log4j HTMLLayout - Learn how log4j framework works starting from environment setup, logging levels, methods, formatting, methods, file logging and database logging. Noun. I'd like to write log to 2 different log files from the same process. Appender Additivity. public interface Appender. Log4j2 is the updated version of the popular and influential log4j library, used extensively throughout the Java ecosystem for so many years. appenders. For example, you can append one file to another or you can append a field to a record.