Logging

Objective

use the Java logging facility
discuss advantages and disadvantages of logging in Java

Logging notes

there is little overhead for unused logging in programs
the logging level can be set so less important or debugging logging can be turned off without recompiling
log messages can be output in different formats such as XML or plain text
multiple loggers can be used at the same time
filters can be used with logging to reduce the messages being stored
logging output can be easily redirected
the default log configuration file can be replaced
logging levels (in descending order): SEVERE, WARNING, INFO, CONFIG, FINE, FINER, FINEST
default logger properties are stored in jreHome/lib/logging.properties
you can change logging properties by modifying the logging.properties file
different settings can be set for each logger in the logging.properties file
you can also create your own logger configuration file and specify it at runtime using the -Djava.util.logging.config.file=yourLogFileName option to the java command

Simple logging to console and setting logger output level

Note how setting the log level in code of the console logger only matters if the configuration file allows logging at the specified levels. The default level is INFO.

import java.util.logging.*;

public class Logging1 {
    public static void main(String[] args) {
        Logger log = Logger.getLogger("edu.kishwaukeecollege.Logging1");
        log.setLevel(Level.INFO);
        log.severe("Test of logger SEVERE message");
        log.warning("Test of logger WARNING message");
        log.info("Test of logger INFO message");
        log.config("Test of logger CONFIG message");
        log.fine("Test of logger FINE message");
        log.finer("Test of logger FINER message");
        log.finest("Test of logger FINEST message");
    }
}

C:\>java Logging1
Feb 14, 2010 3:22:52 AM Logging1 main
SEVERE: Test of logger SEVERE message
Feb 14, 2010 3:22:52 AM Logging1 main
WARNING: Test of logger WARNING message
Feb 14, 2010 3:22:52 AM Logging1 main
INFO: Test of logger INFO message

// contents of mylog.props file used in next command
handlers= java.util.logging.ConsoleHandler
java.util.logging.ConsoleHandler.level = FINER
java.util.logging.ConsoleHandler.formatter = java.util.logging.SimpleFormatter

C:\>java -Djava.util.logging.config.file=mylog.props Logging1
Feb 14, 2010 3:23:01 AM Logging1 main
SEVERE: Test of logger SEVERE message
Feb 14, 2010 3:23:01 AM Logging1 main
WARNING: Test of logger WARNING message
Feb 14, 2010 3:23:01 AM Logging1 main
INFO: Test of logger INFO message
Feb 14, 2010 3:23:01 AM Logging1 main
CONFIG: Test of logger CONFIG message
Feb 14, 2010 3:23:01 AM Logging1 main
FINE: Test of logger FINE message
Feb 14, 2010 3:23:01 AM Logging1 main
FINER: Test of logger FINER message
*/

Logging to a file in addition to the console

Note how the file log has a different logging level than the console. Note also that the output to the file defaults to XML format. You can specify your own formatter or use one of the built-in formatters. The console default formatter produces a simple text output. You can have quite a bit of detail recorded in the log entries. Notice that the XML format log entries include the class and method where the log entry came from.

import java.util.logging.*;
import java.io.*;

public class Logging2 {
    public static void main(String[] args) {
        Logger log = null;
        try {
            FileHandler fh = new FileHandler("test.log", true);
            log = Logger.getLogger("edu.kishwaukeecollege.Logging2");
            log.addHandler(fh);
        } catch (IOException e) {
            e.printStackTrace();
            System.exit(1);
        }
        log.setLevel(Level.CONFIG);
        log.severe("Test of logger SEVERE message");
        log.warning("Test of logger WARNING message");
        log.info("Test of logger INFO message");
        log.config("Test of logger CONFIG message");
        log.fine("Test of logger FINE message");
        log.finer("Test of logger FINER message");
        log.finest("Test of logger FINEST message");
    }
}
/*
Screen output:

C:\>java Logging2
Feb 14, 2010 3:29:01 AM Logging2 main
SEVERE: Test of logger SEVERE message
Feb 14, 2010 3:29:01 AM Logging2 main
WARNING: Test of logger WARNING message
Feb 14, 2010 3:29:01 AM Logging2 main
INFO: Test of logger INFO message

Contents of log file:

<?xml version="1.0" encoding="windows-1252" standalone="no"?>
<!DOCTYPE log SYSTEM "logger.dtd">
<log>
<record>
  <date>2010-02-14T03:29:01</date>
  <millis>1266139741531</millis>
  <sequence>0</sequence>
  <logger>edu.kishwaukeecollege.Logging2</logger>
  <level>SEVERE</level>
  <class>Logging2</class>
  <method>main</method>
  <thread>10</thread>
  <message>Test of logger SEVERE message</message>
</record>
<record>
  <date>2010-02-14T03:29:01</date>
  <millis>1266139741546</millis>
  <sequence>1</sequence>
  <logger>edu.kishwaukeecollege.Logging2</logger>
  <level>WARNING</level>
  <class>Logging2</class>
  <method>main</method>
  <thread>10</thread>
  <message>Test of logger WARNING message</message>
</record>
<record>
  <date>2010-02-14T03:29:01</date>
  <millis>1266139741546</millis>
  <sequence>2</sequence>
  <logger>edu.kishwaukeecollege.Logging2</logger>
  <level>INFO</level>
  <class>Logging2</class>
  <method>main</method>
  <thread>10</thread>
  <message>Test of logger INFO message</message>
</record>
<record>
  <date>2010-02-14T03:29:01</date>
  <millis>1266139741546</millis>
  <sequence>3</sequence>
  <logger>edu.kishwaukeecollege.Logging2</logger>
  <level>CONFIG</level>
  <class>Logging2</class>
  <method>main</method>
  <thread>10</thread>
  <message>Test of logger CONFIG message</message>
</record>
</log>

Eliminating logger output to the console

Logger log = Logger.getLogger("");
Handler[] handlers = log.getHandlers();
if (handlers[0] instanceof ConsoleHandler) log.removeHandler(handlers[0]);

Changing the console logging level

import java.util.logging.*;
import java.io.*;

public class Logging3 {
    public static void main(String[] args) {
        Logger log = Logger.getLogger("edu.kishwaukeecollege.Logging3");
        log.setUseParentHandlers(false);
        Handler handler = new ConsoleHandler();
        handler.setLevel(Level.FINE); // changes console log level
        log.addHandler(handler);
        log.setLevel(Level.ALL);  // can reduce what handler sees
        log.severe("Test of logger SEVERE message");
        log.warning("Test of logger WARNING message");
        log.info("Test of logger INFO message");
        log.config("Test of logger CONFIG message");
        log.fine("Test of logger FINE message");
        log.finer("Test of logger FINER message");
        log.finest("Test of logger FINEST message");
    }
}
/*
Sample output:

Feb 14, 2010 11:08:37 AM Logging3 main
SEVERE: Test of logger SEVERE message
Feb 14, 2010 11:08:37 AM Logging3 main
WARNING: Test of logger WARNING message
Feb 14, 2010 11:08:37 AM Logging3 main
INFO: Test of logger INFO message
Feb 14, 2010 11:08:37 AM Logging3 main
CONFIG: Test of logger CONFIG message
Feb 14, 2010 11:08:37 AM Logging3 main
FINE: Test of logger FINE message
*/

Other options

can use logger.log(Level level, String msg) in place of logger.level(String msg)
a set of Logger methods exist for giving precise location where the logging was done (see logp methods)
specific log messages can be generated when entering or exiting a method
specific log messages can be generated when throwing errors
you can also set up log rotation files using patterns for naming the files
log rotation can also set size and time limits for log files