Logger creation in classes in Spring Boot - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to create a logger instance in a Spring Boot class.

Spring Boot

private static final Logger logger = LoggerFactory.[1](MyClass.class);

Drag options to blanks, or click blank then click option'

AgetLogger

BcreateLogger

CnewLogger

DbuildLogger

Attempts:

3 left

💡 Hint

Common Mistakes

Using methods like createLogger or newLogger which do not exist.

Trying to instantiate Logger directly.

The LoggerFactory's getLogger method is used to create a logger instance for the class.

2fill in blank

medium

Complete the code to import the Logger class from the correct package.

Spring Boot

import org.slf4j.[1];

Drag options to blanks, or click blank then click option'

ALoggerFactory

BLogger

CLogManager

DLog

Attempts:

3 left

💡 Hint

Common Mistakes

Importing LoggerFactory instead of Logger.

Using LogManager which is from Log4j, not SLF4J.

The Logger interface is imported from org.slf4j.Logger.

3fill in blank

hard

Fix the error in the logger declaration by completing the code.

Spring Boot

private static final Logger logger = LoggerFactory.getLogger([1]);

Drag options to blanks, or click blank then click option'

Athis

B"MyClass"

CMyClass

DMyClass.class

Attempts:

3 left

💡 Hint

Common Mistakes

Passing a string instead of a class literal.

Passing an instance or 'this' keyword.

The getLogger method requires a Class object, so MyClass.class is correct.

4fill in blank

hard

Fill both blanks to correctly declare and import the logger in a Spring Boot class.

Spring Boot

import org.slf4j.[1];

private static final Logger [2] = LoggerFactory.getLogger(MyService.class);

Drag options to blanks, or click blank then click option'

ALogger

Blogger

CLoggerFactory

Dlog

Attempts:

3 left

💡 Hint

Common Mistakes

Importing LoggerFactory instead of Logger.

Naming the logger variable incorrectly.

The Logger interface is imported, and the logger instance is commonly named logger.

5fill in blank

hard

Fill all three blanks to complete the logger declaration with import and usage in a method.

Spring Boot

import org.slf4j.[1];

private static final Logger [2] = LoggerFactory.getLogger(AppController.class);

public void start() {
    [3].info("Application started");
}

Drag options to blanks, or click blank then click option'

ALogger

Blogger

DLoggerFactory

Attempts:

3 left

💡 Hint

Common Mistakes

Using LoggerFactory as a variable name.

Not importing Logger interface.

Calling info on LoggerFactory instead of logger.

Import the Logger interface, name the logger variable logger, and use it to log info messages.

Practice

1. What is the main purpose of creating a logger in a Spring Boot class?

easy

A. To record messages about the program's behavior for debugging and monitoring

B. To store user data permanently

C. To speed up the program execution

D. To create user interface elements automatically

Solution

1. Step 1: Understand the role of a logger

   A logger records messages about what happens inside the program, helping developers see the flow and errors.

2. Step 2: Identify the correct purpose

   Logging is not for storing data, speeding up code, or UI creation, but for tracking program behavior.

3. Final Answer:

   To record messages about the program's behavior for debugging and monitoring -> Option A

4. Quick Check:

   Logger purpose = record messages [OK]

Hint: Loggers track program events and errors clearly [OK]

Common Mistakes:

• Thinking loggers store data permanently
• Confusing loggers with UI components
• Assuming loggers improve speed

2. Which of the following is the correct way to create a logger in a Spring Boot class named MyService?

easy

A. private Logger logger = LoggerFactory.createLogger(MyService);

B. Logger logger = new Logger(MyService);

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

D. static Logger logger = LoggerFactory.getLogger("MyService");

Solution

1. Step 1: Recall the standard logger creation syntax

   In Spring Boot, the logger is created using LoggerFactory.getLogger(ClassName.class) and declared static final.

2. Step 2: Match the correct syntax

   private static final Logger logger = LoggerFactory.getLogger(MyService.class); matches the correct pattern: private static final Logger logger = LoggerFactory.getLogger(MyService.class); others use wrong methods or missing keywords.

3. Final Answer:

   private static final Logger logger = LoggerFactory.getLogger(MyService.class); -> Option C

4. Quick Check:

   Logger creation = LoggerFactory.getLogger(ClassName.class) [OK]

Hint: Use LoggerFactory.getLogger(ClassName.class) with static final [OK]

Common Mistakes:

• Using new Logger() instead of LoggerFactory
• Missing static or final keywords
• Passing string instead of ClassName.class

3. Given the following code snippet in a Spring Boot class:

private static final Logger logger = LoggerFactory.getLogger(App.class);

public void run() {
    logger.info("Start running");
    logger.debug("Debugging info");
    logger.error("An error occurred");
}

What will be the visible output if the logging level is set to INFO?

medium

A. All three messages will be logged

B. No messages will be logged

C. Only "Debugging info" will be logged

D. Only "Start running" and "An error occurred" messages will be logged

Solution

1. Step 1: Understand logging levels and filtering

   Logging levels filter messages: INFO level shows INFO, WARN, ERROR but hides DEBUG messages.

2. Step 2: Apply level filter to messages

   "Start running" is INFO, "Debugging info" is DEBUG (hidden), "An error occurred" is ERROR (shown).

3. Final Answer:

   Only "Start running" and "An error occurred" messages will be logged -> Option D

4. Quick Check:

   INFO level hides DEBUG but shows INFO and ERROR [OK]

Hint: INFO level shows INFO and above, hides DEBUG [OK]

Common Mistakes:

• Assuming DEBUG messages show at INFO level
• Thinking ERROR messages are hidden
• Confusing log levels order

4. Identify the error in the following logger declaration inside a Spring Boot class named OrderService:

private Logger logger = LoggerFactory.getLogger(OrderService);

medium

A. Logger must be declared inside a method, not as a field

B. Missing .class after OrderService in getLogger method

C. LoggerFactory does not have getLogger method

D. Logger should not be private

Solution

1. Step 1: Check the getLogger argument type

   The getLogger method requires a Class object, so it should be OrderService.class, not OrderService.

2. Step 2: Verify other parts of declaration

   Logger can be private and declared as a field; LoggerFactory has getLogger method; no need to declare inside method.

3. Final Answer:

   Missing .class after OrderService in getLogger method -> Option B

4. Quick Check:

   Use ClassName.class in getLogger [OK]

Hint: Always add .class after class name in getLogger() [OK]

Common Mistakes:

• Omitting .class after class name
• Thinking logger cannot be private
• Assuming getLogger method doesn't exist

5. You want to create a logger in a Spring Boot class UserController that logs a warning message only when a user ID is missing. Which of the following code snippets correctly creates the logger and logs the warning message?

hard

A. private static final Logger logger = LoggerFactory.getLogger(UserController.class);

public void checkUser(String userId) {
if (userId == null || userId.isEmpty()) {
logger.warn("User ID is missing");
}
}

B. Logger logger = LoggerFactory.getLogger(UserController);

public void checkUser(String userId) {
if (userId == null) {
logger.error("User ID is missing");
}
}

C. private Logger logger = LoggerFactory.getLogger(UserController.class);

public void checkUser(String userId) {
if (userId.isEmpty()) {
logger.info("User ID is missing");
}
}

D. static final Logger logger = LoggerFactory.getLogger("UserController");

public void checkUser(String userId) {
if (userId == null) {
logger.debug("User ID is missing");
}
}

Solution

1. Step 1: Verify correct logger creation

   private static final Logger logger = LoggerFactory.getLogger(UserController.class); public void checkUser(String userId) { if (userId == null || userId.isEmpty()) { logger.warn("User ID is missing"); } } uses private static final Logger with LoggerFactory.getLogger(UserController.class), which is the standard pattern.

2. Step 2: Check logging condition and level

   private static final Logger logger = LoggerFactory.getLogger(UserController.class); public void checkUser(String userId) { if (userId == null || userId.isEmpty()) { logger.warn("User ID is missing"); } } checks if userId is null or empty and logs a warning with logger.warn(), which matches the requirement.

3. Final Answer:

   private static final Logger logger = LoggerFactory.getLogger(UserController.class); public void checkUser(String userId) { if (userId == null || userId.isEmpty()) { logger.warn("User ID is missing"); } } -> Option A

4. Quick Check:

   Use static final logger and logger.warn() for warnings [OK]

Hint: Use logger.warn() for warnings and check null or empty [OK]

Common Mistakes:

• Not using static final for logger
• Logging wrong level (info, debug, error) for warnings
• Not checking both null and empty strings