The FileHandler in java.util.logging is used to write log messages to disk files. It supports features like log rotation and appending to existing files.
Key Features
- Writes logs to files.
- Can rotate log files based on size and limit.
- Can append to existing files.
- Works with formatters like
SimpleFormatteror custom ones.
Commonly Used Methods
Simple Program
import java.io.IOException;
import java.util.logging.*;
public class SimpleFileHandlerDemo {
public static void main(String[] args) {
Logger logger = Logger.getLogger("file.logger");
logger.setUseParentHandlers(false);
try {
FileHandler fileHandler = new FileHandler("simplelog.log", true); // append = true
fileHandler.setLevel(Level.ALL);
fileHandler.setFormatter(new SimpleFormatter());
logger.addHandler(fileHandler);
logger.setLevel(Level.ALL);
logger.info("This is an INFO log to file.");
logger.warning("This is a WARNING log to file.");
} catch (IOException e) {
e.printStackTrace();
}
}
}Output: simplelog.log contains:
May 24, 2025 10:45:00 AM SimpleFileHandlerDemo main
INFO: This is an INFO log to file.
May 24, 2025 10:45:00 AM SimpleFileHandlerDemo main
WARNING: This is a WARNING log to file.Code language: CSS (css)Problem Statement
LotusJavaPrince and Mahesh are developing a secure transaction system. Logs should not go to the console. Instead, they must go to a rotating log file system to track transactions and errors while ensuring file size is controlled.
They use FileHandler with log rotation and a custom formatter for readable output.
import java.io.IOException;
import java.util.logging.*;
class TransactionFormatter extends Formatter {
public String format(LogRecord record) {
return String.format("[%s] - %s: %s%n",
record.getLevel(),
record.getLoggerName(),
record.getMessage());
}
}
public class TransactionLogger {
private static final Logger logger = Logger.getLogger("com.bank.transaction");
public static void main(String[] args) {
setupLogger();
processTransaction("LotusJavaPrince", 10000, true);
processTransaction("Mahesh", 25000, false);
processTransaction("Intruder", 50000, false);
}
private static void setupLogger() {
logger.setUseParentHandlers(false);
try {
// Limit = 1MB, Count = 5 rotating files
FileHandler fh = new FileHandler("transactions.log", 1024 * 1024, 5, true);
fh.setFormatter(new TransactionFormatter());
fh.setLevel(Level.INFO);
logger.addHandler(fh);
logger.setLevel(Level.ALL);
} catch (IOException e) {
System.err.println("FileHandler setup failed: " + e.getMessage());
}
}
private static void processTransaction(String user, double amount, boolean success) {
if (success) {
logger.info(user + " transferred $" + amount + " successfully.");
} else if ("Mahesh".equals(user)) {
logger.warning("Transaction failed for Mahesh. Exceeds daily limit.");
} else {
logger.severe("Unauthorized transaction attempt by " + user + "!");
}
}
}Output: transactions.log (rotated if size > 1MB)
[INFO] - com.bank.transaction: LotusJavaPrince transferred $10000.0 successfully.
[WARNING] - com.bank.transaction: Transaction failed for Mahesh. Exceeds daily limit.
[SEVERE] - com.bank.transaction: Unauthorized transaction attempt by Intruder!FileHandler play a vital role in directing log messages to their respective file.
- Ideal for production-grade logging.
- Saves logs to a file on disk.
- Supports log rotation, file size limits, and appending.
- Useful for audit trails, security tracking, and error diagnostics.