Multi-module project structure in Spring Boot

Introduction

A multi-module project helps organize code into smaller parts. It makes big projects easier to manage and reuse.

When your project has different parts like web, service, and database layers.

When you want to share common code between different applications.

When multiple teams work on different parts of the same project.

When you want to build and test parts separately.

When you want to keep your project clean and organized.

Syntax

Spring Boot

root-project/
  ├─ pom.xml (parent)
  ├─ module-web/
  │    └─ pom.xml
  ├─ module-service/
  │    └─ pom.xml
  └─ module-data/
       └─ pom.xml

The root pom.xml lists all modules as children.

Each module has its own pom.xml to define dependencies and build rules.

Examples

This is the parent pom.xml listing all modules.

Spring Boot

<project>
  <modules>
    <module>module-web</module>
    <module>module-service</module>
    <module>module-data</module>
  </modules>
</project>

Example of a module pom.xml that refers to the parent project.

Spring Boot

<project>
  <parent>
    <groupId>com.example</groupId>
    <artifactId>root-project</artifactId>
    <version>1.0</version>
  </parent>
  <artifactId>module-web</artifactId>
  <dependencies>
    <!-- module dependencies -->
  </dependencies>
</project>

Sample Program

This shows a simple multi-module Spring Boot project with a parent pom.xml and two modules: module-web and module-service. The web module depends on the service module.

Spring Boot

<?xml version="1.0" encoding="UTF-8"?>
<project xmlns="http://maven.apache.org/POM/4.0.0"
         xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
         xsi:schemaLocation="http://maven.apache.org/POM/4.0.0
                             http://maven.apache.org/xsd/maven-4.0.0.xsd">
  <modelVersion>4.0.0</modelVersion>
  <groupId>com.example</groupId>
  <artifactId>root-project</artifactId>
  <version>1.0</version>
  <packaging>pom</packaging>
  <modules>
    <module>module-web</module>
    <module>module-service</module>
  </modules>
</project>

--- module-web/pom.xml ---
<project xmlns="http://maven.apache.org/POM/4.0.0"
         xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
         xsi:schemaLocation="http://maven.apache.org/POM/4.0.0
                             http://maven.apache.org/xsd/maven-4.0.0.xsd">
  <modelVersion>4.0.0</modelVersion>
  <parent>
    <groupId>com.example</groupId>
    <artifactId>root-project</artifactId>
    <version>1.0</version>
  </parent>
  <artifactId>module-web</artifactId>
  <dependencies>
    <dependency>
      <groupId>org.springframework.boot</groupId>
      <artifactId>spring-boot-starter-web</artifactId>
    </dependency>
    <dependency>
      <groupId>com.example</groupId>
      <artifactId>module-service</artifactId>
    </dependency>
  </dependencies>
</project>

--- module-service/pom.xml ---
<project xmlns="http://maven.apache.org/POM/4.0.0"
         xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
         xsi:schemaLocation="http://maven.apache.org/POM/4.0.0
                             http://maven.apache.org/xsd/maven-4.0.0.xsd">
  <modelVersion>4.0.0</modelVersion>
  <parent>
    <groupId>com.example</groupId>
    <artifactId>root-project</artifactId>
    <version>1.0</version>
  </parent>
  <artifactId>module-service</artifactId>
  <dependencies>
    <dependency>
      <groupId>org.springframework.boot</groupId>
      <artifactId>spring-boot-starter</artifactId>
    </dependency>
  </dependencies>
</project>

Important Notes

Always keep the parent pom.xml packaging as pom.

Modules can depend on each other by adding dependencies with the module's artifactId.

Use this structure to build and test modules separately or together.

Summary

Multi-module projects split big code into smaller parts.

The parent pom.xml manages all modules.

Modules can share code and depend on each other easily.

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