Log levels (TRACE, DEBUG, INFO, WARN, ERROR) in Spring Boot - Performance & Optimization

Performance: Log levels (TRACE, DEBUG, INFO, WARN, ERROR)

MEDIUM IMPACT

Log levels affect application runtime performance and resource usage by controlling the amount of logging output generated.

Controlling log verbosity in a Spring Boot application

Spring Boot

if (logger.isTraceEnabled()) { logger.trace("Detailed trace info: {}", expensiveMethod()); }

Checks if TRACE is enabled before calling expensiveMethod(), avoiding unnecessary computation.

📈 Performance Gainavoids CPU and memory use when TRACE is off, improving runtime speed

Controlling log verbosity in a Spring Boot application

Spring Boot

logger.trace("Detailed trace info: {}", expensiveMethod());

TRACE logs are always evaluated even if TRACE level is disabled, causing unnecessary CPU and memory use.

📉 Performance Costblocks main thread for expensiveMethod() call even if log is not output

Performance Comparison

Pattern	CPU Usage	I/O Overhead	Memory Usage	Verdict
Unconditional TRACE logging with expensive calls	High (always runs expensive code)	High (many logs written)	High (string creation)	[X] Bad
Conditional TRACE logging with isTraceEnabled()	Low (skips expensive code)	Low (logs only when enabled)	Low (avoids string creation)	[OK] Good
INFO level logging for key events	Moderate	Moderate	Moderate	[!] OK
ERROR level logging only for errors	Low	Low	Low	[OK] Good

Rendering Pipeline

Logging statements are processed during application runtime and can block threads or increase I/O if too verbose or expensive.

→Application Runtime

→I/O Operations

⚠️ BottleneckExcessive logging causes CPU overhead and slows down I/O, impacting app responsiveness.

Optimization Tips

1Use TRACE and DEBUG levels only during development or troubleshooting.

2Guard expensive log calls with level checks like isTraceEnabled().

3Use INFO, WARN, and ERROR levels for regular operation to minimize overhead.

Performance Quiz - 3 Questions

Which log level is the most detailed and can cause the highest performance overhead if used excessively?

A INFO

B TRACE

C WARN

D ERROR

DevTools: Performance

How to check: Record a performance profile while running the app and look for CPU time spent in logging methods.

What to look for: High CPU time in logging calls indicates excessive or expensive logging impacting performance.

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