Log4j

References

• http://www.developer.com/open/article.php/10930_3097221_1
• http://logging.apache.org/log4j/docs/manual.html
• Sun's alternative logging infrastructure for jdk 1.4: http://java.sun.com/j2se/1.4.2/docs/guide/util/logging/
• Email error messages with SMTPAppender: http://www.onjava.com/pub/a/onjava/2004/09/29/smtp-logging.html
• http://java2.5341.com/msg/32707.html
• http://logging.apache.org/log4j/docs/api/
• PatternLayout specs: http://logging.apache.org/log4j/docs/api/org/apache/log4j/PatternLayout.html

Theory

• Logger - the object that takes messages for logging
• Appender - the object that outputs messages to some medium
• Layout - controls the format of the output message

Logging Levels

DEBUG < INFO < WARN < ERROR < FATAL

If a given logger has level WARN, the an INFO logging request will not be processed.

Hierarchy and Inheritance

There is always a root logger. Each logger is associated with some node in the package hierarchy, and its log level is either explicitly assigned or inherited from higher up in the hierarchy. Example: During coding, my root log level is DEBUG. I'm done working on the dc.util package and want to focus on classes in dc and dc.db. I might stifle excessive log messages from the dc.util classes by creating a dc.util logger (in the properties file) and assigning it a level of WARNING or ERROR.

Download

• Make sure log4j.jar is on your classpath. Get it from here:

http://logging.apache.org/log4j/docs/download.html

Properties File

Create a log4j.properties file:

Example 1:

# set root logger level to ERROR and give it an appender
log4j.rootLogger = ERROR, RootAppender
log4j.appender.RootAppender = org.apache.log4j.ConsoleAppender
log4j.appender.RootAppender.layout = org.apache.log4j.PatternLayout
log4j.appender.RootAppender.layout.ConversionPattern = %d{HH:mm:ss} %-5p [%c{1}] %m%n

# set the top level org logger to WARN
log4j.logger.org = WARN

# set the top level Mirc logger to INFO
log4j.logger.org.rsna = INFO

Example 2:

# set root logger level to ERROR and give it an appender
log4j.rootLogger = ERROR, RootAppender
log4j.appender.RootAppender = org.apache.log4j.ConsoleAppender
log4j.appender.RootAppender.layout = org.apache.log4j.PatternLayout
log4j.appender.RootAppender.layout.ConversionPattern = %d{HH:mm:ss} %-5p [%c{1}] %m%n

# set the top level org logger to WARN
log4j.logger.org = WARN

# set the top level Mirc logger to INFO
log4j.logger.org.rsna = INFO

Other kinds of Appenders

• FileAppender
• DailyRollingFileAppender
• RollingFileAppender
• SMTPAppender seems to depend on javax.Mail, which seems to be part of J2EE.

Load in main()

In the main() method, load the log4j properties like this:

initializeLog4J("config/log4j.properties");
...
/**
* Function to configure Log4J from the properties file.
*/
private static void initializeLog4J(String filename) {
  File file = new File(filename);
  if (file.exists()) {
    PropertyConfigurator.configure(filename);
  }
}

Logging

In each class that will include logging statements:

//Import necessary log4j API classes
import org.apache.log4j.*;
...
private static final Logger logger = Logger.getLogger(DicomObjectProcessor.class);

Use log statements wherever it makes sense:

logger.debug();
logger.info();
logger.warn();
logger.error();
logger.fatal();