crux of the issue lies in how SpringDoc OpenAPI processes classes annotated with @RestControllerAdvice. These classes are designed to handle exceptions globally across your application. While incredibly useful for centralized error handling, they can inadvertently expose exception handling methods as API endpoints in Swagger UI. When SpringDoc attempts to document these methods, which are not intended to be part of the public API, it can lead to errors, specifically a 500 Internal Server Error when trying to load the API definition via /v3/api-docs. This error manifests as a failure in the Swagger UI to display your API documentation, hindering development and testing efforts.

The Role of @RestControllerAdvice

To fully grasp the problem, it's essential to understand the role of @RestControllerAdvice. In Spring Boot applications, this annotation allows you to centralize exception handling logic. By creating a class annotated with @RestControllerAdvice, you can define methods to handle specific exceptions or general exception types using the @ExceptionHandler annotation. These handler methods can then return appropriate responses, such as error messages or custom error codes, to the client. This approach promotes cleaner code and reduces redundancy by avoiding the need to implement exception handling logic in every controller.

SpringDoc OpenAPI and API Documentation

SpringDoc OpenAPI is a library that automates the generation of OpenAPI (formerly Swagger) documentation for Spring Boot applications. It analyzes your controllers, their methods, and the data structures they use to create a comprehensive API specification. This specification can then be used by tools like Swagger UI to provide an interactive interface for exploring and testing your API. SpringDoc relies on annotations and reflection to gather information about your API endpoints. However, when it encounters methods within @RestControllerAdvice classes, it may incorrectly interpret them as API endpoints, leading to the aforementioned errors.

Reproducing the 500 Error: A Step-by-Step Guide

To demonstrate the bug, follow these steps to reproduce the 500 Internal Server Error in a Spring Boot application using SpringDoc OpenAPI:

1. Set Up a Spring Boot Project: Start by creating a new Spring Boot project using Spring Initializr or your preferred IDE. Ensure you include the necessary dependencies, namely Spring Web and org.springdoc:springdoc-openapi-starter-webmvc-ui:2.3.0. It is preferable to use Spring Boot version 3.2.x to replicate this error consistently. This setup provides the basic framework for a web application with SpringDoc OpenAPI integrated for API documentation.
2. Create a Global Exception Handler: Implement a global exception handler using @RestControllerAdvice. This handler will define how your application responds to exceptions. For example, create a class named GlobalExceptionHandler with the following code:

@RestControllerAdvice
public class GlobalExceptionHandler {

    @ExceptionHandler(Exception.class)
    public ResponseEntity<String> handleAll(Exception ex) {
        return ResponseEntity.status(500).body("Error occurred");
    }
}

This code snippet defines a class annotated with @RestControllerAdvice. The handleAll method is annotated with @ExceptionHandler(Exception.class), indicating that it should handle all exceptions. It returns a ResponseEntity with a 500 status code and a simple error message.

3. Access Swagger UI: Run your Spring Boot application and navigate to /swagger-ui/index.html in your web browser. This is the standard URL for accessing the Swagger UI, which should display your API documentation.

4. Observe the Error: Upon accessing Swagger UI, you should encounter an error message indicating that the API definition failed to load. The browser's developer console will likely show a 500 Internal Server Error for the /v3/api-docs endpoint. This error confirms that SpringDoc is unable to generate the API documentation due to the presence of the @RestControllerAdvice class.

5. Detailed Error Message: The error message displayed in the Swagger UI will typically state: "Failed to load API definition. Fetch error: Internal Server Error /v3/api-docs". This message provides a clear indication of the problem: SpringDoc is encountering an issue while attempting to retrieve the API definition from the /v3/api-docs endpoint.

Expected Behavior vs. Actual Behavior

The expected behavior is that SpringDoc OpenAPI should either ignore methods within @RestControllerAdvice classes by default or provide a clear mechanism to exclude them from API documentation. Ideally, placing the @Hidden annotation at the class level of the @RestControllerAdvice class should prevent all its methods from being included in the generated OpenAPI specification. This would align with the typical use case of @RestControllerAdvice, which focuses on exception handling rather than defining public API endpoints.

However, the actual behavior deviates from this expectation. SpringDoc incorrectly attempts to process the exception handling methods within @RestControllerAdvice as if they were API endpoints. This leads to errors during API documentation generation, resulting in the 500 Internal Server Error. Moreover, placing @Hidden at the class level does not resolve the issue. Instead, developers are forced to manually add @Hidden to each individual method within the @RestControllerAdvice class, which is a tedious and error-prone workaround.

The Workaround: Applying @Hidden to Each Method

As mentioned earlier, the current workaround involves adding the @Hidden annotation from the io.swagger.v3.oas.annotations package to each method within the @RestControllerAdvice class. This annotation explicitly tells SpringDoc to exclude the annotated method from the generated OpenAPI specification.

For example, to fix the issue in the GlobalExceptionHandler class, you would modify the code as follows:

import io.swagger.v3.oas.annotations.Hidden;
import org.springframework.http.ResponseEntity;
import org.springframework.web.bind.annotation.ExceptionHandler;
import org.springframework.web.bind.annotation.RestControllerAdvice;

@RestControllerAdvice
public class GlobalExceptionHandler {

    @Hidden
    @ExceptionHandler(Exception.class)
    public ResponseEntity<String> handleAll(Exception ex) {
        return ResponseEntity.status(500).body("Error occurred");
    }
}

By adding @Hidden to the handleAll method, you instruct SpringDoc to ignore this method during API documentation generation. This prevents the 500 error and allows Swagger UI to load the API definition correctly. However, remember that this workaround needs to be applied to every method within your @RestControllerAdvice classes to fully resolve the issue.

Potential Solutions and Improvements

To address this issue more effectively, several improvements could be made to SpringDoc OpenAPI:

1. Default Exclusion of @RestControllerAdvice Methods: SpringDoc could be modified to ignore methods within @RestControllerAdvice classes by default. This would align with the intended purpose of these classes, which are primarily for exception handling rather than defining public API endpoints. This change would prevent the need for manual intervention in most cases.
2. Class-Level @Hidden Annotation: SpringDoc should honor the @Hidden annotation at the class level for @RestControllerAdvice classes. If a class is annotated with @Hidden, all its methods should be excluded from API documentation. This would provide a more convenient way to exclude entire exception handler classes without having to annotate each method individually.
3. Clearer Error Messages: When SpringDoc encounters an issue while processing @RestControllerAdvice classes, it should provide a more informative error message. The current 500 Internal Server Error is not specific enough and doesn't guide developers towards the root cause. A message indicating that methods within @RestControllerAdvice classes may be causing the problem would greatly assist in troubleshooting.
4. Documentation Updates: The SpringDoc documentation should explicitly mention this issue and the workaround. This would help developers avoid the problem and quickly resolve it if they encounter it. The documentation could also include best practices for using @RestControllerAdvice with SpringDoc.

Conclusion

The 500 Internal Server Error caused by @RestControllerAdvice in SpringDoc OpenAPI is a recurring issue that can disrupt API documentation generation. While the workaround of adding @Hidden to each method provides a temporary solution, it's not ideal. By understanding the root cause of the bug and implementing the suggested improvements, SpringDoc can better handle @RestControllerAdvice classes and provide a smoother experience for developers. Ultimately, these changes will contribute to more robust and accurate API documentation, facilitating better collaboration and API consumption.

This article has explored the intricacies of this bug, provided a step-by-step guide to reproduce it, outlined the expected and actual behaviors, and offered a practical workaround. Furthermore, it has highlighted potential solutions and improvements that could enhance SpringDoc's handling of @RestControllerAdvice classes. By addressing this issue, SpringDoc can further solidify its position as a leading tool for API documentation in the Spring Boot ecosystem.