Careeroza

Log Levels

The Five Levels

LevelUse forShopEase example
ERRORsomething failed and needs attentionpayment gateway call failed
WARNunexpected but handledstock is low, retrying a call
INFOimportant business events (default level)order 101 placed
DEBUGdetails for developerscomputed cart total and discounts
TRACEvery detailed, step by stepevery field of a request

A logger prints a message only if its level is equal to or more severe than the configured level. With level INFO: ERROR, WARN and INFO are printed; DEBUG and TRACE are not.

Level Filtering

Example

An enum models the order of severity. Changing the configured level changes what gets printed:

import java.util.List;

public class Main {
    enum Level { TRACE, DEBUG, INFO, WARN, ERROR }

    record LogEvent(Level level, String message) {}

    public static void main(String[] args) {
        List<LogEvent> events = List.of(
            new LogEvent(Level.TRACE, "Request headers: {Accept=application/json}"),
            new LogEvent(Level.DEBUG, "Cart total computed: 2998"),
            new LogEvent(Level.INFO, "Order 101 placed"),
            new LogEvent(Level.WARN, "Stock for product 1 is low: 3 left"),
            new LogEvent(Level.ERROR, "Payment failed for order 102: gateway timeout"));

        for (Level configured : List.of(Level.INFO, Level.DEBUG, Level.ERROR)) {
            System.out.println("--- logging.level = " + configured + " ---");
            for (LogEvent e : events) {
                if (e.level().compareTo(configured) >= 0) {
                    System.out.printf("%-5s %s%n", e.level(), e.message());
                }
            }
        }
    }
}

Output

--- logging.level = INFO ---
INFO  Order 101 placed
WARN  Stock for product 1 is low: 3 left
ERROR Payment failed for order 102: gateway timeout
--- logging.level = DEBUG ---
DEBUG Cart total computed: 2998
INFO  Order 101 placed
WARN  Stock for product 1 is low: 3 left
ERROR Payment failed for order 102: gateway timeout
--- logging.level = ERROR ---
ERROR Payment failed for order 102: gateway timeout

Logging Exceptions

try {
    paymentGateway.charge(orderId, amount);
} catch (PaymentException e) {
    // pass the exception as the LAST argument: the full stack trace is logged
    log.error("Payment failed for order {}", orderId, e);
    throw e;
}

Note: Never log passwords, full card numbers, OTPs or tokens. Logs are read by many people and kept for a long time; leaking personal data in logs can break laws such as India's DPDP Act.