1. Logging Logging

2. Introduction • Ships must keep a written log telling speed, direction, destination, etc. • A kind of diary of the ship. • Large programs should keep a written log telling about all the major events in the “life” of the program. • Server programs usually keeps a log telling which clients requested what service from the server – and when – and how the server responded to the request • Ordinary response • Exception(al) response Logging

3. Java logging API • J2SE has included a logging API since version 1.4 • Package: java.util.logging • The logging API is highly configurable • You chose which media the log is written to • You choose which level of messages to write to the log. • Only severe problems, or all kinds of information messages Logging

4. Class Logger • Class Logger is the most central class in the logging API. • Class Logger has no public constructor • You must use a static get method to obtain an object of class Logger. • public static LoggergetLogger(String name) Logging

5. Log levels • A log record has an associated log level telling how “important” the log record is. • The class Level defines 7 log levels • The levels in descending order are: • SEVERE (highest value) • WARNING • INFO • CONFIG • FINE • FINER • FINEST (lowest value) Logging

6. Log methods • Class Logger has a lot of log methods including • a general log method • log(Level level, String message) • a method for each log level • severe(String message) • info(String message) • etc. • These methods are convenience methods • They are not really necessary. • All log methods are thread safe (synchronized) • Can be used simultaneously by many threads. Logging

7. Architecture • A logger object associates a set of handler objects. • A handler object associates a filter and a formatter. Logging

8. Handlers • A logger object is associated with a set of handler objects. • A handler writes log records to a specific media. • Some concrete handlers • ConsoleHandler • Writes log records to the console (i.e. the screen). • FileHandler • Writes log records to a file. • SocketHandler • Writes log records to a network connection. Logging

9. Each handler has a filter. The filter decides whether the log record is loggable or not Whether is should be written by this handler Each handler has a formatter. The formatter decides the format of the log record written by this handler. Different kinds of formatters SimpleFormatter XMLFormatter Make your own formatter by sub-classing Formatter Filters and formatters Logging

10. Logger names • Each logger has a name • Used to create the logger • The names form a hierarchy • dk, dk.rhs, dk.rhs.myproject • Often you use package names as logger names. Logging

11. Design patterns in the logging API • To achieve flexibility the logging API used a number of design patterns • Factory method • getLogger in Logger • Observer – observable • Handler observes the Logger • Whenever a log “event” happens in Logger all associated handlers are “notified”. • You can attach and detach handlers at runtime. • Strategy • Handler.filter and Handler.formatter • Encapsulates part of the algorithm of Handler in separate classes. In this case: How to filter and how to format. Logging

12. References • Sun Microsystems: Java Logging Overview, 2001 • http://java.sun.com/j2se/1.5.0/docs/guide/logging/overview.html • JavaDoc of package java.util.logging • http://java.sun.com/j2se/1.5.0/docs/api/overview-summary.html • Mark Grand: Design Patterns in Java 2nd edition, Volume 1, Wiley 2002 • Factory Method, page 103-117 • Observer, page 387-396 • Strategy, page 413-418 Logging