spring boot容器启动详解

一、前言

spring cloud大行其道的当下，如果不了解基本原理那么是很纠结的(看见的都是约定大于配置，但是原理呢？为什么要这么做？如何串联起来的？)。spring cloud是基于spring boot快速搭建的，今天咱们就看看spring boot容器启动流程(全文基于1.5.9版本)。(本文不讲解如何快速启动spring boot，那些直接官方看即可，官网文档飞机票)

二、容器启动

spring boot一般是指定容器启动main方法，然后以命令行方式启动Jar包，如下图：

1 @SpringBootApplication
2 public class Application {
3     public static void main(String[] args) {
4         SpringApplication.run(Application.class, args);
5     }
6 }

这里核心关注2个东西：

1.@SpringBootApplication注解

2.SpringApplication.run()静态方法

下面我们就分别探究这两块内容。

2.1 @SpringBootApplication注解

源码如下：

 1 @Target(ElementType.TYPE)
 2 @Retention(RetentionPolicy.RUNTIME)
 3 @Documented
 4 @Inherited
 5 @SpringBootConfiguration
 6 @EnableAutoConfiguration
 7 @ComponentScan(excludeFilters = {
 8         @Filter(type = FilterType.CUSTOM, classes = TypeExcludeFilter.class),
 9         @Filter(type = FilterType.CUSTOM, classes = AutoConfigurationExcludeFilter.class) })
10 public @interface SpringBootApplication {

核心注解：

@SpringBootConfiguration（实际就是个@Configuration）：表示这是一个JavaConfig配置类，可以在这个类中自定义bean,依赖关系等。-》这个是spring-boot特有的注解，常用到。
@EnableAutoConfiguration：借助@Import的帮助，将所有符合自动配置条件的bean定义加载到IoC容器（建议放在根包路径下，这样可以扫描子包和类）。-》这个需要详细深挖！
@ComponentScan：spring的自动扫描注解，可定义扫描范围，加载到IOC容器。-》这个不多说，spring的注解大家肯定眼熟

其中@EnableAutoConfiguration这个注解的源码：

1 @SuppressWarnings("deprecation")
2 @Target(ElementType.TYPE)
3 @Retention(RetentionPolicy.RUNTIME)
4 @Documented
5 @Inherited
6 @AutoConfigurationPackage
7 @Import(EnableAutoConfigurationImportSelector.class)
8 public @interface EnableAutoConfiguration {

核心是一个EnableAutoConfigurationImportSelector类图如下：

核心方法在顶级接口ImportSelector的selectImports（），源码如下：

 1 @Override
 2     public String[] selectImports(AnnotationMetadata annotationMetadata) {
 3         if (!isEnabled(annotationMetadata)) {
 4             return NO_IMPORTS;
 5         }
 6         try {         //1.从META-INF/spring-autoconfigure-metadata.properties文件中载入483条配置属性（有一些有默认值），
 7             AutoConfigurationMetadata autoConfigurationMetadata = AutoConfigurationMetadataLoader
 8                     .loadMetadata(this.beanClassLoader);
 9             AnnotationAttributes attributes = getAttributes(annotationMetadata);//2.获取注解属性
10             List<String> configurations = getCandidateConfigurations(annotationMetadata,//3.获取97个自动配置类
11                     attributes);
12             configurations = removeDuplicates(configurations);//4.移除重复的
13             configurations = sort(configurations, autoConfigurationMetadata);//5.排序
14             Set<String> exclusions = getExclusions(annotationMetadata, attributes);//6.获取需要排除的
15             checkExcludedClasses(configurations, exclusions);//7.校验排除类
16             configurations.removeAll(exclusions);//8.删除所有需要排除的
17             configurations = filter(configurations, autoConfigurationMetadata);//9.过滤器OnClassCondition（注解中配置的当存在某类才生效）
18             fireAutoConfigurationImportEvents(configurations, exclusions);//10.触发自动配置导入监听事件
19             return configurations.toArray(new String[configurations.size()]);
20         }
21         catch (IOException ex) {
22             throw new IllegalStateException(ex);
23         }
24     }

这里注意3个核心方法：

1）loadMetadata 加载配置

其实就是用类加载器去加载：META-INF/spring-autoconfigure-metadata.properties（spring-boot-autoconfigure-1.5.9.RELEASE-sources.jar）文件中定义的配置，返回PropertiesAutoConfigurationMetadata（实现了AutoConfigurationMetadata接口，封装了属性的get set方法）

2）getCandidateConfigurations获取默认支持的自动配置类名列表

自动配置灵魂方法，SpringFactoriesLoader.loadFactoryNames 从META-INF/spring.factories（spring-boot-autoconfigure-1.5.9.RELEASE-sources.jar）文件中获取自动配置类key=EnableAutoConfiguration.class的配置。

 1 protected List<String> getCandidateConfigurations(AnnotationMetadata metadata,
 2             AnnotationAttributes attributes) {//话说这里2个入参没啥用啊...谁来给我解释一下...
 3         List<String> configurations = SpringFactoriesLoader.loadFactoryNames(
 4                 getSpringFactoriesLoaderFactoryClass(), getBeanClassLoader());
 5         Assert.notEmpty(configurations,
 6                 "No auto configuration classes found in META-INF/spring.factories. If you "
 7                         + "are using a custom packaging, make sure that file is correct.");
 8         return configurations;
 9     }
10 //返回的是EnableAutoConfiguration类
11 protected Class<?> getSpringFactoriesLoaderFactoryClass() {
12         return EnableAutoConfiguration.class;
13     }

实际获取了什么？spring.factories文件如下，实际获取了# Auto Configure自动配置模块的所有类。

# Initializers
org.springframework.context.ApplicationContextInitializer=\
org.springframework.boot.autoconfigure.SharedMetadataReaderFactoryContextInitializer,\
org.springframework.boot.autoconfigure.logging.AutoConfigurationReportLoggingInitializer

# 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 ===========这里就是全部的自动配置类===============================
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.CassandraRepositoriesAutoConfiguration,\
org.springframework.boot.autoconfigure.data.couchbase.CouchbaseDataAutoConfiguration,\
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.LdapDataAutoConfiguration,\
org.springframework.boot.autoconfigure.data.ldap.LdapRepositoriesAutoConfiguration,\
org.springframework.boot.autoconfigure.data.mongo.MongoDataAutoConfiguration,\
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.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.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.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.flyway.FlywayAutoConfiguration,\
org.springframework.boot.autoconfigure.groovy.template.GroovyTemplateAutoConfiguration,\
org.springframework.boot.autoconfigure.jersey.JerseyAutoConfiguration,\
org.springframework.boot.autoconfigure.jooq.JooqAutoConfiguration,\
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.mobile.DeviceResolverAutoConfiguration,\
org.springframework.boot.autoconfigure.mobile.DeviceDelegatingViewResolverAutoConfiguration,\
org.springframework.boot.autoconfigure.mobile.SitePreferenceAutoConfiguration,\
org.springframework.boot.autoconfigure.mongo.embedded.EmbeddedMongoAutoConfiguration,\
org.springframework.boot.autoconfigure.mongo.MongoAutoConfiguration,\
org.springframework.boot.autoconfigure.mustache.MustacheAutoConfiguration,\
org.springframework.boot.autoconfigure.orm.jpa.HibernateJpaAutoConfiguration,\
org.springframework.boot.autoconfigure.reactor.ReactorAutoConfiguration,\
org.springframework.boot.autoconfigure.security.SecurityAutoConfiguration,\
org.springframework.boot.autoconfigure.security.SecurityFilterAutoConfiguration,\
org.springframework.boot.autoconfigure.security.FallbackWebSecurityAutoConfiguration,\
org.springframework.boot.autoconfigure.security.oauth2.OAuth2AutoConfiguration,\
org.springframework.boot.autoconfigure.sendgrid.SendGridAutoConfiguration,\
org.springframework.boot.autoconfigure.session.SessionAutoConfiguration,\
org.springframework.boot.autoconfigure.social.SocialWebAutoConfiguration,\
org.springframework.boot.autoconfigure.social.FacebookAutoConfiguration,\
org.springframework.boot.autoconfigure.social.LinkedInAutoConfiguration,\
org.springframework.boot.autoconfigure.social.TwitterAutoConfiguration,\
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.DispatcherServletAutoConfiguration,\
org.springframework.boot.autoconfigure.web.EmbeddedServletContainerAutoConfiguration,\
org.springframework.boot.autoconfigure.web.ErrorMvcAutoConfiguration,\
org.springframework.boot.autoconfigure.web.HttpEncodingAutoConfiguration,\
org.springframework.boot.autoconfigure.web.HttpMessageConvertersAutoConfiguration,\
org.springframework.boot.autoconfigure.web.MultipartAutoConfiguration,\
org.springframework.boot.autoconfigure.web.ServerPropertiesAutoConfiguration,\
org.springframework.boot.autoconfigure.web.WebClientAutoConfiguration,\
org.springframework.boot.autoconfigure.web.WebMvcAutoConfiguration,\
org.springframework.boot.autoconfigure.websocket.WebSocketAutoConfiguration,\
org.springframework.boot.autoconfigure.websocket.WebSocketMessagingAutoConfiguration,\
org.springframework.boot.autoconfigure.webservices.WebServicesAutoConfiguration
============================================end================================================
# 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

# 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.JspTemplateAvailabilityProvider

3）filter过滤器根据OnClassCondition注解把不满足条件的过滤掉

 1 private List<String> filter(List<String> configurations,
 2             AutoConfigurationMetadata autoConfigurationMetadata) {
 3         long startTime = System.nanoTime();
 4         String[] candidates = configurations.toArray(new String[configurations.size()]);
 5         boolean[] skip = new boolean[candidates.length];
 6         boolean skipped = false;
           //获取需要过滤的自动配置导入拦截器，spring.factories配置中就一个：org.springframework.boot.autoconfigure.condition.OnClassCondition

 7         for (AutoConfigurationImportFilter filter : getAutoConfigurationImportFilters()) {
 8             invokeAwareMethods(filter);
 9             boolean[] match = filter.match(candidates, autoConfigurationMetadata);
10             for (int i = 0; i < match.length; i++) {
11                 if (!match[i]) {
12                     skip[i] = true;
13                     skipped = true;
14                 }
15             }
16         }
17         if (!skipped) {//多条件只要有一个不匹配->skipped = true，全部匹配-》skipped = false->直接返回
18             return configurations;
19         }
20         List<String> result = new ArrayList<String>(candidates.length);
21         for (int i = 0; i < candidates.length; i++) {
22             if (!skip[i]) {//匹配-》不跳过-》添加进result
23                 result.add(candidates[i]);
24             }
25         }
26         if (logger.isTraceEnabled()) {
27             int numberFiltered = configurations.size() - result.size();
28             logger.trace("Filtered " + numberFiltered + " auto configuration class in "
29                     + TimeUnit.NANOSECONDS.toMillis(System.nanoTime() - startTime)
30                     + " ms");
31         }
32         return new ArrayList<String>(result);
33     }

2.2 SpringApplication.run()静态方法

SpringApplication.run

 1 public ConfigurableApplicationContext run(String... args) {
 2         StopWatch stopWatch = new StopWatch();
 3         stopWatch.start();
 4         ConfigurableApplicationContext context = null;
 5         FailureAnalyzers analyzers = null;
 6         configureHeadlessProperty();
 7         SpringApplicationRunListeners listeners = getRunListeners(args);//1.获取监听器
 8         listeners.starting();-->启动！
 9         try {
10             ApplicationArguments applicationArguments = new DefaultApplicationArguments(
11                     args);
12             ConfigurableEnvironment environment = prepareEnvironment(listeners,//2.准备好环境，触发ApplicationEnvironmentPreparedEvent事件
13                     applicationArguments);
14             Banner printedBanner = printBanner(environment);//打印启动提示字符，默认spring的字符图
15             context = createApplicationContext();//实例化一个可配置应用上下文
16             analyzers = new FailureAnalyzers(context);
17             prepareContext(context, environment, listeners, applicationArguments,//3.准备上下文
18                     printedBanner);
19             refreshContext(context);//4.刷新上下文
20             afterRefresh(context, applicationArguments);//5.刷新上下文后
21             listeners.finished(context, null);--关闭！
22             stopWatch.stop();
23             if (this.logStartupInfo) {
24                 new StartupInfoLogger(this.mainApplicationClass)
25                         .logStarted(getApplicationLog(), stopWatch);
26             }
27             return context;
28         }
29         catch (Throwable ex) {
30             handleRunFailure(context, listeners, analyzers, ex);
31             throw new IllegalStateException(ex);
32         }
33     }

如上图,容器启动流程可以分为5个主要步骤：

1.getRunListeners获取监听器（SpringApplicationRunListeners ）

实际是SpringApplicationRunListener类

 1 private SpringApplicationRunListeners getRunListeners(String[] args) {
 2     Class<?>[] types = new Class<?>[] { SpringApplication.class, String[].class };
 3     return new SpringApplicationRunListeners(logger, getSpringFactoriesInstances(
 4             SpringApplicationRunListener.class, types, this, args));
 5 }
 6 
 7 private <T> Collection<? extends T> getSpringFactoriesInstances(Class<T> type) {
 8     return getSpringFactoriesInstances(type, new Class<?>[] {});
 9 }
10 
11 private <T> Collection<? extends T> getSpringFactoriesInstances(Class<T> type,
12         Class<?>[] parameterTypes, Object... args) {
13     ClassLoader classLoader = Thread.currentThread().getContextClassLoader();
14     // 使用Set确保的字符串的唯一性
15     Set<String> names = new LinkedHashSet<String>(
16             SpringFactoriesLoader.loadFactoryNames(type, classLoader));// 1.载入工厂名称集合
17     List<T> instances = createSpringFactoriesInstances(type, parameterTypes,// 2.创建工厂实例
18             classLoader, args, names);
19     AnnotationAwareOrderComparator.sort(instances);// 排序
20     return instances;
21 }

载入工厂名称（loadFactoryNames）

当前类的类加载器从META-INF/spring.factories文件中获取SpringApplicationRunListener类的配置

 1 public static List<String> loadFactoryNames(Class<?> factoryClass, ClassLoader classLoader) {
 2         String factoryClassName = factoryClass.getName();
 3         try {
 4             Enumeration<URL> urls = (classLoader != null ? classLoader.getResources(FACTORIES_RESOURCE_LOCATION) :
 5                     ClassLoader.getSystemResources(FACTORIES_RESOURCE_LOCATION));
 6             List<String> result = new ArrayList<String>();
 7             while (urls.hasMoreElements()) {
 8                 URL url = urls.nextElement();
 9                 Properties properties = PropertiesLoaderUtils.loadProperties(new UrlResource(url));
10                 String factoryClassNames = properties.getProperty(factoryClassName);
11                 result.addAll(Arrays.asList(StringUtils.commaDelimitedListToStringArray(factoryClassNames)));
12             }
13             return result;
14         }
15         catch (IOException ex) {
16             throw new IllegalArgumentException("Unable to load [" + factoryClass.getName() +
17                     "] factories from location [" + FACTORIES_RESOURCE_LOCATION + "]", ex);
18         }
19     }

上图，获取到工厂类名后，下面来看看META-INF/spring.factories中定义了啥：

# PropertySource Loaders
org.springframework.boot.env.PropertySourceLoader=\
org.springframework.boot.env.PropertiesPropertySourceLoader,\
org.springframework.boot.env.YamlPropertySourceLoader

# Run Listeners  这里呢，看这里！！！！
org.springframework.boot.SpringApplicationRunListener=\
org.springframework.boot.context.event.EventPublishingRunListener

# Application Context Initializers
org.springframework.context.ApplicationContextInitializer=\
org.springframework.boot.context.ConfigurationWarningsApplicationContextInitializer,\
org.springframework.boot.context.ContextIdApplicationContextInitializer,\
org.springframework.boot.context.config.DelegatingApplicationContextInitializer,\
org.springframework.boot.context.embedded.ServerPortInfoApplicationContextInitializer

# Application Listeners
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.liquibase.LiquibaseServiceLocatorApplicationListener,\
org.springframework.boot.logging.ClasspathLoggingApplicationListener,\
org.springframework.boot.logging.LoggingApplicationListener

# Environment Post Processors
org.springframework.boot.env.EnvironmentPostProcessor=\
org.springframework.boot.cloud.CloudFoundryVcapEnvironmentPostProcessor,\
org.springframework.boot.env.SpringApplicationJsonEnvironmentPostProcessor

# Failure Analyzers
org.springframework.boot.diagnostics.FailureAnalyzer=\
org.springframework.boot.diagnostics.analyzer.BeanCurrentlyInCreationFailureAnalyzer,\
org.springframework.boot.diagnostics.analyzer.BeanNotOfRequiredTypeFailureAnalyzer,\
org.springframework.boot.diagnostics.analyzer.BindFailureAnalyzer,\
org.springframework.boot.diagnostics.analyzer.ConnectorStartFailureAnalyzer,\
org.springframework.boot.diagnostics.analyzer.NoUniqueBeanDefinitionFailureAnalyzer,\
org.springframework.boot.diagnostics.analyzer.PortInUseFailureAnalyzer,\
org.springframework.boot.diagnostics.analyzer.ValidationExceptionFailureAnalyzer

# FailureAnalysisReporters
org.springframework.boot.diagnostics.FailureAnalysisReporter=\
org.springframework.boot.diagnostics.LoggingFailureAnalysisReporter

哇，都是些类全名称，且key都是接口，value都是实现类。我们根据key=“org.springframework.boot.SpringApplicationRunListener”查询得到实现类value="org.springframework.boot.context.event.EventPublishingRunListener"事件发布启动监听器，一猜也知道肯定要用”反射”根据类名获取类实例，下面很快得到验证...

创建spring工厂实例（createSpringFactoriesInstances）

根据第一步得到的Set<String> names（SpringApplicationRunListener的唯一实现类EventPublishingRunListener）生成"事件发布启动监听器"工厂实例

 1 @SuppressWarnings("unchecked")
 2     private <T> List<T> createSpringFactoriesInstances(Class<T> type,
 3             Class<?>[] parameterTypes, ClassLoader classLoader, Object[] args,
 4             Set<String> names) {
 5         List<T> instances = new ArrayList<T>(names.size());
 6         for (String name : names) {
 7             try {
 8                 Class<?> instanceClass = ClassUtils.forName(name, classLoader);// 利用反射获取类
 9                 Assert.isAssignable(type, instanceClass);
10                 Constructor<?> constructor = instanceClass
11                         .getDeclaredConstructor(parameterTypes);// 得到构造器
12                 T instance = (T) BeanUtils.instantiateClass(constructor, args);// 根据构造器和参数构造实例
13                 instances.add(instance);
14             }
15             catch (Throwable ex) {
16                 throw new IllegalArgumentException(
17                         "Cannot instantiate " + type + " : " + name, ex);
18             }
19         }
20         return instances;
21     }

2.准备好环境

构造一个ConfigurableEnvironment,这里不多说。

3.准备上下文

 1 private void prepareContext(ConfigurableApplicationContext context,
 2             ConfigurableEnvironment environment, SpringApplicationRunListeners listeners,
 3             ApplicationArguments applicationArguments, Banner printedBanner) {
 4         context.setEnvironment(environment);
 5         postProcessApplicationContext(context);//单例一个BeanNameGenerator，把ResourceLoader设置进应用上下文
 6         applyInitializers(context);//执行初始化器
 7         listeners.contextPrepared(context);// 监听器执行上下文"已准备好"方法
 8         if (this.logStartupInfo) {
 9             logStartupInfo(context.getParent() == null);
10             logStartupProfileInfo(context);
11         }
12 
13         // 添加spring boot特殊单例bean
14         context.getBeanFactory().registerSingleton("springApplicationArguments",
15                 applicationArguments);
16         if (printedBanner != null) {
17             context.getBeanFactory().registerSingleton("springBootBanner", printedBanner);
18         }
19 
20         // 载入资源
21         Set<Object> sources = getSources();
22         Assert.notEmpty(sources, "Sources must not be empty");
23         load(context, sources.toArray(new Object[sources.size()]));
24         listeners.contextLoaded(context);// 监听器执行"上下文已加载"方法
25     }

4.刷新上下文

 1 private void refreshContext(ConfigurableApplicationContext context) {
 2     refresh(context);//核心类
 3     if (this.registerShutdownHook) {
 4         try {
 5             context.registerShutdownHook();//注册关闭钩子，容器关闭时执行
 6         }
 7         catch (AccessControlException ex) {
 8             // Not allowed in some environments.
 9         }
10     }
11 }
12 
13 protected void refresh(ApplicationContext applicationContext) {
14     Assert.isInstanceOf(AbstractApplicationContext.class, applicationContext);
15     ((AbstractApplicationContext) applicationContext).refresh();
16 }

最终执行的是AbstractApplicationContext抽象类的refresh方法。

 1 public void refresh() throws BeansException, IllegalStateException {
 2         synchronized (this.startupShutdownMonitor) {
 3             //准备刷新的上下文环境，例如对系统属性或者环境变量进行准备及验证。
 4             prepareRefresh();
 5 
 6             //启动子类的refreshBeanFactory方法.解析xml
 7             ConfigurableListableBeanFactory beanFactory = obtainFreshBeanFactory();
 8 
 9             //为BeanFactory配置容器特性，例如类加载器、事件处理器等.
10             prepareBeanFactory(beanFactory);
11 
12             try {
13                 //设置BeanFactory的后置处理. 空方法，留给子类拓展用。 
14                 postProcessBeanFactory(beanFactory);
15 
16                 //调用BeanFactory的后处理器, 这些后处理器是在Bean定义中向容器注册的.  
17                 invokeBeanFactoryPostProcessors(beanFactory);
18 
19                 //注册Bean的后处理器, 在Bean创建过程中调用.  
20                 registerBeanPostProcessors(beanFactory);
21 
22                 //初始化上下文中的消息源，即不同语言的消息体进行国际化处理  
23                 initMessageSource();
24 
25                 //初始化ApplicationEventMulticaster bean,应用事件广播器
26                 initApplicationEventMulticaster();
27 
28                 //初始化其它特殊的Bean， 空方法，留给子类拓展用。 
29                 onRefresh();
30 
31                 //检查并向容器注册监听器Bean
32                 registerListeners();
33 
34                 //实例化所有剩余的(non-lazy-init) 单例Bean.
35                 finishBeanFactoryInitialization(beanFactory);
36 
37                 //发布容器事件, 结束refresh过程. 
38                 finishRefresh();
39             }
40 
41             catch (BeansException ex) {
42                 if (logger.isWarnEnabled()) {
43                     logger.warn("Exception encountered during context initialization - " +
44                             "cancelling refresh attempt: " + ex);
45                 }
46 
47                 //销毁已经创建的单例Bean, 以避免资源占用.
48                 destroyBeans();
49 
50                 //取消refresh操作, 重置active标志. 
51                 cancelRefresh(ex);
52 
53                 // Propagate exception to caller.
54                 throw ex;
55             }
56 
57             finally {
58                 //重置Spring的核心缓存
59                 resetCommonCaches();
60             }
61         }
62     }

5.刷新完上下文后

spring boot提供的2个供用户自己拓展的接口：ApplicationRunner和CommandLineRunner。可以在容器启动完毕后（上下文刷新后）执行，做一些类似数据初始化的操作。

 1 private void callRunners(ApplicationContext context, ApplicationArguments args) {
 2         List<Object> runners = new ArrayList<Object>();
 3         runners.addAll(context.getBeansOfType(ApplicationRunner.class).values());//从上下文中获取ApplicationRunner类型的bean
 4         runners.addAll(context.getBeansOfType(CommandLineRunner.class).values());//从上下文中获取CommandLineRunner类型的bean
 5         AnnotationAwareOrderComparator.sort(runners);//排序
 6         for (Object runner : new LinkedHashSet<Object>(runners)) {
 7             if (runner instanceof ApplicationRunner) {
 8                 callRunner((ApplicationRunner) runner, args);//执行
 9             }
10             if (runner instanceof CommandLineRunner) {
11                 callRunner((CommandLineRunner) runner, args);
12             }
13         }
14     }

两个区别在于入参不同，根据实际情况自己选择。

 1 public interface CommandLineRunner {
 8     void run(String... args) throws Exception;
10 }
11 
12 public interface ApplicationRunner {
19     void run(ApplicationArguments args) throws Exception;
20 
21 }