ClassCastException in case of parametrized types inside ResponseResult when I use @RestControllerAdvice and implement ResponseBodyAdvice.

[breezefaith]

Description

In my project, I use @RestControllerAdvice annotation and implement ResponseBodyAdvice interface in order to obtain a unified structural response data.

I cannot use Spring Doc in my project since it fails with a ClassCastException.

But when I delete my ResponseBodyAdvice, Spring Doc works properly.

Environments

• Java 21
• SpringBoot 3.1.2
• springdoc-openapi-starter-webmvc-ui 2.2.0

Source Code

1. GlobalResponseAdvice

@RestControllerAdvice
public class GlobalResponseAdvice implements ResponseBodyAdvice<Object> {
    @Override
    public boolean supports(MethodParameter returnType, Class<? extends HttpMessageConverter<?>> converterType) {
        return !returnType.hasMethodAnnotation(IgnoreRestControllerResponseAdvice.class)
                && !returnType.getParameterType().isAssignableFrom(ResponseResult.class);
    }

    @Override
    public Object beforeBodyWrite(Object body, MethodParameter returnType, MediaType selectedContentType, Class<? extends HttpMessageConverter<?>> selectedConverterType, ServerHttpRequest request, ServerHttpResponse response) {
        if (returnType.getParameterType().isAssignableFrom(void.class)) {
            return ResponseResult.success();
        }
        if (!(body instanceof ResponseResult)) {
            if (body instanceof String) {
                try {
                    return JsonUtil.toJSON(ResponseResult.success(body));
                } catch (JsonProcessingException e) {
                    throw new RuntimeException(e);
                }
            }
            return ResponseResult.success(body);
        }
        return body;
    }
}

2. ApiResourceController

@RestController
@RequestMapping("/api-resource")
@Tag(name = "Api资源管理")
public class ApiResourceController {
    @Resource
    private ApiResourceService apiResourceService;

    @PostMapping("/page")
    public Page<ApiResource> page(@RequestBody @Nullable ApiResource searchVo,
                                  @RequestParam(name = "no", defaultValue = "0") int no,
                                  @RequestParam(name = "size", defaultValue = "10") int size
    ) {
        return apiResourceService.search(searchVo, PageRequest.of(no, size));
    }

    @GetMapping("/retrieve/{id}")
    public ApiResource retrieve(@PathVariable("id") Long id) {
        return apiResourceService.retrieve(id);
    }

    @PostMapping("/create")
    public Long create(@RequestBody ApiResource role) {
        return apiResourceService.create(role).getId();
    }

    @PutMapping("/update")
    public void update(@RequestBody ApiResource role) {
        apiResourceService.update(role);
    }

    @DeleteMapping("/remove")
    public void remove(@RequestParam("ids") List<Long> ids) {
        apiResourceService.remove(ids);
    }
}

Exception

1. The exception When I use the default configuration

pom.xml:

<?xml version="1.0" encoding="UTF-8"?>
<project xmlns="http://maven.apache.org/POM/4.0.0" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
         xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 https://maven.apache.org/xsd/maven-4.0.0.xsd">
    <modelVersion>4.0.0</modelVersion>
    ……
    <properties>
        <java.version>21</java.version>
        <project.build.sourceEncoding>UTF-8</project.build.sourceEncoding>
        <project.reporting.outputEncoding>UTF-8</project.reporting.outputEncoding>
        <spring-boot.version>3.1.2</spring-boot.version>
    </properties>
    <dependencies>
        ……
        <dependency>
            <groupId>org.springframework.boot</groupId>
            <artifactId>spring-boot-devtools</artifactId>
            <scope>runtime</scope>
            <optional>true</optional>
        </dependency>
        <dependency>
            <groupId>org.springdoc</groupId>
            <artifactId>springdoc-openapi-starter-webmvc-ui</artifactId>
            <version>2.2.0</version>
        </dependency>
        ……
    </dependencies>
    <dependencyManagement>
        <dependencies>
            <dependency>
                <groupId>org.springframework.boot</groupId>
                <artifactId>spring-boot-dependencies</artifactId>
                <version>${spring-boot.version}</version>
                <type>pom</type>
                <scope>import</scope>
            </dependency>
        </dependencies>
    </dependencyManagement>
    ……
</project>

url:

http://localhost:8080/v3/api-docs

response:

{
   "code": 500,
   "message": "class com.fast.alden.entity.dto.ResponseResult cannot be cast to class [B (com.fast.alden.entity.dto.ResponseResult is in unnamed module of loader org.springframework.boot.devtools.restart.classloader.RestartClassLoader @b6971fd; [B is in module java.base of loader 'bootstrap')",
   "data": null
}

details:

2023-11-07T17:03:49.909+08:00 TRACE 28448 --- [nio-8080-exec-1] o.s.web.method.HandlerMethod             : Arguments: [java.lang.ClassCastException: class com.fast.alden.entity.dto.ResponseResult cannot be cast to class [B (com.fast.alden.entity.dto.ResponseResult is in unnamed module of loader org.springframework.boot.devtools.restart.classloader.RestartClassLoader @b6971fd; [B is in module java.base of loader 'bootstrap'), SecurityContextHolderAwareRequestWrapper[ org.springframework.security.web.header.HeaderWriterFilter$HeaderWriterRequest@10a176bf], org.springframework.security.web.header.HeaderWriterFilter$HeaderWriterResponse@5f0173f6]
2023-11-07T17:03:49.909+08:00 ERROR 28448 --- [nio-8080-exec-1] c.f.a.a.GlobalExceptionHandlerAdvice     : class com.fast.alden.entity.dto.ResponseResult cannot be cast to class [B (com.fast.alden.entity.dto.ResponseResult is in unnamed module of loader org.springframework.boot.devtools.restart.classloader.RestartClassLoader @b6971fd; [B is in module java.base of loader 'bootstrap')

java.lang.ClassCastException: class com.fast.alden.entity.dto.ResponseResult cannot be cast to class [B (com.fast.alden.entity.dto.ResponseResult is in unnamed module of loader org.springframework.boot.devtools.restart.classloader.RestartClassLoader @b6971fd; [B is in module java.base of loader 'bootstrap')
	at org.springframework.http.converter.ByteArrayHttpMessageConverter.getContentLength(ByteArrayHttpMessageConverter.java:38) ~[spring-web-6.0.11.jar:6.0.11]
	at org.springframework.http.converter.AbstractHttpMessageConverter.addDefaultHeaders(AbstractHttpMessageConverter.java:258) ~[spring-web-6.0.11.jar:6.0.11]
	at org.springframework.http.converter.AbstractHttpMessageConverter.write(AbstractHttpMessageConverter.java:210) ~[spring-web-6.0.11.jar:6.0.11]
	at org.springframework.web.servlet.mvc.method.annotation.AbstractMessageConverterMethodProcessor.writeWithMessageConverters(AbstractMessageConverterMethodProcessor.java:300) ~[spring-webmvc-6.0.11.jar:6.0.11]
	at org.springframework.web.servlet.mvc.method.annotation.RequestResponseBodyMethodProcessor.handleReturnValue(RequestResponseBodyMethodProcessor.java:194) ~[spring-webmvc-6.0.11.jar:6.0.11]
	at org.springframework.web.method.support.HandlerMethodReturnValueHandlerComposite.handleReturnValue(HandlerMethodReturnValueHandlerComposite.java:78) ~[spring-web-6.0.11.jar:6.0.11]
	at org.springframework.web.servlet.mvc.method.annotation.ServletInvocableHandlerMethod.invokeAndHandle(ServletInvocableHandlerMethod.java:136) ~[spring-webmvc-6.0.11.jar:6.0.11]
	at org.springframework.web.servlet.mvc.method.annotation.RequestMappingHandlerAdapter.invokeHandlerMethod(RequestMappingHandlerAdapter.java:884) ~[spring-webmvc-6.0.11.jar:6.0.11]
	at org.springframework.web.servlet.mvc.method.annotation.RequestMappingHandlerAdapter.handleInternal(RequestMappingHandlerAdapter.java:797) ~[spring-webmvc-6.0.11.jar:6.0.11]
	at org.springframework.web.servlet.mvc.method.AbstractHandlerMethodAdapter.handle(AbstractHandlerMethodAdapter.java:87) ~[spring-webmvc-6.0.11.jar:6.0.11]
	at org.springframework.web.servlet.DispatcherServlet.doDispatch(DispatcherServlet.java:1081) ~[spring-webmvc-6.0.11.jar:6.0.11]
	at org.springframework.web.servlet.DispatcherServlet.doService(DispatcherServlet.java:974) ~[spring-webmvc-6.0.11.jar:6.0.11]
	at org.springframework.web.servlet.FrameworkServlet.processRequest(FrameworkServlet.java:1011) ~[spring-webmvc-6.0.11.jar:6.0.11]
	at org.springframework.web.servlet.FrameworkServlet.doGet(FrameworkServlet.java:903) ~[spring-webmvc-6.0.11.jar:6.0.11]
	at jakarta.servlet.http.HttpServlet.service(HttpServlet.java:564) ~[tomcat-embed-core-10.1.11.jar:6.0]
	at org.springframework.web.servlet.FrameworkServlet.service(FrameworkServlet.java:885) ~[spring-webmvc-6.0.11.jar:6.0.11]
	at jakarta.servlet.http.HttpServlet.service(HttpServlet.java:658) ~[tomcat-embed-core-10.1.11.jar:6.0]
	at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:205) ~[tomcat-embed-core-10.1.11.jar:10.1.11]
	at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:149) ~[tomcat-embed-core-10.1.11.jar:10.1.11]
	at org.apache.tomcat.websocket.server.WsFilter.doFilter(WsFilter.java:51) ~[tomcat-embed-websocket-10.1.11.jar:10.1.11]
	at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:174) ~[tomcat-embed-core-10.1.11.jar:10.1.11]
	at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:149) ~[tomcat-embed-core-10.1.11.jar:10.1.11]
	at org.springframework.web.filter.OncePerRequestFilter.doFilter(OncePerRequestFilter.java:110) ~[spring-web-6.0.11.jar:6.0.11]
	at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:174) ~[tomcat-embed-core-10.1.11.jar:10.1.11]
	at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:149) ~[tomcat-embed-core-10.1.11.jar:10.1.11]
	at org.springframework.security.web.FilterChainProxy.lambda$doFilterInternal$3(FilterChainProxy.java:231) ~[spring-security-web-6.1.2.jar:6.1.2]
	at org.springframework.security.web.FilterChainProxy$VirtualFilterChain.doFilter(FilterChainProxy.java:365) ~[spring-security-web-6.1.2.jar:6.1.2]
	at org.springframework.security.web.access.intercept.AuthorizationFilter.doFilter(AuthorizationFilter.java:100) ~[spring-security-web-6.1.2.jar:6.1.2]
	at org.springframework.security.web.FilterChainProxy$VirtualFilterChain.doFilter(FilterChainProxy.java:374) ~[spring-security-web-6.1.2.jar:6.1.2]
	at org.springframework.security.web.access.ExceptionTranslationFilter.doFilter(ExceptionTranslationFilter.java:126) ~[spring-security-web-6.1.2.jar:6.1.2]
	at org.springframework.security.web.access.ExceptionTranslationFilter.doFilter(ExceptionTranslationFilter.java:120) ~[spring-security-web-6.1.2.jar:6.1.2]
	at org.springframework.security.web.FilterChainProxy$VirtualFilterChain.doFilter(FilterChainProxy.java:374) ~[spring-security-web-6.1.2.jar:6.1.2]
	at org.springframework.security.web.session.SessionManagementFilter.doFilter(SessionManagementFilter.java:131) ~[spring-security-web-6.1.2.jar:6.1.2]
	at org.springframework.security.web.session.SessionManagementFilter.doFilter(SessionManagementFilter.java:85) ~[spring-security-web-6.1.2.jar:6.1.2]
	at org.springframework.security.web.FilterChainProxy$VirtualFilterChain.doFilter(FilterChainProxy.java:374) ~[spring-security-web-6.1.2.jar:6.1.2]
	at org.springframework.security.web.authentication.AnonymousAuthenticationFilter.doFilter(AnonymousAuthenticationFilter.java:100) ~[spring-security-web-6.1.2.jar:6.1.2]
	at org.springframework.security.web.FilterChainProxy$VirtualFilterChain.doFilter(FilterChainProxy.java:374) ~[spring-security-web-6.1.2.jar:6.1.2]
	at org.springframework.security.web.servletapi.SecurityContextHolderAwareRequestFilter.doFilter(SecurityContextHolderAwareRequestFilter.java:179) ~[spring-security-web-6.1.2.jar:6.1.2]
	at org.springframework.security.web.FilterChainProxy$VirtualFilterChain.doFilter(FilterChainProxy.java:374) ~[spring-security-web-6.1.2.jar:6.1.2]
	at org.springframework.security.web.savedrequest.RequestCacheAwareFilter.doFilter(RequestCacheAwareFilter.java:63) ~[spring-security-web-6.1.2.jar:6.1.2]
	at org.springframework.security.web.FilterChainProxy$VirtualFilterChain.doFilter(FilterChainProxy.java:374) ~[spring-security-web-6.1.2.jar:6.1.2]
	……

2. Exception When I disable spring-boot-devtools in pom.xml

pom.xml:

<?xml version="1.0" encoding="UTF-8"?>
<project xmlns="http://maven.apache.org/POM/4.0.0" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
         xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 https://maven.apache.org/xsd/maven-4.0.0.xsd">
    ……
    <properties>
        <java.version>21</java.version>
        <project.build.sourceEncoding>UTF-8</project.build.sourceEncoding>
        <project.reporting.outputEncoding>UTF-8</project.reporting.outputEncoding>
        <spring-boot.version>3.1.2</spring-boot.version>
    </properties>
    <dependencies>
        ……
<!--        <dependency>-->
<!--            <groupId>org.springframework.boot</groupId>-->
<!--            <artifactId>spring-boot-devtools</artifactId>-->
<!--            <scope>runtime</scope>-->
<!--            <optional>true</optional>-->
<!--        </dependency>-->
        <dependency>
            <groupId>org.springdoc</groupId>
            <artifactId>springdoc-openapi-starter-webmvc-ui</artifactId>
            <version>2.2.0</version>
            <exclusions>
                <exclusion>
                    <groupId>org.springframework.boot</groupId>
                    <artifactId>spring-boot-devtools</artifactId>
                </exclusion>
            </exclusions>
        </dependency>
        ……
    </dependencies>
    ……
</project>

url:

http://localhost:8080/v3/api-docs

response:

{
  "code": 500,
  "message": "class com.fast.alden.entity.dto.ResponseResult cannot be cast to class [B (com.fast.alden.entity.dto.ResponseResult is in unnamed module of loader 'app'; [B is in module java.base of loader 'bootstrap')",
  "data": null
}

details:

2023-11-07T17:26:15.975+08:00 TRACE 3684 --- [nio-8080-exec-1] o.s.web.method.HandlerMethod             : Arguments: [java.lang.ClassCastException: class com.fast.alden.entity.dto.ResponseResult cannot be cast to class [B (com.fast.alden.entity.dto.ResponseResult is in unnamed module of loader 'app'; [B is in module java.base of loader 'bootstrap'), SecurityContextHolderAwareRequestWrapper[ org.springframework.security.web.header.HeaderWriterFilter$HeaderWriterRequest@54afe8b9], org.springframework.security.web.header.HeaderWriterFilter$HeaderWriterResponse@57bbfe04]
2023-11-07T17:26:15.976+08:00 ERROR 3684 --- [nio-8080-exec-1] c.f.a.a.GlobalExceptionHandlerAdvice     : class com.fast.alden.entity.dto.ResponseResult cannot be cast to class [B (com.fast.alden.entity.dto.ResponseResult is in unnamed module of loader 'app'; [B is in module java.base of loader 'bootstrap')

java.lang.ClassCastException: class com.fast.alden.entity.dto.ResponseResult cannot be cast to class [B (com.fast.alden.entity.dto.ResponseResult is in unnamed module of loader 'app'; [B is in module java.base of loader 'bootstrap')
	at org.springframework.http.converter.ByteArrayHttpMessageConverter.getContentLength(ByteArrayHttpMessageConverter.java:38) ~[spring-web-6.0.11.jar:6.0.11]
	at org.springframework.http.converter.AbstractHttpMessageConverter.addDefaultHeaders(AbstractHttpMessageConverter.java:258) ~[spring-web-6.0.11.jar:6.0.11]
	at org.springframework.http.converter.AbstractHttpMessageConverter.write(AbstractHttpMessageConverter.java:210) ~[spring-web-6.0.11.jar:6.0.11]
	at org.springframework.web.servlet.mvc.method.annotation.AbstractMessageConverterMethodProcessor.writeWithMessageConverters(AbstractMessageConverterMethodProcessor.java:300) ~[spring-webmvc-6.0.11.jar:6.0.11]
	at org.springframework.web.servlet.mvc.method.annotation.RequestResponseBodyMethodProcessor.handleReturnValue(RequestResponseBodyMethodProcessor.java:194) ~[spring-webmvc-6.0.11.jar:6.0.11]
	at org.springframework.web.method.support.HandlerMethodReturnValueHandlerComposite.handleReturnValue(HandlerMethodReturnValueHandlerComposite.java:78) ~[spring-web-6.0.11.jar:6.0.11]
	at org.springframework.web.servlet.mvc.method.annotation.ServletInvocableHandlerMethod.invokeAndHandle(ServletInvocableHandlerMethod.java:136) ~[spring-webmvc-6.0.11.jar:6.0.11]
	at org.springframework.web.servlet.mvc.method.annotation.RequestMappingHandlerAdapter.invokeHandlerMethod(RequestMappingHandlerAdapter.java:884) ~[spring-webmvc-6.0.11.jar:6.0.11]
	at org.springframework.web.servlet.mvc.method.annotation.RequestMappingHandlerAdapter.handleInternal(RequestMappingHandlerAdapter.java:797) ~[spring-webmvc-6.0.11.jar:6.0.11]
	at org.springframework.web.servlet.mvc.method.AbstractHandlerMethodAdapter.handle(AbstractHandlerMethodAdapter.java:87) ~[spring-webmvc-6.0.11.jar:6.0.11]
	at org.springframework.web.servlet.DispatcherServlet.doDispatch(DispatcherServlet.java:1081) ~[spring-webmvc-6.0.11.jar:6.0.11]
	at org.springframework.web.servlet.DispatcherServlet.doService(DispatcherServlet.java:974) ~[spring-webmvc-6.0.11.jar:6.0.11]
	at org.springframework.web.servlet.FrameworkServlet.processRequest(FrameworkServlet.java:1011) ~[spring-webmvc-6.0.11.jar:6.0.11]
	at org.springframework.web.servlet.FrameworkServlet.doGet(FrameworkServlet.java:903) ~[spring-webmvc-6.0.11.jar:6.0.11]
	at jakarta.servlet.http.HttpServlet.service(HttpServlet.java:564) ~[tomcat-embed-core-10.1.11.jar:6.0]
	at org.springframework.web.servlet.FrameworkServlet.service(FrameworkServlet.java:885) ~[spring-webmvc-6.0.11.jar:6.0.11]
	at jakarta.servlet.http.HttpServlet.service(HttpServlet.java:658) ~[tomcat-embed-core-10.1.11.jar:6.0]
	at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:205) ~[tomcat-embed-core-10.1.11.jar:10.1.11]
	at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:149) ~[tomcat-embed-core-10.1.11.jar:10.1.11]
	at org.apache.tomcat.websocket.server.WsFilter.doFilter(WsFilter.java:51) ~[tomcat-embed-websocket-10.1.11.jar:10.1.11]
	at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:174) ~[tomcat-embed-core-10.1.11.jar:10.1.11]
	at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:149) ~[tomcat-embed-core-10.1.11.jar:10.1.11]
	at org.springframework.web.filter.OncePerRequestFilter.doFilter(OncePerRequestFilter.java:110) ~[spring-web-6.0.11.jar:6.0.11]
	at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:174) ~[tomcat-embed-core-10.1.11.jar:10.1.11]
	at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:149) ~[tomcat-embed-core-10.1.11.jar:10.1.11]
	at org.springframework.security.web.FilterChainProxy.lambda$doFilterInternal$3(FilterChainProxy.java:231) ~[spring-security-web-6.1.2.jar:6.1.2]
	at org.springframework.security.web.FilterChainProxy$VirtualFilterChain.doFilter(FilterChainProxy.java:365) ~[spring-security-web-6.1.2.jar:6.1.2]
	at org.springframework.security.web.access.intercept.AuthorizationFilter.doFilter(AuthorizationFilter.java:100) ~[spring-security-web-6.1.2.jar:6.1.2]
	at org.springframework.security.web.FilterChainProxy$VirtualFilterChain.doFilter(FilterChainProxy.java:374) ~[spring-security-web-6.1.2.jar:6.1.2]
	at org.springframework.security.web.access.ExceptionTranslationFilter.doFilter(ExceptionTranslationFilter.java:126) ~[spring-security-web-6.1.2.jar:6.1.2]
	at org.springframework.security.web.access.ExceptionTranslationFilter.doFilter(ExceptionTranslationFilter.java:120) ~[spring-security-web-6.1.2.jar:6.1.2]
	at org.springframework.security.web.FilterChainProxy$VirtualFilterChain.doFilter(FilterChainProxy.java:374) ~[spring-security-web-6.1.2.jar:6.1.2]
	at org.springframework.security.web.session.SessionManagementFilter.doFilter(SessionManagementFilter.java:131) ~[spring-security-web-6.1.2.jar:6.1.2]
	at org.springframework.security.web.session.SessionManagementFilter.doFilter(SessionManagementFilter.java:85) ~[spring-security-web-6.1.2.jar:6.1.2]
	at org.springframework.security.web.FilterChainProxy$VirtualFilterChain.doFilter(FilterChainProxy.java:374) ~[spring-security-web-6.1.2.jar:6.1.2]
	at org.springframework.security.web.authentication.AnonymousAuthenticationFilter.doFilter(AnonymousAuthenticationFilter.java:100) ~[spring-security-web-6.1.2.jar:6.1.2]
	at org.springframework.security.web.FilterChainProxy$VirtualFilterChain.doFilter(FilterChainProxy.java:374) ~[spring-security-web-6.1.2.jar:6.1.2]
	at org.springframework.security.web.servletapi.SecurityContextHolderAwareRequestFilter.doFilter(SecurityContextHolderAwareRequestFilter.java:179) ~[spring-security-web-6.1.2.jar:6.1.2]
	at org.springframework.security.web.FilterChainProxy$VirtualFilterChain.doFilter(FilterChainProxy.java:374) ~[spring-security-web-6.1.2.jar:6.1.2]
	at org.springframework.security.web.savedrequest.RequestCacheAwareFilter.doFilter(RequestCacheAwareFilter.java:63) ~[spring-security-web-6.1.2.jar:6.1.2]
	at org.springframework.security.web.FilterChainProxy$VirtualFilterChain.doFilter(FilterChainProxy.java:374) ~[spring-security-web-6.1.2.jar:6.1.2]
	……

[frudder]

body is byte[]

[breezefaith]

已收到您的来信，非常感谢

[bnasslahsen]

Not reproducible.
Provide a Minimal, Reproducible Example - with HelloController that reproduces the problem

[breezefaith]

已收到您的来信，非常感谢

[DavidSonoda]

Hi, @bnasslahsen @breezefaith

I can confirm this issue still exists. After some investigation, the issue appears to be that the api-docs endpoint uses byte[] as return type while using application/json as return mediatype. Correct me if I'm wrong, but below is my investigation.

Cause of the issue

springdoc-openapi/springdoc-openapi-starter-webmvc-api/src/main/java/org/springdoc/webmvc/api/OpenApiWebMvcResource.java

Lines 100 to 115 in e14f975

	/**
	* Openapi json string.
	*
	* @param request the request
	* @param apiDocsUrl the api docs url
	* @param locale the locale
	* @return the string
	* @throws JsonProcessingException the json processing exception
	*/
	@Operation(hidden = true)
	@GetMapping(value = API_DOCS_URL, produces = MediaType.APPLICATION_JSON_VALUE)
	@Override
	public byte[] openapiJson(HttpServletRequest request, @Value(API_DOCS_URL) String apiDocsUrl, Locale locale)
	throws JsonProcessingException {
	return super.openapiJson(request, apiDocsUrl, locale);
	}

This will cause the AbstractMessageConverterMethodProcessor to select ByteArrayHttpMessageConverter as the message converter. Because by default, ByteArrayHttpMessageConverter supports */* and application/octet-stream as MIMEs.

Class ByteArrayHttpMessageConverter
...
By default, this converter supports all media types (/), and writes with a Content-Type of application/octet-stream. This can be overridden by setting the supportedMediaTypes property.

While ByteArrayHttpMessageConverter is calculating content-length for the response header, it calls the overriden method of getContentLength in it, which is of byte[] type. However, on AbstractHttpMessageConverter:269 (where the exception is thrown)

Long contentLength = getContentLength(t, headers.getContentType());

Here, t is whatever wrapper class instance that's created in the implemented method of ResponseBodyAdvice#beforeBodyWrite, then it's passed over to the message converter, thus causing the exception.

As the name suggests, beforeBodyWrite is applied before message converter write the response body, so I believe it's users responsibility to make any custom implemented ResponseBodyAdivce to be aware of any selected message converters by AbstractMessageConverterMethodProcessor to avoid any issues. Specifically, custom beforeBodyWrite method should handle possible message converter candidates' inability to process your custom response wrapper class.

My workaround

Since I had almost identical issue as OP, in my implemented beforeBodyWrite, I added the following logic to avoid any custom response wrapping when ByteArrayHttpMessageConverter is selected.

  if (selectedConverterType == ByteArrayHttpMessageConverter::class.java) {
      return body
  }

My question

While I think this is not a bug from springdoc-openapi and I'm not a spring expert, but my question to the developer is, why using byte[] as the return type of the endpoints? Any particular considerations?

Any insights are appreciated!