Logger creation in classes in Spring Boot - Practice Problems & Coding Challenges

Challenge - 5 Problems

📝 Syntax

intermediate

Correct Logger Declaration in a Spring Boot Class

Which option correctly declares a logger in a Spring Boot class using SLF4J?

Spring Boot

public class MyService {
    // Logger declaration here
}

A private Logger logger = LoggerFactory.getLogger();

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

C public static Logger logger = new Logger(MyService.class);

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

💡 Hint

Remember to use LoggerFactory and pass the class type for proper logger creation.

Option B correctly uses LoggerFactory.getLogger with the class type and declares the logger as private static final, which is the recommended pattern in Spring Boot.

❓ component_behavior

intermediate

Logger Output Behavior in Spring Boot

Given a Spring Boot class with a logger declared as in option A of the previous question, what will be the output when calling logger.info("Hello World") if the logging level is set to WARN?

Spring Boot

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

public void greet() {
    logger.info("Hello World");
}

A An error will occur because the logger is not initialized.

B The message will be logged but only in the debug log file.

C The message 'Hello World' will be printed to the console.

D No output will be shown because INFO is lower than WARN level.

💡 Hint

Check the logging level hierarchy: ERROR > WARN > INFO > DEBUG.

Since the logging level is set to WARN, INFO messages are ignored and not printed.

🔧 Debug

advanced

Identify the Logger Initialization Error

What error will occur when running this Spring Boot class code snippet?

Spring Boot

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

    public void process() {
        logger.info("Processing user");
    }
}

A No error, logs 'Processing user' correctly.

B NullPointerException at runtime because logger is null.

C Compilation error: getLogger() requires a Class or String argument.

D IllegalArgumentException because logger name is empty.

💡 Hint

Check the LoggerFactory.getLogger method signature.

LoggerFactory.getLogger() requires a parameter to specify the logger name or class. Omitting it causes a compilation error.

❓ state_output

advanced

Logger Instance Identity in Multiple Class Instances

If you create two instances of a Spring Boot class with a static logger declared as private static final Logger logger = LoggerFactory.getLogger(MyClass.class);, how many distinct logger instances exist in memory?

Spring Boot

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

    public void log() {
        logger.info("Logging");
    }
}

MyClass a = new MyClass();
MyClass b = new MyClass();

A One single logger instance shared by both class instances.

B No logger instances until log() is called.

C Two separate logger instances, one per class instance.

D A new logger instance is created each time logger.info is called.

💡 Hint

Static fields belong to the class, not to instances.

Static final logger means one logger instance per class, shared by all instances.

🧠 Conceptual

expert

Best Practice for Logger Declaration in Spring Boot

Which statement best explains why logger fields are declared as private static final in Spring Boot classes?

A To ensure a single shared logger per class, improve performance, and prevent modification.

B To allow each instance to have its own logger with different configurations.

C To enable logger injection by Spring's dependency injection container.

D To make the logger accessible from other classes without creating an instance.

💡 Hint

Think about memory usage and immutability of logger objects.

Declaring logger as private static final ensures one immutable logger per class, improving performance and consistency.