SpringBoot启动过程

创建 SpringApplication
- 保存一些信息。
- 判定当前应用的类型。ClassUtils。Servlet
- bootstrappers：初始启动引导器（List）：去spring.factories文件中找 org.springframework.boot.Bootstrapper
- 找 ApplicationContextInitializer；去spring.factories找 ApplicationContextInitializer
- List<ApplicationContextInitializer<?>> initializers
- 找 ApplicationListener ；应用监听器。去spring.factories找 ApplicationListener
- List<ApplicationListener<?>> listeners
运行 SpringApplication
- 1. StopWatch
- 2. 记录应用的启动时间
- 3. 创建引导上下文（Context环境）createBootstrapContext()
  - 获取到所有之前的 bootstrappers 挨个执行 intitialize() 来完成对引导启动器上下文环境设置
- 4. 让当前应用进入****headless模式。java.awt.headless
- 5. 获取所有 RunListener（运行监听器）【为了方便所有Listener进行事件感知】
- getSpringFactoriesInstances 去spring.factories找 SpringApplicationRunListener.
```
**遍历** **SpringApplicationRunListener 调用 starting 方法；**
```
- 相当于通知所有感兴趣系统正在启动过程的人，项目正在 starting。
6. 保存命令行参数；ApplicationArguments
准备环境 prepareEnvironment（）;
- 返回或者创建基础环境信息对象。StandardServletEnvironment
- 配置环境信息对象。
- 读取所有的配置源的配置属性值。
绑定环境信息
- 监听器调用 listener.environmentPrepared()；通知所有的监听器当前环境准备完成
7. 打印Banner图
8. 创建IOC容器（createApplicationContext（））
- 根据项目类型（Servlet）创建容器，
- 当前会创建 AnnotationConfigServletWebServerApplicationContext
9. 准备ApplicationContext IOC容器的基本信息 prepareContext()
- 保存环境信息
- IOC容器的后置处理流程。
- 应用初始化器；applyInitializers；
- 遍历所有的 ApplicationContextInitializer 。调用 initialize.。来对ioc容器进行初始化扩展功能
  - 遍历所有的 listener 调用 contextPrepared。EventPublishRunListenr；通知所有的监听器contextPrepared
- 所有的监听器调用 contextLoaded。通知所有的监听器 contextLoaded；
10. 刷新IOC容器。refreshContext
- 创建容器中的所有组件（Spring注解）
11. 容器刷新完成后工作？afterRefresh
12. StopWatch会显示用了多久
- 所有监听器调用 listeners.started(context); 通知所有的监听器 started
13 调用所有runners；callRunners()
获取容器中的 ApplicationRunner
- 获取容器中的 CommandLineRunner
- 合并所有runner并且按照@Order进行排序
- 遍历所有的runner。调用 run 方法
如果以上有异常，
- 调用Listener 的 failed
调用所有监听器的 running 方法 listeners.running(context); 通知所有的监听器 running
running如果有问题。继续通知 failed 。调用所有 Listener 的 failed；通知所有的监听器 failed

常见的5个重要的类

/**
 * @author WGR
 * @create 2021/3/8 -- 13:25
 */
public class MyApplicationContextInitializer implements ApplicationContextInitializer {


    @Override
    public void initialize(ConfigurableApplicationContext applicationContext) {
        System.out.println("MyApplicationContextInitializer ....initialize.... ");
    }
}

/**
 * @author WGR
 * @create 2021/3/8 -- 13:29
 */
public class MyApplicationListener implements ApplicationListener {
    @Override
    public void onApplicationEvent(ApplicationEvent event) {
        System.out.println("MyApplicationListener.....onApplicationEvent...");
    }
}

/**
 * @author WGR
 * @create 2021/3/8 -- 13:32
 */
@Component
public class MyApplicationRunner implements ApplicationRunner {
    @Override
    public void run(ApplicationArguments args) throws Exception {
        System.out.println("MyApplicationRunner....run....");
    }
}

/**
 * @author WGR
 * @create 2021/3/8 -- 13:33
 */
@Component
public class MyCommandLineRunner implements CommandLineRunner {
    @Override
    public void run(String... args) throws Exception {
        System.out.println("MyCommandLineRunner....run....");
    }
}


/**
 * @author WGR
 * @create 2021/3/8 -- 13:30
 */
public class MySpringApplicationRunListener implements SpringApplicationRunListener {

    private final SpringApplication application;

    public MySpringApplicationRunListener(SpringApplication application, String[] args) {
        this.application = application;
    }

    @Override
    public void starting(ConfigurableBootstrapContext bootstrapContext) {
        System.out.println("MySpringApplicationRunListener....starting....");

    }


    @Override
    public void environmentPrepared(ConfigurableBootstrapContext bootstrapContext, ConfigurableEnvironment environment) {
        System.out.println("MySpringApplicationRunListener....environmentPrepared....");
    }


    @Override
    public void contextPrepared(ConfigurableApplicationContext context) {
        System.out.println("MySpringApplicationRunListener....contextPrepared....");

    }

    @Override
    public void contextLoaded(ConfigurableApplicationContext context) {
        System.out.println("MySpringApplicationRunListener....contextLoaded....");
    }

    @Override
    public void started(ConfigurableApplicationContext context) {
        System.out.println("MySpringApplicationRunListener....started....");
    }

    @Override
    public void running(ConfigurableApplicationContext context) {
        System.out.println("MySpringApplicationRunListener....running....");
    }

    @Override
    public void failed(ConfigurableApplicationContext context, Throwable exception) {
        System.out.println("MySpringApplicationRunListener....failed....");
    }
}

注意：如果MySpringApplicationRunListener类没有构造器的话，会报下面这个错

运行的周期如下：

MyApplicationListener.....onApplicationEvent...
MySpringApplicationRunListener....starting....
MyApplicationListener.....onApplicationEvent...
MySpringApplicationRunListener....environmentPrepared....

  .   ____          _            __ _ _
 /\ / ___'_ __ _ _(_)_ __  __ _    
( ( )\___ | '_ | '_| | '_ / _` |    
 \/  ___)| |_)| | | | | || (_| |  ) ) ) )
  '  |____| .__|_| |_|_| |_\__, | / / / /
 =========|_|==============|___/=/_/_/_/
 :: Spring Boot ::                (v2.4.2)

MyApplicationContextInitializer ....initialize.... 
MyApplicationListener.....onApplicationEvent...
MySpringApplicationRunListener....contextPrepared....
2021-03-08 13:47:46.528  INFO 7736 --- [           main] com.dalianpai.admin.AdminApplication     : Starting AdminApplication using Java 1.8.0_131 on WGR-PC with PID 7736 (E:IdeaSpacedemospring-boot-admin	argetclasses started by asus in E:IdeaSpacedemo)
2021-03-08 13:47:46.532  INFO 7736 --- [           main] com.dalianpai.admin.AdminApplication     : No active profile set, falling back to default profiles: default
MyApplicationListener.....onApplicationEvent...
MySpringApplicationRunListener....contextLoaded....
2021-03-08 13:47:48.519  INFO 7736 --- [           main] o.s.b.w.embedded.tomcat.TomcatWebServer  : Tomcat initialized with port(s): 8888 (http)
2021-03-08 13:47:48.528  INFO 7736 --- [           main] o.apache.catalina.core.StandardService   : Starting service [Tomcat]
2021-03-08 13:47:48.528  INFO 7736 --- [           main] org.apache.catalina.core.StandardEngine  : Starting Servlet engine: [Apache Tomcat/9.0.41]
2021-03-08 13:47:48.622  INFO 7736 --- [           main] o.a.c.c.C.[Tomcat].[localhost].[/]       : Initializing Spring embedded WebApplicationContext
2021-03-08 13:47:48.622  INFO 7736 --- [           main] w.s.c.ServletWebServerApplicationContext : Root WebApplicationContext: initialization completed in 1969 ms
2021-03-08 13:47:48.871  INFO 7736 --- [           main] o.s.s.concurrent.ThreadPoolTaskExecutor  : Initializing ExecutorService 'applicationTaskExecutor'
2021-03-08 13:47:48.972  WARN 7736 --- [           main] ion$DefaultTemplateResolverConfiguration : Cannot find template location: classpath:/templates/ (please add some templates or check your Thymeleaf configuration)
2021-03-08 13:47:49.366  INFO 7736 --- [           main] o.s.b.a.e.web.EndpointLinksResolver      : Exposing 2 endpoint(s) beneath base path '/actuator'
2021-03-08 13:47:49.403  INFO 7736 --- [           main] o.s.b.w.embedded.tomcat.TomcatWebServer  : Tomcat started on port(s): 8888 (http) with context path ''
MyApplicationListener.....onApplicationEvent...
MyApplicationListener.....onApplicationEvent...
2021-03-08 13:47:49.417  INFO 7736 --- [           main] com.dalianpai.admin.AdminApplication     : Started AdminApplication in 3.504 seconds (JVM running for 6.046)
MyApplicationListener.....onApplicationEvent...
MyApplicationListener.....onApplicationEvent...
MySpringApplicationRunListener....started....
MyApplicationRunner....run....
MyCommandLineRunner....run....
MyApplicationListener.....onApplicationEvent...
MyApplicationListener.....onApplicationEvent...
MySpringApplicationRunListener....running....