1. Logging in Java applications Sean C. Sullivan July 23, 2002 Portland Java Users Group

2. In the beginning… public class HelloWorld { public static void main(String[] args) { System.out.println(“Hello world!”); }

3. Real-world applications are complex applications are multi-threaded and multi- user multiple web applications per application server each web application may communicate with one-or-more backend systems

4. Why logging? Logs provide precise context about a run of the application Logs can be saved to a persistent medium to be studied at a later time

5. When to use logging In your development phase: –logging can help you debug the code In your production environment: –helps you troubleshoot problems

6. Hello Log4j import org.apache.log4j.*; public class HelloLog4j { private static Logger logger = Logger.getLogger(HelloLog4j.class); public static void main(String[] args) { BasicConfigurator.configure(); logger.debug(“In the main method"); logger.info("What a beautiful day."); logger.error(“This is an error message.”); }

7. Output from HelloLog4j 0 [main] DEBUG HelloLog4j - In the main method 0 [main] INFO HelloLog4j - What a beautiful day. 10 [main] ERROR HelloLog4j - This is an error message.

8. Hello java.util.logging import java.util.logging.*; public class HelloJDKLogging { private static Logger logger = Logger.getLogger("com. foo.HelloJDKLogging"); public static void main(String argv[]) { logger.setLevel(Level.ALL); logger.fine(“Program started"); logger.info(“This app uses java.util.logging”); logger.warning(“Oops, I did it again"); }

9. Output from HelloJDKLogging Jul 23, 2002 12:12:43 PM HelloJDKLogging main INFO: This app uses java.util.logging Jul 23, 2002 12:12:43 PM HelloJDKLogging main WARNING: Oops, I did it again

10. Logging concepts named loggers levels destination for log messages message log format

11. Log4j features logging via JMS logging to a database via JDBC logging to Windows NT event log logging to Unix syslog GUI log viewer (“chainsaw”)

12. Comparison: log4j & java.util.logging org.apache.log4jjava.util.logging LogManager class Logger class Named loggers Hierarchical namespaces Appender’sHandler’s Layout’sFormatter’s Level class Filter classFilter interface

13. org.apache.log4j.Level Other levels: –Level.ALL –Level.OFF Order: DEBUG < INFO < WARN < ERROR < FATAL

14. java.util.Logging.Level Other levels: –Level.ALL –Level.OFF Order: FINEST < FINER < FINE < CONFIG < INFO < WARNING < SEVERE

15. Named loggers in Log4j Logger “com.foo” is a parent of “com.foo.Bar” children inherit a Level value from their parent

16. Log4j Appender’s AsyncAppender ConsoleAppender DailyRollingFileAppender JMSAppender NTEventLogAppender NullAppender RollingFileAppender SMTPAppender SocketAppender SyslogAppender

17. java.util.logging Handlers StreamHandler ConsoleHandler FileHandler SocketHandler MemoryHandler

18. java.util.logging Formatters SimpleFormatter XMLFormatter

19. Log4j log viewer: Chainsaw

20. Logging: Best practices use the appropriate message level roll your log files daily / weekly review your error log on a daily basis

21. Logging: Worst practices System.out.println / System.err.println logging passwords to a log file logging informational messages to STDERR logging a message for every single HTTP request multiple applications sending log messages to a single log file ignoring error messages that appear in your application error log misleading log messages

22. Related projects… Jakarta Commons Logging –http://jakarta.apache.org/commons/ Protomatter Syslog library –http://protomatter.sourceforge.net/

23. Summary Stop using System.out.println Start using Log4j or java.util.logging