Add something to log if there is an exception in logback - logback

Question

Due to reasons outside of my control, newlines aren't handled properly by some logging infrastructure I have to use.
A workaround is to replace the \n with another character, e.g. _newline_
This can be done in logback by configuring the pattern:
<appender name="STDOUT" class="ch.qos.logback.core.ConsoleAppender">
<encoder>
<!-- Standard pattern -->
<!-- <pattern>%coloredLevel - %logger - %message%n%xException{15}</pattern> -->
<!-- With newlines removed -->
<pattern>%coloredLevel - %logger - %replace(%message){'\n', '_newline_'}_newline_%replace(%xException){'\n', '_newline_'}%nopex%n</pattern>
</encoder>
</appender>
However, this adds a superfluous _newline_ in logslines when there isn't an exception. (and adds an extra newline to stack traces, but this isn't a large problem)
Is there a way to only output the _newline_ in between the message and the if there is an exception?

Answer 1

Found a bit easier solution to conditionally output newline between log message and exception message. It seems that exception message is always followed by newline, no matter how many lines of stack trace you print. What could you do is print an exception with 0 lines of stack trace (%ex{0}), and then truncate from left all the characters until the length is no more than 1 (%.1ex{0}). This effectively leaves a \n if there's an exception and produces empty string in case there's no exception.
So this:
// logger.error("Log message", new RuntimeException("Exception message", new RuntimeException("Cause")));
// logger.error("Log message without exception");
<pattern>%-5p %m%.1ex{0}%ex%nopex%n</pattern>
Results in:
ERROR Log message
java.lang.RuntimeException: Exception message
[ redacted ]
at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:624)
at java.lang.Thread.run(Thread.java:748)
Caused by: java.lang.RuntimeException: Cause
... 10 common frames omitted
ERROR Log message without exception
Now the fun thing. Remember the fact that exceptions are always followed by newline? This makes the log events with exceptions to be followed by empty line (one newline from exception and one from the pattern), unless you know a trick how to print something only when there are no exceptions...
P.S. you can wrap everything in replace(){} to escape newlines, too:
<pattern>%-5p %replace(%m%.1ex{0}%ex){'[\r\n]+', 'LF'}%nopex%n</pattern>

Answer 2

I solved it very hackily, by replace an extra exception stack trace with a single _newline_
<pattern>%coloredLevel - %logger - %replace(%message){'\n', '_newline_'} %replace(%replace(%ex{0}){'\n',''}){'.+', '<nl>'}%replace(%xException){'\n', '_newline_'}%nopex%n</pattern>