Whenever a ch.qos.logback.classic.spi.LoggingEvent is constructed, MessageFormatter.arrayFormat(...) is called, and the formattedMessage member variable assigned. This involves calling toString() on all arguments in argArray, which can be slow, and occurs even if the formattedMessage is not part of any appender layouts.

For my project's purposes, I have a custom formatter which only outputs an abbreviated string form of certain objects with expensive toString() methods; however, I am still paying the overhead in the toString() calls, since they're made even when getFormattedMessage() is never called. Note that I don't control the source of the objects with expensive toString() methods, nor do I control the source everywhere they're logged, so I can't simply change them directly.

formattedMessage is already evaluated lazily in getFormattedMessage(), so the only side effect of this change is the introspection of argsArray to look for a Throwable at the end, which is a trivial operation to perform separately.