Embedded server concept in Spring Boot - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ component_behavior

intermediate

What happens when you run a Spring Boot app with an embedded server?

Consider a Spring Boot application with the default embedded Tomcat server. What is the behavior when you run the app?

A The app starts and listens on port 8080 without needing an external server.

B The app fails to start because an external Tomcat server is required.

C The app starts but does not listen on any port until manually configured.

D The app starts but only serves static files, no dynamic content.

💡 Hint

Think about what embedded means in this context.

Spring Boot includes an embedded Tomcat server by default, so the app runs standalone and listens on port 8080 without needing an external server.

📝 Syntax

intermediate

Which property sets the embedded server port in Spring Boot?

In application.properties, which property correctly sets the embedded server port to 9090?

A spring.server.port=9090

B server.embedded.port=9090

C embedded.server.port=9090

D server.port=9090

💡 Hint

Look for the standard prefix for server settings in Spring Boot.

The correct property to set the embedded server port is server.port. Other options are invalid keys.

🔧 Debug

advanced

Why does this Spring Boot app fail to start with embedded server?

Given this Spring Boot main class, why does the app fail to start the embedded server?

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);
    }
}

Spring Boot

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);
    }
}

A No embedded server dependency included in the project.

B Missing args parameter in run method causes failure.

C The @SpringBootApplication annotation is missing.

D The main method should return void, not static.

💡 Hint

Consider the dependencies required to auto-configure an embedded server.

The main class is correct and SpringApplication.run(MyApp.class, args) works fine, but without a dependency like spring-boot-starter-web, no embedded server (Tomcat) is included or auto-configured, so none starts.

🧠 Conceptual

advanced

What is the main advantage of using an embedded server in Spring Boot?

Why do developers prefer embedded servers in Spring Boot applications?

A It disables HTTP support to improve security.

B It improves the app's runtime performance by using native OS servers.

C It allows running the app as a standalone jar without installing a separate server.

D It forces the app to use only Tomcat as the server.

💡 Hint

Think about deployment simplicity and portability.

Embedded servers let you package the app with the server inside, so you can run it anywhere without extra setup.

❓ state_output

expert

What is the output when running this Spring Boot app with embedded server and custom port?

Given this application.properties and main class, what port will the embedded server listen on?

application.properties:
server.port=7070

Main class:
import org.springframework.boot.SpringApplication;
import org.springframework.boot.autoconfigure.SpringBootApplication;

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

Spring Boot

application.properties:
server.port=7070

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

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

A The embedded server listens on default port 8080.

B The embedded server listens on port 7070.

C The app fails to start due to port conflict.

D The embedded server listens on port 80.

💡 Hint

Check how Spring Boot reads properties for server configuration.

The server.port property overrides the default 8080 port, so the embedded server listens on 7070.

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