How to send a stacktrace to log4j?

You can also get stack trace as string via ExceptionUtils.getStackTrace.

See: ExceptionUtils.java

I use it only for log.debug, to keep log.error simple.

If you want to log a stacktrace without involving an exception just do this:

String message = "";

for(StackTraceElement stackTraceElement : Thread.currentThread().getStackTrace()) {                         
    message = message + System.lineSeparator() + stackTraceElement.toString();
}   
log.warn("Something weird happened. I will print the the complete stacktrace even if we have no exception just to help you find the cause" + message);

You pass the exception directly to the logger, e.g.

try {
   ...
} catch (Exception e) {
    log.error( "failed!", e );
}

It's up to log4j to render the stack trace.