Transcript Logging
Logging Logging 1 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 2 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 3 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 Logger getLogger(String name) Logging 4 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 5 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 6 Architecture • A logger object associates a set of handler objects. • A handler object associates a filter and a formatter. Logging 7 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 8 Filters and formatters • 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 Logging 9 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 10 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 11 References • Sun Microsystems: Java Logging Overview, 2001 – http://java.sun.com/j2se/1.5.0/docs/guide/logging/over view.html • JavaDoc of package java.util.logging – http://java.sun.com/j2se/1.5.0/docs/api/overviewsummary.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 12