Multi-Module Project With Spring Boot

Last Updated : 04 Jun, 2024

Multi-Module project with Spring Boot refers to a project structure where multiple modules or subprojects are organized under a single parent project. Each module can represent a distinct component, functionality, or layer of the application, allowing for better organization, maintainability, and code reuse. In simple, we can say a Spring Boot project that comprises nested Maven projects is commonly referred to as a multi-module project.

In a Spring Boot Multi-Module Project, the application is divided into multiple modules, where each module plays an important role in the certain functionality of an application. A module can be considered an independent project or sub-project. In this article, we will discuss how to create a multi-module project with Spring Boot.

Note: Multi-Module Project is just a set of multiple projects where each project has its respective function.

Working of Spring Boot Multi-Module Project

Create a Parent Module
- First, you have to create a Parent module.
- The parent module acts as a container of sub-modules.
- Also, you can use the Parent module for bootstrapping of the application ( main() method ).
- Alternatively, you can also use any of the sub-module by implementing the main() method for bootstrapping the application and deleting the main() method from the parent module.
Configure Parent Module
- Your Parent module should have pom packaging instead of jar and war.
- When you create sub-modules, all of them get listed in the <modules> tag in pom.xml of the parent module.
Inheritance
- All the dependencies you have enlisted in the pom.xml of the parent module will be automatically inherited by sub-modules directly.
- Therefore, you don't need to add dependencies to sub-module's pom.xml to use them.

Steps to Create a Multi-Module Project in Spring Boot

Step 1: Create a simple Spring Starter Project (File -> New -> Spring Starter Project -> Next -> ( Select Dependencies ) -> Next -> Finish)
Step 2: Change or add <packaging>pom</packaging>
Step 3: Add a sub-module (Right click on Parent Module -> New -> other -> Maven -> Maven Module -> Next ( Select both checkboxes ) -> Next -> jar or war -> Finish).
Step 4: After creating sub-modules, the parent module will contain the folders of sub-modules, and also independent projects of the already created respective sub-modules will be created.

Step 1: Create the Parent Module

ParentGFG - Parent Module

Now, we will create a simple Parent module using Maven dependencies with pom packaging to the aggregate library and application modules.

pom packaging:

  <groupId>sia</groupId>
     <artifactId>ParentGFG</artifactId>
     <version>0.0.1-SNAPSHOT</version>
  <packaging>pom</packaging>

pom.xml (Configurations):

XML

<?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                              https://maven.apache.org/xsd/maven-4.0.0.xsd">     <modelVersion>4.0.0</modelVersion>     <parent>         <groupId>org.springframework.boot</groupId>         <artifactId>spring-boot-starter-parent</artifactId>         <version>2.6.3</version>         <relativePath/> <!-- lookup parent from repository -->     </parent>     <groupId>sia</groupId>     <artifactId>ParentGFG</artifactId>     <version>0.0.1-SNAPSHOT</version>     <packaging>pom</packaging>     <name>ParentGFG</name>     <description>Multi Module Project</description>     <properties>         <java.version>11</java.version>     </properties>     <dependencies>         <dependency>             <groupId>org.springframework.boot</groupId>             <artifactId>spring-boot-starter-thymeleaf</artifactId>         </dependency>         <dependency>             <groupId>org.springframework.boot</groupId>             <artifactId>spring-boot-starter-web</artifactId>         </dependency>          <dependency>             <groupId>org.springframework.boot</groupId>             <artifactId>spring-boot-devtools</artifactId>             <scope>runtime</scope>             <optional>true</optional>         </dependency>         <dependency>             <groupId>org.projectlombok</groupId>             <artifactId>lombok</artifactId>             <optional>true</optional>         </dependency>         <dependency>             <groupId>org.springframework.boot</groupId>             <artifactId>spring-boot-starter-test</artifactId>             <scope>test</scope>         </dependency>     </dependencies>      <build>         <plugins>             <plugin>                 <groupId>org.springframework.boot</groupId>                 <artifactId>spring-boot-maven-plugin</artifactId>                 <configuration>                     <excludes>                         <exclude>                             <groupId>org.projectlombok</groupId>                             <artifactId>lombok</artifactId>                         </exclude>                     </excludes>                 </configuration>             </plugin>         </plugins>     </build>      <modules>         <module>GFG-Module1</module>         <module>GFG-Module2</module>     </modules> </project>

Note:

After creating Spring Starter Project with a jar or war packaging, make sure that you add or change the 'packaging' tag with 'pom' -> '<packaging>pom</packaging>'.
When you will create sub-modules, the 'modules' tag will automatically get generated with added respective modules.
You can also remove the 'src' folder from Parent Module after implementing the main() method in any of the sub-module, as the Parent Module can also act just like a container.

Step 2: Create the Sub Modules

GFG-Module1:

Now, we will create one sub module named GFG-Module1.

pom.xml (Configurations):

Declares a parent module of which this sub-module will be a part.
Information tags of this sub-module are also initialized as well.

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                               https://maven.apache.org/xsd/maven-4.0.0.xsd">   <modelVersion>4.0.0</modelVersion>   <parent>     <groupId>sia</groupId>     <artifactId>ParentGFG</artifactId>     <version>0.0.1-SNAPSHOT</version>   </parent>   <artifactId>GFG-Module1</artifactId>   <name>GFG-Module1</name>   <description>GeeksforGeeks</description> </project>

Step 3: Create Main Application Class

Main.java (Bootstrapping of the Application)

Java

package controller;  import org.springframework.boot.SpringApplication; import org.springframework.boot.autoconfigure.SpringBootApplication;  @SpringBootApplication public class Main {      public static void main(String[] args)     {         SpringApplication.run(Main.class, args);     } }

Step 4: Create a REST Controller

RestApi.java (Endpoint of the Application)

This class uses 'Starter Web' dependency which is automatically inherited from the parent module's pom.xml.
This controller class accepts HTTP requests.
Here, the HTTP GET request is accepted by get() method.
This method uses an entity class that is created and declared in a sub-module ( GFG-Module2 ).
To import classes etc from different modules, you have to hover on the red error underline.
Click -> 'fix project setup' and select the respective sub-module where you have created the class in.

Java

package controller;  import entity.UserModel; import org.springframework.web.bind.annotation.GetMapping; import org.springframework.web.bind.annotation.RequestMapping; import org.springframework.web.bind.annotation.RestController;  @RestController @RequestMapping("/get") public class RestApi {      @GetMapping public UserModel get()     {          UserModel entity = new UserModel();         entity.setId("1");         entity.setName("Darshan.G.Pawar");         entity.setEmail("geek@geek");         entity.setPincode("422 009");          return entity;     } }

GFG-Module2:

pom.xml (Configurations)

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                               https://maven.apache.org/xsd/maven-4.0.0.xsd">   <modelVersion>4.0.0</modelVersion>   <parent>     <groupId>sia</groupId>     <artifactId>ParentGFG</artifactId>     <version>0.0.1-SNAPSHOT</version>   </parent>   <artifactId>GFG-Module2</artifactId>   <name>GFG-Module2</name>   <description>GeeksforGeeks</description> </project>

Step 5: Create Entity Class in Another Module

UserModel.java (User data entity class)

This class represents a user entity class, which has been used by the get() method of the RestApi.java class.
This class requires an important set of Getter and Setter methods of all instance variables fields.
This class uses 'Lombok' dependency which is automatically inherited from the parent module's pom.xml.
Therefore, the '@Data' annotation of the Lombok library is used to automatically generate Getter and Setter methods at runtime.
The annotation '@RequiredArgsConstructor' is used to automatically generate a constructor for required ( where the constrain is '@NonNull' ) or final fields of a class.
If a class doesn't contain fields, then the '@RequiredArgsConstructor' annotation acts as a '@NoArgsConstructor' annotation which creates zero parameter constructor.

Java

package entity;  import lombok.Data; import lombok.RequiredArgsConstructor;  @Data @RequiredArgsConstructor public class UserModel {      String id;     String name;     String email;     String pincode; }

Output:

RestApi.java of GFG-Module1

GFG-Module2 (sub-module) declares an entity (Data) object.
This data is used in the body of the response which is returned by the RESTful API of GFG-Module1 ( sub-module ).

Default JSON body output ( UserModel object ) from @RestController

Note: You will have to run the GFG-Module1 as the main() method and the controller class is in this sub-module.

Advantages of a Multi-Module Project

It provides a great ability to build all sub-modules only with a single command.
We can run the build command from the parent module.
While building the application, the build system takes care of the build order.
Deployment of the applications gets very convenient and flexible.
Also, the code from the various modules across different projects can be re-used.
Last but not the least, with the help of Multi-Module Project architecture, we can gain benefits from the microservices architecture.

Multi-Module Project With Spring Boot

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Java

Multi-Module Project With Spring Boot

Working of Spring Boot Multi-Module Project

Steps to Create a Multi-Module Project in Spring Boot

Step 1: Create the Parent Module

ParentGFG - Parent Module

Step 2: Create the Sub Modules

GFG-Module1:

Step 3: Create Main Application Class

Step 4: Create a REST Controller

GFG-Module2:

Step 5: Create Entity Class in Another Module

Output:

RestApi.java of GFG-Module1

Advantages of a Multi-Module Project

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