Java Logging API - Tutorial

The JDK contains the "Java Logging API". Via a logger you can save text to a central place to report on errors, provide additional information about your program, etc. This logging API allows to configure how messages are written by which class with which priority.

1.2. Create a logger

The package java.util.logging provides the logging capabilities via the class Logger.

To create a logger in your Java coding.

				
import java.util.logging.Logger;

private final static Logger LOGGER = Logger.getLogger(MyClass.class .getName());

1.3. Level

The log levels define the severity of a message. The class Level is used to define which messages should be written to the log.

Die Log Levels in descending order are:

SEVERE (highest)
WARNING
INFO
CONFIG
FINE
FINER
FINEST

In addition to that you have also the levels OFF and ALL to turn the logging of or to log everything.

For example the following will set the logger to level info which means all messages with severe, warning and info will be logged.

				
LOGGER.setLevel(Level.INFO);

1.4. Handler

Each logger can have access to several handler.

The handler receives the log message from the logger and exports it to a certain target

A handler can be turn off with setLevel(Level.OFF) and turned on with setLevel(...)

You have several standard handler, e.g.

ConsoleHandler: Write the log message to console
FileHandler: Writes the log message to file

Log Levels INFO and higher will be automatically written to the console.

1.5. Formatter

Each handlers output can be configured with a formatter

Available formatter

SimpleFormatter Generate all messages as text
XMLFormatter Generates XML output for the log messages

You can also build your own formatter. The following is an example of a formatter which will use create HTML output.

				
package logging;

import java.text.SimpleDateFormat;
import java.util.Date;
import java.util.logging.Formatter;
import java.util.logging.Handler;
import java.util.logging.Level;
import java.util.logging.LogRecord;

//This custom formatter formats parts of a log record to a single line
class MyHtmlFormatter extends Formatter
{
	// This method is called for every log records
	public String format(LogRecord rec)
	{
		StringBuffer buf = new StringBuffer(1000);
		// Bold any levels >= WARNING
		buf.append("<tr>");
		buf.append("<td>");

		if (rec.getLevel().intValue() >= Level.WARNING.intValue())
		{
			buf.append("<b>");
			buf.append(rec.getLevel());
			buf.append("</b>");
		} else
		{
			buf.append(rec.getLevel());
		}
		buf.append("</td>");
		buf.append("<td>");
		buf.append(calcDate(rec.getMillis()));
		buf.append(' ');
		buf.append(formatMessage(rec));
		buf.append('\n');
		buf.append("<td>");
		buf.append("</tr>\n");
		return buf.toString();
	}

	private String calcDate(long millisecs)
	{
		SimpleDateFormat date_format = new SimpleDateFormat("MMM dd,yyyy HH:mm");
		Date resultdate = new Date(millisecs);
		return date_format.format(resultdate);
	}

	// This method is called just after the handler using this
	// formatter is created
	public String getHead(Handler h)
	{
		return "<HTML>\n<HEAD>\n" + (new Date()) + "\n</HEAD>\n<BODY>\n<PRE>\n"
				+ "<table border>\n  "
				+ "<tr><th>Time</th><th>Log Message</th></tr>\n";
	}

	// This method is called just after the handler using this
	// formatter is closed
	public String getTail(Handler h)
	{
		return "</table>\n  </PRE></BODY>\n</HTML>\n";
	}
}

1.6. Log Manager

The log manager is responsible for creating and managing the logger and the maintenance of the configuration.

We could set the logging level for a package, or even a set of packages, by calling the LogManager.setLevel(String name, Level level) method. So, for example, we could set the logging level of all loggers "logging" to Level.FINE by making this call:

				
LogManager.getLogManager().setLevel("logging", Level.FINE)

1.7. Best Practices

It is common practice to use the fully qualified name of each class whose activity is being logged as a message category because this allows developers to fine-tune log settings for each class.

Using the fully qualified class name of your class as the name of your Logger is the approach recommended by the Logging API documentation.

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