SLF4J with Logback: parameterized logging to avoid string concatenation overhead

Using string concatenation in log statements evaluates the string even when the log level is disabled, wasting CPU and memory on log messages that will never be written.

Use SLF4J parameterized logging with {} placeholders:

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

@Service
public class OrderService {
    private static final Logger log = LoggerFactory.getLogger(OrderService.class);
    // or with Lombok: @Slf4j

    public void processOrder(Order order) {
        // BAD — concatenation always evaluated
        log.debug("Processing order: " + order.getId() + " for user " + order.getUserId());

        // GOOD — deferred evaluation, no string built if DEBUG disabled
        log.debug("Processing order: {} for user {}", order.getId(), order.getUserId());

        // For expensive toString, use Supplier (SLF4J 2.x)
        log.debug("State: {}", () -> expensiveComputation());
    }
}

Configure per-package levels in logback-spring.xml or application.yml:

logging:
  level:
    root: warn
    com.myapp: info
    com.myapp.repository: debug

SLF4J checks whether the log level is enabled before constructing the message string. With {} placeholders, the arguments are passed as varargs and only formatted if the message will actually be logged.