Embedded server concept in Spring Boot - Step-by-Step Execution

Concept Flow - Embedded server concept

Start Spring Boot Application

↓

Create Embedded Server Instance

↓

Configure Server Settings

↓

Start Server Listening on Port

↓

Handle Incoming HTTP Requests

↓

Process Requests with Controllers

↓

Send HTTP Responses

↓

Application Running with Embedded Server

This flow shows how Spring Boot starts an embedded server, configures it, listens for requests, and processes them within the application.

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 a Spring Boot app with an embedded server that listens for HTTP requests.

Execution Table

Step	Action	Result	Server State
1	SpringApplication.run() called	Spring Boot app starts	Server instance created but not started
2	Embedded server (e.g., Tomcat) initialized	Server configured with default port 8080	Server ready to start
3	Server starts listening on port 8080	Server is running and waiting for requests	Server running
4	HTTP request received at /hello	Request passed to controller	Server running, processing request
5	Controller processes request and returns response	Response sent to client	Server running
6	No more requests	Server continues listening	Server running
7	Application stopped	Server shuts down	Server stopped

💡 Application stops or is terminated, embedded server shuts down

Variable Tracker

Variable	Start	After Step 2	After Step 3	After Step 5	Final
serverInstance	null	created	running	running	stopped
port	null	8080	8080	8080	null
request	null	null	received	processed	null
response	null	null	null	sent	null

Key Moments - 3 Insights

Why does Spring Boot create an embedded server instead of requiring a separate server?

Spring Boot creates an embedded server automatically (see Step 2 in execution_table) so you don't need to install or configure a separate server. This makes running your app simpler and self-contained.

How does the embedded server handle HTTP requests?

When a request arrives (Step 4), the embedded server passes it to the app's controllers to process and send back a response (Step 5). The server stays running to handle more requests.

What happens if the port 8080 is already in use?

The server will fail to start at Step 3 because it cannot bind to port 8080. You would see an error and need to change the port configuration before restarting.

Visual Quiz - 3 Questions

Look at the execution_table, what is the server state after Step 3?

A Server running and listening on port 8080

B Server created but not started

C Server stopped

D Server processing request

💡 Hint

Check the 'Server State' column for Step 3 in the execution_table

At which step does the embedded server start listening for HTTP requests?

A Step 1

B Step 3

C Step 2

D Step 4

💡 Hint

Look for the action 'Server starts listening on port 8080' in the execution_table

If the application is stopped, what is the final state of the serverInstance variable?

A null

B created

C stopped

D running

💡 Hint

Check the variable_tracker row for serverInstance at the 'Final' column

Concept Snapshot

Spring Boot Embedded Server Concept:
- Spring Boot auto-creates an embedded server (like Tomcat) inside your app.
- The server starts when you run SpringApplication.run().
- It listens on a default port (8080) for HTTP requests.
- Requests are handled by your controllers inside the app.
- No need to install or configure an external server.
- Server runs until the app stops.

Full Transcript

When you start a Spring Boot application using SpringApplication.run(), it creates an embedded server instance such as Tomcat inside your app. This server is configured automatically and starts listening on port 8080 by default. When an HTTP request arrives, the embedded server passes it to your application's controllers to process and send back a response. The server keeps running and handling requests until the application is stopped, at which point the server shuts down. This embedded server concept means you don't need to install or manage a separate server, making development and deployment simpler and more self-contained.

Practice

1. What is the main advantage of using an embedded server in a Spring Boot application?

easy

A. It requires manual installation of a web server before running the app.

B. It allows the application to run standalone without needing an external server.

C. It disables HTTP support in the application.

D. It forces the use of XML configuration files.

Solution

1. Step 1: Understand embedded server purpose

   Embedded servers are included inside the app, so no external server setup is needed.

2. Step 2: Compare options

   Only It allows the application to run standalone without needing an external server. correctly states the standalone running advantage. Others describe incorrect or unrelated facts.

3. Final Answer:

   It allows the application to run standalone without needing an external server. -> Option B

4. Quick Check:

   Embedded server = standalone app [OK]

Hint: Embedded server means no external server needed [OK]

Common Mistakes:

• Thinking external server is still required
• Confusing embedded server with disabling HTTP
• Assuming XML config is mandatory

2. Which dependency is commonly used to include an embedded Tomcat server in a Spring Boot project?

easy

A. spring-boot-starter-security

B. spring-boot-starter-data-jpa

C. spring-boot-starter-test

D. spring-boot-starter-web

Solution

1. Step 1: Identify embedded server dependency

   The embedded Tomcat server comes with the spring-boot-starter-web dependency.

2. Step 2: Eliminate unrelated dependencies

   Other options relate to data, security, or testing, not embedded servers.

3. Final Answer:

   spring-boot-starter-web -> Option D

4. Quick Check:

   Web starter includes embedded Tomcat [OK]

Hint: Web starter includes embedded Tomcat by default [OK]

Common Mistakes:

• Choosing data or security starters for embedded server
• Confusing test starter with server dependency

3. Given this Spring Boot application.properties setting:
server.port=8085
What will happen when you run the app with embedded Tomcat?

medium

A. The embedded server will start on port 8085.

B. The server will ignore this and use default port 8080.

C. The app will fail to start due to invalid port.

D. The server will start on port 80.

Solution

1. Step 1: Understand server.port property

   Setting server.port changes the embedded server's listening port.

2. Step 2: Apply property effect

   Since 8085 is valid, the embedded Tomcat will start on port 8085.

3. Final Answer:

   The embedded server will start on port 8085. -> Option A

4. Quick Check:

   server.port sets embedded server port [OK]

Hint: server.port config changes embedded server port [OK]

Common Mistakes:

• Assuming default port always used
• Thinking invalid port causes failure (8085 is valid)
• Confusing port 80 as default

4. Consider this Spring Boot main class snippet:

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

If you remove the embedded server dependency, what will happen when you run this app?

medium

A. The app will start with an external server automatically.

B. The app will fail to start with a missing server error.

C. The app will start but no web server will run.

D. The app will start and embed Jetty server by default.

Solution

1. Step 1: Understand dependency role

   spring-boot-starter-web provides the embedded server. Without it, Spring Boot runs as a non-web application.

2. Step 2: Effect of removing embedded server dependency

   The app starts successfully, but no web server runs since no web dependencies are present.

3. Final Answer:

   The app will start but no web server will run. -> Option C

4. Quick Check:

   No web starter = app starts, no server [OK]

Hint: No web starter = app starts without server [OK]

Common Mistakes:

• Thinking app fails to start without server dependency
• Believing external server auto-starts
• Assuming Jetty starts by default without dependency

5. You want to switch your Spring Boot app's embedded server from Tomcat to Jetty. Which change is required?

hard

A. Exclude spring-boot-starter-tomcat and add spring-boot-starter-jetty dependency.

B. Add spring-boot-starter-jetty without excluding Tomcat.

C. Change server.port to 9090 in application.properties.

D. Add spring-boot-starter-undertow dependency only.

Solution

1. Step 1: Understand default embedded server

   Spring Boot uses Tomcat by default with spring-boot-starter-web.

2. Step 2: How to switch embedded server

   You must exclude Tomcat dependency and add Jetty starter to replace it.

3. Final Answer:

   Exclude spring-boot-starter-tomcat and add spring-boot-starter-jetty dependency. -> Option A

4. Quick Check:

   Exclude Tomcat + add Jetty = switch server [OK]

Hint: Exclude Tomcat before adding Jetty starter [OK]

Common Mistakes:

• Adding Jetty without excluding Tomcat causes conflicts
• Changing port does not switch server
• Adding Undertow starter switches to Undertow, not Jetty