Log levels (TRACE, DEBUG, INFO, WARN, ERROR) in Spring Boot - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is the purpose of the TRACE log level in Spring Boot?

TRACE is the most detailed log level. It records very fine-grained information, useful for diagnosing problems or understanding detailed program flow.

beginner

Which log level is typically used to log general information about application progress or state?

INFO level is used to log general, important runtime events that highlight the progress of the application without being too detailed.

intermediate

What kind of events should be logged at WARN level?

WARN level logs events that are unusual or unexpected but do not stop the application from working. They signal potential problems.

intermediate

How does the ERROR log level differ from WARN in Spring Boot logging?

ERROR logs serious problems that cause parts of the application to fail or stop working correctly. It indicates failures that need immediate attention.

intermediate

Why should DEBUG level logs be used carefully in production environments?

DEBUG logs provide detailed information useful for developers but can produce large log files and expose sensitive information, so they are usually disabled in production.

Which log level provides the most detailed information in Spring Boot?

A TRACE

B INFO

C WARN

D ERROR

TRACE is the most detailed log level, capturing very fine-grained information.

What log level should you use to record a recoverable problem that might need attention?

A DEBUG

B WARN

C ERROR

D INFO

WARN is used for unusual or unexpected events that do not stop the application but may need attention.

Which log level is best for general messages about application progress?

A TRACE

B ERROR

C INFO

D DEBUG

INFO logs general important runtime events without too much detail.

Why might DEBUG logs be turned off in production?

A They are not useful for developers

B They can slow down the application

C They produce too little information

D They can expose sensitive data and create large log files

DEBUG logs can expose sensitive info and create large logs, so they are often disabled in production.

Which log level indicates a serious failure that needs immediate attention?

A ERROR

B INFO

C WARN

D TRACE

ERROR logs serious problems causing failures that require immediate fixing.

Explain the differences between TRACE, DEBUG, INFO, WARN, and ERROR log levels in Spring Boot.

Think about how much detail each level provides and the severity of events they represent.

When and why would you adjust log levels in a Spring Boot application?

Consider balancing detail with performance and security.

Practice

1. Which log level in Spring Boot shows the most detailed information, typically used for diagnosing problems during development?

easy

A. ERROR

B. INFO

C. WARN

D. TRACE

Solution

1. Step 1: Understand log level hierarchy

   TRACE is the most detailed log level, showing fine-grained events.

2. Step 2: Compare with other levels

   INFO, WARN, and ERROR show less detail and focus on general info or problems.

3. Final Answer:

   TRACE -> Option D

4. Quick Check:

   Most detailed log = TRACE [OK]

Hint: TRACE logs show the tiniest details for debugging [OK]

Common Mistakes:

• Confusing INFO with TRACE as most detailed
• Thinking ERROR shows all details
• Assuming WARN is more detailed than DEBUG

2. Which of the following is the correct way to set the log level to WARN in application.properties for Spring Boot?

easy

A. logging.level.root=WARN

B. log.level=WARN

C. logging.level=WARN

D. logging.root.level=warn

Solution

1. Step 1: Recall Spring Boot logging property format

   The correct property is logging.level.root to set root log level.

2. Step 2: Check option syntax

   logging.level.root=WARN uses correct key and uppercase level, matching Spring Boot conventions.

3. Final Answer:

   logging.level.root=WARN -> Option A

4. Quick Check:

   Correct property key = logging.level.root [OK]

Hint: Use logging.level.root=LEVEL to set global log level [OK]

Common Mistakes:

• Using log.level instead of logging.level.root
• Incorrect property key like logging.root.level
• Using lowercase level names incorrectly

3. Given this Spring Boot code snippet:

logger.debug("Debug message");
logger.info("Info message");
logger.error("Error message");

If the log level is set to INFO, which messages will appear in the logs?

medium

A. Info message, Error message

B. Only Error message

C. Debug message, Info message, Error message

D. Only Debug message

Solution

1. Step 1: Understand log level filtering

   Setting level to INFO shows logs at INFO and higher severity (WARN, ERROR).

2. Step 2: Check which messages qualify

   DEBUG is lower than INFO, so it is hidden; INFO and ERROR show.

3. Final Answer:

   Info message, Error message -> Option A

4. Quick Check:

   INFO level shows INFO and ERROR logs [OK]

Hint: Logs show messages at set level and above [OK]

Common Mistakes:

• Assuming DEBUG logs show at INFO level
• Thinking ERROR logs are hidden at INFO level
• Confusing INFO with DEBUG level

4. You set logging.level.com.example=DEBUG in application.properties, but DEBUG messages from com.example.service do not appear. What is the most likely cause?

medium

A. DEBUG level is not supported in Spring Boot

B. The root log level is set higher than DEBUG, blocking DEBUG logs

C. The package name is misspelled in the property

D. You must restart the application to apply log level changes

Solution

1. Step 1: Check property correctness

   If the package name is misspelled, the setting won't apply to the intended classes.

2. Step 2: Evaluate other options

   DEBUG is supported; root level does not block specific package settings; restart is usually required but not the main cause.

3. Final Answer:

   The package name is misspelled in the property -> Option C

4. Quick Check:

   Correct package name is critical for log level settings [OK]

Hint: Check package spelling carefully in logging properties [OK]

Common Mistakes:

• Assuming root level overrides package level always
• Believing DEBUG is unsupported
• Ignoring typos in package names

5. You want to see detailed logs only for the com.example.repository package but keep the rest of the application logging at WARN level. Which application.properties configuration achieves this?

hard

A. logging.level.root=TRACE
logging.level.com.example.repository=WARN

B. logging.level.root=WARN
logging.level.com.example.repository=TRACE

C. logging.level.root=INFO
logging.level.com.example.repository=ERROR

D. logging.level.root=DEBUG
logging.level.com.example.repository=INFO

Solution

1. Step 1: Set global log level to WARN

   Set logging.level.root=WARN to limit logs globally to warnings and errors.

2. Step 2: Override package log level to TRACE

   Set logging.level.com.example.repository=TRACE to get detailed logs only for that package.

3. Final Answer:

   logging.level.root=WARN logging.level.com.example.repository=TRACE -> Option B

4. Quick Check:

   Global WARN + package TRACE = detailed logs only there [OK]

Hint: Set root level high, override package level low for details [OK]

Common Mistakes:

• Setting root level too low, flooding logs
• Overriding package with higher level than root
• Confusing package and root property keys