Spring Boot Starters

Introduction

When you begin developing applications with Spring Boot, one of the first concepts you'll encounter is "Spring Boot Starters." These are a set of convenient dependency descriptors that greatly simplify your build configuration and allow you to quickly get started with specific functionality in your application.

Think of Spring Boot Starters as pre-packaged bundles of dependencies that address common use cases. Instead of manually adding and configuring multiple individual dependencies, you can add a single starter that pulls in everything you need for a particular type of functionality.

What Are Spring Boot Starters?

Spring Boot Starters are essentially dependency descriptors that combine:

1. The core Spring modules you need for a specific functionality
2. Third-party libraries that work well with Spring
3. Auto-configuration properties to make everything work together seamlessly

By including a starter in your project, Spring Boot automatically configures the necessary beans, properties, and infrastructure components based on sensible defaults, allowing you to focus on your business logic rather than configuration.

How Spring Boot Starters Work

Spring Boot Starters work through a combination of:

1. Dependency Management: Each starter includes a curated set of dependencies that work well together
2. Auto-configuration: Built-in mechanisms detect what's on your classpath and configure appropriate beans
3. Sensible defaults: Pre-configured properties that can be overridden if needed

This approach follows Spring Boot's "convention over configuration" philosophy, reducing boilerplate code and configuration while giving you the flexibility to customize when required.

Common Spring Boot Starters

Here are some of the most commonly used Spring Boot Starters:

Starter	Purpose
spring-boot-starter	Core starter with auto-config and logging
spring-boot-starter-web	For building web applications including RESTful apps
spring-boot-starter-data-jpa	For using Spring Data JPA with Hibernate
spring-boot-starter-security	For adding Spring Security
spring-boot-starter-test	For testing Spring Boot applications
spring-boot-starter-thymeleaf	For using Thymeleaf template engine

Using Spring Boot Starters in Your Project

Let's look at how to add starters to your project using different build systems:

Maven

Add the desired starter to your pom.xml file:

xml

<dependencies>
    <dependency>
        <groupId>org.springframework.boot</groupId>
        <artifactId>spring-boot-starter-web</artifactId>
    </dependency>
</dependencies>

If you're using the Spring Boot parent POM, you don't need to specify the version:

xml

<parent>
    <groupId>org.springframework.boot</groupId>
    <artifactId>spring-boot-starter-parent</artifactId>
    <version>2.7.1</version>
</parent>

Gradle

Add the starter to your build.gradle file:

groovy

dependencies {
    implementation 'org.springframework.boot:spring-boot-starter-web'
}

If you're using the Spring Boot plugin:

groovy

plugins {
    id 'org.springframework.boot' version '2.7.1'
    id 'io.spring.dependency-management' version '1.0.11.RELEASE'
    id 'java'
}

Practical Examples

Let's explore some practical examples of how Spring Boot Starters simplify development:

Example 1: Building a Simple REST API

By using spring-boot-starter-web, you can quickly create a REST API:

java

import org.springframework.boot.SpringApplication;
import org.springframework.boot.autoconfigure.SpringBootApplication;
import org.springframework.web.bind.annotation.GetMapping;
import org.springframework.web.bind.annotation.RestController;

@SpringBootApplication
public class SimpleApiApplication {
    public static void main(String[] args) {
        SpringApplication.run(SimpleApiApplication.class, args);
    }
}

@RestController
class HelloController {
    @GetMapping("/hello")
    public String sayHello() {
        return "Hello from Spring Boot!";
    }
}

What happens behind the scenes:

1. The spring-boot-starter-web pulls in dependencies for Spring MVC, embedded Tomcat, and JSON processing
2. Spring Boot auto-configures a dispatcher servlet, default error pages, and JSON conversion
3. Your application is ready to handle HTTP requests with minimal configuration

When you run this application and navigate to http://localhost:8080/hello in your browser, you'll see:

Hello from Spring Boot!

Example 2: Setting Up Database Connectivity

Using spring-boot-starter-data-jpa makes database interactions straightforward:

java

import javax.persistence.Entity;
import javax.persistence.GeneratedValue;
import javax.persistence.Id;
import org.springframework.data.jpa.repository.JpaRepository;
import org.springframework.stereotype.Repository;

@Entity
public class Product {
    @Id
    @GeneratedValue
    private Long id;
    private String name;
    private double price;
    
    // Constructors, getters, and setters
    public Product() {}
    
    public Product(String name, double price) {
        this.name = name;
        this.price = price;
    }
    
    // Getters and setters
    public Long getId() { return id; }
    public void setId(Long id) { this.id = id; }
    public String getName() { return name; }
    public void setName(String name) { this.name = name; }
    public double getPrice() { return price; }
    public void setPrice(double price) { this.price = price; }
}

@Repository
public interface ProductRepository extends JpaRepository<Product, Long> {
}

Configure the database connection in application.properties:

properties

spring.datasource.url=jdbc:h2:mem:testdb
spring.datasource.driverClassName=org.h2.Driver
spring.jpa.database-platform=org.hibernate.dialect.H2Dialect
spring.h2.console.enabled=true

Now you can use the repository in your service or controller:

java

import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.web.bind.annotation.*;
import java.util.List;

@RestController
@RequestMapping("/products")
public class ProductController {
    @Autowired
    private ProductRepository repository;
    
    @GetMapping
    public List<Product> getAllProducts() {
        return repository.findAll();
    }
    
    @PostMapping
    public Product addProduct(@RequestBody Product product) {
        return repository.save(product);
    }
}

What happens behind the scenes:

1. Spring Boot configures a data source and connection pool
2. It sets up JPA and Hibernate with sensible defaults
3. Spring Data repositories automatically implement database operations
4. H2 in-memory database is used (which is included in the starter)

Creating Custom Starters

As your projects grow, you might want to create custom starters to share common functionality across multiple applications. Here's how to create a basic custom starter:

1. Create an Autoconfiguration Module

First, create a module for your auto-configuration:

java

@Configuration
@ConditionalOnClass(YourService.class)
public class YourServiceAutoConfiguration {

    @Bean
    @ConditionalOnMissingBean
    public YourService yourService() {
        return new YourServiceImpl();
    }
}

2. Register Your Auto-configuration

Create a file at src/main/resources/META-INF/spring.factories:

properties

org.springframework.boot.autoconfigure.EnableAutoConfiguration=\
com.yourpackage.YourServiceAutoConfiguration

3. Create the Starter POM

Create a separate module that depends on your auto-configuration module:

xml

<dependencies>
    <dependency>
        <groupId>com.yourpackage</groupId>
        <artifactId>your-service-autoconfigure</artifactId>
        <version>1.0.0</version>
    </dependency>
    <!-- Other necessary dependencies -->
</dependencies>

Best Practices for Using Spring Boot Starters

1. Start small: Begin with just the starters you need to avoid bloating your application
2. Understand what each starter includes: Be aware of the dependencies each starter pulls in
3. Override only what you need: Spring Boot's defaults are sensible for most use cases
4. Explore the documentation: Check the official documentation to see available configuration properties
5. Inspect the dependencies: Use mvn dependency:tree or Gradle's dependencies task to see what's included

Common Issues and Solutions

Multiple Web Starters Conflict

Problem: Including both spring-boot-starter-web and spring-boot-starter-webflux can cause conflicts.

Solution: Choose either the servlet-based spring-boot-starter-web or the reactive spring-boot-starter-webflux based on your needs, not both.

Excluding Unwanted Dependencies

Problem: A starter includes dependencies you don't want.

Solution: Use exclusions in your build file:

xml

<dependency>
    <groupId>org.springframework.boot</groupId>
    <artifactId>spring-boot-starter-web</artifactId>
    <exclusions>
        <exclusion>
            <groupId>org.springframework.boot</groupId>
            <artifactId>spring-boot-starter-tomcat</artifactId>
        </exclusion>
    </exclusions>
</dependency>

Version Conflicts

Problem: Third-party libraries conflict with versions managed by Spring Boot.

Solution: Let Spring Boot manage versions whenever possible, or explicitly override versions with care:

xml

<properties>
    <library.version>2.5.0</library.version>
</properties>

Summary

Spring Boot Starters provide an efficient way to add sets of related dependencies to your project, greatly simplifying the process of building Spring applications. They embody Spring Boot's philosophy of "convention over configuration" by automatically configuring common functionality while still allowing for customization.

Key takeaways:

• Starters are pre-packaged dependency descriptors that simplify build configuration
• They provide auto-configuration and sensible defaults
• The Spring Boot ecosystem offers many official starters for common use cases
• Custom starters can be created to share common functionality across projects
• Using starters dramatically reduces setup time and boilerplate code

Additional Resources

• Official Spring Boot Starters Documentation
• Complete List of Spring Boot Starters
• Guide to Creating a Custom Starter

Exercises

1. Create a simple REST API using spring-boot-starter-web that returns a list of products.
2. Modify a Spring Boot application to use MongoDB instead of JPA by switching starters.
3. Create a custom Spring Boot starter that provides a standardized way to handle error responses in your organization.
4. Build a Spring Boot application that combines both web and messaging functionality using appropriate starters.
5. Examine the dependency tree of a Spring Boot application and identify which dependencies come from which starters.

Happy coding with Spring Boot Starters!