spring-010: spring-boot-bootstrapping-behind-the-scenes

When you start a Spring Boot application, a series of steps happen behind the scenes to bootstrap and initialize the application. Here's a breakdown of the logical execution order with a step-by-step explanation.

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1. Steps in Spring Boot Application Startup

1.1 Bootstrap the JVM and Load Main Class

• The Java Virtual Machine (JVM) starts and loads the main class containing the public static void main(String[] args) method.

1.2 Run the SpringApplication.run() method

• The SpringApplication.run() method is executed, which starts the Spring Boot application.
• This is the entry point for all Spring Boot applications.

1.3 Initialize SpringApplication

• SpringApplication is initialized by performing several setup steps, including:
  • Determining whether the application is a web or non-web application.
  • Loading application configuration and environment properties.
  • Preparing the application context.

1.4 Prepare the ApplicationContext

• The Spring Application Context is created, which acts as a container to manage beans, configurations, and dependencies.
• If it's a web application, a Servlet-based context (like AnnotationConfigServletWebServerApplicationContext) is used.

1.5 Load ApplicationContextInitializers

• Any registered ApplicationContextInitializer beans (which modify the context before it starts) are loaded.

1.6 Detect and Apply SpringApplicationRunListeners

• Spring scans and loads any SpringApplicationRunListener implementations that allow external listeners to be notified about application startup events.

1.7 Prepare the Environment (Properties, Profiles, and Configurations)

• The environment variables, application.properties or application.yml, and command-line arguments are loaded.
• Active and default profiles are determined.

1.8 Refresh the Application Context

• The Application Context is fully refreshed, which includes:
  • Scanning for @Component, @Service, @Repository, and @Controller annotated classes.
  • Instantiating beans and managing their lifecycle.
  • Applying dependency injection and resolving dependencies.

1.9 Start Embedded Web Server (if applicable)

• If the application is a web application, Spring Boot automatically starts an embedded web server (Tomcat, Jetty, or Undertow).
• The web server listens for incoming requests on a specified port.

1.10 Execute CommandLineRunner and ApplicationRunner Beans

• If any beans implementing CommandLineRunner or ApplicationRunner are present, they are executed.
• These are useful for running initialization logic after the application has started.

1.11 Trigger Application Ready Events

• The ApplicationReadyEvent is triggered, signaling that the application is fully started and ready to handle requests.

1.12 Keep Application Running (for Web Apps)

• For web applications, the application keeps running, waiting for incoming HTTP requests.
• For non-web applications, it will exit after running initialization logic.

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2. Logical Execution Order: Simple Overview

To remember the startup flow, think of it as three major phases:

1️⃣ Bootstrapping Phase

• JVM starts the application.
• SpringApplication.run() is executed.
• Environment and configuration are prepared.

2️⃣ Context Initialization Phase

• Application Context is created and refreshed.
• Beans are scanned, created, and injected.
• If a web app, the embedded server starts.

3️⃣ Post-Startup Phase

• CommandLineRunner and ApplicationRunner execute.
• Application is now fully running and waiting for requests. 🚀