Java : Formatter (logging) with Examples

final var logger = Logger.getLogger("com.example.logging");
logger.setUseParentHandlers(false);

final var handler = new ConsoleHandler();
logger.addHandler(handler);

// SimpleFormatter
System.out.println(handler.getFormatter()); // java.util.logging.SimpleFormatter@d76b01a

logger.info("TEST LOG!");

// Result
// ↓
//Mar 17, 2022 1:04:31 AM com.example.logging.FormatterTest$1 method
//INFO: TEST LOG!

// XMLFormatter
handler.setFormatter(new XMLFormatter());

logger.info("TEST LOG!");

// Result
// ↓
//<record>
//  <date>2022-03-17T08:04:31.426182700Z</date>
//  <millis>1647504271426</millis>
//  <nanos>182700</nanos>
//  <sequence>2</sequence>
//  <logger>com.example.logging</logger>
//  <level>INFO</level>
//  <class>com.example.logging.FormatterTest$1</class>
//  <method>method</method>
//  <thread>1</thread>
//  <message>TEST LOG!</message>
//</record>

// A customized formatter.
class SingleLineFormatter extends Formatter {
    @Override
    public String format(LogRecord logRecord) {
        final var message = formatMessage(logRecord);
        return "%s (%s) : %s%n".formatted(
                logRecord.getInstant(), logRecord.getLevel().getName(), message);
    }
}

final var logger = Logger.getLogger("com.example.logging");
logger.setUseParentHandlers(false);

final var handler = new ConsoleHandler();
logger.addHandler(handler);

handler.setFormatter(new SingleLineFormatter());

logger.info("TEST LOG : 1");
logger.info("TEST LOG : 2");

// Result
// ↓
//2022-03-17T05:10:41.581937800Z (INFO) : TEST LOG : 1
//2022-03-17T05:10:41.584935200Z (INFO) : TEST LOG : 2

Formatter ()

Construct a new formatter.

Please see format(LogRecord record).

abstract String format (LogRecord record)

Format the given log record and return the formatted string.

class MyFormatter extends Formatter {
    @Override
    public String format(LogRecord logRecord) {
        final var message = formatMessage(logRecord);
        return "%s: %s%n".formatted(logRecord.getLevel().getName(), message);
    }
}

final var logger = Logger.getLogger("com.example.logging");
logger.setUseParentHandlers(false);

final var handler = new ConsoleHandler();
logger.addHandler(handler);

handler.setFormatter(new MyFormatter());

logger.info("TEST LOG : 1");
logger.info("TEST LOG : 2");

// Result
// ↓
//INFO: TEST LOG : 1
//INFO: TEST LOG : 2

String formatMessage (LogRecord record)

Localize and format the message string from a log record.

package com.example.logging;

import java.util.ListResourceBundle;

public class LoggerEnResourceBundle extends ListResourceBundle {

    @Override
    protected Object[][] getContents() {
        return new Object[][]{
                {"SAMPLE", "XXX"}
        };
    }
}

class MyFormatter extends Formatter {
    @Override
    public String format(LogRecord logRecord) {
        final var message = formatMessage(logRecord);
        return "%s: %s%n".formatted(logRecord.getLevel().getName(), message);
    }
}

final var logger = Logger.getLogger(
        "com.example.logging",
        "com.example.logging.LoggerEnResourceBundle");
logger.setUseParentHandlers(false);

final var handler = new ConsoleHandler();
logger.addHandler(handler);

handler.setFormatter(new MyFormatter());

logger.info("SAMPLE");

// Result
// ↓
//INFO: XXX

String getHead (Handler h)

Return the header string for a set of formatted records.

class MyFormatter extends Formatter {
    @Override
    public String format(LogRecord logRecord) {
        final var message = formatMessage(logRecord);
        return "%s (%s) : %s%n".formatted(
                logRecord.getInstant(), logRecord.getLevel().getName(), message);
    }

    @Override
    public String getHead(Handler h) {
        return "--- head ---%n".formatted();
    }

    @Override
    public String getTail(Handler h) {
        return "--- tail ---%n".formatted();
    }
}

final var logger = Logger.getLogger("com.example.logging");
logger.setUseParentHandlers(false);

final var pattern = Path.of("R:", "java-work", "log.txt");
final var handler = new FileHandler(pattern.toString());
logger.addHandler(handler);

handler.setFormatter(new MyFormatter());

logger.info("TEST LOG : 1");
logger.info("TEST LOG : 2");

logger.removeHandler(handler);
handler.close();

System.out.print(Files.readString(pattern));

// Result
// ↓
//--- head ---
//2022-03-17T05:18:12.308480200Z (INFO) : TEST LOG : 1
//2022-03-17T05:18:12.310478600Z (INFO) : TEST LOG : 2
//--- tail ---

String getTail (Handler h)

Return the tail string for a set of formatted records.

Please see getHead(Handler h).