How to enable debug in slf4j Logger?

Programmatically, with logback:

setLoggingLevel(ch.qos.logback.classic.Level.DEBUG);

where

public static void setLoggingLevel(ch.qos.logback.classic.Level level) {
    ch.qos.logback.classic.Logger root = (ch.qos.logback.classic.Logger) org.slf4j.LoggerFactory.getLogger(ch.qos.logback.classic.Logger.ROOT_LOGGER_NAME);
    root.setLevel(level);
}

exists various capability to switch debug log on:
this article have good explanation all of those. to me good fit is:

Using slf4j with Log4j logger
create file src/main/resources/log4j.properties

log4j.rootLogger=DEBUG, STDOUT
log4j.logger.deng=INFO
log4j.appender.STDOUT=org.apache.log4j.ConsoleAppender
log4j.appender.STDOUT.layout=org.apache.log4j.PatternLayout
log4j.appender.STDOUT.layout.ConversionPattern=%5p [%t] (%F:%L) - %m%n