Logging Jan 14 2019 What is logging Logging is producing messages that tell you what your program is doing It s not much different than using System out println Log messages can go to the console to a file or one of several other places e g sent over the Internet You can use logging to help you debug a program You can use logging to produce a file when the user runs your program Why use logging Reality Large programs always have bugs Your program may go to customers users Typical error report from customer It doesn t work JUnit testing can greatly reduce this problem but it s impossible to see in advance all potentially useful tests We love it when we have a customer who tries assorted things and gives us a detailed scenario in which the program fails Such customers are rare Here s what you want to tell the typical customer Send us the log it s in such and such a place Basic use import java util logging private static Logger myLogger Logger getLogger myPackage myLogger log Level SEVERE Bad news Aug 15 2004 10 51 09 AM myPackage Sync main SEVERE Bad news Logging levels and methods Level SEVERE Level WARNING Level INFO Level CONFIG Level FINE Level FINER Level FINEST myLogger severe String msg myLogger warning String msg myLogger info String msg myLogger config String msg myLogger fine String msg myLogger finer String msg myLogger finest String msg These levels are ordered so that you can set the level of severity that results in log messages However the levels have no inherent meaning they are what you make of them Controlling logging levels public void Logger setLevel Level newLevel Additional settings Sets the logger to log all messages at newLevel or above Logger calls at lower levels don t do anything Example logger setLevel Level WARNING logger setLevel Level ALL logger setLevel Level OFF public Level getLevel Note that this returns a Level not an int Level has intValue and toString methods Additional Logger methods void entering String sourceClass String sourceMethod void entering String sourceClass String sourceMethod Object param1 void entering String sourceClass String sourceMethod Object params void exiting String sourceClass String sourceMethod void exiting String sourceClass String sourceMethod Object result These log messages at level FINER Logging flow of control application Logger Filter Handler destination Filter Formatter You send your message to a Logger The Logger checks a Filter to see whether to ignore the message The Logger sends the message to a Handler to put the message somewhere The Handler checks another Filter to see whether to ignore this kind of message sent to this destination The Handler calls a Formatter to decide what kind of a text string to produce The Handler sends the formatted message somewhere Filters Filter is an interface it defines the single method boolean isLoggable LogRecord record A LogRecord is another class in java util logging it provides numerous methods for examining the proposed logging message We won t go into the details of LogRecord in this lecture see the API if you need to use this Logging formatting and destinations The JDK defines five Handlers It also defines two ways to format messages StreamHandler sends messages to an OutputStream ConsoleHandler sends messages to System err default FileHandler sends messages to a file SocketHandler sends messages to a TCP port MemoryHandler buffers messages in memory SimpleFormatter default XMLFormatter And of course you can define your own Handlers and Formatters As you can tell from the Basic Use slide earlier you can ignore all of this and just use the defaults Using a FileHandler try logger addHandler new FileHandler myLogFile catch SecurityException e e printStackTrace The default Formatter for a FileHandler is XMLFormatter FileHandler results xml version 1 0 encoding MacRoman standalone no DOCTYPE log SYSTEM logger dtd log record date 2004 08 15T13 21 26 date millis 1092590486248 millis sequence 0 sequence logger myPackage logger level SEVERE level class myClass class method main method thread 10 thread message Bad news message record log Reading log files Don t like XML See the SimpleFormatter class When you log your log messages must be exceptionally easy to understand Remember you might see a log file months or even years after you wrote the program Messages such as at1 12 37 5 may no longer make any sense to you What to log Here s what you don t want to log Everything Specific methods only after an error has occurred Here s what you do want to log Critical events such as updating a database Results of a long sequence of operations Interactions with other components of a large system Actions known to be complex or error prone When logging goes bad Here s an all too common error Create log files on every run of the program Never get rid of old log files Solution Use the FileHandler class The FileHandler can either write to a specified file or it can write to a rotating set of files For a rotating set of files as each file reaches a given size limit it is closed rotated out and a new file opened Successively older files are named by adding 0 1 2 etc into the base filename The rest of the story As usual I ve only given you a few features beyond the most basic usage If you want to know about Filters other destinations than the console or a file etc it s all in the Java API You should also know that there is an alternative open source logging package log4j log4j has been around longer than Sun s logging package Many people feel that log4j is better I didn t cover it because it isn t installed in the labs here The End
View Full Document
Unlocking...