Building a Custom Spring Boot Starter for Reddit API

Spring Boot Starters are one of the things that make Spring Boot so convenient. When you add spring-boot-starter-web to your project, you get an embedded Tomcat server, Jackson for JSON, and everything configured automatically. But have you ever wondered how these starters actually work? Or how you can build your own?

In this post, we’ll build a custom Spring Boot Starter that wraps Reddit’s public JSON API. By the end, you’ll understand how auto-configuration works and how to create reusable components for your own projects.

Why Build a Custom Starter?

Let’s say you have a piece of functionality that you use across multiple projects. Maybe it’s a custom logging setup, a specific API client, or some internal library. Instead of copying the same configuration code everywhere, you can package it as a Spring Boot Starter.

Benefits of creating a starter:

• Zero configuration for consumers (just add the dependency)
• Sensible defaults that work out of the box
• Customizable through application.properties
• Reusable across multiple projects

Project Structure

Our Reddit starter will be a multi-module Maven project with three modules:

spring-boot-starter-reddit/
├── reddit-spring-boot-autoconfigure/    # The brain - auto-configuration logic
├── reddit-spring-boot-starter/          # The convenience - pulls everything together
└── reddit-spring-boot-sample/           # Demo app showing how to use it

Module	Purpose
reddit-spring-boot-autoconfigure	Contains the auto-configuration logic and Reddit client
reddit-spring-boot-starter	Dependency aggregator that pulls everything together
reddit-spring-boot-sample	Demo application showing how to use the starter

Building the Reddit Client

First, let’s create the core component: a simple Java class that calls Reddit’s API using Spring’s RestClient.

public class RedditClient {
    private final RestClient restClient;
    private final RedditProperties properties;

    public RedditClient(RestClient.Builder restClientBuilder, RedditProperties properties) {
        this.properties = properties;
        this.restClient = restClientBuilder
                .baseUrl("https://www.reddit.com")
                .defaultHeader("User-Agent", properties.getUserAgent())
                .build();
    }

    public List<RedditPost> getPosts(String subreddit) {
        return getPosts(subreddit, properties.getDefaultLimit());
    }

    public List<RedditPost> getPosts(String subreddit, int limit) {
        RedditResponse response = restClient.get()
                .uri("/r/{subreddit}/hot.json?limit={limit}", subreddit, limit)
                .retrieve()
                .body(RedditResponse.class);

        return response.getData().getChildren().stream()
                .map(child -> child.getData())
                .toList();
    }
}

This is just a regular Java class. Nothing special about it yet.

Configuration Properties

Next, we need a way to let users customize the client’s behavior. Spring Boot’s @ConfigurationProperties is perfect for this.

@ConfigurationProperties(prefix = "reddit")
public class RedditProperties {

    private String userAgent = "SpringBootRedditClient:v1.0.0";
    private String defaultSubreddit = "programming";
    private int defaultLimit = 25;

    // getters and setters
}

Now users can configure the client in their application.properties:

reddit.user-agent=MyApp:v1.0.0 (by /u/username)
reddit.default-subreddit=java
reddit.default-limit=50

The Auto-Configuration Class

This is where the magic happens. The auto-configuration class tells Spring Boot how to create and configure our beans automatically.

@AutoConfiguration
@ConditionalOnClass(RestClient.class)
@EnableConfigurationProperties(RedditProperties.class)
public class RedditAutoConfiguration {

    @Bean
    @ConditionalOnMissingBean
    public RedditClient redditClient(
            RestClient.Builder restClientBuilder,
            RedditProperties properties) {
        return new RedditClient(restClientBuilder, properties);
    }
}

Let’s break down the annotations:

Annotation	Purpose
@AutoConfiguration	Marks this class for auto-configuration discovery
@ConditionalOnClass	Only activate if RestClient is on the classpath
@EnableConfigurationProperties	Registers our properties class
@ConditionalOnMissingBean	Only create the bean if user hasn’t defined their own

Spring Boot will detect this bean and skip creating the default one. This is the “opinionated defaults with easy customization” philosophy in action!

Registering the Auto-Configuration

For Spring Boot 3.x and 4.x, we need to register our auto-configuration in:

META-INF/spring/org.springframework.boot.autoconfigure.AutoConfiguration.imports

org.example.redditspringbootautoconfigure.RedditAutoConfiguration

For backward compatibility with Spring Boot 2.x, you can also include:

META-INF/spring.factories

org.springframework.boot.autoconfigure.EnableAutoConfiguration=\
org.example.redditspringbootautoconfigure.RedditAutoConfiguration

This tells Spring Boot: “Hey, when you’re scanning for auto-configurations, include this class!”

The Starter Module

The starter module is simple. It’s just a pom.xml that pulls in the autoconfigure module and any required dependencies.

<dependencies>
    <dependency>
        <groupId>com.example</groupId>
        <artifactId>reddit-spring-boot-autoconfigure</artifactId>
        <version>${project.version}</version>
    </dependency>
    <dependency>
        <groupId>org.springframework.boot</groupId>
        <artifactId>spring-boot-starter-web</artifactId>
    </dependency>
</dependencies>

Using the Starter

This demonstrates how easy it is to use our starter:

1. Add the Dependency

<dependency>
    <groupId>org.example</groupId>
    <artifactId>reddit-spring-boot-starter</artifactId>
    <version>0.0.1-SNAPSHOT</version>
</dependency>

2. Use the Auto-Configured Bean

@RestController
@RequestMapping("/api/reddit")
public class RedditController {

    private final RedditClient redditClient;

    public RedditController(RedditClient redditClient) {
        this.redditClient = redditClient;  // Auto-injected!
    }

    @GetMapping("/posts/{subreddit}")
    public List<RedditPost> getPosts(@PathVariable String subreddit) {
        return redditClient.getPosts(subreddit);
    }

    @GetMapping("/search")
    public List<RedditPost> search(@RequestParam String q) {
        return redditClient.search(q);
    }
}

That’s it! No configuration needed. The RedditClient is automatically available for injection.

3. Optional: Customize via Properties

spring.application.name=reddit-spring-boot-sample

# Customize the Reddit client
reddit.user-agent=MyAwesomeApp:v1.0.0 (by /u/myusername)
reddit.default-subreddit=programming
reddit.default-limit=50

server.port=8080

Testing the Sample Application

Build and run the sample application:

mvn clean install
cd reddit-spring-boot-sample
mvn spring-boot:run

Test the endpoint:

curl http://localhost:8080/api/reddit/posts/java

You should see a list of posts from the Java subreddit.

Key Takeaways

Building a custom Spring Boot Starter comes down to a few key concepts:

1. Create your core components as regular Java classes
2. Use @ConfigurationProperties to allow customization through properties
3. Write an auto-configuration class with conditional annotations
4. Register the auto-configuration in the imports file
5. Package it as a starter for easy consumption

The philosophy behind Spring Boot Starters is opinionated defaults with easy customization. Your starter should work out of the box with sensible defaults, but allow users to customize everything through properties or by providing their own beans.

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You can find the complete source code for this project on GitHub.