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.

Practice

1. What is the main method used for in a Spring Boot application?

easy

A. To configure the database connection

B. To start the Spring Boot application using SpringApplication.run()

C. To define REST endpoints

D. To create HTML views

Solution

1. Step 1: Understand the role of the main method

   The main method is the entry point of any Java application, including Spring Boot apps.

2. Step 2: Identify what SpringApplication.run() does

   SpringApplication.run() starts the Spring Boot application by initializing the Spring context and components.

3. Final Answer:

   To start the Spring Boot application using SpringApplication.run() -> Option B

4. Quick Check:

   Main method starts app = To start the Spring Boot application using SpringApplication.run() [OK]

Hint: Main method always starts Spring Boot with SpringApplication.run() [OK]

Common Mistakes:

• Confusing main method with configuration tasks
• Thinking main method defines endpoints
• Assuming main method creates UI views

2. Which of the following is the correct way to run a Spring Boot application from the command line using Maven?

easy

A. mvn spring-boot:run

B. mvn run spring-boot

C. mvn start boot

D. mvn execute springboot

Solution

1. Step 1: Recall Maven command for Spring Boot

   The standard Maven command to run a Spring Boot app is 'mvn spring-boot:run'.

2. Step 2: Check other options for correctness

   Other options do not follow Maven or Spring Boot plugin syntax and will cause errors.

3. Final Answer:

   mvn spring-boot:run -> Option A

4. Quick Check:

   Maven run command = mvn spring-boot:run [OK]

Hint: Use 'mvn spring-boot:run' to run Spring Boot apps with Maven [OK]

Common Mistakes:

• Mixing up command order
• Using invalid Maven goals
• Typing incomplete commands

3. Given this main class, what happens when you run it?

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

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

medium

A. The application starts and initializes Spring Boot context

B. The application throws a compilation error

C. The application runs but does not start Spring Boot

D. The application starts but immediately exits without running

Solution

1. Step 1: Analyze the main method code

   The main method calls SpringApplication.run() with the class annotated by @SpringBootApplication.

2. Step 2: Understand @SpringBootApplication effect

   This annotation enables auto-configuration and component scanning, so Spring Boot context starts properly.

3. Final Answer:

   The application starts and initializes Spring Boot context -> Option A

4. Quick Check:

   SpringApplication.run() starts app = The application starts and initializes Spring Boot context [OK]

Hint: SpringApplication.run() with @SpringBootApplication starts app [OK]

Common Mistakes:

• Thinking @SpringBootApplication is optional
• Assuming code throws error without extra config
• Believing app runs but does not start Spring context

4. You try to run your Spring Boot app but get this error: java.lang.NoClassDefFoundError: org/springframework/boot/SpringApplication. What is the most likely cause?

medium

A. The Java version is too new for Spring Boot

B. The main method is missing in the application class

C. The Spring Boot dependency is missing from the project

D. The application.properties file is empty

Solution

1. Step 1: Understand the error meaning

   NoClassDefFoundError means the JVM cannot find the SpringApplication class at runtime.

2. Step 2: Identify cause of missing class

   This usually happens if the Spring Boot dependency is not included or not properly loaded in the project.

3. Final Answer:

   The Spring Boot dependency is missing from the project -> Option C

4. Quick Check:

   NoClassDefFoundError = missing dependency [OK]

Hint: NoClassDefFoundError usually means missing dependency [OK]

Common Mistakes:

• Blaming missing main method for this error
• Thinking empty config files cause class errors
• Assuming Java version causes missing class

5. You want to run your Spring Boot app as a standalone jar. Which command correctly runs the jar file named app.jar?

medium

A. mvn run app.jar

B. java run app.jar

C. gradle run app.jar

D. java -jar app.jar

Solution

1. Step 1: Recall how to run a jar file in Java

   The standard command to run an executable jar is 'java -jar filename.jar'.

2. Step 2: Check other options for correctness

   The mvn and gradle commands are for build tools, not for running jar files directly. 'java run app.jar' is invalid Java command syntax.

3. Final Answer:

   java -jar app.jar -> Option D

4. Quick Check:

   Run jar with 'java -jar' = java -jar app.jar [OK]

Hint: Run jar files with 'java -jar filename.jar' [OK]

Common Mistakes:

• Using build tool commands to run jar directly
• Omitting '-jar' option
• Typing invalid Java commands