use DeepSeek for one week, omg, really powerful for our tech guys.

Every answer exceeded my expectations and I could easily access and understand details that I didn’t know before or would have taken me a lot of time to know, now with deep seek, I feel like I have the world at my fingertips.

List one example I asked to DeepSeek.

Q: Spring Boot startup flow

here are the full output from DeepSeek.

Thought for 26 seconds

Okay, the user asked about the Spring Boot startup flow. Let me think about how to explain this clearly.

First, I should start by mentioning that when a Spring Boot app runs, it begins with the main method. The main method uses SpringApplication.run(). That’s the entry point.

Next, I need to outline the key steps in the startup process. Let me recall the order: loading the application context, running the application runners, and so on.

Wait, maybe I should break it down step by step. Let’s see. The main method triggers SpringApplication.run(), which initializes the application. Then, Spring Boot starts setting up the environment, like reading properties and profiles.

After that, it creates the ApplicationContext. The type of context depends on whether it’s a web app or not. For web apps, it’s AnnotationConfigServletWebServerApplicationContext, otherwise AnnotationConfigApplicationContext.

Then there’s the part about @SpringBootApplication. The @EnableAutoConfiguration triggers auto-configuration, which sets up beans based on dependencies. Component scanning happens here too, looking for @Component, @Service, etc.

Don’t forget about the application runners, like CommandLineRunner and ApplicationRunner. They execute after the context is loaded but before the app starts accepting requests.

Oh, and the embedded server setup. Spring Boot automatically starts Tomcat, Jetty, or another server if it’s a web app. The server starts on the specified port, and then the app is ready to handle requests.

I should also mention events and listeners. Spring Boot publishes events during startup, and developers can hook into these for custom logic.

Wait, maybe I should structure this as a numbered list for clarity. Let me check the order again: main method, environment setup, context creation, auto-configuration, bean registration, runners, server start.

Also, the application.yml or application.properties are loaded early on, so properties are available during context setup.

I should explain each step briefly but clearly. Make sure to highlight the auto-configuration and component scanning parts since they’re key features of Spring Boot.

Maybe include an example code snippet for the main class. That could help illustrate the starting point.

Let me also think about any common extensions or hooks, like initializing beans, PostConstruct, or using ApplicationContextInitializer. But maybe keep it simple for now unless the user asks for more details.

Okay, putting it all together: start with the main method, then the steps in order, explaining each part. Highlight auto-configuration, component scanning, runners, and embedded server. Maybe a flowchart would help, but since it’s text, describe the flow step-by-step.

The Spring Boot application startup flow follows a specific sequence to initialize the application context, load configurations, and start the embedded server. Here’s a step-by-step breakdown:

1. Entry Point: main() Method

The application starts from the main method, which calls SpringApplication.run():

1
2
3
4
5
6
@SpringBootApplication
public class MyApp {
  public static void main(String[] args) {
    SpringApplication.run(MyApp.class, args);
  }
}

2. Initialize SpringApplication

  • Loads Defaults: Configures default settings (e.g., banner, logging).
  • Infers Application Type: Determines if it’s a web application (Servlet, Reactive, or non-web).
  • Loads ApplicationContextInitializers: Customizes the context before it’s refreshed.
  • Loads ApplicationListeners: Listens to Spring Boot events (e.g., ApplicationStartingEvent).

3. Prepare the Environment

  • Load Properties: Reads application.properties, application.yml, and profiles (e.g., @Profile).
  • Set Active Profiles: Configures environments (e.g., dev, prod).

4. Create ApplicationContext

  • Web Apps: Uses AnnotationConfigServletWebServerApplicationContext.
  • Non-Web Apps: Uses AnnotationConfigApplicationContext.

5. Run ApplicationContext

  • Component Scanning: Scans packages for @Component, @Service, @Repository, etc.
  • Process @Configuration Classes: Loads beans defined in configuration classes.
  • Auto-Configuration: Enabled via @EnableAutoConfiguration (part of @SpringBootApplication):
    • Checks classpath dependencies (e.g., spring-boot-starter-web).
    • Configures beans automatically (e.g., DataSource, JdbcTemplate).

6. Execute CommandLineRunner/ApplicationRunner

Runs custom logic after the context is loaded but before the app fully starts:

1
2
3
4
@Bean
public CommandLineRunner demo() {
  return args -> System.out.println("App started!");
}

7. Start Embedded Web Server

  • Tomcat/Jetty/Undertow: Starts automatically for web applications.
  • Port Configuration: Listens on the specified port (default: 8080).

8. Application Ready

  • Publishes ApplicationReadyEvent.
  • The app is now ready to handle HTTP requests (if applicable).

Key Annotations

  • @SpringBootApplication: Combines @Configuration, @ComponentScan, and @EnableAutoConfiguration.
  • @ConditionalOnClass: Drives auto-configuration based on classpath dependencies.

Flow Summary

1
2
3
4
5
6
7
main() → SpringApplication.run()
  → Environment Setup
  → ApplicationContext Creation
  → Component Scan & Auto-Configuration
  → Run CommandLineRunner/ApplicationRunner
  → Start Embedded Server
  → Application Ready

This streamlined flow allows Spring Boot to “just work” with minimal configuration while providing hooks for customization.