Running a Spring Boot application in Spring Boot - Step-by-Step Execution

Concept Flow - Running a Spring Boot application

Start Application

↓

Spring Boot Main Method

↓

SpringApplication.run()

↓

Spring Boot Context Initialized

↓

Embedded Server Starts

↓

Application Ready to Serve Requests

This flow shows how starting a Spring Boot app runs the main method, initializes the context, starts the embedded server, and makes the app ready.

Execution Sample

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);
  }
}

This code starts the Spring Boot application by running the main method which calls SpringApplication.run.

Execution Table

Step	Action	Method Called	Result	Next Step
1	Start main method	main()	Entered main method	Call SpringApplication.run()
2	Run Spring Boot app	SpringApplication.run()	Spring context initializing	Initialize beans
3	Initialize beans	Context initialization	Beans created and wired	Start embedded server
4	Start embedded server	Tomcat/Jetty start	Server listening on port 8080	Application ready
5	Application ready	N/A	App ready to accept requests	Wait for requests

💡 Application runs continuously until stopped by user or error

Variable Tracker

Variable	Start	After Step 2	After Step 3	After Step 4	Final
Application Context	null	initializing	initialized	initialized	running
Embedded Server	stopped	stopped	starting	running	running

Key Moments - 2 Insights

Why does the application keep running after main method finishes?

Because the embedded server (like Tomcat) runs in the background, keeping the app alive as shown in steps 4 and 5 of the execution_table.

What does SpringApplication.run() actually do?

It initializes the Spring context, creates beans, and starts the embedded server, as detailed from step 2 to step 4 in the execution_table.

Visual Quiz - 3 Questions

Look at the execution_table, what is the state of the Application Context after step 3?

A null

B initialized

C initializing

D running

💡 Hint

Check the variable_tracker row for Application Context after Step 3

At which step does the embedded server start listening on port 8080?

A Step 2

B Step 3

C Step 4

D Step 5

💡 Hint

Look at the 'Result' column in execution_table for server listening info

If SpringApplication.run() fails to initialize beans, what step would fail?

A Step 3

B Step 4

C Step 2

D Step 5

💡 Hint

Bean creation happens during context initialization in Step 3

Concept Snapshot

Spring Boot apps start by running the main method.
Inside main, SpringApplication.run() initializes the app.
It creates the Spring context and beans.
Then it starts an embedded server (like Tomcat).
The app stays running, ready to handle requests.
Stop the app to end the server and process.

Full Transcript

Running a Spring Boot application begins with the main method. This method calls SpringApplication.run(), which initializes the Spring context and creates all necessary beans. After that, an embedded server such as Tomcat or Jetty starts listening on the default port 8080. The application remains running, ready to serve incoming requests until it is stopped by the user or an error occurs.

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