Reviewing the ch.qos.logback.core.WriterAppender class, I see that the attributes are 'protected' (not 'private'). However, there are set and get methods for most of them (but not all).

I suggest making the attributes private and create setters and getters for all of them (there is no getWriter() for instance). This would prevent subclasses from incorrectly working directly against the attributes (for instance, not using the setWriter() would cause errors). If some 'public' getters are not desirable, at least 'protected' ones should be provided.