Update `Content-Type` based on `encoding` in MVC `FreeMarkerView`

[sbrannen]

Overview

As I mentioned in #33071 (comment), if you set the default encoding in FreeMarkerConfigurer and use FreeMarkerViewResolver to resolve a FreeMarkerView, Spring MVC currently does not update the charset of the Content-Type header of the HTTP response to reflect the actual type of encoding used to read the template file and render the body of the response.

Specifically, if you don't specify the content type via the FreeMarkerViewResolver, "text/html;charset=ISO-8859-1" will be used, which is the default set in AbstractView, and that can cause issues if templates are read and rendered using UTF-8 or some other explicit encoding.

As discussed in #33071, users may reasonably expect that an explicit encoding (set in FreeMarkerConfigurer) will be used as the charset of the Content-Type header of the HTTP response by default, unless the user explicitly overrides that via FreeMarkerViewResolver#setContentType.

Luckily, FreeMarker 2.3.26 introduced Configuration#isDefaultEncodingExplicitlySet() which "Tells if setDefaultEncoding(String) (or equivalent) was already called on this instance, or it just holds the default value." By checking that, we can therefore reliably update the charset of the Content-Type header of the HTTP response to reflect the actual type of encoding used to read the template file and render the body of the response, as long as the user explicitly set the desired default encoding via FreeMarkerConfigurer.

Note, however, that the same logic cannot be reliably applied to the FreeMarker support in WebFlux since the WebFlux infrastructure automatically sets the default encoding to UTF-8.

Related Issues

• Improve documentation regarding encoding in FreeMarker support #33071
• Set output_encoding in FreeMarkerView implementations #33106

Deliverables

• Update Content-Type based on explicit default encoding in FreeMarkerView in Spring MVC.
• Update documentation to point out that FreeMarker 2.3.26 is the new minimum baseline for Spring Framework 6.2.