SLF4J - Quick Reference
When to Use This Skill
- Standard logging API for Java
- Integration with Logback, Log4j2
- Java logging best practices
Deep Knowledge: Use mcp__documentation__fetch_docs with technology: slf4j for comprehensive documentation.
Essential Patterns
Logger Declaration
import org.slf4j.Logger;
import org.slf4j.LoggerFactory;
public class UserService {
private static final Logger log = LoggerFactory.getLogger(UserService.class);
// Or with Lombok
// @Slf4j on class
}
Logging Levels
log.trace("Detailed debug info: {}", details);
log.debug("Debug info for development");
log.info("Normal operation: user {} logged in", userId);
log.warn("Potential problem: {} retries remaining", retries);
log.error("Error occurred: {}", message, exception);
Parameterized Logging
// Good - lazy evaluation
log.debug("Processing order {} for user {}", orderId, userId);
// Avoid - always evaluates
log.debug("Processing order " + orderId + " for user " + userId);
// Multiple parameters
log.info("User {} performed {} on resource {}", userId, action, resourceId);
Exception Logging
try {
processOrder(order);
} catch (OrderException e) {
// Exception is always last parameter
log.error("Failed to process order {}: {}", order.getId(), e.getMessage(), e);
}
MDC (Mapped Diagnostic Context)
import org.slf4j.MDC;
// Set context
MDC.put("requestId", requestId);
MDC.put("userId", userId);
try {
// All logs in this scope will include requestId and userId
log.info("Processing request");
} finally {
MDC.clear();
}
Conditional Logging
if (log.isDebugEnabled()) {
log.debug("Expensive computation result: {}", expensiveMethod());
}
// Better with lambdas (SLF4J 2.0+)
log.atDebug().log(() -> "Result: " + expensiveMethod());
Fluent API (SLF4J 2.0+)
log.atInfo()
.addKeyValue("orderId", orderId)
.addKeyValue("amount", amount)
.log("Order processed successfully");
When NOT to Use This Skill
- Logging framework configuration: Use
logback or Log4j2 skills for XML/config
- Performance tuning: Configuration-level optimization is in implementation skills
- Transport/appender setup: That's implementation-specific (Logback/Log4j2)
- Non-Java projects: Use language-appropriate logging APIs
- Direct implementation usage: Always code to SLF4J API, not Logback/Log4j2 directly
Anti-Patterns
| Anti-Pattern |
Why It's Bad |
Solution |
| String concatenation in logs |
Always evaluated, performance hit |
Use parameterized logging: log.info("User {}", id) |
| Logging without level check for expensive ops |
Wastes CPU even when disabled |
Use if (log.isDebugEnabled()) before expensive calls |
| Catching exceptions without logging |
Silent failures, hard to debug |
Always log with log.error("msg", exception) |
| Not using MDC for request context |
Loses correlation across logs |
Use MDC.put() with try-finally |
| Using wrong log level |
Too much noise or missing issues |
Follow conventions: ERROR=requires action, WARN=potential issue |
| Logging sensitive data |
Security/compliance violation |
Mask PII, passwords, tokens before logging |
Quick Troubleshooting
| Issue |
Cause |
Solution |
| NoClassDefFoundError: StaticLoggerBinder |
No SLF4J implementation on classpath |
Add Logback or Log4j2 dependency |
| Multiple bindings warning |
Multiple SLF4J implementations |
Keep only one: Logback OR Log4j2 |
| Logs not appearing |
Wrong log level |
Check implementation config (logback.xml) |
| Parameters not replaced |
Wrong placeholder syntax |
Use {} not %s or other formats |
| MDC values empty |
MDC cleared or wrong thread |
Ensure MDC.put() before logging, clear in finally |
| Exception stack trace missing |
Exception not last parameter |
Put exception as last param: log.error("msg", e) |
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