How to Implement Pre‑warming in the Spring Ecosystem: Extension Points and Their Execution Order

Question: In the Spring ecosystem, which extension points should be used to implement a pre‑warming feature (such as loading a local cache in advance) and why?

Answer:

1. Implement org.springframework.context.SmartLifecycle : This runs in the finishRefresh phase, just before the web server actually starts. It must be ordered before WebServerStartStopLifecycle to ensure the cache is ready.

Note: The SmartLifecycle bean is executed prior to WebServerStartStopLifecycle by default.

2. Implement org.springframework.context.ApplicationListener<ContextRefreshedEvent> : Listen for the ContextRefreshedEvent . The event is published after all SmartLifecycle beans have completed, allowing you to perform additional initialization.

Note: The listener runs in the same thread as the event publication; it should not be asynchronous to avoid dead‑locks.

3. Implement org.springframework.beans.factory.InitializingBean : Its afterPropertiesSet() method is called after the bean’s properties are set, providing another hook for early initialization.

4. Implement org.springframework.boot.ApplicationRunner or org.springframework.boot.CommandLineRunner : These run in the final stage of Spring Boot startup, just before the application is marked healthy.

5. Use the @PostConstruct annotation: The annotated method is invoked after the bean’s dependencies are injected, offering a simple way to execute pre‑warming logic.

6. Combine the above as needed, but avoid asynchronous execution for points 5 and 6 to prevent potential dead‑locks.

The overall goal is to execute the pre‑warming logic after the container has finished its startup preparation but before any incoming traffic reaches the application.

Execution timing details:

• SmartLifecycle runs at the end of the container’s refresh phase (the finishRefresh stage) and must be ordered before WebServerStartStopLifecycle .

• ContextRefreshedEvent is published after all SmartLifecycle beans have completed, providing a natural point to trigger further initialization.

• ApplicationRunner and CommandLineRunner are invoked by SpringApplication#callRunners during the last phase of Spring Boot startup, just before the health status is set to healthy.

Relevant code snippets:

org.springframework.context.support.AbstractApplicationContext#refresh

org.springframework.context.support.DefaultLifecycleProcessor#startBeans

org.springframework.boot.SpringApplication#run(java.lang.String...)

org.springframework.boot.SpringApplication#callRunners

These snippets illustrate where the lifecycle callbacks occur within the Spring bootstrapping process.