Support escape chars in PropertyPlaceholderHelper

[philwebb]

We have a Spring Boot pull request open that uses PropertyPlaceholderHelper to resolve {variable} items in a message source. We'd like to also support escaping so that user can write \{notreplaced} to bypass expansion.

[jhoeller]

In terms of its design, PropertyPlaceholderHelper is not really meant to go to that degree of sophisticated parsing. We got ignoreUnresolvablePlaceholders there, as a sort-of alternative to escaping. That seems good enough for its original purpose of parsing configuration values. I wonder whether message subtitution isn't better off with some other parser, even a custom one...

[philwebb]

Thanks @jhoeller, I'll close this one and consider at an alternative parser.