How to load OpenAPI YAML specification from external JAR in Spring Boot 3

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Summary

To load an OpenAPI YAML specification from an external JAR in Spring Boot 3, you need to configure Springdoc OpenAPI to use the external specification instead of generating it automatically from the code. Key takeaways include understanding how to override the default behavior of Springdoc OpenAPI and correctly referencing the external OpenAPI YAML file.

Root Cause

The root cause of the issue is that Springdoc OpenAPI automatically generates the OpenAPI specification from the code by default. To use an external specification, you need to override this default behavior. The reasons for this include:

  • The external JAR library contains the desired OpenAPI YAML specification.
  • Springdoc OpenAPI does not automatically detect the external specification.
  • Configuration is required to point Springdoc OpenAPI to the external specification.

Why This Happens in Real Systems

This issue occurs in real systems because:

  • Modularity and reuse are important in software design, leading to the use of external libraries.
  • Customization and flexibility are required in many applications, making it necessary to override default behaviors.
  • Complexity in modern software systems means that multiple configurations and overrides are often necessary.

Real-World Impact

The impact of not being able to load the OpenAPI YAML specification from an external JAR includes:

  • Inaccurate or incomplete documentation because the automatically generated specification may not reflect the desired API documentation.
  • Increased maintenance costs due to the need to manually synchronize the external specification with the generated one.
  • Reduced usability of the API due to potential discrepancies between the intended API documentation and what is actually generated.

Example or Code

To configure Springdoc OpenAPI to use the external OpenAPI YAML specification, you can use the following approach:

import org.springdoc.core.GroupedOpenApi
import org.springdoc.core.SpringDocConfiguration
import org.springframework.context.annotation.Configuration

@Configuration
class OpenApiConfig {
    @Bean
    fun groupedOpenApi(): GroupedOpenApi {
        return GroupedOpenApi.builder()
           .group("service-integration")
           .pathsToMatch("/rest-api/**")
           .build()
    }

    @Bean
    fun springDocConfig(): SpringDocConfiguration {
        return SpringDocConfiguration.builder()
           .openApiunksFromExternalResource("classpath:/openapi.yml")
           .build()
    }
}

How Senior Engineers Fix It

Senior engineers fix this issue by:

  • Understanding the default behavior of Springdoc OpenAPI and how it can be overridden.
  • Configuring Springdoc OpenAPI to use the external OpenAPI YAML specification.
  • Testing the configuration to ensure that the correct specification is being used.

Why Juniors Miss It

Junior engineers may miss this solution because:

  • Lack of experience with configuring Springdoc OpenAPI and overriding its default behavior.
  • Insufficient understanding of how to reference external resources in a Spring Boot application.
  • Overlooking the need to configure Springdoc OpenAPI to use an external specification.