The Formatter class in java.util.logging is used to control the output format of log messages. When a Logger sends a log record to a Handler, the handler uses a Formatter to convert the LogRecord into a human-readable string.
Common Subclasses of Formatter
Commomly Used Methods
Simple Example with Built-in Formatters
import java.util.logging.*;
public class SimpleFormatterExample {
public static void main(String[] args) throws Exception {
Logger logger = Logger.getLogger(SimpleFormatterExample.class.getName());
logger.setUseParentHandlers(false); // Disable default console output
FileHandler fh = new FileHandler("simple_log.log");
// Use built-in SimpleFormatter
fh.setFormatter(new SimpleFormatter());
logger.addHandler(fh);
logger.info("Logging with SimpleFormatter");
}
}
Output in simple_log.log:
May 24, 2025 12:00:00 PM SimpleFormatterExample main
INFO: Logging with SimpleFormatterCode language: CSS (css)Problem Statement:
LotusJavaPrince and Mahesh need a clean, one-line-per-entry log format for their payment gateway system. The default format adds timestamps and log level on separate lines, which is hard to parse for dashboards. They want a custom formatter that logs messages as:
[LEVEL] – MESSAGE
import java.util.logging.*;
import java.io.IOException;
// Custom Formatter
class OneLineFormatter extends Formatter {
@Override
public String format(LogRecord record) {
return "[" + record.getLevel() + "] - " + record.getMessage() + "\n";
}
}
class PaymentGateway {
private static final Logger logger = Logger.getLogger(PaymentGateway.class.getName());
public void processPayment(String user, double amount) {
logger.info("Processing payment of $" + amount + " for user: " + user);
if (amount > 10000) {
logger.warning("Large payment detected for " + user);
} else {
logger.info("Payment processed successfully for " + user);
}
}
}
public class PaymentLoggerCaseStudy {
public static void main(String[] args) {
try {
setupCustomFormatter();
} catch (IOException e) {
System.err.println("Logger setup failed: " + e.getMessage());
return;
}
PaymentGateway gateway = new PaymentGateway();
gateway.processPayment("LotusJavaPrince", 5000); // Normal
gateway.processPayment("Mahesh", 15000); // Large
}
private static void setupCustomFormatter() throws IOException {
Logger rootLogger = Logger.getLogger("");
rootLogger.setUseParentHandlers(false);
FileHandler fh = new FileHandler("payments.log", true);
fh.setFormatter(new OneLineFormatter());
rootLogger.addHandler(fh);
rootLogger.setLevel(Level.INFO);
}
}[INFO] - Processing payment of $5000.0 for user: LotusJavaPrince
[INFO] - Payment processed successfully for LotusJavaPrince
[INFO] - Processing payment of $15000.0 for user: Mahesh
[WARNING] - Large payment detected for MaheshFormatter allows full control over how logs are formatted.
- You can use
SimpleFormatterorXMLFormatter, or write your own. - Great for creating readable logs, dashboard feeds, or structured outputs.
- Combine formatters with
FileHandler,StreamHandler, orSocketHandlerfor powerful logging solutions.