How do I create a custom logger Formatter?

By Wayan in Logging 5 Comments

To create a custom Formatter we need to extend 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.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.

08/10/2021 07:55:55.153 - [org.kodejava.util.logging.LogCustomFormatter.main] - [INFO] - Example of creating custom formatter.
08/10/2021 07:55:55.164 - [org.kodejava.util.logging.LogCustomFormatter.main] - [WARNING] - A warning message.
08/10/2021 07:55:55.164 - [org.kodejava.util.logging.LogCustomFormatter.main] - [SEVERE] - A severe message.
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5 Comments

  2. Why did you explicitly create a new ConsoleHandler? Why can’t you re-use the handler it has? for example:

    for (Handler handler : logger.getHandlers()) {
    handler.setFormatter(formatter);
}

    ^ this doesn’t work for me. but I don’t know why. 🙁

    Reply

  3. I see some bad practices here. DateFormat is not thread-safe – use DateTimeFormatter (Java 8) or create new instance every time you need it (which probabably isn’t too good either 😉 ).

    Use System.lineSeparator() instead of “\n”! You don’t need (and shouldn’t) override getHead() and getTail() methods. StringBuilder can be used in a lot more “streamlined” manner (append().append().apend()) – you do it, but only partially.

    Last, but not least, as Rodrigo mentioned, the formatter itself should be in separate class file.

    By the way, for case of this example it probably would be good to add handling throwables as well.

    Reply

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