Printing methods and the basic selection rule

By definition, the printing method determines the level of a logging request. For example, if L is a logger instance, then the statement L.info("..") is a logging statement of level INFO.

A logging request is said to be enabled if its level is higher than or equal to the effective level of its logger. Otherwise, the request is said to be disabled. As described previously, a logger without an assigned level will inherit one from its nearest ancestor. This rule is summarized below.

Basic Selection Rule
A log request of level p issued to a logger having an effective level q, is enabled if p >= q.

This rule is at the heart of logback. It assumes that levels are ordered as follows: TRACE < DEBUG < INFO < WARN < ERROR.

In a more graphic way, here is how the selection rule works. In the following table, the vertical header shows the level of the logging request, designated by p, while the horizontal header shows effective level of the logger, designated by q. The intersection of the rows (level request) and columns (effective level) is the boolean resulting from the basic selection rule.

Here is an example of the basic selection rule.

import ch.qos.logback.classic.Level;
import org.slf4j.Logger;
import org.slf4j.LoggerFactory;
....

// get a logger instance named "com.foo". Let us further assume that the
// logger is of type  ch.qos.logback.classic.Logger so that we can
// set its level
ch.qos.logback.classic.Logger logger = 
        (ch.qos.logback.classic.Logger) LoggerFactory.getLogger("com.foo");
//set its Level to INFO. The setLevel() method requires a logback logger
logger.setLevel(Level. INFO);

Logger barlogger = LoggerFactory.getLogger("com.foo.Bar");

// This request is enabled, because WARN >= INFO
logger.warn("Low fuel level.");

// This request is disabled, because DEBUG < INFO. 
logger.debug("Starting search for nearest gas station.");

// The logger instance barlogger, named "com.foo.Bar", 
// will inherit its level from the logger named 
// "com.foo" Thus, the following request is enabled 
// because INFO >= INFO. 
barlogger.info("Located nearest gas station.");

// This request is disabled, because DEBUG < INFO. 
barlogger.debug("Exiting gas station search");