spring-: how-spring-boot-determines-application-context

Spring Boot automatically determines the implementation of Application Context

When calling , Spring Boot will automatically determine the correct ApplicationContext implementation according to the following factors.

: SpringApplication.run(MySpringBootApp.class, args); Class path (dependencies in the project)

1. running application type (web or non -web)
2. OK process SpringApplication internally use the following logic to select the appropriate ApplicationContext:

If Spring MVC or Spring Webflux (Spring-Boot-Starter-Web or Spring-Boot-Starter-WEBFLUX):

It will initialize a
• Web -based application context

  : Based on the service (Spring-Boot-Starter-Web):

  →
  (Spring MVC applications for embedded Tomcat, Jetty or Undertow).
  • Reacting (Spring-Boot-Starter-Webflux): →
  (for WebFlux applications).
  • AnnotationConfigServletWebServerApplicationContext
  If there is neither Spring-Boot-Starter-Web nor Spring-Boot-Starter-weight:
  • AnnotationConfigReactiveWebServerApplicationContext It will initialize a
  non -web application above and below
  : →
.

• WEB application context as examples
  • If it contains Spring-Boot-Starter-Web, the output will be: AnnotationConfigApplicationContext
  Non -web application context examples
If you remove the Spring-Boot-Starter-Web, the output will become:

@SpringBootApplication
public class WebApplication {
    public static void main(String[] args) {
        ConfigurableApplicationContext context = SpringApplication.run(WebApplication.class, args);
        System.out.println("Context Type: " + context.getClass().getName());
    }
}

The importance of initialization of Application Context

<程> Application below

<code>Context Type: org.springframework.boot.web.servlet.context.AnnotationConfigServletWebServerApplicationContext</code>

is

Core container

, it manages the life cycle and configuration of Bean in the Spring Boot application. Initialization is important for the following reasons:

1. Bean management

• Application context Registration and management bean , allowing dependency injection (@Autowired).
• Without application context, Spring will not know how to instance and inject dependencies.

2. Automatic configuration

• The mechanism @EnableAutoConfiguration depends on the application context.
It scan the path and configure the
• Spring component according to the dependencies.
3. Life cycle and event management

Application context publishes the life cycle event (

,
• ). ApplicationReadyEvent ApplicationStartedEvent It listens to the shutdown signal and manage the resource correctly.
4. Embedded web server support

For web applications, the application context starts

embedded server
• (Tomcat, Jetty, Undertow). Without it, Spring Boot
Can't handle HTTP request
• .
5. Environment and attribute management

context from or

load
• configuration attribute application.properties. application.yml It manages configuration files () and settings specific to the environment.
• @Profile
The actual impact of the correct Application Context

---

Choosing the correct application above and below will affect the behavior of

Application

, as follows:

1. Determine whether to start the embedded web server

• If you choose the wrong context, your application may not be able to start as a web application.
• Web application requires AnnotationConfigServletWebServerApplicationContext, it guides Tomcat/Jetty.

2. Control component scanning and dependence injection

• The context is initialized and injected by the dependencies within its range.
• For example, non -web context will not scan or initialize the controller (@RestController will not work).

3. Enable or disable automatic configuration

• Spring Boot Based on the selected context Automatic application configuration .
• For example: If the web context is selected, the Spring Boot will automatically configure the MVC component.

4. Affecting the management and loading method of bean

• Web context pre -configured DispatcherServlet, it handles the HTTP request.
• Non -web context does not have, which means that If there is no additional configuration, you will not be able to handle the web request .

---

Summary

---

方面	Application Context 的影响
Bean 管理	初始化和管理依赖项 (`@Autowired`)
Web 服务器	启动嵌入式 Tomcat/Jetty（如果为 Web 上下文）
自动配置	根据类路径应用自动配置
生命周期管理	处理启动/关机事件
配置文件和环境	加载属性，管理配置文件 (`@Profile`)
依赖注入范围	确定哪些 Bean 和控制器可用

The last idea

Spring Boot will automatically select the correct application above and below according to the class path

.

• Web application Need to be based on the context of web ().
• Non -web application The standard -based context using standard (). AnnotationConfigServletWebServerApplicationContext
The correct context of initialization
• ensure the correct dependency injection, automatic configuration and life cycle management. ?

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