Multi-module project structure in Spring Boot - Mini Project: Build & Apply

Multi-module project structure

📖 Scenario: You are building a Spring Boot application that has two modules: core for business logic and web for the web interface. This helps keep your code organized like departments in a company.

🎯 Goal: Create a multi-module Spring Boot project with a core module containing a service class and a web module that depends on core and exposes a REST endpoint.

📋 What You'll Learn

Create a parent pom.xml with modules core and web

In core module, create a service class GreetingService with a method getGreeting() returning "Hello from Core!"

In web module, create a Spring Boot application class WebApplication

In web module, create a REST controller GreetingController that uses GreetingService to return the greeting at /greet

💡 Why This Matters

🌍 Real World

Large Spring Boot applications often split code into modules to keep code organized and reusable, like departments in a company.

💼 Career

Understanding multi-module projects is important for working on enterprise Java applications and collaborating in teams.

1

Create parent pom.xml with modules

Create a parent pom.xml file that defines packaging as pom and includes two modules: core and web. Set the groupId to com.example and version to 1.0.0.

Spring Boot

Hint

The parent pom.xml should have packaging set to pom and list core and web as modules.

2

Create core module with GreetingService

In the core module, create a Spring Boot pom.xml with packaging jar and parent set to the root project. Then create a class GreetingService in package com.example.core with a method getGreeting() that returns the string "Hello from Core!".

Spring Boot

// Create GreetingService.java in package com.example.core
// Add method getGreeting() returning "Hello from Core!"

Hint

Make sure the core module pom.xml sets the parent and packaging correctly. The GreetingService class should have a method returning the exact string.

3

Create web module with Spring Boot application

In the web module, create a pom.xml with packaging jar, parent set to the root project, and a dependency on the core module. Then create a Spring Boot application class WebApplication in package com.example.web with the @SpringBootApplication annotation and a main method.

Spring Boot

// Create WebApplication.java in package com.example.web
// Annotate with @SpringBootApplication and add main method

Hint

Remember to add the core module as a dependency in the web module pom.xml. The WebApplication class needs the @SpringBootApplication annotation and a main method to start.

4

Create GreetingController using GreetingService

In the web module, create a REST controller class GreetingController in package com.example.web. Inject GreetingService from the core module using constructor injection. Add a @GetMapping for /greet that returns the greeting string from GreetingService.getGreeting().

Spring Boot

// Create GreetingController.java in package com.example.web
// Inject GreetingService via constructor
// Add @RestController annotation
// Add @GetMapping("/greet") method returning greetingService.getGreeting()

Hint

Use constructor injection to get GreetingService into GreetingController. Annotate the class with @RestController and add a method with @GetMapping("/greet") that returns the greeting.

Practice

1. What is the main purpose of a multi-module project in Spring Boot?

easy

A. To create multiple databases for one application

B. To run multiple Spring Boot applications simultaneously

C. To split a large project into smaller, manageable parts

D. To avoid using pom.xml files

Solution

1. Step 1: Understand project organization

   Multi-module projects break a big codebase into smaller modules for easier management.

2. Step 2: Identify the main goal

   This splitting helps teams work on parts independently and share code efficiently.

3. Final Answer:

   To split a large project into smaller, manageable parts -> Option C

4. Quick Check:

   Multi-module = smaller parts [OK]

Hint: Think of dividing work into smaller folders [OK]

Common Mistakes:

• Confusing multi-module with running multiple apps
• Thinking it removes the need for pom.xml
• Assuming it creates multiple databases

2. Which file typically manages all modules in a Spring Boot multi-module project?

easy

A. application.properties

B. module-specific application.yml

C. settings.gradle

D. parent pom.xml

Solution

1. Step 1: Identify the build management file

   In Maven-based Spring Boot projects, the parent pom.xml controls all modules.

2. Step 2: Understand file roles

   Other files like application.properties configure app settings, not modules.

3. Final Answer:

   parent pom.xml -> Option D

4. Quick Check:

   Parent pom manages modules [OK]

Hint: Parent pom.xml controls modules [OK]

Common Mistakes:

• Confusing application.properties with module management
• Thinking settings.gradle is used in Maven projects
• Assuming module-specific files manage all modules

3. Given a multi-module project with modules core and web, where web depends on core, what happens if core is not built before web?

medium

A. The web module build will fail due to missing dependencies

B. The web module will build successfully without core

C. Both modules will build independently without errors

D. The core module will build automatically after web

Solution

1. Step 1: Understand module dependencies

   If web depends on core, it needs core built first to access its code.

2. Step 2: Consequence of build order

   Building web without core causes missing dependency errors and build failure.

3. Final Answer:

   The web module build will fail due to missing dependencies -> Option A

4. Quick Check:

   Dependent module needs dependency built first [OK]

Hint: Build dependencies before dependents [OK]

Common Mistakes:

• Assuming modules build independently
• Thinking build order does not matter
• Believing dependent modules build automatically

4. You have a multi-module Spring Boot project but the web module cannot find classes from the core module. What is the most likely cause?

medium

A. The core module is not listed as a dependency in web's pom.xml

B. The web module has no application.properties

C. The parent pom.xml is missing the modules section

D. The core module is using a different Java version

Solution

1. Step 1: Check module dependencies

   If web cannot find core classes, it likely lacks a dependency declaration on core.

2. Step 2: Verify dependency declaration

   Adding core as a dependency in web's pom.xml fixes class visibility.

3. Final Answer:

   The core module is not listed as a dependency in web's pom.xml -> Option A

4. Quick Check:

   Missing dependency causes class not found [OK]

Hint: Always declare module dependencies in pom.xml [OK]

Common Mistakes:

• Ignoring dependency declarations
• Thinking application.properties affects class visibility
• Assuming Java version mismatch causes missing classes

5. In a multi-module Spring Boot project, you want to share common utility classes across modules without duplicating code. Which approach is best?

hard

A. Copy utility classes into each module's source folder

B. Create a separate common module and add it as a dependency to other modules

C. Put all utility classes in the parent pom.xml

D. Use application.properties to share utility classes

Solution

1. Step 1: Identify code sharing strategy

   To avoid duplication, create a dedicated common module for shared utilities.

2. Step 2: Use module dependencies

   Other modules add common as a dependency to access shared code cleanly.

3. Final Answer:

   Create a separate common module and add it as a dependency to other modules -> Option B

4. Quick Check:

   Shared code in common module [OK]

Hint: Use a common module for shared code [OK]

Common Mistakes:

• Copying code causes duplication and maintenance issues
• Trying to put code in pom.xml (not for code)
• Using properties files to share classes