Running a Spring Boot application in Spring Boot

Introduction

Running a Spring Boot application starts your program so it can listen for requests and do its work.

When you want to test your web app locally on your computer.

When you want to see if your backend service is working.

When you want to run your app to handle real user requests.

When you want to debug or develop new features in your Spring Boot project.

Syntax

Spring Boot

mvn spring-boot:run

# or if using Gradle
./gradlew bootRun

# Or run the main class directly
java -jar target/your-app.jar

You can use Maven or Gradle commands to start the app during development.

After building, you can run the packaged jar file with java -jar.

Examples

This command runs the Spring Boot app using Maven. It compiles and starts the app.

Spring Boot

mvn spring-boot:run

This command runs the Spring Boot app using Gradle. It builds and starts the app.

Spring Boot

./gradlew bootRun

This runs the packaged Spring Boot app jar file directly after building.

Spring Boot

java -jar target/myapp-0.0.1-SNAPSHOT.jar

Sample Program

This is a minimal Spring Boot application. The @SpringBootApplication annotation sets up the app. The main method starts it.

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);
    }
}

Important Notes

Make sure you have Java installed and your build tool (Maven or Gradle) set up.

The console shows logs when the app starts, including a message that it started successfully.

You can stop the app anytime by pressing Ctrl+C in the terminal.

Summary

Running a Spring Boot app means starting it so it can do its work.

You can run it using Maven, Gradle, or by running the jar file.

The main method with SpringApplication.run() is the entry point.

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