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Log4j is one of the most widely used logging frameworks in Java applications. Whether you’re a developer, DevOps engineer, or tester, understanding Log4j is crucial for debugging and monitoring applications.

In this guide, we’ll cover the top 20 Log4j interview questions with detailed answers to help you crack your next technical interview.

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What is Log4j and Why is it Used?

Answer:
Log4j is a reliable, fast, and flexible logging framework for Java applications. It helps developers track application behavior, debug issues, and maintain logs efficiently.

Key Features of Log4j:

• Supports multiple log levels (DEBUG, INFO, WARN, ERROR, FATAL).
• Configurable via XML, JSON, YAML, or properties files.
• Supports asynchronous logging for better performance.

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20 Most Common Log4j Interview Questions and Answers

1. What are the Main Components of Log4j?

Answer:
Log4j consists of three main components:

• Loggers – Capture log messages.
• Appenders – Define where logs are written (Console, File, Database).
• Layouts – Format log messages in a readable structure.

2. How to Change Log4j Log Level?

Answer:
You can change the log level in the log4j2.xml or log4j.properties file:

<Root level="debug">  
    <AppenderRef ref="Console"/>  
</Root>  

Or in log4j.properties:

log4j.rootLogger=DEBUG, Console  

3. What is Log4j vs Log4j2?

Answer:

Feature	Log4j 1.x	Log4j2
Performance	Slower due to synchronization	Faster with async logging
Configuration	Properties/XML	XML, JSON, YAML
Plugins	Limited	Extensive plugin support

Log4j2 is the upgraded version with better performance and features.

4. How to Configure Log4j in a Java Application?

Answer:

1. Add Log4j dependency in pom.xml (Maven):

<dependency>  
    <groupId>org.apache.logging.log4j</groupId>  
    <artifactId>log4j-core</artifactId>  
    <version>2.20.0</version>  
</dependency>  

2. Create a log4j2.xml file in src/main/resources.

5. What are Log4j Appenders?

Answer:
Appenders define where logs are stored. Common appenders:

• ConsoleAppender – Logs to console.
• FileAppender – Logs to a file.
• RollingFileAppender – Rotates logs based on size/date.

6. What is the Difference Between SLF4J and Log4j?

Answer:

• SLF4J is a logging facade (abstraction layer).
• Log4j is an actual logging implementation.
  SLF4J can use Log4j as its backend.

7. How to Debug Log4j Configuration Issues?

Answer:
Enable internal Log4j debugging by adding:

-Dlog4j.debug=true  

Or in log4j2.xml:

<Configuration status="TRACE">  

8. What is MDC in Log4j?

Answer:
Mapped Diagnostic Context (MDC) helps store contextual information (e.g., user ID, session ID) for logging.

MDC.put("user", "JohnDoe");  
logger.info("User logged in");  

9. How to Disable Log4j Logging?

Answer:
Set the root logger level to OFF:

log4j.rootLogger=OFF  

10. What is Async Logging in Log4j2?

Answer:
Async logging improves performance by writing logs in a separate thread. Enable it in log4j2.xml:

<AsyncLogger name="com.example" level="debug"/>  

11. How to Log Exceptions in Log4j?

Answer:
Use logger.error() with the exception:

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

12. What is a Log4j Filter?

Answer:
Filters control which log events get processed. Example:

<ThresholdFilter level="ERROR" onMatch="ACCEPT" onMismatch="DENY"/>  

13. How to Use Log4j in Spring Boot?

Answer:
Spring Boot uses Logback by default. To use Log4j2:

1. Exclude Logback in pom.xml:

<exclusions>  
    <exclusion>  
        <groupId>org.springframework.boot</groupId>  
        <artifactId>spring-boot-starter-logging</artifactId>  
    </exclusion>  
</exclusions>  

2. Add Log4j2 dependency.

14. What is the Log4j Vulnerability (Log4Shell)?

Answer:
CVE-2021-44228 (Log4Shell) was a critical RCE vulnerability in Log4j 2.x. Mitigation:

• Upgrade to Log4j 2.17.0 or later.
• Disable JNDI lookups: -Dlog4j2.formatMsgNoLookups=true

15. How to Use Multiple Loggers in Log4j?

Answer:
Define multiple loggers in log4j2.xml:

<Logger name="com.example.dao" level="debug"/>  
<Logger name="com.example.service" level="info"/>  

16. What is a Log4j Pattern Layout?

Answer:
It formats log messages. Example:

log4j.appender.Console.layout.ConversionPattern=%d{yyyy-MM-dd} %-5p %c{1}:%L - %m%n  

17. How to Log in JSON Format Using Log4j?

Answer:
Use JsonLayout in log4j2.xml:

<JsonLayout compact="true" eventEol="true"/>  

18. What is the Difference Between TRACE and DEBUG?

Answer:

• TRACE – Very detailed logs (for deep debugging).
• DEBUG – General debugging logs.

19. How to Rotate Logs in Log4j?

Answer:
Use RollingFileAppender:

<RollingFileAppender  
    filePattern="logs/app-%d{yyyy-MM-dd}.log.gz"  
    strategy="timeBased"/>  

20. How to Migrate from Log4j 1.x to Log4j2?

Answer:

1. Replace dependencies with Log4j2.
2. Update configuration files (log4j2.xml instead of log4j.properties).
3. Use the Log4j 1.x bridge if needed:

<dependency>  
    <groupId>org.apache.logging.log4j</groupId>  
    <artifactId>log4j-1.2-api</artifactId>  
</dependency>  

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People Also Ask

Q1. Is Log4j still used in 2024?

Yes, Log4j2 is widely used due to its performance and security improvements.

Q2. What replaced Log4j?

Log4j2 is the successor, but alternatives include Logback and SLF4J.

Q3. How to check Log4j version?

java -jar log4j-core-*.jar --version