Logging Frameworks

Logging is essential for debugging and monitoring applications.

Why Not System.out.println()?

• No log levels (INFO, DEBUG, ERROR)
• No timestamps
• No file output
• Hard to disable in production

SLF4J (Simple Logging Facade for Java)

SLF4J is a facade/abstraction for various logging frameworks.

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

public class MyClass {
    private static final Logger logger = LoggerFactory.getLogger(MyClass.class);
    
    public void doSomething() {
        logger.info("Info message");
        logger.debug("Debug message");
        logger.error("Error message");
    }
}

Log Levels

Logback Configuration

logback.xml:

<configuration>
    <appender name="STDOUT" class="ch.qos.logback.core.ConsoleAppender">
        <encoder>
            <pattern>%d{HH:mm:ss.SSS} [%thread] %-5level %logger{36} - %msg%n</pattern>
        </encoder>
    </appender>
    
    <root level="INFO">
        <appender-ref ref="STDOUT" />
    </root>
</configuration>

Best Practices

1. Use Parameterized Logging

// Bad
logger.info("User " + username + " logged in");

// Good
logger.info("User {} logged in", username);

2. Log Exceptions Properly

try {
    // code
} catch (Exception e) {
    logger.error("Error occurred", e); // Include exception
}

3. Use Appropriate Log Levels

logger.debug("Entering method");  // Development only
logger.info("User created");      // Important events
logger.warn("Deprecated API used"); // Warnings
logger.error("Failed to connect", e); // Errors

Quiz