Running a Spring Boot application in Spring Boot - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ component_behavior

intermediate

What happens when you run this Spring Boot main class?

Consider this Spring Boot application main class. What will be the output in the console when you run it?

Spring Boot

import org.springframework.boot.SpringApplication;
import org.springframework.boot.autoconfigure.SpringBootApplication;

@SpringBootApplication
public class DemoApplication {
    public static void main(String[] args) {
        SpringApplication.run(DemoApplication.class, args);
        System.out.println("Application started");
    }
}

A The console shows Spring Boot startup logs followed by 'Application started'.

B Only 'Application started' is printed, no Spring Boot logs appear.

C The application starts but does not print anything to the console.

D The application fails to start due to missing @EnableAutoConfiguration annotation.

💡 Hint

SpringApplication.run starts the Spring Boot context and logs startup info before the next line runs.

SpringApplication.run launches the Spring Boot application and prints startup logs. The following System.out.println runs after startup, so both outputs appear.

📝 Syntax

intermediate

Identify the syntax error in this Spring Boot main class

Which option correctly fixes the syntax error in this Spring Boot main class?

Spring Boot

import org.springframework.boot.SpringApplication;
import org.springframework.boot.autoconfigure.SpringBootApplication;

@SpringBootApplication
public class DemoApplication {
    public static void main(String[] args) {
        SpringApplication.run(DemoApplication.class args);
    }
}

A Add a comma between DemoApplication.class and args: SpringApplication.run(DemoApplication.class, args);

B Remove args completely: SpringApplication.run(DemoApplication.class);

C Replace args with new String[]{}: SpringApplication.run(DemoApplication.class new String[]{});

D Add a semicolon after DemoApplication.class: SpringApplication.run(DemoApplication.class; args);

💡 Hint

Check the method call syntax and parameter separation.

The method call requires a comma between parameters. Missing comma causes a syntax error.

❓ state_output

advanced

What is the value of 'running' after this Spring Boot application starts?

Given this code snippet, what will be the value of the 'running' variable after the application starts?

Spring Boot

import org.springframework.boot.SpringApplication;
import org.springframework.boot.autoconfigure.SpringBootApplication;
import org.springframework.context.ConfigurableApplicationContext;

@SpringBootApplication
public class DemoApplication {
    public static void main(String[] args) {
        ConfigurableApplicationContext context = SpringApplication.run(DemoApplication.class, args);
        boolean running = context.isRunning();
        System.out.println(running);
    }
}

A false

B true

C null

D The code throws an exception

💡 Hint

Check what isRunning() returns after SpringApplication.run completes.

After SpringApplication.run returns, the application context is running, so isRunning() returns true.

🔧 Debug

advanced

Why does this Spring Boot application fail to start?

This Spring Boot application fails with an error. What is the cause?

Spring Boot

import org.springframework.boot.SpringApplication;
import org.springframework.boot.autoconfigure.SpringBootApplication;

@SpringBootApplication
public class DemoApplication {
    public static void main(String[] args) {
        SpringApplication.run();
    }
}

A The main method must return a value; void is invalid.

B The @SpringBootApplication annotation is missing required parameters.

C SpringApplication.run() requires at least one argument; missing arguments cause a compile error.

D The class must extend SpringBootServletInitializer to run.

💡 Hint

Check the method signature of SpringApplication.run.

SpringApplication.run requires the main class and args parameters. Calling it without arguments causes a compile-time error.

🧠 Conceptual

expert

What is the role of the @SpringBootApplication annotation when running a Spring Boot app?

Which statement best describes what @SpringBootApplication does when you run a Spring Boot application?

A @SpringBootApplication only marks the class as a REST controller for handling HTTP requests.

B @SpringBootApplication is optional and has no effect on application startup.

C @SpringBootApplication disables auto-configuration and requires manual bean registration.

D @SpringBootApplication combines @Configuration, @EnableAutoConfiguration, and @ComponentScan to configure and start the app automatically.

💡 Hint

Think about what annotations @SpringBootApplication groups together.

@SpringBootApplication is a convenience annotation that includes @Configuration, @EnableAutoConfiguration, and @ComponentScan, enabling automatic configuration and component scanning.