SpringBoot应用启动过程分析

真的好奇害死猫！之前写过几个SpringBoot应用，但是一直没搞明白应用到底是怎么启动的，心里一直有点膈应。好吧，趁有空去看了下源码，写下这篇博客作为学习记录吧！

个人拙见，若哪里有理解不对的地方，请各路大神指正，小弟不胜感激！

一.应用启动类

@SpringBootApplication
public class DemoApplication {
	public static void main(String[] args) {
    	SpringApplication.run(DemoApplication.class, args);
	}
}

开发SpirngBoot应用时，入口类就这简单的几行。但是却完成了N多服务的初始化、加载和发布。那么这几行代码究竟干了什么呢，SpringBoot应用到底是怎么启动的。

二.@SpringBootApplication注解

2.1.SpringBootApplication注解

@Target(ElementType.TYPE)
@Retention(RetentionPolicy.RUNTIME)
@Documented
@Inherited
@SpringBootConfiguration
@EnableAutoConfiguration
@ComponentScan(excludeFilters = {
      @Filter(type = FilterType.CUSTOM, classes = TypeExcludeFilter.class),
      @Filter(type = FilterType.CUSTOM, classes = AutoConfigurationExcludeFilter.class) })
public @interface SpringBootApplication {

@SpringBootApplication=@SpringBootConfiguration+@EnableAutoConfiguration+@ComponentScan

2.2.@SpringBootConfiguration

/**
 * Indicates that a class Spring Boot application
 * {@link Configuration @Configuration}. Can be used as an alternative to the Spring's
 * standard {@code @Configuration} annotation so that configuration can be found
 * automatically (for example in tests).
 */
@Target(ElementType.TYPE)
@Retention(RetentionPolicy.RUNTIME)
@Documented
@Configuration
public @interface SpringBootConfiguration {

}

SpringBootConfiguration注解和Spring的@Configuration注解作用一样。标注当前类是配置类，并会将当前类内声明的一个或多个以@Bean注解标记的方法的实例纳入到spring容器中。比如容器加载时，会生成Hello的Bean加载到IOC容器中。

@SpringBootConfiguration
public class ExampleConfig {
	@Bean
	public void Hello(){
		System.out.println("hello");
	}
}

2.3.@EnableAutoConfiguration

@SuppressWarnings("deprecation")
@Target(ElementType.TYPE)
@Retention(RetentionPolicy.RUNTIME)
@Documented
@Inherited
@AutoConfigurationPackage
@Import(EnableAutoConfigurationImportSelector.class)
public @interface EnableAutoConfiguration {

}

这个注解是SpringBoot能进行自动配置的关键。@Import注解用于导入配置类，我们看下导入类EnableAutoConfigurationImportSelector。容器刷新时，会调用AutoConfigurationImportSelector类的selectImports方法，扫描META-INF/spring.factories文件自动配置类（key为EnableAutoConfiguration），然后Spring容器处理配置类。（对Spring的一些加载过程不清晰，我是相当的迷啊）

2.4.@ComponentScan

@ComponentScan(excludeFilters = {
      @Filter(type = FilterType.CUSTOM, classes = TypeExcludeFilter.class),
      @Filter(type = FilterType.CUSTOM, classes = AutoConfigurationExcludeFilter.class) })
/**
 * Configures component scanning directives for use with @{@link Configuration} classes.
 * Provides support parallel with Spring XML's {@code <context:component-scan>} element.
 *
 * <p>Either {@link #basePackageClasses} or {@link #basePackages} (or its alias
 * {@link #value}) may be specified to define specific packages to scan. If specific
 * packages are not defined, scanning will occur from the package of the
 * class that declares this annotation.
 */
@Retention(RetentionPolicy.RUNTIME)
@Target(ElementType.TYPE)
@Documented
@Repeatable(ComponentScans.class)
public @interface ComponentScan

@ComponentScan扫描指定的包路径，若未指定包路径，则以声明这个注解的类作为基本包路径。比如@SpringBootApplication就没有指定包路径，则DemoApplication的包路径将作为扫描的基本包路径，因此强烈建议将主类放在顶层目录下。

excludeFilters属性指定哪些类型不符合组件扫描的条件，会在扫描的时候过滤掉。

@Filter(type = FilterType.CUSTOM, classes = AutoConfigurationExcludeFilter.class)

比如上面这段代码。@Filter声明了过滤器类型类为自定义类型（需要实现TypeFilter接口），过滤器为AutoConfigurationExcludeFilter。当match方法为true，返回扫描类对象，否则过滤掉。但是要注意@ComponentScan的key为excludeFilters，因此这些类型将在包扫描的时候过滤掉，也就是说，ComponentScan在扫描时，发现当前扫描类满足macth的条件(match返回true)，是不会将该类加载到容器的。

	//metadataReader  表示读取到的当前正在扫描的类的信息
	//metadataReaderFactory 表示可以获得到其他任何类的信息
	@Override
	public boolean match(MetadataReader metadataReader,
			MetadataReaderFactory metadataReaderFactory) throws IOException {
		return isConfiguration(metadataReader) && isAutoConfiguration(metadataReader);
	}
	//该类是带有Configuration注解的配置类
	private boolean isConfiguration(MetadataReader metadataReader) {
		return metadataReader.getAnnotationMetadata()
				.isAnnotated(Configuration.class.getName());
	}
	//该类是否为spring.factory配置的自动配置类
	private boolean isAutoConfiguration(MetadataReader metadataReader) {
		return getAutoConfigurations()
				.contains(metadataReader.getClassMetadata().getClassName());
	}

三.run(DemoApplication.class, args)解析

3.1.进入SpringApplication

public static ConfigurableApplicationContext run(Class<?>[] primarySources,
      String[] args) {
   return new SpringApplication(primarySources).run(args);
}

我们根据DemoApplication跟进代码，发现其调用的SpringApplication类的run方法。这个方法就干了2件事：一是创建SpringApplication对象，二是启动SpringApplication。

3.2.SpringApplication构造器分析

1.构造器

public SpringApplication(Class<?>... primarySources) {
	this(null, primarySources);
}
/**
* Create a new {@link SpringApplication} instance. The application context will load
* beans from the specified primary sources
*/
public SpringApplication(ResourceLoader resourceLoader, Class<?>... primarySources) {
   this.resourceLoader = resourceLoader;
   Assert.notNull(primarySources, "PrimarySources must not be null");
   this.primarySources = new LinkedHashSet<>(Arrays.asList(primarySources));
  //根据应用是否存在某些类推断应用类型，分为响应式web应用，servlet类型web应用和非web应用，在后面用于确定实例化applicationContext的类型
   this.webApplicationType = WebApplicationType.deduceFromClasspath();
   //设置初始化器，读取spring.factories文件key ApplicationContextInitializer对应的value并实例化
   //ApplicationContextInitializer接口用于在Spring上下文被刷新之前进行初始化的操作
   setInitializers((Collection) getSpringFactoriesInstances(
         ApplicationContextInitializer.class));

   //设置监听器，读取spring.factories文件key ApplicationListener对应的value并实例化
   // interface ApplicationListener<E extends ApplicationEvent> extends EventListener
   //ApplicationListener继承EventListener，实现了观察者模式。对于Spring框架的观察者模式实现，它限定感兴趣的事件类型需要是ApplicationEvent类型事件

   setListeners((Collection) getSpringFactoriesInstances(ApplicationListener.class));
   //没啥特别作用，仅用于获取入口类class对象
   this.mainApplicationClass = deduceMainApplicationClass();
}

在构造器里主要干了2件事，一个设置初始化器，二是设置监听器。

2.设置初始化器

setInitializers((Collection) getSpringFactoriesInstances(
      ApplicationContextInitializer.class));

private <T> Collection<T> getSpringFactoriesInstances(Class<T> type) {
   return getSpringFactoriesInstances(type, new Class<?>[] {});
}

private <T> Collection<T> getSpringFactoriesInstances(Class<T> type,
      Class<?>[] parameterTypes, Object... args) {
   ClassLoader classLoader = getClassLoader();
   // Use names and ensure unique to protect against duplicates
   Set<String> names = new LinkedHashSet<>(
    //从类路径的META-INF处读取相应配置文件spring.factories，然后进行遍历，读取配置文件中Key(type)对应的value
         SpringFactoriesLoader.loadFactoryNames(type, classLoader));
   //将names的对象实例化
   List<T> instances = createSpringFactoriesInstances(type, parameterTypes,
         classLoader, args, names);
   AnnotationAwareOrderComparator.sort(instances);
   return instances;
}

根据入参type类型ApplicationContextInitializer.class从类路径的META-INF处读取相应配置文件spring.factories并实例化对应Initializer。上面这2个函数后面会反复用到。

org.springframework.context.ApplicationContextInitializer=\
org.springframework.boot.context.ConfigurationWarningsApplicationContextInitializer,\
org.springframework.boot.context.ContextIdApplicationContextInitializer,\
org.springframework.boot.context.config.DelegatingApplicationContextInitializer,\
org.springframework.boot.web.context.ServerPortInfoApplicationContextInitializer

3.设置监听器

setListeners((Collection) getSpringFactoriesInstances(ApplicationListener.class));

和设置初始化器一个套路，通过getSpringFactoriesInstances函数实例化监听器。

org.springframework.context.ApplicationListener=\
org.springframework.boot.ClearCachesApplicationListener,\
org.springframework.boot.builder.ParentContextCloserApplicationListener,\
org.springframework.boot.context.FileEncodingApplicationListener,\
org.springframework.boot.context.config.AnsiOutputApplicationListener,\
org.springframework.boot.context.config.ConfigFileApplicationListener,\
org.springframework.boot.context.config.DelegatingApplicationListener,\
org.springframework.boot.context.logging.ClasspathLoggingApplicationListener,\
org.springframework.boot.context.logging.LoggingApplicationListener,\
org.springframework.boot.liquibase.LiquibaseServiceLocatorApplicationListener

3.3.run(String... args)解析

1.run函数

/**
* Run the Spring application, creating and refreshing a new ApplicationContext
*/

public ConfigurableApplicationContext run(String... args) {
   //计时器
   StopWatch stopWatch = new StopWatch();
   stopWatch.start();

   ConfigurableApplicationContext context = null;
   Collection<SpringBootExceptionReporter> exceptionReporters = new ArrayList<>();

   //设置java.awt.headless系统属性为true，Headless模式是系统的一种配置模式。
   // 在该模式下，系统缺少了显示设备、键盘或鼠标。但是服务器生成的数据需要提供给显示设备等使用。
   // 因此使用headless模式，一般是在程序开始激活headless模式，告诉程序，现在你要工作在Headless  	  mode下，依靠系统的计算能力模拟出这些特性来
   configureHeadlessProperty();

   //获取监听器集合对象
   SpringApplicationRunListeners listeners = getRunListeners(args);

   //发出开始执行的事件。
   listeners.starting();

   try {
      //根据main函数传入的参数，创建DefaultApplicationArguments对象
      ApplicationArguments applicationArguments = new DefaultApplicationArguments(
            args);
      //根据扫描到的监听器对象和函数传入参数，进行环境准备。
      ConfigurableEnvironment environment = prepareEnvironment(listeners,
            applicationArguments);

      configureIgnoreBeanInfo(environment);
      Banner printedBanner = printBanner(environment);

      context = createApplicationContext();

      //和上面套路一样，读取spring.factories文件key SpringBootExceptionReporter对应的value
      exceptionReporters = getSpringFactoriesInstances(
            SpringBootExceptionReporter.class,
            new Class[] { ConfigurableApplicationContext.class }, context);

      prepareContext(context, environment, listeners, applicationArguments,
            printedBanner);

      //和上面的一样，context准备完成之后，将触发SpringApplicationRunListener的contextPrepared执行
      refreshContext(context);

      //其实啥也没干。但是老版本的callRunners好像是在这里执行的。
      afterRefresh(context, applicationArguments);

      stopWatch.stop();
      if (this.logStartupInfo) {
         new StartupInfoLogger(this.mainApplicationClass)
               .logStarted(getApplicationLog(), stopWatch);
      }
      //发布ApplicationStartedEvent事件，发出结束执行的事件
      listeners.started(context);
      //在某些情况下，我们希望在容器bean加载完成后执行一些操作，会实现ApplicationRunner或者CommandLineRunner接口
      //后置操作，就是在容器完成刷新后，依次调用注册的Runners，还可以通过@Order注解设置各runner的执行顺序。
      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;
}

2.获取run listeners

SpringApplicationRunListeners listeners = getRunListeners(args);

和构造器设置初始化器一个套路，根据传入type SpringApplicationRunListener去扫描spring.factories文件，读取type对应的value并实例化。然后利用实例化对象创建SpringApplicationRunListeners对象。

org.springframework.boot.SpringApplicationRunListener=\
org.springframework.boot.context.event.EventPublishingRunListener

EventPublishingRunListener的作用是发布SpringApplicationEvent事件。

EventPublishingRunListener更像是被监听对象，这个命名让我有点迷。

public class EventPublishingRunListener implements SpringApplicationRunListener, Ordered {
	......
   @Override
   public void starting() {
      this.initialMulticaster.multicastEvent(
            new ApplicationStartingEvent(this.application, this.args));
   }

   @Override
   public void environmentPrepared(ConfigurableEnvironment environment) {
      this.initialMulticaster.multicastEvent(new ApplicationEnvironmentPreparedEvent(
            this.application, this.args, environment));
   }
	........

}

3.发出开始执行的事件

listeners.starting();

继续跟进starting函数，

public void starting() {
   this.initialMulticaster.multicastEvent(
         new ApplicationStartingEvent(this.application, this.args));
}
//获取ApplicationStartingEvent类型的事件后，发布事件
	@Override
	public void multicastEvent(final ApplicationEvent event, @Nullable ResolvableType eventType) {
		ResolvableType type = (eventType != null ? eventType : resolveDefaultEventType(event));
		for (final ApplicationListener<?> listener : getApplicationListeners(event, type)) {
			Executor executor = getTaskExecutor();
			if (executor != null) {
				executor.execute(() -> invokeListener(listener, event));
			}
			else {
				invokeListener(listener, event);
			}
		}
	}
//继续跟进invokeListener方法,最后调用ApplicationListener监听者的onApplicationEvent处理事件
	private void doInvokeListener(ApplicationListener listener, ApplicationEvent event) {
		try {
			listener.onApplicationEvent(event);
		}
		catch (ClassCastException ex) {
			.....
		}
	}

这个后面也会反复遇到，比如listeners.running(context)。

这里是典型的观察者模式。

//观察者：监听<E extends ApplicationEvent>类型事件
ApplicationListener<E extends ApplicationEvent> extends EventListener

//事件类型：
Event extends SpringApplicationEvent  extends ApplicationEvent extends EventObject

//被观察者：发布事件
EventPublishingRunListener implements SpringApplicationRunListener

SpringApplication根据当前事件Event类型，比如ApplicationStartingEvent，查找到监听ApplicationStartingEvent的观察者EventPublishingRunListener，调用观察者的onApplicationEvent处理事件。

4.环境准备

//根据main函数传入的参数，创建DefaultApplicationArguments对象
ApplicationArguments applicationArguments = new DefaultApplicationArguments(
      args);
//根据扫描到的listeners对象和函数传入参数，进行环境准备。
ConfigurableEnvironment environment = prepareEnvironment(listeners,
      applicationArguments);

ApplicationArguments提供运行application的参数，后面会作为一个Bean注入到容器。这里重点说下prepareEnvironment方法做了些什么。

private ConfigurableEnvironment prepareEnvironment(
      SpringApplicationRunListeners listeners,
      ApplicationArguments applicationArguments) {

   // Create and configure the environment
   ConfigurableEnvironment environment = getOrCreateEnvironment();

   configureEnvironment(environment, applicationArguments.getSourceArgs());

    //和listeners.starting一样的流程
   listeners.environmentPrepared(environment);

   //上述完成了环境的创建和配置，传入的参数和资源加载到environment

   //绑定环境到SpringApplication
   bindToSpringApplication(environment);
   if (!this.isCustomEnvironment) {
      environment = new EnvironmentConverter(getClassLoader())
            .convertEnvironmentIfNecessary(environment, deduceEnvironmentClass());
   }
   ConfigurationPropertySources.attach(environment);
   return environment;
}

这段代码核心有3个。

1. configureEnvironment，用于基本运行环境的配置。
2. 发布事件ApplicationEnvironmentPreparedEvent。和发布ApplicationStartingEvent事件的流程一样。
3. 绑定环境到SpringApplication

5.创建ApplicationContext

context = createApplicationContext();

传说中的IOC容器终于来了。

在实例化context之前，首先需要确定context的类型，这个是根据应用类型确定的。应用类型webApplicationType在构造器已经推断出来了。

protected ConfigurableApplicationContext createApplicationContext() {
   Class<?> contextClass = this.applicationContextClass;
   if (contextClass == null) {
      try {
         switch (this.webApplicationType) {
         case SERVLET:
            //应用为servlet类型的web应用
            contextClass = Class.forName(DEFAULT_SERVLET_WEB_CONTEXT_CLASS);
            break;
         case REACTIVE:
            //应用为响应式web应用
            contextClass = Class.forName(DEFAULT_REACTIVE_WEB_CONTEXT_CLASS);
            break;
         default:
            //应用为非web类型的应用
            contextClass = Class.forName(DEFAULT_CONTEXT_CLASS);
         }
      }
      catch (ClassNotFoundException ex) {
         throw new IllegalStateException(
               "Unable create a default ApplicationContext, "
                     + "please specify an ApplicationContextClass",
               ex);
      }
   }
   return (ConfigurableApplicationContext) BeanUtils.instantiateClass(contextClass);
}

获取context类型后，进行实例化，这里根据class类型获取无参构造器进行实例化。

public static <T> T instantiateClass(Class<T> clazz) throws BeanInstantiationException {
   Assert.notNull(clazz, "Class must not be null");
   if (clazz.isInterface()) {
      throw new BeanInstantiationException(clazz, "Specified class is an interface");
   }
   try {
       //clazz.getDeclaredConstructor()获取无参的构造器，然后进行实例化
      return instantiateClass(clazz.getDeclaredConstructor());
   }
   catch (NoSuchMethodException ex) {
	.......
}

比如web类型为servlet类型，就会实例化org.springframework.boot.web.servlet.context.

AnnotationConfigServletWebServerApplicationContext类型的context。

6.context前置处理阶段

private void prepareContext(ConfigurableApplicationContext context,
      ConfigurableEnvironment environment, SpringApplicationRunListeners listeners,
      ApplicationArguments applicationArguments, Banner printedBanner) {
   //关联环境
   context.setEnvironment(environment);

   //ApplicationContext预处理，主要配置Bean生成器以及资源加载器
   postProcessApplicationContext(context);

   //调用初始化器，执行initialize方法，前面set的初始化器终于用上了
   applyInitializers(context);
   //发布contextPrepared事件，和发布starting事件一样，不多说
   listeners.contextPrepared(context);
   if (this.logStartupInfo) {
      logStartupInfo(context.getParent() == null);
      logStartupProfileInfo(context);
   }

   // Add boot specific singleton beans
   ConfigurableListableBeanFactory beanFactory = context.getBeanFactory();
   //bean, springApplicationArguments,用于获取启动application所需的参数
   beanFactory.registerSingleton("springApplicationArguments", applicationArguments);

   //加载打印Banner的Bean
   if (printedBanner != null) {
      beanFactory.registerSingleton("springBootBanner", printedBanner);
   }

   if (beanFactory instanceof DefaultListableBeanFactory) {
      ((DefaultListableBeanFactory) beanFactory)
            .setAllowBeanDefinitionOverriding(this.allowBeanDefinitionOverriding);
   }
   // Load the sources，根据primarySources加载resource。primarySources:一般为主类的class对象
   Set<Object> sources = getAllSources();
   Assert.notEmpty(sources, "Sources must not be empty");
   //构造BeanDefinitionLoader并完成定义的Bean的加载
   load(context, sources.toArray(new Object[0]));
   //发布ApplicationPreparedEvent事件，表示application已准备完成
   listeners.contextLoaded(context);
}

7.刷新容器

private void refreshContext(ConfigurableApplicationContext context) {
   refresh(context);
   // 注册一个关闭容器时的钩子函数,在jvm关闭时调用
   if (this.registerShutdownHook) {
      try {
         context.registerShutdownHook();
      }
      catch (AccessControlException ex) {
         // Not allowed in some environments.
      }
   }
}

调用父类AbstractApplicationContext刷新容器的操作，具体的还没看。

@Override
public void refresh() throws BeansException, IllegalStateException {
   synchronized (this.startupShutdownMonitor) {
      // Prepare this context for refreshing.
      prepareRefresh();

      // Tell the subclass to refresh the internal bean factory.
      ConfigurableListableBeanFactory beanFactory = obtainFreshBeanFactory();

      // Prepare the bean factory for use in this context.
      prepareBeanFactory(beanFactory);

      try {
         // Allows post-processing of the bean factory in context subclasses.
         postProcessBeanFactory(beanFactory);

         // Invoke factory processors registered as beans in the context.
         invokeBeanFactoryPostProcessors(beanFactory);

         // Register bean processors that intercept bean creation.
         registerBeanPostProcessors(beanFactory);

         // Initialize message source for this context.
         initMessageSource();

         // Initialize event multicaster for this context.
         initApplicationEventMulticaster();

         // Initialize other special beans in specific context subclasses.
         onRefresh();

         // Check for listener beans and register them.
         registerListeners();

         // Instantiate all remaining (non-lazy-init) singletons.
         finishBeanFactoryInitialization(beanFactory);

         // Last step: publish corresponding event.
         finishRefresh();
      }
       ......
}

8.后置操作，调用Runners

后置操作，就是在容器完成刷新后，依次调用注册的Runners，还可以通过@Order注解设置各runner的执行顺序。

Runner可以通过实现ApplicationRunner或者CommandLineRunner接口。

	private void callRunners(ApplicationContext context, ApplicationArguments args) {
		List<Object> runners = new ArrayList<>();
		runners.addAll(context.getBeansOfType(ApplicationRunner.class).values());
		runners.addAll(context.getBeansOfType(CommandLineRunner.class).values());
		AnnotationAwareOrderComparator.sort(runners);
		for (Object runner : new LinkedHashSet<>(runners)) {
			if (runner instanceof ApplicationRunner) {
				callRunner((ApplicationRunner) runner, args);
			}
			if (runner instanceof CommandLineRunner) {
				callRunner((CommandLineRunner) runner, args);
			}
		}
	}

根据源码可知，runners收集从容器获取的ApplicationRunner和CommandLineRunner类型的Bean，然后依次执行。

9.发布ApplicationReadyEvent事件

listeners.running(context);

应用启动完成，可以对外提供服务了，在这里发布ApplicationReadyEvent事件。流程还是和starting时一样。