Java: Logging API
Java Logging
This article describes how to use the Logging API in Java programs. It includes an example for creating a 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 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 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(""); buf.append("");
if (rec.getLevel().intValue() >= Level.WARNING.intValue()) { buf.append(""); buf.append(rec.getLevel()); buf.append(""); } else { buf.append(rec.getLevel()); }
buf.append(""); buf.append("");
buf.append(calcDate(rec.getMillis())); buf.append(' '); buf.append(formatMessage(rec)); buf.append('\n');
buf.append(""); buf.append("\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
\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 </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. 2. Example
This example is stored in the project "de.vogella.logger".
Create your own formatter.
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(""); buf.append("");
if (rec.getLevel().intValue() >= Level.WARNING.intValue()) { buf.append(""); buf.append(rec.getLevel()); buf.append(""); } else { buf.append(rec.getLevel()); }
buf.append(""); buf.append("");
buf.append(calcDate(rec.getMillis())); buf.append(' '); buf.append(formatMessage(rec)); buf.append('\n');
buf.append(""); buf.append("\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
\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 </BODY>\n</HTML>\n";
} }
Initialize the logger.
package logging;
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 { // Create Logger Logger logger = Logger.getLogger(""); 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); } }
Use the logger.
package logging;
import java.io.IOException; import java.util.logging.Level; import java.util.logging.Logger;
public class UseLogger { // Always use the classname, this way you can refactor private final static Logger LOGGER = Logger.getLogger(UseLogger.class .getName());
public void writeLog() { // 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 logger = new UseLogger(); try { MyLogger.setup(); } catch (IOException e) { e.printStackTrace(); throw new RuntimeException("Problems with creating the log files"); } logger.writeLog(); } }