Running a Spring Boot application in Spring Boot - Mini Project: Build & Apply

Running a Spring Boot Application

📖 Scenario: You want to create a simple Spring Boot application that starts up and shows a welcome message on the home page. This is like opening a shop and putting a welcome sign on the door.

🎯 Goal: Build a minimal Spring Boot application that runs and displays "Welcome to Spring Boot!" on the home page.

📋 What You'll Learn

Create a Spring Boot main application class with the correct annotation

Add a configuration variable for the welcome message

Create a controller with a method to handle the home page request

Complete the application by running it with the Spring Boot run method

💡 Why This Matters

🌍 Real World

Spring Boot is widely used to build web applications and REST APIs quickly. Knowing how to run a basic app is the first step to creating real services.

💼 Career

Many Java developer jobs require knowledge of Spring Boot to build and maintain backend services. This project teaches the essential startup pattern.

1

Create the Spring Boot main application class

Create a class called DemoApplication in package com.example.demo. Add the @SpringBootApplication annotation and a main method that calls SpringApplication.run(DemoApplication.class, args).

Spring Boot

package com.example.demo;

// Your code here

Hint

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

2

Add a configuration variable for the welcome message

Inside the DemoApplication class, add a public static final String variable called WELCOME_MESSAGE and set it to "Welcome to Spring Boot!".

Spring Boot

package com.example.demo;

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);
    }

    // Add the WELCOME_MESSAGE variable below
    // Your code here
}

Hint

Define a public static final String named WELCOME_MESSAGE with the exact text.

3

Create a controller to handle the home page request

Create a new class called HomeController in package com.example.demo. Add the @RestController annotation. Add a method called home that returns the DemoApplication.WELCOME_MESSAGE string. Annotate this method with @GetMapping("/").

Spring Boot

package com.example.demo;

// Add imports and create HomeController class below
// Your code here

Hint

Use @RestController on the class and @GetMapping("/") on the method that returns the welcome message.

4

Run the Spring Boot application

Ensure the DemoApplication class has the main method that calls SpringApplication.run(DemoApplication.class, args). This method starts the application.

Spring Boot

package com.example.demo;

// Confirm the main method is present and runs the application
// Your code here

Hint

The main method must call SpringApplication.run to start the app.

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