SpringDoc OpenAPI: ControllerAdvice Response Injection Issues

Introduction: The Challenge of Consistent OpenAPI Specs

Generating a consistent and reliable OpenAPI specification is crucial for any API project. It serves as the single source of truth for your API, guiding developers and facilitating integrations. However, maintaining this consistency can be tricky, especially when dealing with features like @ControllerAdvice in Spring Boot, which is designed to handle exceptions globally. When exception handling logic is centrally managed using @ControllerAdvice, you expect these global error responses to be reflected in your OpenAPI specifications, ensuring that all consumers of your API are aware of all possible responses. This article delves into the intricacies of how SpringDoc injects responses declared in a @ControllerAdvice into generated OpenAPI specs and explores the observed inconsistencies, especially when migrating between OpenAPI versions. The goal is to provide clarity and guidance to produce stable, predictable OpenAPI output across different SpringDoc versions.

Understanding the Core Problem: Inconsistent Response Injection

The central issue revolves around the inconsistent behavior of SpringDoc when injecting responses defined in @ControllerAdvice into the generated OpenAPI specifications. The core problem lies in the varying treatment of different exception types and their corresponding HTTP status codes. This inconsistency leads to unpredictable API documentation, where some error responses are globally included, while others are only present if explicitly declared in the operation.

Specifically, the observed behavior highlights a divergence between how SpringDoc handles different types of exceptions. When using OpenAPI 3.0.1, the specifications included responses from the @ControllerAdvice for all operations, including the 405 Method Not Allowed response. However, when migrating to OpenAPI 3.1.0, the 405 response is excluded from operations unless the operation explicitly declares the corresponding exception. In contrast, other responses, such as 400, 422, and 500, continue to be globally included, even if the corresponding exceptions are not explicitly declared. This creates a confusing and inconsistent rule for how exception responses are incorporated into the OpenAPI documentation. This inconsistency makes it difficult to predict the generated documentation and can lead to misunderstandings among API consumers.

Exploring the Code and Configuration: Key Elements

To understand the root cause of these inconsistencies, it's essential to examine the key elements involved. The core of the problem lies in the interaction between your @ControllerAdvice exception handlers, the springdoc.override-with-generic-response=true property, and the SpringDoc library itself. Let's break down these elements:

• @ControllerAdvice: This annotation is the cornerstone of global exception handling in Spring Boot. It allows you to centralize error handling logic, ensuring that common exceptions are handled consistently across your application. You define exception handler methods within your @ControllerAdvice classes, each annotated with @ExceptionHandler to handle specific exception types. These methods typically return a custom response body, including information about the error.
• @ExceptionHandler: This annotation is used within your @ControllerAdvice classes to specify which exceptions a particular method handles. It's crucial for mapping exceptions to the appropriate error responses. For instance, the example code provided handles ConstraintViolationException and HttpRequestMethodNotSupportedException and maps them to BAD_REQUEST (400) and METHOD_NOT_ALLOWED (405) status codes, respectively.
• @ResponseStatus: This annotation is used to set the HTTP status code for an exception handler method. It ensures that the correct status code is returned to the client when an exception is handled. For instance, using @ResponseStatus(HttpStatus.BAD_REQUEST) with an @ExceptionHandler for ConstraintViolationException will return a 400 status code.
• springdoc.override-with-generic-response=true: This SpringDoc property is vital to the issue. This setting is intended to override existing responses in your API documentation with generic responses, ensuring that the responses defined in your @ControllerAdvice are reflected in the generated OpenAPI specification. By setting this property to true, you're effectively telling SpringDoc to prioritize the responses defined in your @ControllerAdvice, providing a more comprehensive view of your API's error handling capabilities. However, the inconsistent behavior arises when SpringDoc interprets these responses differently based on the exception type and OpenAPI version.
• SpringDoc Library: The SpringDoc library is responsible for generating the OpenAPI specification from your Spring Boot application. It analyzes your code, including your controllers, request mappings, and @ControllerAdvice annotations, to create the specification. The behavior of SpringDoc, specifically how it handles @ControllerAdvice responses, is the source of the observed inconsistencies. The different versions of SpringDoc (e.g., 2.1.0 and 2.8.3) and the underlying OpenAPI specifications (3.0.1 and 3.1.0) contribute to the varying output. The changes made in the library, especially in how it parses and interprets the @ControllerAdvice configurations, lead to the divergent behavior.

Diving into the Specific Behaviors: OAS 3.0.1 vs. OAS 3.1.0

The most significant aspect of the problem is the difference in behavior between OpenAPI 3.0.1 and OpenAPI 3.1.0. Understanding these differences is vital to finding a solution or workaround.

• OAS 3.0.1: With OAS 3.0.1, the SpringDoc library injected responses from the @ControllerAdvice into all operations. This means that all operations in your API documentation included the responses defined in your exception handlers, such as the 405 Method Not Allowed response. This behavior provided a comprehensive and consistent view of the API's error handling capabilities. It ensured that all potential error responses were explicitly documented, allowing consumers to anticipate and handle them.
• OAS 3.1.0: Under OAS 3.1.0, the behavior changes. The 405 response is excluded from operations unless explicitly declared in the operation. This means that the 405 response is only included in the API documentation if the corresponding exception is explicitly declared or thrown by the operation handler. However, other responses, such as 400, 422, and 500, continue to appear globally. This discrepancy creates the inconsistency that the article focuses on. This inconsistent behavior can cause confusion among developers consuming the API. If the documentation shows that 400, 422, and 500 responses are possible, but 405 is not, it might lead them to believe that the API does not handle 405 errors, which is incorrect.

Analyzing the Root Causes: Possible Explanations for the Inconsistencies

Several factors might contribute to the observed inconsistencies, but pinpointing the exact cause requires delving into the SpringDoc library's source code and understanding its internal logic.

• Exception Type Classification: SpringDoc might internally classify exception types differently. Some exception types, such as HttpRequestMethodNotSupportedException (405), might be treated as