SLF4J and Logback basics in Spring Boot - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ component_behavior

intermediate

What is the output of this SLF4J logging code snippet?

Consider this Spring Boot component using SLF4J for logging:

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

public class MyService {
  private static final Logger logger = LoggerFactory.getLogger(MyService.class);

  public void process() {
    logger.info("Process started");
    logger.debug("Debugging process");
    logger.error("Error occurred");
  }
}

If the logging level is set to INFO, which messages will appear in the console?

Spring Boot

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

public class MyService {
  private static final Logger logger = LoggerFactory.getLogger(MyService.class);

  public void process() {
    logger.info("Process started");
    logger.debug("Debugging process");
    logger.error("Error occurred");
  }
}

A All three messages will appear: 'Process started', 'Debugging process', and 'Error occurred'.

B No messages will appear because INFO level disables all logging.

C Only 'Debugging process' and 'Error occurred' messages will appear.

D Only 'Process started' and 'Error occurred' messages will appear.

💡 Hint

Remember that INFO level shows messages at INFO and higher severity.

SLF4J logging respects the configured level. INFO level shows INFO, WARN, ERROR but hides DEBUG and TRACE messages.

📝 Syntax

intermediate

Which Logback configuration snippet correctly sets the root logger to WARN level?

Given these Logback XML configuration options, which one correctly sets the root logger level to WARN?

Spring Boot

<configuration>
  <!-- options below -->
</configuration>

A

<root level="warn">
  <appender-ref ref="STDOUT" />
</root>

B

<root level="Warn">
  <appender-ref ref="STDOUT" />
</root>

C

<root level="WARN">
  <appender-ref ref="STDOUT" />
</root>

D

<root level="warning">
  <appender-ref ref="STDOUT" />
</root>

💡 Hint

Logback levels are case-sensitive and must be uppercase.

Logback requires log levels in uppercase: TRACE, DEBUG, INFO, WARN, ERROR. 'warn' or 'Warn' or 'warning' are invalid.

🔧 Debug

advanced

Why does this SLF4J logging statement cause an error?

Examine this code snippet:

logger.info("User {} logged in at {}", userName);

What error will this cause and why?

Spring Boot

logger.info("User {} logged in at {}", userName);

A Syntax error because of missing semicolon.

B Runtime error due to missing argument for second placeholder.

C No error; logs message with 'null' for second placeholder.

D Compile-time error due to wrong method signature.

💡 Hint

Count the placeholders and arguments carefully.

SLF4J requires matching arguments for each '{}' placeholder. Missing arguments cause runtime exceptions.

❓ state_output

advanced

What is the effect of setting additivity to false in Logback logger configuration?

Given this Logback snippet:

<logger name="com.example" level="DEBUG" additivity="false">
  <appender-ref ref="FILE" />
</logger>

What happens to log messages from 'com.example' package?

Spring Boot

<logger name="com.example" level="DEBUG" additivity="false">
  <appender-ref ref="FILE" />
</logger>

A Messages are logged only to FILE appender, not to parent appenders.

B Messages are logged only to parent appenders, ignoring FILE appender.

C Messages are ignored because additivity=false disables logging.

D Messages are logged to FILE and all parent appenders.

💡 Hint

Additivity controls if logs bubble up to parent loggers.

Setting additivity=false stops logs from propagating to parent appenders, so only specified appenders receive logs.

🧠 Conceptual

expert

Which statement best describes the relationship between SLF4J and Logback in a Spring Boot application?

Choose the most accurate description of how SLF4J and Logback work together in Spring Boot logging.

A SLF4J is a logging facade that delegates logging calls to Logback, which is the actual logging implementation.

B Logback is a facade that delegates logging calls to SLF4J, which handles the actual logging.

C SLF4J and Logback are independent logging frameworks that cannot be used together.

D SLF4J replaces Logback entirely and provides all logging features by itself.

💡 Hint

Think about the role of a facade versus an implementation.

SLF4J provides a simple interface for logging. Logback is a concrete logging framework that SLF4J calls delegate to.