A custom Spring Boot starter is a reusable module that bundles common dependencies and auto-configuration so multiple applications can pull in shared functionality with minimal setup.
Key Points: • Create a separate Maven or Gradle module containing the shared dependencies the starter should provide. • Write @Configuration classes annotated with conditions like @ConditionalOnMissingBean or @ConditionalOnProperty to auto-configure beans sensibly. • Register the auto-configuration class in META-INF/spring/org.springframework.boot.autoconfigure.AutoConfiguration.imports (or spring.factories on older Boot versions). • Publish the starter to an internal artifact repository so other teams can depend on it like any other library. • Useful for standardizing cross-cutting concerns such as logging, security defaults, or a shared HTTP client across many microservices.
Example: An organization builds an internal "acme-logging-starter" that auto-configures structured JSON logging and correlation IDs, so every microservice just adds one dependency instead of copy-pasting logging configuration.
Interview Tip: A concise interview answer is:
"A custom starter is its own module with the shared dependencies and an auto-configuration class guarded by conditional annotations like @ConditionalOnMissingBean, registered so Spring Boot picks it up automatically. It's useful for standardizing things like logging or security defaults across many microservices without repeating configuration in each one."