Spring Boot - Multi-Module Project

A multi-module project in Maven helps in breaking down a large project into smaller, manageable modules or sub-projects which can be built independently or together. Spring Boot can be seamlessly integrated into a multi-module Maven project. Here's how you can set it up:

1. Parent POM

Start by creating a parent project, which is essentially a project with packaging of type pom. This project will contain common configurations and dependencies for all sub-modules.

<groupId>com.example</groupId>
<artifactId>parent-project</artifactId>
<version>1.0.0</version>
<packaging>pom</packaging>

<modules>
    <module>module1</module>
    <module>module2</module>
</modules>

<parent>
    <groupId>org.springframework.boot</groupId>
    <artifactId>spring-boot-starter-parent</artifactId>
    <version>2.5.4</version>
</parent>

The <modules> element lists all the sub-modules, and we have set the Spring Boot Starter Parent as the parent.

2. Create Sub-Modules

For each sub-module, you will have a directory at the same level as the parent project. Each of these will contain its own pom.xml. The sub-modules can be Spring Boot applications, libraries, or any valid Maven project.

Example for module1:

<parent>
    <groupId>com.example</groupId>
    <artifactId>parent-project</artifactId>
    <version>1.0.0</version>
</parent>

<groupId>com.example</groupId>
<artifactId>module1</artifactId>
<version>1.0.0</version>

<dependencies>
    <!-- Spring Boot and other dependencies specific to this module -->
</dependencies>

3. Dependency Management in Parent POM

In the parent pom.xml, you can define common dependencies and properties for all sub-modules using the <dependencyManagement> element. This ensures consistency across all modules.

<dependencyManagement>
    <dependencies>
        <!-- Define common dependencies here -->
    </dependencies>
</dependencyManagement>

4. Inter-module Dependencies

If one module depends on another, you can add a dependency to the pom.xml of the dependent module:

<dependencies>
    <dependency>
        <groupId>com.example</groupId>
        <artifactId>module2</artifactId>
        <version>1.0.0</version>
    </dependency>
    <!-- other dependencies -->
</dependencies>

5. Building the Project

When you build the parent project using mvn clean install, all sub-modules will be built in the order defined in the parent pom.xml. If you navigate to a sub-module's directory, you can build it individually using the same Maven commands.

Benefits:

Modularization: Helps in modularizing a large project, ensuring that each module is focused on a single responsibility.
Reusability: Common functionalities can be isolated into a module and be reused across multiple modules or projects.
Parallel Development: Different teams can work on different modules simultaneously without much interference.

Conclusion:

Setting up a multi-module project in Spring Boot using Maven can greatly improve the maintainability and structure of large projects. This structure also promotes the separation of concerns and modularity, enabling teams to work more efficiently and systematically.

Configuring dependencies and parent-child relationships in multi-module projects:
- Description: In a multi-module project, dependencies and parent-child relationships are defined in the pom.xml files to manage project structure and build order.
- Code:
```

<modules>
    <module>module1</module>
    <module>module2</module>
</modules>


<parent>
    <groupId>com.example</groupId>
    <artifactId>parent</artifactId>
    <version>1.0.0</version>
</parent>
```
Sharing resources and configuration across modules in Spring Boot:
- Description: Sharing resources and configurations involves defining common properties or configurations in a shared module.
- Code:
```
# Shared configuration in a common module
shared.property=value
```
Building and packaging multi-module projects with Maven in Spring Boot:
- Description: Building a multi-module project with Maven involves using the mvn install command in the parent project to build and install all modules.
- Code: Run the following command in the parent project:
```
mvn clean install
```

Handling dependencies and versioning in multi-module projects with Spring Boot:

Description: Managing dependencies and versions is critical. Define dependencies and versions in the parent pom.xml to ensure consistency.

Code:

<!-- Parent POM -->
<properties>
    <spring-boot.version>2.5.4</spring-boot.version>
</properties>

<dependencies>
    <dependency>
        <groupId>org.springframework.boot</groupId>
        <artifactId>spring-boot-starter</artifactId>
        <version>${spring-boot.version}</version>
    </dependency>
</dependencies>