When I added a GetMapping for /v3/api-docs, it caused an exception.

[ok1996]

Describe the bug

When there are two possible URLs, such as /v3/api-docs and /${spring.application.name}/v3/api-docs,it might lead to an exception during the requestUrl.substring operation if the requestUrl used during access doesn't include ${spring.application.name}.
In such cases, the length of the prefix would exceed the length of the requestUrl, causing an issue.

To Reproduce
Steps to reproduce the behavior:

I added the following code for forwarding in the service layer:

@Controller
public class AggregatedDocGatewayForward {

    @GetMapping("/${spring.application.name}/v3/api-docs/{param}")
    public String forwardV3ApiDocsParam(@PathVariable String param) {
        return "forward:/v3/api-docs/" + param;
    }

    @GetMapping(value = {"/${spring.application.name}/v3/api-docs"
            , "/${spring.application.name}/v3/api-docs/default"})
    public String forwardV3ApiDocs() {
        return "forward:/v3/api-docs";
    }

}

The following code may occasionally result in an exception:

org.springdoc.webmvc.api.OpenApiWebMvcResource:
  
	@Override
	protected String getServerUrl(HttpServletRequest request, String apiDocsUrl) {
		String requestUrl = decode(request.getRequestURL().toString());
		Optional<SpringWebProvider> springWebProviderOptional = springDocProviders.getSpringWebProvider();
		String prefix = StringUtils.EMPTY;
		if (springWebProviderOptional.isPresent())
			prefix = springWebProviderOptional.get().findPathPrefix(springDocConfigProperties);
		return requestUrl.substring(0, requestUrl.length() - apiDocsUrl.length() - prefix.length());
	}

Screenshots

[ok1996]

I avoided this issue by trying a different approach.

@Configuration
public class CustomUrlMappingConfiguration {

    @Value("${spring.application.name}")
    private String applicationName;

    @Bean
    public SimpleUrlHandlerMapping customUrlHandlerMapping() {
        SimpleUrlHandlerMapping mapping = new SimpleUrlHandlerMapping();
        mapping.setOrder(1);
        Map<String, Object> urlMap = new HashMap<>(4);
        urlMap.put("/" + applicationName + "/v3/api-docs/{param}", parameterizableViewController("v3/api-docs/{param}"));
        urlMap.put("/" + applicationName + "/v3/api-docs", parameterizableViewController("v3/api-docs"));
        urlMap.put("/" + applicationName + "/v3/api-docs/default", parameterizableViewController("v3/api-docs"));
        mapping.setUrlMap(urlMap);
        return mapping;
    }

    private ParameterizableViewController parameterizableViewController(String viewName) {
        ParameterizableViewController controller = new ParameterizableViewController();
        controller.setViewName("forward:/" + viewName);
        return controller;
    }
}

[uc4w6c]

I didn't feel the need to modify springdoc as it was a customized feature issue and there was a workaround.
Also, I think redirect is better than forward.