JsonTemplateLayout uses a static list of resolvers. While this makes it impossible 1 to update the set of resolvers used for the layout, e.g., adding new user-provided custom resolvers, it is also old fashioned. Log4j has a great plugin system and it is leveraged by PatternLayout exactly for the same thing. This story aims to bring this improvement to JsonTemplateLayout too.
1 We can still allow the injection of a custom resolver factory in the layout configuration, but again, there is a better way to do it: plugins!