How do I log a stacktrace using java's Logger class

You need to understand that void is actually nothingness. You cannot convert what is nothing. You might end up printing void as a string, but (trust me), you don't want that.

I think what you are looking for is

// assuming ex is your Exception object
logger.error(ex.getMessage(), ex);
// OR
Logger.log(errorLogLevel, ex.getMessage(), ex)

This will print the error message using the logger that you have configured. For more details, you can take a look at the java docs for Exception#getMessage()

Use java.util.logging.Logger#log(Level, String, Throwable) and pass in ex as third argument like this:

LOGGER.log(Level.INFO, ex.getMessage(), ex);

Also another alternative would be:

import org.apache.commons.lang3.exception.ExceptionUtils;

log.error("Exception : "+ExceptionUtils.getStackTrace(exception));