SpringBoot源码分析之—SpringBoot项目启动类SpringApplication浅析

  • 2019 年 10 月 15 日
  • 笔记

源码版本说明

本文源码采用版本为SpringBoot 2.1.0BUILD,对应的SpringFramework 5.1.0.RC1

注意：本文只是从整体上梳理流程，不做具体深入分析

SpringBoot入口类

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

　　这是我们日常使用springboot开发见到次数最多的引导类了，完成这个类的编写，就完成了一个springboot项目的框架，springboot就回自动为我们完成一些默认配置，并帮我们初始化上下文容器，但细节我们是不知道的，下面我们就一起探索下SpringApplication.run(DemoApplication .class, args);这行代码背后的故事！

SpringApplication初始化阶段

public SpringApplication(Class<?>... primarySources) {      this(null, primarySources);  }    // 初始化准备阶段  @SuppressWarnings({ "unchecked", "rawtypes" })  public SpringApplication(ResourceLoader resourceLoader, Class<?>... primarySources) {      this.resourceLoader = resourceLoader;      Assert.notNull(primarySources, "PrimarySources must not be null");      // 参数初始化      this.primarySources = new LinkedHashSet<>(Arrays.asList(primarySources));      // 推断应用类型      this.webApplicationType = deduceWebApplicationType();      // 加载ApplicationContextInitializer系列初始化器（从spring.factories文件加载，并实例化和排序后存到this.initializers）      setInitializers((Collection) getSpringFactoriesInstances(              ApplicationContextInitializer.class));      // 加载ApplicationListener系列监听器（从spring.factories文件加载，并实例化和排序后存到this.listeners）      setListeners((Collection) getSpringFactoriesInstances(ApplicationListener.class));      // 推断应用入口类（main函数所在类）      this.mainApplicationClass = deduceMainApplicationClass();  }

推断应用类型

// 推断应用类型  this.webApplicationType = deduceWebApplicationType();
private WebApplicationType deduceWebApplicationType() {      if (ClassUtils.isPresent(REACTIVE_WEB_ENVIRONMENT_CLASS, null)              && !ClassUtils.isPresent(MVC_WEB_ENVIRONMENT_CLASS, null)              && !ClassUtils.isPresent(JERSEY_WEB_ENVIRONMENT_CLASS, null)) {          return WebApplicationType.REACTIVE;      }      for (String className : WEB_ENVIRONMENT_CLASSES) {          if (!ClassUtils.isPresent(className, null)) {              return WebApplicationType.NONE;          }      }      return WebApplicationType.SERVLET;  }
private static final String REACTIVE_WEB_ENVIRONMENT_CLASS = "org.springframework."          + "web.reactive.DispatcherHandler";    private static final String MVC_WEB_ENVIRONMENT_CLASS = "org.springframework."          + "web.servlet.DispatcherServlet";    private static final String JERSEY_WEB_ENVIRONMENT_CLASS = "org.glassfish.jersey.server.ResourceConfig";    private static final String[] WEB_ENVIRONMENT_CLASSES = { "javax.servlet.Servlet",          "org.springframework.web.context.ConfigurableWebApplicationContext" };

根据当前应用ClassPath下是否存在相关类，来确定应用类型。

加载ApplicationContextInitializer系列初始化器

// 加载ApplicationContextInitializer系列初始化器（从spring.factories文件加载，并实例化和排序后存到this.initializers）  setInitializers((Collection) getSpringFactoriesInstances(          ApplicationContextInitializer.class));
public void setInitializers(          Collection<? extends ApplicationContextInitializer<?>> initializers) {      this.initializers = new ArrayList<>();      this.initializers.addAll(initializers);  }
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 = Thread.currentThread().getContextClassLoader();      // Use names and ensure unique to protect against duplicates      Set<String> names = new LinkedHashSet<>(              SpringFactoriesLoader.loadFactoryNames(type, classLoader));      List<T> instances = createSpringFactoriesInstances(type, parameterTypes,              classLoader, args, names);      AnnotationAwareOrderComparator.sort(instances);      return instances;  }
@SuppressWarnings("unchecked")  private <T> List<T> createSpringFactoriesInstances(Class<T> type,          Class<?>[] parameterTypes, ClassLoader classLoader, Object[] args,          Set<String> names) {      List<T> instances = new ArrayList<>(names.size());      for (String name : names) {          try {              Class<?> instanceClass = ClassUtils.forName(name, classLoader);              Assert.isAssignable(type, instanceClass);              Constructor<?> constructor = instanceClass                      .getDeclaredConstructor(parameterTypes);              T instance = (T) BeanUtils.instantiateClass(constructor, args);              instances.add(instance);          }          catch (Throwable ex) {              throw new IllegalArgumentException(                      "Cannot instantiate " + type + " : " + name, ex);          }      }      return instances;  }

　　利用Spring工厂加载机制，实例化ApplicationContextInitializer接口的实现类，被加载的实现类都配置在MATE-INF/spring.factories文件中，getSpringFactoriesInstances(Class<T> type,Class<?>[] parameterTypes, Object... args)这个方法就负责加载配置类并实例化和排序后返回，后面监听器、异常收集器和Runner等也是通过这个类实现实例化对应实现类的。下面是spring-boot-autoconfiguresrcmainresourcesMETA-INFspring.factories文件的配置内容。

# Initializers  org.springframework.context.ApplicationContextInitializer=  org.springframework.boot.autoconfigure.SharedMetadataReaderFactoryContextInitializer,  org.springframework.boot.autoconfigure.logging.ConditionEvaluationReportLoggingListener    # Application Listeners  org.springframework.context.ApplicationListener=  org.springframework.boot.autoconfigure.BackgroundPreinitializer    # Auto Configuration Import Listeners  org.springframework.boot.autoconfigure.AutoConfigurationImportListener=  org.springframework.boot.autoconfigure.condition.ConditionEvaluationReportAutoConfigurationImportListener    # Auto Configuration Import Filters  org.springframework.boot.autoconfigure.AutoConfigurationImportFilter=  org.springframework.boot.autoconfigure.condition.OnClassCondition    # Auto Configure  org.springframework.boot.autoconfigure.EnableAutoConfiguration=  org.springframework.boot.autoconfigure.admin.SpringApplicationAdminJmxAutoConfiguration,  org.springframework.boot.autoconfigure.aop.AopAutoConfiguration,  org.springframework.boot.autoconfigure.amqp.RabbitAutoConfiguration,  org.springframework.boot.autoconfigure.batch.BatchAutoConfiguration,  org.springframework.boot.autoconfigure.cache.CacheAutoConfiguration,  org.springframework.boot.autoconfigure.cassandra.CassandraAutoConfiguration,  org.springframework.boot.autoconfigure.cloud.CloudAutoConfiguration,  org.springframework.boot.autoconfigure.context.ConfigurationPropertiesAutoConfiguration,  org.springframework.boot.autoconfigure.context.MessageSourceAutoConfiguration,  org.springframework.boot.autoconfigure.context.PropertyPlaceholderAutoConfiguration,  org.springframework.boot.autoconfigure.couchbase.CouchbaseAutoConfiguration,  org.springframework.boot.autoconfigure.dao.PersistenceExceptionTranslationAutoConfiguration,  org.springframework.boot.autoconfigure.data.cassandra.CassandraDataAutoConfiguration,  org.springframework.boot.autoconfigure.data.cassandra.CassandraReactiveDataAutoConfiguration,  org.springframework.boot.autoconfigure.data.cassandra.CassandraReactiveRepositoriesAutoConfiguration,  org.springframework.boot.autoconfigure.data.cassandra.CassandraRepositoriesAutoConfiguration,  org.springframework.boot.autoconfigure.data.couchbase.CouchbaseDataAutoConfiguration,  org.springframework.boot.autoconfigure.data.couchbase.CouchbaseReactiveDataAutoConfiguration,  org.springframework.boot.autoconfigure.data.couchbase.CouchbaseReactiveRepositoriesAutoConfiguration,  org.springframework.boot.autoconfigure.data.couchbase.CouchbaseRepositoriesAutoConfiguration,  org.springframework.boot.autoconfigure.data.elasticsearch.ElasticsearchAutoConfiguration,  org.springframework.boot.autoconfigure.data.elasticsearch.ElasticsearchDataAutoConfiguration,  org.springframework.boot.autoconfigure.data.elasticsearch.ElasticsearchRepositoriesAutoConfiguration,  org.springframework.boot.autoconfigure.data.jpa.JpaRepositoriesAutoConfiguration,  org.springframework.boot.autoconfigure.data.ldap.LdapRepositoriesAutoConfiguration,  org.springframework.boot.autoconfigure.data.mongo.MongoDataAutoConfiguration,  org.springframework.boot.autoconfigure.data.mongo.MongoReactiveDataAutoConfiguration,  org.springframework.boot.autoconfigure.data.mongo.MongoReactiveRepositoriesAutoConfiguration,  org.springframework.boot.autoconfigure.data.mongo.MongoRepositoriesAutoConfiguration,  org.springframework.boot.autoconfigure.data.neo4j.Neo4jDataAutoConfiguration,  org.springframework.boot.autoconfigure.data.neo4j.Neo4jRepositoriesAutoConfiguration,  org.springframework.boot.autoconfigure.data.solr.SolrRepositoriesAutoConfiguration,  org.springframework.boot.autoconfigure.data.redis.RedisAutoConfiguration,  org.springframework.boot.autoconfigure.data.redis.RedisReactiveAutoConfiguration,  org.springframework.boot.autoconfigure.data.redis.RedisRepositoriesAutoConfiguration,  org.springframework.boot.autoconfigure.data.rest.RepositoryRestMvcAutoConfiguration,  org.springframework.boot.autoconfigure.data.web.SpringDataWebAutoConfiguration,  org.springframework.boot.autoconfigure.elasticsearch.jest.JestAutoConfiguration,  org.springframework.boot.autoconfigure.elasticsearch.rest.RestClientAutoConfiguration,  org.springframework.boot.autoconfigure.flyway.FlywayAutoConfiguration,  org.springframework.boot.autoconfigure.freemarker.FreeMarkerAutoConfiguration,  org.springframework.boot.autoconfigure.gson.GsonAutoConfiguration,  org.springframework.boot.autoconfigure.h2.H2ConsoleAutoConfiguration,  org.springframework.boot.autoconfigure.hateoas.HypermediaAutoConfiguration,  org.springframework.boot.autoconfigure.hazelcast.HazelcastAutoConfiguration,  org.springframework.boot.autoconfigure.hazelcast.HazelcastJpaDependencyAutoConfiguration,  org.springframework.boot.autoconfigure.http.HttpMessageConvertersAutoConfiguration,  org.springframework.boot.autoconfigure.http.codec.CodecsAutoConfiguration,  org.springframework.boot.autoconfigure.influx.InfluxDbAutoConfiguration,  org.springframework.boot.autoconfigure.info.ProjectInfoAutoConfiguration,  org.springframework.boot.autoconfigure.integration.IntegrationAutoConfiguration,  org.springframework.boot.autoconfigure.jackson.JacksonAutoConfiguration,  org.springframework.boot.autoconfigure.jdbc.DataSourceAutoConfiguration,  org.springframework.boot.autoconfigure.jdbc.JdbcTemplateAutoConfiguration,  org.springframework.boot.autoconfigure.jdbc.JndiDataSourceAutoConfiguration,  org.springframework.boot.autoconfigure.jdbc.XADataSourceAutoConfiguration,  org.springframework.boot.autoconfigure.jdbc.DataSourceTransactionManagerAutoConfiguration,  org.springframework.boot.autoconfigure.jms.JmsAutoConfiguration,  org.springframework.boot.autoconfigure.jmx.JmxAutoConfiguration,  org.springframework.boot.autoconfigure.jms.JndiConnectionFactoryAutoConfiguration,  org.springframework.boot.autoconfigure.jms.activemq.ActiveMQAutoConfiguration,  org.springframework.boot.autoconfigure.jms.artemis.ArtemisAutoConfiguration,  org.springframework.boot.autoconfigure.groovy.template.GroovyTemplateAutoConfiguration,  org.springframework.boot.autoconfigure.jersey.JerseyAutoConfiguration,  org.springframework.boot.autoconfigure.jooq.JooqAutoConfiguration,  org.springframework.boot.autoconfigure.jsonb.JsonbAutoConfiguration,  org.springframework.boot.autoconfigure.kafka.KafkaAutoConfiguration,  org.springframework.boot.autoconfigure.ldap.embedded.EmbeddedLdapAutoConfiguration,  org.springframework.boot.autoconfigure.ldap.LdapAutoConfiguration,  org.springframework.boot.autoconfigure.liquibase.LiquibaseAutoConfiguration,  org.springframework.boot.autoconfigure.mail.MailSenderAutoConfiguration,  org.springframework.boot.autoconfigure.mail.MailSenderValidatorAutoConfiguration,  org.springframework.boot.autoconfigure.mongo.embedded.EmbeddedMongoAutoConfiguration,  org.springframework.boot.autoconfigure.mongo.MongoAutoConfiguration,  org.springframework.boot.autoconfigure.mongo.MongoReactiveAutoConfiguration,  org.springframework.boot.autoconfigure.mustache.MustacheAutoConfiguration,  org.springframework.boot.autoconfigure.orm.jpa.HibernateJpaAutoConfiguration,  org.springframework.boot.autoconfigure.quartz.QuartzAutoConfiguration,  org.springframework.boot.autoconfigure.reactor.core.ReactorCoreAutoConfiguration,  org.springframework.boot.autoconfigure.security.servlet.SecurityAutoConfiguration,  org.springframework.boot.autoconfigure.security.servlet.UserDetailsServiceAutoConfiguration,  org.springframework.boot.autoconfigure.security.servlet.SecurityFilterAutoConfiguration,  org.springframework.boot.autoconfigure.security.reactive.ReactiveSecurityAutoConfiguration,  org.springframework.boot.autoconfigure.security.reactive.ReactiveUserDetailsServiceAutoConfiguration,  org.springframework.boot.autoconfigure.sendgrid.SendGridAutoConfiguration,  org.springframework.boot.autoconfigure.session.SessionAutoConfiguration,  org.springframework.boot.autoconfigure.security.oauth2.client.servlet.OAuth2ClientAutoConfiguration,  org.springframework.boot.autoconfigure.security.oauth2.client.reactive.ReactiveOAuth2ClientAutoConfiguration,  org.springframework.boot.autoconfigure.solr.SolrAutoConfiguration,  org.springframework.boot.autoconfigure.thymeleaf.ThymeleafAutoConfiguration,  org.springframework.boot.autoconfigure.transaction.TransactionAutoConfiguration,  org.springframework.boot.autoconfigure.transaction.jta.JtaAutoConfiguration,  org.springframework.boot.autoconfigure.validation.ValidationAutoConfiguration,  org.springframework.boot.autoconfigure.web.client.RestTemplateAutoConfiguration,  org.springframework.boot.autoconfigure.web.embedded.EmbeddedWebServerFactoryCustomizerAutoConfiguration,  org.springframework.boot.autoconfigure.web.reactive.HttpHandlerAutoConfiguration,  org.springframework.boot.autoconfigure.web.reactive.ReactiveWebServerFactoryAutoConfiguration,  org.springframework.boot.autoconfigure.web.reactive.WebFluxAutoConfiguration,  org.springframework.boot.autoconfigure.web.reactive.error.ErrorWebFluxAutoConfiguration,  org.springframework.boot.autoconfigure.web.reactive.function.client.WebClientAutoConfiguration,  org.springframework.boot.autoconfigure.web.servlet.DispatcherServletAutoConfiguration,  org.springframework.boot.autoconfigure.web.servlet.ServletWebServerFactoryAutoConfiguration,  org.springframework.boot.autoconfigure.web.servlet.error.ErrorMvcAutoConfiguration,  org.springframework.boot.autoconfigure.web.servlet.HttpEncodingAutoConfiguration,  org.springframework.boot.autoconfigure.web.servlet.MultipartAutoConfiguration,  org.springframework.boot.autoconfigure.web.servlet.WebMvcAutoConfiguration,  org.springframework.boot.autoconfigure.websocket.reactive.WebSocketReactiveAutoConfiguration,  org.springframework.boot.autoconfigure.websocket.servlet.WebSocketServletAutoConfiguration,  org.springframework.boot.autoconfigure.websocket.servlet.WebSocketMessagingAutoConfiguration,  org.springframework.boot.autoconfigure.webservices.WebServicesAutoConfiguration,  org.springframework.boot.autoconfigure.webservices.client.WebServiceTemplateAutoConfiguration    # Failure analyzers  org.springframework.boot.diagnostics.FailureAnalyzer=  org.springframework.boot.autoconfigure.diagnostics.analyzer.NoSuchBeanDefinitionFailureAnalyzer,  org.springframework.boot.autoconfigure.jdbc.DataSourceBeanCreationFailureAnalyzer,  org.springframework.boot.autoconfigure.jdbc.HikariDriverConfigurationFailureAnalyzer,  org.springframework.boot.autoconfigure.session.NonUniqueSessionRepositoryFailureAnalyzer    # Template availability providers  org.springframework.boot.autoconfigure.template.TemplateAvailabilityProvider=  org.springframework.boot.autoconfigure.freemarker.FreeMarkerTemplateAvailabilityProvider,  org.springframework.boot.autoconfigure.mustache.MustacheTemplateAvailabilityProvider,  org.springframework.boot.autoconfigure.groovy.template.GroovyTemplateAvailabilityProvider,  org.springframework.boot.autoconfigure.thymeleaf.ThymeleafTemplateAvailabilityProvider,  org.springframework.boot.autoconfigure.web.servlet.JspTemplateAvailabilityProvider

加载ApplicationListener系列监听器

// 加载ApplicationListener系列监听器（从spring.factories文件加载，并实例化和排序后存到this.listeners）  setListeners((Collection) getSpringFactoriesInstances(ApplicationListener.class));
public void setListeners(Collection<? extends ApplicationListener<?>> listeners) {      this.listeners = new ArrayList<>();      this.listeners.addAll(listeners);  }
private <T> Collection<T> getSpringFactoriesInstances(Class<T> type) {      return getSpringFactoriesInstances(type, new Class<?>[] {});  }

　　如上面的初始化器，ApplicationListener系列监听器也是通过getSpringFactoriesInstances(Class<T> type, Class<?>[] parameterTypes, Object... args)这个方法完成加载、排序及实例化的，而后存入到this.listeners中。

推断应用入口类

// 推断应用入口类（main函数所在类）  this.mainApplicationClass = deduceMainApplicationClass();
private Class<?> deduceMainApplicationClass() {      try {          // 通过new一个运行时异常获取堆栈信息          StackTraceElement[] stackTrace = new RuntimeException().getStackTrace();          for (StackTraceElement stackTraceElement : stackTrace) {              // 找到main函数所在的入口类              if ("main".equals(stackTraceElement.getMethodName())) {                  return Class.forName(stackTraceElement.getClassName());              }          }      }      catch (ClassNotFoundException ex) {          // Swallow and continue      }      return null;  }

　　推断应用入口类这部分比较有意思，他是通过new了一个运行时异常来拿到main线程的堆栈信息，遍历所有方法找到main方法所在的类。

运行阶段

// 运行阶段  public ConfigurableApplicationContext run(String... args) {      // 初始化容器启动计时器      StopWatch stopWatch = new StopWatch();      // 开始计时      stopWatch.start();      // 初始化上下文ConfigurableApplicationContext      ConfigurableApplicationContext context = null;      // 初始化异常收集器      Collection<SpringBootExceptionReporter> exceptionReporters = new ArrayList<>();      // 配置系统参数"java.awt.headless"      configureHeadlessProperty();      // 获取SpringApplicationRunListener系列监听器（从spring.factories文件加载，并实例化和排序）      SpringApplicationRunListeners listeners = getRunListeners(args);      // 遍历所有SpringApplicationRunListener系列监听器，广播ApplicationStartingEvent      listeners.starting();      try {          // 处理args参数          ApplicationArguments applicationArguments = new DefaultApplicationArguments(                  args);          // 准备环境（创建、配置、绑定环境、广播ApplicationEnvironmentPreparedEvent）          ConfigurableEnvironment environment = prepareEnvironment(listeners,                  applicationArguments);          configureIgnoreBeanInfo(environment);          // 打印Banner          Banner printedBanner = printBanner(environment);          // 根据应用类型创建上下文          context = createApplicationContext();          // 获取SpringBootExceptionReporter系列异常收集器（从spring.factories文件加载，并实例化和排序）          exceptionReporters = getSpringFactoriesInstances(                  SpringBootExceptionReporter.class,                  new Class[] { ConfigurableApplicationContext.class }, context);          // 上下文前置处理（执行ApplicationContextInitializer系列初始化器、加载资源、广播ApplicationPreparedEvent）          prepareContext(context, environment, listeners, applicationArguments,                  printedBanner);          // 刷新上下文（）          refreshContext(context);          // 上下文后置处理(目前啥也没干)          afterRefresh(context, applicationArguments);          // 启动完成，打印用时          stopWatch.stop();          if (this.logStartupInfo) {              new StartupInfoLogger(this.mainApplicationClass)                      .logStarted(getApplicationLog(), stopWatch);          }          // 遍历前面设置的ConfigurableApplicationContext监听器，发布ApplicationStartedEvent          listeners.started(context);          // 按顺序回调实现了ApplicationRunner或CommandLineRunner接口的Runners          callRunners(context, applicationArguments);      }      catch (Throwable ex) {          // 处理异常（发布ExitCodeEvent和ApplicationFailedEvent事件、异常收集器处理异常）          handleRunFailure(context, ex, exceptionReporters, listeners);          throw new IllegalStateException(ex);      }        try {          // 遍历前面设置好的SpringApplicationRunListener监听器，发布ApplicationReadyEvent          listeners.running(context);      }      catch (Throwable ex) {          handleRunFailure(context, ex, exceptionReporters, null);          throw new IllegalStateException(ex);      }      return context;  }

获取SpringApplicationRunListener系列监听器

// 获取SpringApplicationRunListener系列监听器（从spring.factories文件加载，并实例化和排序）  SpringApplicationRunListeners listeners = getRunListeners(args);
private SpringApplicationRunListeners getRunListeners(String[] args) {      Class<?>[] types = new Class<?>[] { SpringApplication.class, String[].class };      return new SpringApplicationRunListeners(logger, getSpringFactoriesInstances(              SpringApplicationRunListener.class, types, this, args));  }

　　这里我们再一次见到getSpringFactoriesInstances(Class<T> type,Class<?>[] parameterTypes, Object... args)这个方法，功能同上。

广播ApplicationStartingEvent事件

// 遍历所有SpringApplicationRunListener系列监听器，广播ApplicationStartingEvent  listeners.starting();
public void starting() {      for (SpringApplicationRunListener listener : this.listeners) {          listener.starting();      }  }
public void starting() {      this.initialMulticaster.multicastEvent(              new ApplicationStartingEvent(this.application, this.args));  }

　　这里会遍历上面拿到的排序好的所有SpringApplicationRunListener系列监听器，广播ApplicationStartingEvent事件，这代表Spring应用开始启动，在这之前只进行了注册化初始化器和监听器。

准备环境、广播ApplicationEnvironmentPreparedEvent事件

// 准备环境（创建、配置、绑定环境、广播ApplicationEnvironmentPreparedEvent）  ConfigurableEnvironment environment = prepareEnvironment(listeners,          applicationArguments);
private ConfigurableEnvironment prepareEnvironment(          SpringApplicationRunListeners listeners,          ApplicationArguments applicationArguments) {      // Create and configure the environment      ConfigurableEnvironment environment = getOrCreateEnvironment();      configureEnvironment(environment, applicationArguments.getSourceArgs());      listeners.environmentPrepared(environment);      bindToSpringApplication(environment);      if (this.webApplicationType == WebApplicationType.NONE) {          environment = new EnvironmentConverter(getClassLoader())                  .convertToStandardEnvironmentIfNecessary(environment);      }      ConfigurationPropertySources.attach(environment);      return environment;  }
public void environmentPrepared(ConfigurableEnvironment environment) {      for (SpringApplicationRunListener listener : this.listeners) {          listener.environmentPrepared(environment);      }  }
@Override  public void environmentPrepared(ConfigurableEnvironment environment) {      this.initialMulticaster.multicastEvent(new ApplicationEnvironmentPreparedEvent(              this.application, this.args, environment));  }

　　这里开始创建、配置和绑定ConfigurableEnvironment环境，环境准备好之后开始遍历SpringApplicationRunListener系列监听器，广播ApplicationEnvironmentPreparedEvent事件，代表环境已准备好。

开始打印Banner

// 打印Banner  Banner printedBanner = printBanner(environment);
private Banner printBanner(ConfigurableEnvironment environment) {      // Mode.OFF：不打印banner      if (this.bannerMode == Banner.Mode.OFF) {          return null;      }      // 加载banner资源，如果自定义了banner样式，在这里加载，否则加载默认banner      ResourceLoader resourceLoader = (this.resourceLoader != null)              ? this.resourceLoader : new DefaultResourceLoader(getClassLoader());      // 初始化bannerPrinter      SpringApplicationBannerPrinter bannerPrinter = new SpringApplicationBannerPrinter(              resourceLoader, this.banner);      // Mode.LOG：通过日志打印banner      if (this.bannerMode == Mode.LOG) {          return bannerPrinter.print(environment, this.mainApplicationClass, logger);      }      // 默认通过控制台打印banner      return bannerPrinter.print(environment, this.mainApplicationClass, System.out);  }

　　关于Banner，SpringBoot支持关闭banner打印、打印到log日志和打印到system日志三种方式；同时支持自定义banner，自定义banner又有图片和txt文本两种（同时存在时先打印图片banner，在打印文本banner），图片banner又支持gif, jpg, png这三种类型的图片格式banner（git优先于jpg优先于png），自定义banner非常简单，只需要将banner文件放到classpath:下就好了（resources目录下），如果存在多个banner文件，想指定某一个文件，只需要在application.properties文件加入如下配置就好了，非常方便。

spring.banner.image.location=banner.png  spring.banner.location=banner.txt

根据应用类型创建上下文

// 根据应用类型创建上下文  context = createApplicationContext();
protected ConfigurableApplicationContext createApplicationContext() {      Class<?> contextClass = this.applicationContextClass;      if (contextClass == null) {          try {              switch (this.webApplicationType) {              case SERVLET:                  contextClass = Class.forName(DEFAULT_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) {              throw new IllegalStateException(                      "Unable create a default ApplicationContext, "                              + "please specify an ApplicationContextClass",                      ex);          }      }      return (ConfigurableApplicationContext) BeanUtils.instantiateClass(contextClass);  }
public static final String DEFAULT_WEB_CONTEXT_CLASS = "org.springframework.boot."          + "web.servlet.context.AnnotationConfigServletWebServerApplicationContext";    public static final String DEFAULT_REACTIVE_WEB_CONTEXT_CLASS = "org.springframework."          + "boot.web.reactive.context.AnnotationConfigReactiveWebServerApplicationContext";    public static final String DEFAULT_CONTEXT_CLASS = "org.springframework.context."          + "annotation.AnnotationConfigApplicationContext";

　　开始创建ConfigurableApplicationContext上下文，其中Servlet类型的web应用会创建AnnotationConfigServletWebServerApplicationContext类型的上下文，Reactive类型的web应用会创建AnnotationConfigReactiveWebServerApplicationContext类型的上下文，非web应用会创建AnnotationConfigApplicationContext类型的上下文。

获取SpringBootExceptionReporter系列异常收集器

// 获取SpringBootExceptionReporter系列异常收集器（从spring.factories文件加载，并实例化和排序）  exceptionReporters = getSpringFactoriesInstances(          SpringBootExceptionReporter.class,          new Class[] { ConfigurableApplicationContext.class }, context);
private <T> Collection<T> getSpringFactoriesInstances(Class<T> type,          Class<?>[] parameterTypes, Object... args) {      ClassLoader classLoader = Thread.currentThread().getContextClassLoader();      // Use names and ensure unique to protect against duplicates      Set<String> names = new LinkedHashSet<>(              SpringFactoriesLoader.loadFactoryNames(type, classLoader));      List<T> instances = createSpringFactoriesInstances(type, parameterTypes,              classLoader, args, names);      AnnotationAwareOrderComparator.sort(instances);      return instances;  }

　　可以看到，加载异常收集器与上面初始化器和监听器如出一辙，不做过多阐述。

上下文前置处理、广播ApplicationPreparedEvent事件

// 上下文前置处理（执行ApplicationContextInitializer系列初始化器、加载资源、广播ApplicationPreparedEvent）  prepareContext(context, environment, listeners, applicationArguments,              printedBanner);
private void prepareContext(ConfigurableApplicationContext context,          ConfigurableEnvironment environment, SpringApplicationRunListeners listeners,          ApplicationArguments applicationArguments, Banner printedBanner) {      // 关联上下文和环境      context.setEnvironment(environment);      //      postProcessApplicationContext(context);      // 去重并排序前面获取好的ApplicationContextInitializer初始化器，执行初始化      applyInitializers(context);      // 遍历前面设置好的SpringApplicationRunListener，但并没有发布（目前什么都没做，貌似为了以后扩展）      listeners.contextPrepared(context);      if (this.logStartupInfo) {          logStartupInfo(context.getParent() == null);          logStartupProfileInfo(context);      }        // Add boot specific singleton beans      // 添加启动特定的单例bean      ConfigurableListableBeanFactory beanFactory = context.getBeanFactory();      beanFactory.registerSingleton("springApplicationArguments", applicationArguments);      if (printedBanner != null) {          beanFactory.registerSingleton("springBootBanner", printedBanner);      }        if (beanFactory instanceof DefaultListableBeanFactory) {          ((DefaultListableBeanFactory) beanFactory)                  .setAllowBeanDefinitionOverriding(this.allowBeanDefinitionOverriding);      }      // Load the sources      // 加载sources资源      Set<Object> sources = getAllSources();      Assert.notEmpty(sources, "Sources must not be empty");        load(context, sources.toArray(new Object[0]));      // 广播ApplicationPreparedEvent      listeners.contextLoaded(context);  }

　　在启动上下文之前，会调用前面初始化好的ApplicationContextInitializer接口实现类，当然也包含我们自定义的，所以我们可以自定义初始化器，在上下文启动前做一些操作。之后会广播ApplicationPreparedEvent事件，通知SpringApplicationRunListener监听器ConfigurableApplicationContext上下文已准备好（框架中用listeners.contextLoaded(context);方法广播了ApplicationPreparedEvent事件，而ApplicationLoadedEvent事件并没有发布，感觉这里以后还会变动。。。）。

刷新上下文

// 刷新上下文（）  refreshContext(context);
private void refreshContext(ConfigurableApplicationContext context) {      refresh(context);      if (this.registerShutdownHook) {          try {              context.registerShutdownHook();          }          catch (AccessControlException ex) {              // Not allowed in some environments.          }      }  }

发布ApplicationStartedEvent事件

// 遍历前面设置的ConfigurableApplicationContext监听器，发布ApplicationStartedEvent  listeners.started(context);
public void started(ConfigurableApplicationContext context) {      for (SpringApplicationRunListener listener : this.listeners) {          listener.started(context);      }  }
@Override  public void started(ConfigurableApplicationContext context) {      context.publishEvent(              new ApplicationStartedEvent(this.application, this.args, context));  }

　　发布ApplicationStartedEvent事件，通知　SpringApplicationRunListener系列监听器ConfigurableApplicationContext上下文已启动完成，Spring Bean 已初始化完成。

回调实现了ApplicationRunner或CommandLineRunner接口的Runners

// 按顺序回调实现了ApplicationRunner或CommandLineRunner接口的Runners  callRunners(context, applicationArguments);
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);          }      }  }

　　这里是有顺序的，ApplicationRunner的实现类优先于CommandLineRunner的实现类被回调

发布ApplicationReadyEvent事件

try {      // 遍历前面设置好的SpringApplicationRunListener监听器，发布ApplicationReadyEvent      listeners.running(context);  }
public void running(ConfigurableApplicationContext context) {      for (SpringApplicationRunListener listener : this.listeners) {          listener.running(context);      }  }
@Override  public void running(ConfigurableApplicationContext context) {      context.publishEvent(              new ApplicationReadyEvent(this.application, this.args, context));  }

　　发布ApplicationReadyEvent事件,通知SpringApplicationRunListener系列监听器ConfigurableApplicationContext上下文已经在运行，整个容器已经准备好。

发布ExitCodeEvent和ApplicationFailedEvent事件和异常收集器收集异常信息

catch (Throwable ex) {      handleRunFailure(context, ex, exceptionReporters, listeners);      throw new IllegalStateException(ex);  }
private void handleRunFailure(ConfigurableApplicationContext context,          Throwable exception,          Collection<SpringBootExceptionReporter> exceptionReporters,          SpringApplicationRunListeners listeners) {      try {          try {              handleExitCode(context, exception);              if (listeners != null) {                  listeners.failed(context, exception);              }          }          finally {              reportFailure(exceptionReporters, exception);              if (context != null) {                  context.close();              }          }      }      catch (Exception ex) {          logger.warn("Unable to close ApplicationContext", ex);      }      ReflectionUtils.rethrowRuntimeException(exception);  }

　　如果启动过程中出现异常，springboot将会发布ExitCodeEvent事件通知上下文停止或重启，并发布ApplicationFailedEvent事件通知SpringApplicationRunListener系列监听器，最后SpringBootExceptionReporter异常收集器收集打印异常。

总结

SpringBoot启动过程大致脉络

  • 准备阶段
    • 参数初始化
    • 推断应用类型
    • 加载ApplicationContextInitializer系列初始化器
    • 加载ApplicationListener系列监听器
    • 推断应用入口类（main函数所在类）
  • 运行阶段
    • 初始化容器启动计时器，开始计时
    • 初始化上下文ConfigurableApplicationContext、异常收集器
    • 配置系统参数"java.awt.headless"
    • 获取SpringApplicationRunListener系列监听器
    • 遍历所有SpringApplicationRunListener系列监听器，广播ApplicationStartingEvent
    • 处理args参数
    • 准备环境（创建、配置、绑定环境、广播ApplicationEnvironmentPreparedEvent）
    • 配置忽略Bean信息
    • 打印Banner
    • 根据应用类型创建上下文
    • 获取SpringBootExceptionReporter系列异常收集器
    • 上下文前置处理（执行ApplicationContextInitializer系列初始化器、加载资源、广播ApplicationPreparedEvent）
    • 刷新上下文
    • 启动完成，打印用时
    • 遍历前面设置的ConfigurableApplicationContext监听器，发布ApplicationStartedEvent
    • 回调实现了ApplicationRunner或CommandLineRunner接口的Runners
    • 遍历前面设置好的SpringApplicationRunListener监听器，发布ApplicationReadyEvent

　　至此，整个SpringBoot项目已经启动完成，我们可以看到，整个过程中Spring的事件驱动机制起着举足轻重的作用，有了这个机制我们可以知晓容器的启动过程，并且可以监听到某些事件，对容器中我们关心的实例做进一步处理，我们深入理解事件驱动机制很有必要，它将帮助我们更好的理解和使用这个Spring框架体系。如果想要文中中文版SpringBoot注释源码，可以在我的github下载，如果发现哪里写的不对，烦请留言通知我。

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