How Spring Boot Application Internally Works | Let's Debug and Understand run() Method Step by Step

Summary

In this video, Ramesh delves into the internals of a Spring Boot application, explaining the execution flow from the entry point class. He walks through how Spring Boot automatically configures the application context, prepares the environment, and triggers various listeners and runners. By examining the main method and the `run` method of `SpringApplication`, he shows how the application context is created, how Spring Boot handles dependencies, and how it initializes the IoC container. The video also highlights debugging techniques to better understand the bootstrapping process, helping viewers grasp how Spring Boot simplifies application setup behind the scenes.

Takeaways

• 😀 Spring Boot simplifies the application setup process, automating tasks that would otherwise require manual configuration.
• 😀 The main entry point of a Spring Boot application is the class annotated with `@SpringBootApplication`, which combines multiple annotations.
• 😀 The `run()` method in Spring Boot is the core method responsible for starting the application and handling various tasks behind the scenes.
• 😀 When you run a Spring Boot application, it starts by creating an application context (Spring IoC container) and setting up listeners.
• 😀 Spring Boot supports different application types, including servlet-based and reactive applications, which determine the type of application context created.
• 😀 The application context is created based on the application's web type (e.g., servlet or reactive), ensuring proper configuration for each use case.
• 😀 Spring Boot uses command-line arguments passed to the application to configure listeners and other internal components.
• 😀 A stopwatch is used to measure the time taken for the Spring Boot application to start, helping to monitor startup performance.
• 😀 The environment of the application (production, development, or test) is configured automatically based on the settings in `application.properties`.
• 😀 Spring Boot prints a banner upon application startup, which can be customized or turned off if desired.
• 😀 After setting up the environment and printing the banner, Spring Boot triggers any configured runners (e.g., `CommandLineRunner` or `ApplicationRunner`) to execute logic during startup.

Q & A

What is the main purpose of the Spring Boot entry point class?

The main purpose of the Spring Boot entry point class is to serve as the starting point for the application. It contains the `main()` method that calls the `run()` method of `SpringApplication`, initiating the execution process of the Spring Boot application.

What is the role of the `run()` method in Spring Boot?

The `run()` method in Spring Boot is responsible for bootstrapping the application. It performs several tasks, such as starting a stopwatch for execution time, configuring the environment, initializing the application context, triggering listeners, and executing any defined runners (e.g., `CommandLineRunner`).

What happens during the execution of the Spring Boot application when `run()` is called?

When the `run()` method is called, several key actions occur: it starts a stopwatch, prepares the environment, prints the startup banner, creates the application context, refreshes the context, and triggers the application's runners. Afterward, it returns the application context, which holds the Spring IoC container.

What is the purpose of the `stopwatch` in the `run()` method?

The `stopwatch` in the `run()` method is used to measure and log the time taken for the Spring Boot application to execute. This helps track the application's startup time.

How does Spring Boot determine the application environment?

Spring Boot determines the application environment based on the configurations in the `application.properties` file. It can configure environments such as production, test, or development, depending on the settings defined in the application configuration.

What is the significance of the banner in Spring Boot application startup?

The banner is printed during the startup of a Spring Boot application to provide visual feedback. It can be customized or turned off, and it appears after the environment is set up, giving users an indication that the application is starting.

How does Spring Boot create the application context (IoC container)?

Spring Boot creates the application context by determining the web application type, such as servlet or reactive, based on the dependencies included in the project (like `spring-boot-starter-web`). Based on this, it creates the appropriate context class (e.g., `AnnotationConfigServletWebServerApplicationContext` for servlet applications).

What are `ApplicationRunner` and `CommandLineRunner` used for in Spring Boot?

Both `ApplicationRunner` and `CommandLineRunner` are used to run specific logic only once during the application startup. These runners are triggered by the `run()` method after the application context has been refreshed, allowing developers to execute tasks at the beginning of the application lifecycle.

How does Spring Boot handle the creation and refreshing of the application context?

Spring Boot automatically creates the application context based on the web application type and dependencies. Once the context is created, it is refreshed to initialize all the beans and configurations. The context is then ready to be used by the application.

What is the importance of the `@SpringBootApplication` annotation in Spring Boot?

The `@SpringBootApplication` annotation is a combination of three annotations: `@SpringBootConfiguration`, `@EnableAutoConfiguration`, and `@ComponentScan`. This annotation enables auto-configuration, component scanning, and Java-based configuration, making it a core feature for setting up Spring Boot applications.