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Logging in Java applications

Logging in Java applications. Sean C. Sullivan July 23, 2002 Portland Java Users Group. In the beginning…. public class HelloWorld { public static void main(String[] args) { System.out.println(“Hello world!”); } }. Real-world applications are complex.

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Logging in Java applications

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  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

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

  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

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