WebMvcConfigurationSupport currently includes the AllEncompassingFormHttpMessageConverter, see https://github.com/spring-projects/spring-framework/blob/6a8f0d6d7d4379860e0197f321f0784f8b46e5df/spring-webmvc/src/main/java/org/springframework/web/servlet/config/annotation/WebMvcConfigurationSupport.java#L895.

Including a form converter on the server side makes no sense, as the servlet environment already provides form support. Effectively, including this converter can result in controllers returning form data, which makes no sense. So, the AllEncompassingFormHttpMessageConverter should be removed from WebMvcConfigurationSupport.

Comment From: snicoll

Here is a project that attempts to demonstrate the problem of writing a Map: https://github.com/snicoll-scratches/spring-framework-32917

There are four tests: one local using MockMvc and three using a server with TestRestTemplate, RestTemplate and RestClient.

The MockMvc test fails with: 

MockHttpServletRequest:
      HTTP Method = GET
      Request URI = /data
       Parameters = {}
          Headers = []
             Body = null
    Session Attrs = {}

Handler:
             Type = com.example.DemoController
           Method = com.example.DemoController#plain()

Async:
    Async started = false
     Async result = null

Resolved Exception:
             Type = null

ModelAndView:
        View name = null
             View = null
            Model = null

FlashMap:
       Attributes = null

MockHttpServletResponse:
           Status = 200
    Error message = null
          Headers = [Content-Type:"application/x-www-form-urlencoded", Content-Length:"20"]
     Content type = application/x-www-form-urlencoded
             Body = name=John+Doe&age=42
    Forwarded URL = null
   Redirected URL = null
          Cookies = []

MockHttpServletRequest:
      HTTP Method = GET
      Request URI = /data
       Parameters = {}
          Headers = []
             Body = null
    Session Attrs = {}

Handler:
             Type = com.example.DemoController
           Method = com.example.DemoController#plain()

Async:
    Async started = false
     Async result = null

Resolved Exception:
             Type = null

ModelAndView:
        View name = null
             View = null
            Model = null

FlashMap:
       Attributes = null

MockHttpServletResponse:
           Status = 200
    Error message = null
          Headers = [Content-Type:"application/x-www-form-urlencoded", Content-Length:"20"]
     Content type = application/x-www-form-urlencoded
             Body = name=John+Doe&age=42
    Forwarded URL = null
   Redirected URL = null
          Cookies = []

Content type [application/x-www-form-urlencoded] is not compatible with [application/json]
java.lang.AssertionError: Content type [application/x-www-form-urlencoded] is not compatible with [application/json]
    at org.springframework.test.util.AssertionErrors.fail(AssertionErrors.java:39)
    at org.springframework.test.util.AssertionErrors.assertTrue(AssertionErrors.java:73)
    at org.springframework.test.web.servlet.result.ContentResultMatchers.lambda$contentTypeCompatibleWith$1(ContentResultMatchers.java:107)
    at org.springframework.test.web.servlet.MockMvc$1.andExpect(MockMvc.java:214)
    at com.example.DemoControllerTests.getData(DemoControllerTests.java:29)
    at java.base/java.lang.reflect.Method.invoke(Method.java:568)
    at java.base/java.util.ArrayList.forEach(ArrayList.java:1511)
    at java.base/java.util.ArrayList.forEach(ArrayList.java:1511)

The test with the RestClient fails as follows:

Could not extract response: no suitable HttpMessageConverter found for response type [interface java.util.Map] and content type [application/x-www-form-urlencoded]
org.springframework.web.client.UnknownContentTypeException: Could not extract response: no suitable HttpMessageConverter found for response type [interface java.util.Map] and content type [application/x-www-form-urlencoded]
    at org.springframework.web.client.DefaultRestClient.readWithMessageConverters(DefaultRestClient.java:229)
    at org.springframework.web.client.DefaultRestClient$DefaultResponseSpec.readBody(DefaultRestClient.java:689)
    at org.springframework.web.client.DefaultRestClient$DefaultResponseSpec.toEntityInternal(DefaultRestClient.java:659)
    at org.springframework.web.client.DefaultRestClient$DefaultResponseSpec.toEntity(DefaultRestClient.java:648)
    at com.example.DemoControllerRestClientTests.getData(DemoControllerRestClientTests.java:46)
    at java.base/java.lang.reflect.Method.invoke(Method.java:568)
    at java.base/java.util.ArrayList.forEach(ArrayList.java:1511)
    at java.base/java.util.ArrayList.forEach(ArrayList.java:1511)

Switching to Spring Framework 6.2.0-M3 and they all work.

Comment From: rstoyanchev

It's not uncommon to access form data through an @RequestBody parameter like in this test I think, so I'm not sure we can just exclude FormHttpMessageConverter from server configuration. On the response side, the converter enables writing multipart that I believe is also a valid use case.

Comment From: poutsma

It's not uncommon to access form data through an @RequestBody parameter like in this test I think, so I'm not sure we can just exclude FormHttpMessageConverter from server configuration. On the response side, the converter enables writing multipart that I believe is also a valid use case.

Understood. I guess we have no other way out that to revert #32826, as supporting maps will break too many applications. Fortunately there is still #32832, which we can keep.