Adding SLF4J Logging to a Class

Direct Answer

Add org.slf4j.Logger and LoggerFactory, create a class-named logger, and write a parameterized event such as log.info("User {} created", userId). Spring Boot applications commonly already have SLF4J and a provider through a starter; a plain Java project must declare the SLF4J API and exactly one compatible runtime provider.

Deep Dive & Architecture

Application code should depend on the SLF4J API. A typical class declares private static final Logger log = LoggerFactory.getLogger(UserService.class) and emits a parameterized event.

In a Spring Boot application, an ordinary starter usually supplies the SLF4J API and the default Logback provider. A minimal or deliberately reconfigured project may not, so check the dependency graph rather than assuming it.

A plain Java project needs slf4j-api plus one compatible provider at runtime. More than one provider can produce warnings and an unexpected selection; no provider means events are not handled as intended.

Code Example

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

public class UserService {
    private static final Logger log = LoggerFactory.getLogger(UserService.class);

    public User createUser(String email) {
        log.info("Creating user with email={}", email); // your first log line
        User user = repo.save(new User(email));
        log.debug("Saved user id={}", user.id());
        return user;
    }
}

// Lombok shortcut — generates the same 'log' field automatically
import lombok.extern.slf4j.Slf4j;

@Slf4j
public class UserService {
    public void demo() { log.info("log field generated by @Slf4j"); }
}

Common Mistakes & Pitfalls

• Wrong imports: importing a logging implementation instead of org.slf4j.*.
• Per-instance logger: dropping static, creating a logger object per instance.
• Concatenation: log.info("email=" + email) instead of log.info("email={}", email).