Junior Level

Starter is a ready-made set of dependencies for specific functionality.

Without Starter:

<!-- Need to add 5+ dependencies manually -->
<dependency><groupId>org.springframework</groupId>...</dependency>
<dependency><groupId>org.hibernate</groupId>...</dependency>
...

With Starter:

<!-- One dependency - everything works! -->
<dependency>
    <groupId>org.springframework.boot</groupId>
    <artifactId>spring-boot-starter-data-jpa</artifactId>
</dependency>

Popular starters:

• spring-boot-starter-web -> REST API
• spring-boot-starter-data-jpa -> Databases
• spring-boot-starter-test -> Testing

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Middle Level

Starter Architecture

mylib-spring-boot-starter
  -> mylib-autoconfigure (configuration)
    -> @ConditionalOnClass(...)
    -> @Bean methods
  -> mylib (main library)

Naming Convention

spring-boot-starter-*     -> Official starters
*-spring-boot-starter     -> Third-party starters

BOM (Bill of Materials)

<!-- Manages versions of all dependencies -->
<parent>
    <groupId>org.springframework.boot</groupId>
    <artifactId>spring-boot-dependencies</artifactId>
</parent>

-> No "Jar Hell" -> all versions are compatible

spring-boot-dependencies is a parent POM with <dependencyManagement> that fixes versions of hundreds of libraries. You inherit it via <parent> or import it into your <dependencyManagement>.

Customization

# Starter provides default settings
# You can override via application.yaml
mylib:
  enabled: true
  timeout: 5000

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Senior Level

Starter Limitations

Starters can pull unnecessary transitive dependencies. If you need a minimal classpath (GraalVM Native Image) - use <exclusions> or add dependencies manually.

Creating Your Own Starter

// In autoconfigure module:
@AutoConfiguration
@ConditionalOnClass(MyService.class)
@ConditionalOnMissingBean(MyService.class)
public class MyLibAutoConfiguration {

    @Bean
    public MyService myService() {
        return new MyService();
    }
}

// Spring Boot 2.7+: META-INF/spring/org.springframework.boot.autoconfigure.AutoConfiguration.imports
// Before 2.7: META-INF/spring.factories
com.example.MyLibAutoConfiguration

ConfigurationProperties

@ConfigurationProperties("mylib")
public class MyLibProperties {
    private boolean enabled = true;
    private int timeout = 5000;
}

-> IDE auto-completion from spring-configuration-metadata.json

Native Image Support

Spring Boot 3 + GraalVM:
  -> Hint files for reflection
  -> AOT compilation
  -> Starter should support Native Image

Production Experience

Real scenario: corporate starter

50 microservices -> same logging configuration
-> Created mycompany-logging-starter
-> All services got a unified log format

Best Practices

1. Separate autoconfigure and starter modules
2. @ConditionalOnMissingBean -> allow overriding
3. Naming -> *-spring-boot-starter for third-party
4. BOM -> version management
5. ConfigurationProperties -> YAML configuration
6. Native Image -> GraalVM support

Summary for Senior

• Starter = dependencies + auto-configuration
• BOM -> compatible versions
• @ConditionalOnMissingBean -> overriding
• Naming -> *-spring-boot-starter for third-party
• ConfigurationProperties -> YAML configuration
• Native Image -> support in Spring Boot 3

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Interview Cheat Sheet

Must know:

• Starter = one dependency instead of many transitive dependencies
• spring-boot-dependencies (BOM) fixes versions of all libraries - no “Jar Hell”
• Starter = autoconfigure module (@ConditionalOnClass, @Bean) + dependencies
• @ConditionalOnMissingBean allows users to override beans
• Third-party starters named -spring-boot-starter, official - spring-boot-starter-
• ConfigurationProperties gives YAML configuration and IDE auto-completion
• Your own starter: @AutoConfiguration + META-INF/spring/…AutoConfiguration.imports (Spring Boot 2.7+)

Common follow-up questions:

• How to create your own starter? - Autoconfigure module with @AutoConfiguration + registration in AutoConfiguration.imports.
• Why is BOM needed? - Fixes compatible versions of hundreds of libraries via dependencyManagement.
• Can you override a bean from a starter? - Yes, via @ConditionalOnMissingBean - your bean replaces the default.
• What to do with unnecessary transitive dependencies? - Use in Maven or assemble classpath manually for GraalVM.

Red flags (DO NOT say):

• “Starter is just a jar with dependencies” - no, there is also auto-configuration.
• “You can name your starter spring-boot-starter-*” - reserved for official ones.
• “BOM is a regular dependency” - no, it is dependencyManagement for versions.
• “Starters work without auto-configuration” - without it, it is just a set of dependencies.

Related topics:

• [[21. How SpringBootApplication works]]
• [[24. What is a Configuration class]]
• [[29. What are profiles in Spring]]
• [[26. What does the Autowired annotation do]]