Why logging matters in Spring Boot - See It in Action

Why logging matters

📖 Scenario: You are building a simple Spring Boot application that processes user requests. To understand what happens inside your app and to find problems quickly, you need to add logging. Logging is like keeping a diary of your app's actions and errors.

🎯 Goal: Learn how to add basic logging to a Spring Boot application and see how logs help track app behavior and errors.

📋 What You'll Learn

Create a Spring Boot application class

Add a logger instance using LoggerFactory

Write log messages at different levels (INFO, ERROR)

Run the app and observe the log output

💡 Why This Matters

🌍 Real World

In real applications, logging helps developers and operators understand app behavior and quickly fix issues.

💼 Career

Knowing how to add and use logging is a key skill for developers and DevOps engineers to maintain reliable software.

1

Create a Spring Boot application class

Create a class called LoggingDemoApplication with the @SpringBootApplication annotation and a main method that runs SpringApplication.run(LoggingDemoApplication.class, args).

Spring Boot

# Create the LoggingDemoApplication class with @SpringBootApplication and main method
# Your code here

Need a hint?

Use @SpringBootApplication on the class and write a main method that calls SpringApplication.run.

2

Add a logger instance

Inside the LoggingDemoApplication class, add a private static final logger variable called logger using LoggerFactory.getLogger(LoggingDemoApplication.class). Import org.slf4j.Logger and org.slf4j.LoggerFactory.

Spring Boot

import org.springframework.boot.SpringApplication;
import org.springframework.boot.autoconfigure.SpringBootApplication;
import org.slf4j.Logger;
import org.slf4j.LoggerFactory;

@SpringBootApplication
public class LoggingDemoApplication {
    // Add logger variable here
    
    public static void main(String[] args) {
        SpringApplication.run(LoggingDemoApplication.class, args);
    }
}

Need a hint?

Use LoggerFactory.getLogger(LoggingDemoApplication.class) to create the logger.

3

Write log messages at INFO and ERROR levels

Inside the main method, add a log message using logger.info("Application started"). Then add a try-catch block that throws and catches an exception, and inside the catch block log the error with logger.error("An error occurred", e).

Spring Boot

import org.springframework.boot.SpringApplication;
import org.springframework.boot.autoconfigure.SpringBootApplication;
import org.slf4j.Logger;
import org.slf4j.LoggerFactory;

@SpringBootApplication
public class LoggingDemoApplication {
    private static final Logger logger = LoggerFactory.getLogger(LoggingDemoApplication.class);

    public static void main(String[] args) {
        SpringApplication.run(LoggingDemoApplication.class, args);
        // Add logger.info and try-catch with logger.error here
        
    }
}

Need a hint?

Use logger.info for a start message and a try-catch block to log an error with logger.error.

4

Run the app and observe the log output

Run the LoggingDemoApplication class. Observe the console output showing the INFO message "Application started" and the ERROR message with the exception stack trace.

Spring Boot

import org.springframework.boot.SpringApplication;
import org.springframework.boot.autoconfigure.SpringBootApplication;
import org.slf4j.Logger;
import org.slf4j.LoggerFactory;

@SpringBootApplication
public class LoggingDemoApplication {
    private static final Logger logger = LoggerFactory.getLogger(LoggingDemoApplication.class);

    public static void main(String[] args) {
        SpringApplication.run(LoggingDemoApplication.class, args);
        logger.info("Application started");
        try {
            throw new RuntimeException("Test exception");
        } catch (Exception e) {
            logger.error("An error occurred", e);
        }
    }
}

// Run the app and check console output

Need a hint?

Run the app and look at the console. You should see the INFO and ERROR log messages with the exception details.