【Spring】原来SpringBoot是这样玩的

菜瓜：我自己去调Mvc的源码差点没给Spring的逻辑秀死。。。难受

水稻：那今天咱们看一个简单易用的SpringBoot吧

菜瓜：可以，这个我熟悉

水稻：熟悉？

菜瓜：当我没说，请开始你的表演

水稻：我没有别的意思，就是单纯的反问（手动狗头）。平时工作中用多了SpringBoot。咱们今天带着几个问题来看看它的操作吧

1. 如何启动Spring容器
2. 如何内嵌Tomcat容器
3. 如何完成自动装配，就是0配置

菜瓜：你确定这是我熟悉的SpringBoot？？？

水稻：。。。看过来

• 启动类点进去

• public ConfigurableApplicationContext run(String... args) {
     StopWatch stopWatch = new StopWatch();
     stopWatch.start();
     ...
     listeners.starting();
     try {
        ...
        // ①创建Spring上下文容器对象 - 默认Servlet容器
        context = createApplicationContext();
        ...
        // ②调用refresh方法 - 回到熟悉的容器启动流程
        refreshContext(context);
        afterRefresh(context, applicationArguments);
        ...
     ...
     return context;
  }

• ① 创建上下文对象

  • 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启动容器

  •     @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 内嵌tomcat容器
                    onRefresh();
                    ...
    
        }
    
        @Override
        protected void onRefresh() {
            super.onRefresh();
            try {
          // ②创建Servlet容器 默认tomcat
                createWebServer();
            }
            ...
        }      
    
        private void createWebServer() {
            WebServer webServer = this.webServer;
            ServletContext servletContext = getServletContext();
            if (webServer == null && servletContext == null) {
                ServletWebServerFactory factory = getWebServerFactory();
          // ③看进去 回到mvc集成tomcat的场景
                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：就这？？？

水稻：是不是极其简单，令人发指。重头戏是后面的自动装配

• 回到咱们启动类的注解上

• ...
  // 标记自身被扫描
  @SpringBootConfiguration
  // 下一步 - 自动装配入口
  @EnableAutoConfiguration
  // 扫描bean路径 - 约定是启动类所在的包：所以没事别把启动类挪走(都是泪)
  @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) {
       ...
       // 获取以EnableAutoConfiguration命名的/META-INF/Spring.factories文件中的value去重
       AutoConfigurationEntry autoConfigurationEntry = ((AutoConfigurationImportSelector) deferredImportSelector).getAutoConfigurationEntry(getAutoConfigurationMetadata(),            annotationMetadata);
       // 启动的时候断点可以看到
       this.autoConfigurationEntries.add(autoConfigurationEntry);
       for (String importClassName : autoConfigurationEntry.getConfigurations()) {
          this.entries.putIfAbsent(importClassName, annotationMetadata);
       }
    }
  }

• AutoConfigurationImportSelector 中的process是被ConfigurationClassPostProcessor通过processConfigBeanDefinitions方法调用（调用链如下）
  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(...);
  --
  搜集到需要自动装配的类，封装成BeanDefinition后续实例化，实现自动装配功能
  譬如引入WebMvcAutoConfiguration类 - webmvc功能自动集成
  org.springframework.boot.autoconfigure.web.servlet.WebMvcAutoConfiguration 

菜瓜：原来如此。你把调用链拎出来就简单了很多。自动装配就是通过SPI加载org.springframework.boot.autoconfigure包下的class，封装成BeanDefinition后交给容器加载

总结：SpringBoot只需要一行代码便能启动一个Java应用。完全解放开发者复杂的配置

• 内嵌Servlet容器，默认tomcat
• 启动SpringWeb容器
• 自动装配了简单web应用需要的工具和组建