After upgrading from Logback 1.1.3 to version 1.1.5 I've noticed that it isn't creating the log file any more.
RollingFileAppender result = logFileName ? new RollingFileAppender() : null
result.file = logFileName
def rollingPolicy = new FixedWindowRollingPolicy( )
rollingPolicy.fileNamePattern = "${logFileName}.%i"
rollingPolicy.setContext(loggerContext)
rollingPolicy.setParent(result)
rollingPolicy.setMinIndex(1)
rollingPolicy.setMaxIndex(9)
result.rollingPolicy = rollingPolicy
result.encoder = createEncoder()
result.setContext(loggerContext)
result.setTriggeringPolicy(new RollOnStartupPolicy())