Creating and Using a Logger with SLF4J

Direct Answer

Create a logger with LoggerFactory.getLogger(MyClass.class) and call parameterized methods such as log.info("Created order {}", orderId). A private static final field is the common one-logger-per-class convention, not a correctness requirement. Passing the declaring class gives the logger a stable class/package name for configuration.

Deep Dive & Architecture

SLF4J loggers are obtained from LoggerFactory. Passing OrderService.class names the logger after that class, so package or class-level configuration can select it.

A common declaration is private static final Logger log = LoggerFactory.getLogger(OrderService.class);. static final avoids storing an identical logger reference in every instance and prevents reassignment, but SLF4J also supports instance logger fields. The important part is a stable, accurate logger name.

Use parameterized calls such as log.info("Created order {}", orderId) instead of concatenating values. Pass an exception object when its stack trace belongs on the event: log.error("Order {} failed", orderId, ex).

Code Example

import org.slf4j.Logger;
import org.slf4j.LoggerFactory;

public class PaymentService {
    // one logger per class, named com.app.PaymentService
    private static final Logger log = LoggerFactory.getLogger(PaymentService.class);

    public void pay(long orderId, double amount) {
        log.info("Charging order {} for {}", orderId, amount); // parameterized, no '+'
        if (amount <= 0) {
            log.warn("Non-positive amount {} for order {}", amount, orderId);
            return;
        }
        try {
            gateway.charge(amount);
        } catch (Exception e) {
            log.error("Charge failed for order {}", orderId, e); // exception last = stack trace
        }
    }
}

Common Mistakes & Pitfalls

• Non-static logger: private final Logger log = ... per instance creates needless objects; use static.
• Hardcoded name: getLogger("PaymentService") as a string breaks on rename and loses package config; use PaymentService.class.
• Concatenation: log.debug("x=" + obj) builds the string even when DEBUG is off; use log.debug("x={}", obj).