Chapter 142: Logging in Java

Logging is your flight recorder: timestamps, severity, context, and enough detail to reconstruct failures without drowning operators in noise. SLF4J is the common API; Logback or Log4j2 sits behind it as the implementation in most Spring Boot apps.

1. Topic title

Logging: choose level, add correlation, ship machine-parseable lines to your log stack

2. What it means

Levels — ERROR (fix now), WARN (degraded), INFO (lifecycle), DEBUG (dev detail), TRACE (very chatty).

MDC (Mapped Diagnostic Context) — thread-local map for requestId, userId—auto-appended by appenders when configured.

Structured JSON — one JSON object per line—Elasticsearch, Loki, CloudWatch parse reliably.

3. Why it is used

Post-mortems without SSH guesswork.

Alerting on ERROR rate or missing heartbeat logs.

4. Mental sketch

println is shouting in a crowded room. Logging is radio call signs with frequency discipline—dispatch hears you when it matters.

5. Java code example

import org.slf4j.Logger;
import org.slf4j.LoggerFactory;
 
public class LogDemo {
    private static final Logger log = LoggerFactory.getLogger(LogDemo.class);
 
    public static void main(String[] args) {
        log.info("starting demo");
        try {
            int x = 1 / 0;
        } catch (ArithmeticException ex) {
            log.error("math failed", ex);
        }
    }
}

Pass the throwable as the last argument—stack traces land in appenders correctly.

6. Explanation of code

{} placeholders defer toString() until the level is enabled—cheaper than string concat for DEBUG guarded paths.

7. Common mistakes

Logging secrets—Authorization headers, SQL with raw passwords, JWT bodies.

INFO spam in hot loops—disk and cost burn.

8. Best practices

One line per event in JSON pipelines; multi-line only for stack traces with escape rules understood by your collector.

Sample DEBUG in prod for subset of traffic when needed.

9. Small practice task

Add MDC.put("requestId", UUID.randomUUID().toString()) at filter entry and MDC.clear() in finally—confirm IDs appear in Logback pattern.

Beginner tip

If logs disappear in Docker, stdout is correct—collect from the runtime, not only files inside the container.