How do you implement a custom Spring Boot starter?

Table of Contents

• Introduction
  • 1. Setting Up a Custom Spring Boot Starter Project
    • Create a Maven Project
  • 2. Implementing Auto-Configuration
    • Step 1: Create an Auto-Configuration Class
    • Step 2: Create a Service or Component
  • 3. Registering Auto-Configuration in spring.factories
  • 4. Creating the META-INF/spring.factories File
  • 5. Publishing the Custom Starter
    • a. Build the Starter
    • b. Publish to a Repository
    • Example Maven deploy command:
  • 6. Using the Custom Starter in Your Spring Boot Application
  • Practical Example
• Conclusion

Introduction

In Spring Boot, starters are a set of convenient dependency descriptors that you can include in your application to get up and running quickly with specific functionality. These starters come pre-configured with commonly used dependencies, but sometimes, you may want to create a custom starter that encapsulates your application's specific configurations, services, or libraries.

A custom Spring Boot starter can simplify the integration of commonly used code or third-party libraries across multiple applications, promoting modularity and reusability. This guide explains how to implement a custom Spring Boot starter, including the following:

• Setting up a custom Spring Boot starter project.
• Creating auto-configuration for your starter.
• Creating the required META-INF/spring.factories file.
• Publishing and using the custom starter in your applications.

1. Setting Up a Custom Spring Boot Starter Project

The first step in creating a custom starter is to set up a Maven or Gradle project that will act as the starter.

Create a Maven Project

In your pom.xml, define the following:

• Group ID: This should represent your company or organization (e.g., com.example).
• Artifact ID: This is the name of the starter (e.g., my-custom-starter).
• Packaging: Set it to jar, as Spring Boot starters are typically packaged as JAR files.

Example of pom.xml for a custom starter:

This creates the skeleton for your Spring Boot starter project.

2. Implementing Auto-Configuration

Spring Boot starters typically use auto-configuration to apply specific configurations automatically when the starter is added to an application.

Step 1: Create an Auto-Configuration Class

An auto-configuration class is the heart of the starter. It defines what happens when the starter is included in an application.

Here is an example of a basic auto-configuration class:

• @Configuration: Marks this as a configuration class.
• @EnableAutoConfiguration: Tells Spring Boot to attempt to configure beans automatically based on the classpath and other conditions.
• The myCustomService() method creates a bean of type MyCustomService.

Step 2: Create a Service or Component

The service or component provided by your starter should implement the desired functionality. Here’s an example of a simple service:

This service simply prints the custom property that is injected during auto-configuration.

3. Registering Auto-Configuration in spring.factories

Spring Boot uses the spring.factories file to discover and load auto-configuration classes. You need to place a spring.factories file inside the src/main/resources/META-INF directory of your project.

Here is an example of the spring.factories file:

This tells Spring Boot to load the MyCustomAutoConfiguration class during application startup if the starter is included.

4. Creating the META-INF/spring.factories File

To ensure Spring Boot discovers and applies your auto-configuration, create the spring.factories file under src/main/resources/META-INF/:

This file registers the auto-configuration class so Spring Boot can discover and apply it.

5. Publishing the Custom Starter

Once you have developed the custom Spring Boot starter, you can publish it to a Maven repository (either a private repository or Maven Central) to make it available for other projects. You can publish it using the following steps:

a. Build the Starter

Build your starter JAR by running:

b. Publish to a Repository

To publish it to a remote repository, use the Maven deploy plugin to push the artifact to a repository like Maven Central, Nexus, or Artifactory.

Example Maven deploy command:

6. Using the Custom Starter in Your Spring Boot Application

After publishing the custom starter, you can include it as a dependency in other Spring Boot applications.

To use your starter, simply add the following dependency to the consuming application’s pom.xml:

Spring Boot will automatically discover and apply the auto-configuration provided by your starter. Any beans, services, or configurations defined in your starter will be available in the consuming application.

Practical Example

Here’s how to use the custom starter:

1. Add the dependency in your Spring Boot application's pom.xml:

2. In your Spring Boot application's configuration or main class, you can access the service that the starter provides:

   When you run the application, it will print the custom property defined in the starter.

Conclusion

Creating a custom Spring Boot starter provides a way to package your application's common configurations, services, or libraries into reusable modules. By implementing auto-configuration, using **spring.factories**, and following the steps outlined in this guide, you can streamline the development process, increase modularity, and easily share functionality across multiple Spring Boot applications.

Key steps in creating a custom starter:

1. Set up a new Maven project with Spring Boot dependencies.
2. Implement the auto-configuration class and related services.
3. Register the auto-configuration using spring.factories.
4. Publish your starter to a repository.
5. Include and use the starter in other Spring Boot applications.

By following these steps, you can create modular, reusable Spring Boot starters that save time and reduce redundancy across projects.