How do I create a custom logger Formatter?

To create a custom Formatter we need to extends the java.util.logging.Formatter abstract class and implements the format(LogRecord) method. In the method then we can format the log message stored in the LogRecord to match our need.

The java.util.logging.Formatter class also have the getHead(Handler) and getTail(Handler) which can be overridden to add a head and a tail to our log message.

package org.kodejava.example.util.logging;

import java.text.DateFormat;
import java.text.SimpleDateFormat;
import java.util.Date;
import java.util.logging.*;

public class LogCustomFormatter {
    public static void main(String[] args) {
        Logger logger = Logger.getLogger(LogCustomFormatter.class.getName());
        logger.setUseParentHandlers(false);

        MyFormatter formatter = new MyFormatter();
        ConsoleHandler handler = new ConsoleHandler();
        handler.setFormatter(formatter);

        logger.addHandler(handler);
        logger.info("Example of creating custom formatter.");
        logger.warning("A warning message.");
        logger.severe("A severe message.");
    }
}

class MyFormatter extends Formatter {
    // Create a DateFormat to format the logger timestamp.
    private static final DateFormat df = new SimpleDateFormat("dd/MM/yyyy hh:mm:ss.SSS");

    public String format(LogRecord record) {
        StringBuilder builder = new StringBuilder(1000);
        builder.append(df.format(new Date(record.getMillis()))).append(" - ");
        builder.append("[").append(record.getSourceClassName()).append(".");
        builder.append(record.getSourceMethodName()).append("] - ");
        builder.append("[").append(record.getLevel()).append("] - ");
        builder.append(formatMessage(record));
        builder.append("\n");
        return builder.toString();
    }

    public String getHead(Handler h) {
        return super.getHead(h);
    }

    public String getTail(Handler h) {
        return super.getTail(h);
    }
}

Below is an output produced by the custom formatter above.

01/05/2009 06:22:09.372 - [org.kodejava.example.util.logging.LogCustomFormatter.main] - [INFO] - Example of creating custom formatter.
01/05/2009 06:22:09.374 - [org.kodejava.example.util.logging.LogCustomFormatter.main] - [WARNING] - A warning message.
01/05/2009 06:22:09.374 - [org.kodejava.example.util.logging.LogCustomFormatter.main] - [SEVERE] - A severe message.

How do I set the formatter of logger handlers?

In this example we’ll see how to set the formatter for the logger handlers. We set the formatter by calling the Handler.setFormatter() method. In the code below we set a SimpleFormatter for our FileHandler handler and XMLFormatter for the ConsoleHandler handler.

This SimpleFormatter format the log in a plain text information while on the other side the XMLFormatter format the log record in XML format.

package org.kodejava.example.util.logging;

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

public class LogFormatter {
    public static void main(String[] args) {
        Logger logger = Logger.getLogger(LogFormatter.class.getName());

        try {
            // Create a FileHanlder that will log to mylog.txt with a
            // SimpleFormatter.
            FileHandler simpleHandler = new FileHandler("mylog.txt", true);
            simpleHandler.setFormatter(new SimpleFormatter());
            logger.addHandler(simpleHandler);

            // Create a ConsoleHanlder that will log to the console with
            // an XMLFormatter.
            ConsoleHandler consoleHandler = new ConsoleHandler();
            consoleHandler.setFormatter(new XMLFormatter());
            logger.addHandler(consoleHandler);

            // Do not sent the message to parent handlers. 
            logger.setUseParentHandlers(false);
        } catch (IOException e) {
            logger.log(Level.SEVERE, "Fail to create logger file handler.", e);
        }

        logger.info("Logging application information message.");
        logger.warning("Logging application warning message.");
    }
}

How do I use logger ConsoleHandler?

In this example you’ll see how to add ConsoleHandler to the Logger instance. It is simple, just create an instance of ConsoleHandler and add it to the Logger using the Logger.addHandler() method.

package org.kodejava.example.util.logging;

import java.util.logging.Logger;
import java.util.logging.ConsoleHandler;
import java.util.logging.Level;

public class LogConsoleHandler {
    public static void main(String[] args) {
        // Obtains Logger instance
        Logger logger = Logger.getLogger(LogConsoleHandler.class.getName());

        // Add ConsoleHandler to Logger.
        ConsoleHandler consoleHandler = new ConsoleHandler();
        logger.addHandler(consoleHandler);

        if (logger.isLoggable(Level.INFO)) {
            logger.info("This is information message for testing ConsoleHandler");
        }
    }
}