菜瓜：我自己去調Mvc的原始碼差點沒給Spring的邏輯秀死。。。難受

水稻：那今天咱們看一個簡單易用的SpringBoot吧

菜瓜：可以，這個我熟悉

水稻：熟悉？

菜瓜：當我沒說，請開始你的表演

水稻：我沒有別的意思，就是單純的反問（手動狗頭）。平時工作中用多了SpringBoot。咱們今天帶著幾個問題來看看它的操作吧

1. 如何啟動Spring容器
2. 如何內嵌Tomcat容器
3. 如何完成自動裝配，就是0配置

菜瓜：你確定這是我熟悉的SpringBoot？？？

水稻：。。。看過來

• 啟動類點進去

• public ConfigurableApplicationContext run(String... args) {
     StopWatch stopWatch = new StopWatch();
     stopWatch.start();
     ...
     listeners.starting();
     try {
        ...
        // ①建立Spring上下文容器物件 - 預設Servlet容器
        context = createApplicationContext();
        ...
        // ②呼叫refresh方法 - 回到熟悉的容器啟動流程
        refreshContext(context);
        afterRefresh(context, applicationArguments);
        ...   
     ...
     return context;
  }
• ① 建立上下文物件

  • protected ConfigurableApplicationContext createApplicationContext() {
       Class<?> contextClass = this.applicationContextClass;
       if (contextClass == null) {
          try {
             switch (this.webApplicationType) {
             case SERVLET:
                contextClass = Class.forName(DEFAULT_SERVLET_WEB_CONTEXT_CLASS);
                break;
             case REACTIVE:
                contextClass = Class.forName(DEFAULT_REACTIVE_WEB_CONTEXT_CLASS);
                break;
             default:
                contextClass = Class.forName(DEFAULT_CONTEXT_CLASS);
             }
          }
          catch (ClassNotFoundException ex) {
             ...
          }
       }
       return (ConfigurableApplicationContext) BeanUtils.instantiateClass(contextClass);
    }
• ②啟動容器

  •     @Override
        public void refresh() throws BeansException, IllegalStateException {
            synchronized (this.startupShutdownMonitor) {
                prepareRefresh();
                ConfigurableListableBeanFactory beanFactory = obtainFreshBeanFactory();
    
                // Prepare the bean factory for use in this context.
                prepareBeanFactory(beanFactory);
    
                try {
                    ...
                    // ①springboot 內嵌tomcat容器
                    onRefresh();
                    ...
    
        }
    
        @Override
        protected void onRefresh() {
            super.onRefresh();
            try {
          // ②建立Servlet容器 預設tomcat
                createWebServer();
            }
            ...
        }      
         
          
        private void createWebServer() {
            WebServer webServer = this.webServer;
            ServletContext servletContext = getServletContext();
            if (webServer == null && servletContext == null) {
                ServletWebServerFactory factory = getWebServerFactory();
          // ③看進去 回到mvc整合tomcat的場景
                this.webServer = factory.getWebServer(getSelfInitializer());
            }
            ...
            initPropertySources();
        }      
             
        @Override
        public WebServer getWebServer(ServletContextInitializer... initializers) {
            Tomcat tomcat = new Tomcat();
            File baseDir = (this.baseDirectory != null) ? this.baseDirectory
                    : createTempDir("tomcat");
            tomcat.setBaseDir(baseDir.getAbsolutePath());
            Connector connector = new Connector(this.protocol);
            tomcat.getService().addConnector(connector);
            customizeConnector(connector);
            tomcat.setConnector(connector);
            tomcat.getHost().setAutoDeploy(false);
            configureEngine(tomcat.getEngine());
            for (Connector additionalConnector : this.additionalTomcatConnectors) {
                tomcat.getService().addConnector(additionalConnector);
            }
            prepareContext(tomcat.getHost(), initializers);
            return getTomcatWebServer(tomcat);
        }      

水稻：好了，第一步和第二步完成了

菜瓜：就這？？？

水稻：是不是極其簡單，令人髮指。重頭戲是後面的自動裝配

• 回到咱們啟動類的註解上

• ...
  // 標記自身被掃描  
  @SpringBootConfiguration
  // 下一步 - 自動裝配入口 
  @EnableAutoConfiguration
  // 掃描bean路徑 - 約定是啟動類所在的包：所以沒事別把啟動類挪走(都是淚)
  @ComponentScan(excludeFilters = {
          @Filter(type = FilterType.CUSTOM, classes = TypeExcludeFilter.class),
          @Filter(type = FilterType.CUSTOM, classes = AutoConfigurationExcludeFilter.class) })
  public @interface SpringBootApplication 
    
  ->   
  @Import(AutoConfigurationImportSelector.class)
  public @interface EnableAutoConfiguration 
    
  -> 重要
  public class AutoConfigurationImportSelector ... {
    @Override
    public void process(AnnotationMetadata annotationMetadata,
          DeferredImportSelector deferredImportSelector) {
       ...
       // 獲取以EnableAutoConfiguration命名的/META-INF/Spring.factories檔案中的value去重 
       AutoConfigurationEntry autoConfigurationEntry = ((AutoConfigurationImportSelector) deferredImportSelector).getAutoConfigurationEntry(getAutoConfigurationMetadata(),            annotationMetadata);
       // 啟動的時候斷點可以看到
       this.autoConfigurationEntries.add(autoConfigurationEntry);
       for (String importClassName : autoConfigurationEntry.getConfigurations()) {
          this.entries.putIfAbsent(importClassName, annotationMetadata);
       }
    }
  }

• AutoConfigurationImportSelector 中的process是被ConfigurationClassPostProcessor通過processConfigBeanDefinitions方法呼叫（呼叫鏈如下）
  1. this.processConfigBeanDefinitions(registry);
  2. parser.parse(candidates);
  3. this.parse(((AnnotatedBeanDefinition)bd).getMetadata(), holder.getBeanName());
  4. sourceClass = this.doProcessConfigurationClass(configClass, sourceClass);
  5. this.processImports(configClass, sourceClass, this.getImports(sourceClass), true);
  6. this.deferredImportSelectorHandler.handle(configClass, (DeferredImportSelector)selector);  
  7. handler.processGroupImports();
  8. grouping.getImports().forEach...
  9. this.group.process(...);
  -- 
  蒐集到需要自動裝配的類，封裝成BeanDefinition後續例項化，實現自動裝配功能
  譬如引入WebMvcAutoConfiguration類 - webmvc功能自動整合
  org.springframework.boot.autoconfigure.web.servlet.WebMvcAutoConfiguration 

菜瓜：原來如此。你把呼叫鏈拎出來就簡單了很多。自動裝配就是通過SPI載入org.springframework.boot.autoconfigure包下的class，封裝成BeanDefinition後交給容器載入

總結：SpringBoot只需要一行程式碼便能啟動一個Java應用。完全解放開發者複雜的配置

• 內嵌Servlet容器，預設tomcat
• 啟動SpringWeb容器
• 自動裝配了簡單web應用需要的工具和組建

&n