兄弟们跟我来，在入口出打上断点，开始DEBUG：
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stepinto进入SpringApplication
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接下来stepinto会提示我们选择进入SpringApplication构造器还是进入run()方法，IDEA会把他们以紫色标出
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写在前面

创建SpringApplication干了哪些事：先说结论：

  1.     public SpringApplication(ResourceLoader resourceLoader, Class<?>... primarySources) {
  2.         this.resourceLoader = resourceLoader;
  3.         Assert.notNull(primarySources, "PrimarySources must not be null");
  4.         this.primarySources = new LinkedHashSet<>(Arrays.asList(primarySources));
  5.         this.webApplicationType = WebApplicationType.deduceFromClasspath();
  6.         this.bootstrappers = new ArrayList<>(getSpringFactoriesInstances(Bootstrapper.class));
  7.         setInitializers((Collection) getSpringFactoriesInstances(ApplicationContextInitializer.class));
  8.         setListeners((Collection) getSpringFactoriesInstances(ApplicationListener.class));
  9.         this.mainApplicationClass = deduceMainApplicationClass();
  10.     }
  • primarySources：保存当前主启动类信息class com.atguigu.boot.Boot09HelloTestApplication
  • webApplicationType：使用工具类ClassUtils判定当前应用的类型。一般是Servlet类型，非Reactive类型
  • bootstrappers：初始启动引导器List：去spring.factories文件中找 org.springframework.boot.Bootstrapper
  • setInitializers：ApplicationContextInitializer：去spring.factories找 ApplicationContextInitializer
    • List> initializers
  • setListeners：ApplicationListener：去spring.factories找 ApplicationListener
    • List> listeners

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倘若stepinto的这一行代码有多个函数（构造方法也是函数），则IDEA会提示我们选择进入SpringApplication构造器还是进入run()方法，IDEA会把他们以紫色标出，由于这里我们分析SpringApplication创建，所以我们选择进入SpringApplication构造器，点机SpringApplication，来到了这，调用this构造器
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接着stepinto，来到了SpringApplication构造器，我们一步步stepout，跳过类的变量初始化，这是个赋初始默认值的过程
最后终于进入了SpringApplication构造器当中的第一行代码
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this.primarySources

在SpringApplication构造器方法当中stepout到这，this.primarySources = new LinkedHashSet<>(Arrays.asList(primarySources));
给主资源this.primarySources赋值，就是我们的主类class com.atguigu.boot.Boot09HelloTestApplication
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this.webApplicationType

接下来我们stepinto进this.webApplicationType = WebApplicationType.deduceFromClasspath();看看做了什么事情
使用ClassUtils工具类，会先判断我们的应用是响应式reactive吗org.springframework.web.reactive.DispatcherHandler，发现不是
我们的应用是servlet的，所以最终会返回return WebApplicationType.SERVLET;
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一行行stepout，我们最终跳出static WebApplicationType deduceFromClasspath() {}方法，返回到上一级，
发现SpringApplication构造器中的this.webApplicationType终于赋到了值，为SERVLET
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this.bootstrappers

下面我们stepout，this.bootstrappers = new ArrayList<>(getSpringFactoriesInstances(Bootstrapper.class));
给初始启动引导器（List）赋值，赋值为空，我们先不管
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setInitializers

下面我们stepout，setInitializers((Collection) getSpringFactoriesInstances(ApplicationContextInitializer.class));会去找 ApplicationContextInitializer；
去spring.factories找 ApplicationContextInitializer，最终赋值给SpringApplication类的变量,,this.initializers,size=8

  1. List<ApplicationContextInitializer<?>> initializers.size=8

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debug栏的this代表当前SpringApplication构造器，展开this，会印证this.initializers.size=8
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关于setInitializers((Collection) getSpringFactoriesInstances(ApplicationContextInitializer.class));具体做了什么事呢？ stepinto看看

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发现它还是从"META-INF/spring.factories"加载所谓的ApplicationContextInitializer
发现spring-boot-2.4.0.jar下有5个Initializer

  1. # Application Context Initializers
  2. org.springframework.context.ApplicationContextInitializer=\
  3. org.springframework.boot.context.ConfigurationWarningsApplicationContextInitializer,\
  4. org.springframework.boot.context.ContextIdApplicationContextInitializer,\
  5. org.springframework.boot.context.config.DelegatingApplicationContextInitializer,\
  6. org.springframework.boot.rsocket.context.RSocketPortInfoApplicationContextInitializer,\
  7. org.springframework.boot.web.context.ServerPortInfoApplicationContextInitializer

发现spring-boot-autoconfigure-2.3.4.RELEASE.jar下有2个Initializer

  1. # Initializers
  2. org.springframework.context.ApplicationContextInitializer=\
  3. org.springframework.boot.autoconfigure.SharedMetadataReaderFactoryContextInitializer,\
  4. org.springframework.boot.autoconfigure.logging.ConditionEvaluationReportLoggingListener

加起来一共7个ApplicationContextInitializer

setListeners

接着stepout,setListeners((Collection) getSpringFactoriesInstances(ApplicationListener.class));
会去找ApplicationListener ；应用监听器。去spring.factories找 ApplicationListener，赋值给SpringApplication类的变量this.listeners,size=10

  1. private List<ApplicationListener<?>> listeners;

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同理，发现它还是从"META-INF/spring.factories"加载所谓的ApplicationListener ；应用监听器
发现spring-boot-2.4.0.jar下有9个ApplicationListener 

  1. # Application Listeners
  2. org.springframework.context.ApplicationListener=\
  3. org.springframework.boot.ClearCachesApplicationListener,\
  4. org.springframework.boot.builder.ParentContextCloserApplicationListener,\
  5. org.springframework.boot.context.FileEncodingApplicationListener,\
  6. org.springframework.boot.context.config.AnsiOutputApplicationListener,\
  7. org.springframework.boot.context.config.DelegatingApplicationListener,\
  8. org.springframework.boot.context.logging.LoggingApplicationListener,\
  9. org.springframework.boot.env.EnvironmentPostProcessorApplicationListener,\
  10. org.springframework.boot.liquibase.LiquibaseServiceLocatorApplicationListener

发现spring-boot-autoconfigure-2.3.4.RELEASE.jar下有1个ApplicationListener 

  1. # Application Listeners
  2. org.springframework.context.ApplicationListener=\
  3. org.springframework.boot.autoconfigure.BackgroundPreinitializer

加起来一共10个ApplicationContextInitializer

this.mainApplicationClass

接着stepout,this.mainApplicationClass = deduceMainApplicationClass();,找我们的主程序，我们stepinto看看
deduceMainApplicationClass()的大概逻辑：找到第一个带有main方法的程序，就是我们的主程序
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stepout结束返回：
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stepout结束再返回：
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stepout结束再返回：
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到这里，再stepinto就轮到run()方法啦，即运行SpringApplication的分析啦，我们下期再见！