SpringBoot 启动配置原理

几个重要的事件回调机制

• ApplicationContextInitializer
• SpringApplicationRunListener
• ApplicationRunner
• CommandLineRunner

启动流程：

1. 创建SpringApplication对象

   /**
    * 在springboot 2以前：
    * 调用initialize(source);
    * 在springboot 2以后，跳过了initialize()方法，直接调用run()方法
    */
   
   private void initialize(Object[] source){
       // 保存主配置类
       if(sources != null && source.length > 0){
           this.sources.addAll(Arrays.asList(sources));
       }
       // 判断当前是否是一个web应用
       this.webEnvironment = deduceWebEnvironment();
       // 从类路径下找到META-INF/spring.factories配置的所有ApplicationContextInitializer；然后保存起来
       setInitializes((Collection) getSpringFactoriesInstances(ApplicationContextInitializer.class));
       // 从类路径下找到META-INF/spring.factories配置的所有ApplicationListener；然后保存起来
     　setListeners((Collection) getSpringFactoriesInstances(ApplicationListener.class));
       // 从多个配置类中找到有main方法的主配置类
       this.mainApplicationClass = deduceMainApplicationClass();
   }

2. 运行run方法

   public ConfigurableApplicationContext run(String... args) {
           StopWatch stopWatch = new StopWatch();
           stopWatch.start();
           ConfigurableApplicationContext context = null;
           Collection<SpringBootExceptionReporter> exceptionReporters = new ArrayList<>();
           configureHeadlessProperty();
           
           // 获取SpringApplicationRunListener;从类路径下META-INF/spring.factories找到
           SpringApplicationRunListeners listeners = getRunListeners(args);
           // 回调所有的SpringApplicationRunListener.starting()方法
           listeners.starting();
           try {
               // 封装命令行参数
               ApplicationArguments applicationArguments = new DefaultApplicationArguments(args);
               // 准备环境
               ConfigurableEnvironment environment = prepareEnvironment(listeners, applicationArguments);
               // 创建环境完成后回调SpringApplicationRunListener.environmentPrepared();表示环境准备完成
               configureIgnoreBeanInfo(environment);
               Banner printedBanner = printBanner(environment);
               
               // 创建ApplicationContext;决定创建web的ioc还是普通的ioc
               context = createApplicationContext();
               exceptionReporters = getSpringFactoriesInstances(SpringBootExceptionReporter.class,
                       new Class[] { ConfigurableApplicationContext.class }, context);
               // 准备上下文环境；将environment保存到ioc中
               // applyInitializers()；回调之前保存的所有的SpringApplicationRunListener的initializers方法
               
               prepareContext(context, environment, listeners, applicationArguments, printedBanner);
               // 回调所有的SpringApplicationRunListener的contextPrepared();
               // 刷新容器；ioc容器初始化完成
               refreshContext(context);
               afterRefresh(context, applicationArguments);
               stopWatch.stop();
               if (this.logStartupInfo) {
                   new StartupInfoLogger(this.mainApplicationClass).logStarted(getApplicationLog(), stopWatch);
               }
               listeners.started(context);
               callRunners(context, applicationArguments);
           }
           catch (Throwable ex) {
               handleRunFailure(context, ex, exceptionReporters, listeners);
               throw new IllegalStateException(ex);
           }
   
           try {
               listeners.running(context);
           }
           catch (Throwable ex) {
               handleRunFailure(context, ex, exceptionReporters, null);
               throw new IllegalStateException(ex);
           }
           return context;
       }