Presents your 
JAVA E-NEWSLETTER for April 21, 2003

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DEBUG WITH JAVA'S LOGGING PACKAGE

Java 1.4 introduced the logging package java.util.logging. If you've
used the popular logging framework log4j, then Java's built-in logging 
will be familiar territory.

You can begin using logging with no setup. The simplest way to do this
is to create an instance of java.util.logging.Logger. Once you have a
Logger instance, you can start logging. Below is a complete example of 
a logging class:

package tips;

import java.util.logging.Logger;

public class LogTip {
    // create an instance of the Logger class
    private static Logger log = Logger.getLogger("tips.LogTip");

    public static void main(String args[]) {
        log.finest("the finest message");
        log.finer("finer message");
        log.fine("a fine message");
        log.config("some configuration message");
        log.info("a little bit of information");
        log.warning("a warning message");
        log.severe("a severe message");
    }
}

If you compile and run this class, you should get console output like
this:

Apr 1, 2003 11:09:05 PM tips.LogTip main
INFO: a little bit of information
Apr 1, 2003 11:09:05 PM tips.LogTip main
WARNING: a warning message
Apr 1, 2003 11:09:05 PM tips.LogTip main
SEVERE: a severe message

Not all of the log entries make it to the console because Java's logging
package makes use of log levels. A log level allows you to control
logging output. During development, you want to output plenty of debug
information so you can see what's going on in the application.

In production, you probably want to suppress debug messages but not
errors. The logging package gives you this control through the use of 
logging levels.

There are seven hierarchal levels of logging defined in the
java.util.logging.Level class: SEVERE, WARNING, INFO, CONFIG, FINE, FINER, and
FINEST. The SEVERE level is for errors and catastrophic application events. The
FINE, FINER, and FINEST levels are for information that is less
important and often used for debugging.

In addition, you can set the level to its extreme value by setting the
logging level to ALL or OFF. When you set the level to ALL, no log entries
are suppressed. When you set the level to OFF, all entries are suppressed.

Because logging levels are hierarchal, when you set logging to a certain
level, only that level and higher are logged. SEVERE is the highest
priority level and FINEST is the lowest. The default log level is INFO;
that's why we see only the last three log messages in the code above. 
To see all of the messages, we'll need to make some configuration changes.

There are a number of ways to modify or replace the default
configuration including by properties file, programmatically at runtime, or by
supplying your own configuration class. When you use a properties file to
control logging, you can either use the default properties file (which is not
recommended), %YOUR_JRE_PATH%/lib/logging.properties, or you can supply
your own properties file by setting the system property
java.util.logging.config.file to the name of the properties file you 
want to use, like this:

java -Djava.util.logging.config.file=log.properties tips.LogTip

Now logging properties will be read from the log.properties file, which
in this case is a modified version of the default logging.properties
file, as shown below:

handlers= java.util.logging.ConsoleHandler

.level=FINEST

java.util.logging.ConsoleHandler.level = FINEST
java.util.logging.ConsoleHandler.formatter =
java.util.logging.SimpleFormatter

Now when the class is run, the output looks like this:

Apr 1, 2003 11:12:00 PM tips.LogTip main
FINEST: the finest message
Apr 1, 2003 11:12:00 PM tips.LogTip main
FINER: finer message
Apr 1, 2003 11:12:00 PM tips.LogTip main
FINE: a fine message
Apr 1, 2003 11:12:00 PM tips.LogTip main
CONFIG: some configuration message
Apr 1, 2003 11:12:00 PM tips.LogTip main
INFO: a little bit of information
Apr 1, 2003 11:12:00 PM tips.LogTip main
WARNING: a warning message
Apr 1, 2003 11:12:00 PM tips.LogTip main
SEVERE: a severe message

In the real world, you'll probably want to exert more control over
logging. You'll want some classes logging at FINEST, others at SEVERE, and
some not at all. You'll want to log some information to the console, 
some to file, and maybe some to a database.
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