Printer<T> are not autoconfigured when exposed as a bean

[tloist]

Status Quo

The WebMvcAutoConfiguration auto detects beans of type:

• Converter
• GenericConverter
• Formatter

and automatically registers them for the ConversionService. Formatters are in fact only a combination of Printer and Parser. For each Formatter, the formatter is split into these two parts and registered as two serperate conversion services again.
My suggestion is:

• Scanning for Parser and Printer seperately, which means the above list changes to:

  • Converter
  • GenericConverter
  • Printer
  • Parser

  If this introduces performance problems, an alternative would be to:

• Scan for Printer instead, then for each Printer check if the Printer is also a Parser (and thus a Formatter) and register the second conversion service as well. Which would make that list from above into:

  • Converter
  • GenericConverter
  • Printer
    • Parser

Intended advantages

What this would make possible is to have a Printer without a Parser auto registered in the conversion service if exposed as a bean. This is useful for those cases, where you want to represent an object on the UI (e.g. templating in thymeleaf with ${{MyClass}}) but never read it from a String.

If you tend to my 2nd proposal, this leaves the question: Why only checking for a Parser in case it is already a Printer?

2 reasons:

• Compability: Currently Formatter also register conversions String -> MyClass and not doing it, would break these applications relying on this behaviour.
• On the other hand I can't really think of a lot of use cases where registering a Parser without a Printer is realistic. If you know a String representation for MyClass that you can read, what should be the reason, you can't also write it?

Workaround

Currently Printer which are not also Formatter are not registered in the conversion service. Either:

• configure them manually / programmatically with an WebMvcConfigurer or
• just convert them to be a Formatter and throw an Exception for reading them from a String. That turns this case into a runtime exception.

[philwebb]

Closing in favor of PR #17064