Java Logging API - Tutorial

Lars Vogel

Version 1.3

14.01.2013

Revision History
Revision 0.1	01.07.2007	Lars Vogel	Created
Revision 0.2 - 1.3	09.07.2007 - 14.01.2013	Lars Vogel	Bug fixes and Enhancements

Java Logging

This article describes how to use the Logging API in Java programs. It includes an example for creating an HTML logger.

Table of Contents

1. Overview

1.1. Logging in Java
1.2. Create a logger
1.3. Level
1.4. Handler
1.5. Formatter
1.6. Log Manager
1.7. Best Practices

2. Example

2.1. Create the logger
2.2. Use the logger

3. Thank you

4. Questions and Discussion

5. Links and Literature

1. Overview

1.1. Logging in Java

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

To create a logger in your Java coding you can use the following snippet.

import java.util.logging.Logger;

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

The Logger you create a actually a hierarchy of Loggers, and a . (dot) in the hierarchy indicates a level in the hierarchy. So if you get a Logger for the com.example key this Logger is a child of the com Logger and the com Logger is child of the Logger for the empty String. You can configure the main logger and this affects all its children.

1.3. Level

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

The following lists the Log Levels in descending order:

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 example code sets the logger to the info level, 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 the setLevel(Level.OFF) method and turned on with setLevel() method.

You have several standard handler, the following list gives some examples.

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 de.vogella.logger;

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 width=\"100%\" border>\n  "
        + "<tr><th>Level</th>" +
        "<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().getLogger(Logger.GLOBAL_LOGGER_NAME).setLevel(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.

2. Example

2.1. Create the logger

This example is stored in the project called de.vogella.logger.

Create your own formatter class.

package de.vogella.logger;

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 width=\"100%\" border>\n  "
        + "<tr><th>Level</th>" +
        "<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";
  }
}

Initialize the logger via the following code. This Logger class uses the new HTML formater you created.

package de.vogella.logger;

import java.io.IOException;
import java.util.logging.FileHandler;
import java.util.logging.Formatter;
import java.util.logging.Level;
import java.util.logging.Logger;
import java.util.logging.SimpleFormatter;

public class MyLogger {
  static private FileHandler fileTxt;
  static private SimpleFormatter formatterTxt;

  static private FileHandler fileHTML;
  static private Formatter formatterHTML;

  static public void setup() throws IOException {

    // Get the global logger to configure it
    Logger logger = Logger.getLogger(Logger.GLOBAL_LOGGER_NAME);

    logger.setLevel(Level.INFO);
    fileTxt = new FileHandler("Logging.txt");
    fileHTML = new FileHandler("Logging.html");

    // Create txt Formatter
    formatterTxt = new SimpleFormatter();
    fileTxt.setFormatter(formatterTxt);
    logger.addHandler(fileTxt);

    // Create HTML Formatter
    formatterHTML = new MyHtmlFormatter();
    fileHTML.setFormatter(formatterHTML);
    logger.addHandler(fileHTML);
  }
}

2.2. Use the logger

The following example class demonstrates how you can use your MyLogger class to create log messages.

package de.vogella.logger.test;

import java.io.IOException;
import java.util.logging.Level;
import java.util.logging.Logger;

import de.vogella.logger.MyLogger;

public class UseLogger {
  // Always use the classname, this way you can refactor
  private final static Logger LOGGER = Logger.getLogger(UseLogger.class
      .getName());

  public void doSomeThingAndLog() {
    // Image here some real work

    // Now we demo the logging

    // Set the LogLevel to Severe, only severe Messages will be written
    LOGGER.setLevel(Level.SEVERE);
    LOGGER.severe("Info Log");
    LOGGER.warning("Info Log");
    LOGGER.info("Info Log");
    LOGGER.finest("Really not important");

    // Set the LogLevel to Info, severe, warning and info will be written
    // Finest is still not written
    LOGGER.setLevel(Level.INFO);
    LOGGER.severe("Info Log");
    LOGGER.warning("Info Log");
    LOGGER.info("Info Log");
    LOGGER.finest("Really not important");
  }

  public static void main(String[] args) {
    UseLogger tester = new UseLogger();
    try {
      MyLogger.setup();
    } catch (IOException e) {
      e.printStackTrace();
      throw new RuntimeException("Problems with creating the log files");
    }
    tester.doSomeThingAndLog();
  }
}

3. Thank you

Please help me to support this article:

4. Questions and Discussion

Before posting questions, please see the vogella FAQ. If you have questions or find an error in this article please use the www.vogella.com Google Group. I have created a short list how to create good questions which might also help you.

5. Links and Literature

http://www.onjava.com/pub/a/onjava/2002/06/19/log.html

An introduction to the Java Logging API," Brian Gilstrap (OnJava.com, June 2002) talks about the use of the standard logging API.

http://www.fawcette.com/javapro/2002_06/magazine/features/shalloway/

The Java Logging API," Stuart Dabbs Halloway (JavaPro, June 2002) is another good introduction to the API.