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Spring Boot容器启动详解

2021-04-08 分类：精选文章 评论(0)
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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，springboot2基本一致)。(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：

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

这里核心关注2个东西：

strong>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： 

@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 {

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：

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

核心是一个EnableAutoConfigurationImportSelector类图如下：

20210406142558367.png

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

@Override
    public String[] selectImports(AnnotationMetadata annotationMetadata) {
        if (!isEnabled(annotationMetadata)) {
            return NO_IMPORTS;
        }
        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），
            AutoConfigurationMetadata autoConfigurationMetadata = AutoConfigurationMetadataLoader
                    .loadMetadata(this.beanClassLoader);
            AnnotationAttributes attributes = getAttributes(annotationMetadata);//2.获取注解属性
            List<String> configurations = getCandidateConfigurations(annotationMetadata,//3.获取97个自动配置类
                    attributes);
            configurations = removeDuplicates(configurations);//4.移除重复的
            configurations = sort(configurations, autoConfigurationMetadata);//5.排序
            Set<String> exclusions = getExclusions(annotationMetadata, attributes);//6.获取需要排除的
            checkExcludedClasses(configurations, exclusions);//7.校验排除类
            configurations.removeAll(exclusions);//8.删除所有需要排除的
            configurations = filter(configurations, autoConfigurationMetadata);//9.过滤器OnClassCondition（注解中配置的当存在某类才生效）
            fireAutoConfigurationImportEvents(configurations, exclusions);//10.触发自动配置导入监听事件
            return configurations.toArray(new String[configurations.size()]);
        }
        catch (IOException ex) {
            throw new IllegalStateException(ex);
        }
    }

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。 

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

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

private List<String> filter(List<String> configurations,
            AutoConfigurationMetadata autoConfigurationMetadata) {
        long startTime = System.nanoTime();
        String[] candidates = configurations.toArray(new String[configurations.size()]);
        boolean[] skip = new boolean[candidates.length];
        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
        for (AutoConfigurationImportFilter filter : getAutoConfigurationImportFilters()) {
            invokeAwareMethods(filter);
            boolean[] match = filter.match(candidates, autoConfigurationMetadata);
            for (int i = 0; i < match.length; i++) {
                if (!match[i]) {
                    skip[i] = true;
                    skipped = true;
                }
            }
        }
        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
            return configurations;
        }
        List<String> result = new ArrayList<String>(candidates.length);
        for (int i = 0; i < candidates.length; i++) {
            if (!skip[i]) {//匹配-》不跳过-》添加进result
                result.add(candidates[i]);
            }
        }
        if (logger.isTraceEnabled()) {
            int numberFiltered = configurations.size() - result.size();
            logger.trace("Filtered " + numberFiltered + " auto configuration class in "
                    + TimeUnit.NANOSECONDS.toMillis(System.nanoTime() - startTime)
                    + " ms");
        }
        return new ArrayList<String>(result);
    }

2.2 SpringApplication.run()静态方法

SpringApplication.run

public ConfigurableApplicationContext run(String... args) {
        StopWatch stopWatch = new StopWatch();
        stopWatch.start();
        ConfigurableApplicationContext context = null;
        FailureAnalyzers analyzers = null;
        configureHeadlessProperty();
        SpringApplicationRunListeners listeners = getRunListeners(args);//1.获取监听器
        listeners.starting();-->启动！
        try {
            ApplicationArguments applicationArguments = new DefaultApplicationArguments(
                    args);
            ConfigurableEnvironment environment = prepareEnvironment(listeners,//2.准备好环境，触发ApplicationEnvironmentPreparedEvent事件
                    applicationArguments);
            Banner printedBanner = printBanner(environment);//打印启动提示字符，默认spring的字符图
            context = createApplicationContext();//实例化一个可配置应用上下文
            analyzers = new FailureAnalyzers(context);
            prepareContext(context, environment, listeners, applicationArguments,//3.准备上下文
                    printedBanner);
            refreshContext(context);//4.刷新上下文
            afterRefresh(context, applicationArguments);//5.刷新上下文后
            listeners.finished(context, null);--关闭！
            stopWatch.stop();
            if (this.logStartupInfo) {
                new StartupInfoLogger(this.mainApplicationClass)
                        .logStarted(getApplicationLog(), stopWatch);
            }
            return context;
        }
        catch (Throwable ex) {
            handleRunFailure(context, listeners, analyzers, ex);
            throw new IllegalStateException(ex);
        }
    }

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

private SpringApplicationRunListeners getRunListeners(String[] args) {
    Class<?>[] types = new Class<?>[] { SpringApplication.class, String[].class };
    return new SpringApplicationRunListeners(logger, getSpringFactoriesInstances(
            SpringApplicationRunListener.class, types, this, args));
}

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

private <T> Collection<? extends T> getSpringFactoriesInstances(Class<T> type,
        Class<?>[] parameterTypes, Object... args) {
    ClassLoader classLoader = Thread.currentThread().getContextClassLoader();
    // 使用Set确保的字符串的唯一性
    Set<String> names = new LinkedHashSet<String>(
            SpringFactoriesLoader.loadFactoryNames(type, classLoader));// 1.载入工厂名称集合
    List<T> instances = createSpringFactoriesInstances(type, parameterTypes,// 2.创建工厂实例
            classLoader, args, names);
    AnnotationAwareOrderComparator.sort(instances);// 排序
    return instances;
}

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

public static List<String> loadFactoryNames(Class<?> factoryClass, ClassLoader classLoader) {
        String factoryClassName = factoryClass.getName();
        try {
            Enumeration<URL> urls = (classLoader != null ? classLoader.getResources(FACTORIES_RESOURCE_LOCATION) :
                    ClassLoader.getSystemResources(FACTORIES_RESOURCE_LOCATION));
            List<String> result = new ArrayList<String>();
            while (urls.hasMoreElements()) {
                URL url = urls.nextElement();
                Properties properties = PropertiesLoaderUtils.loadProperties(new UrlResource(url));
                String factoryClassNames = properties.getProperty(factoryClassName);
                result.addAll(Arrays.asList(StringUtils.commaDelimitedListToStringArray(factoryClassNames)));
            }
            return result;
        }
        catch (IOException ex) {
            throw new IllegalArgumentException("Unable to load [" + factoryClass.getName() +
                    "] factories from location [" + FACTORIES_RESOURCE_LOCATION + "]", ex);
        }
    }

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 names（SpringApplicationRunListener的唯一实现类EventPublishingRunListener）生成"事件发布启动监听器"工厂实例

@SuppressWarnings("unchecked")
    private <T> List<T> createSpringFactoriesInstances(Class<T> type,
            Class<?>[] parameterTypes, ClassLoader classLoader, Object[] args,
            Set<String> names) {
        List<T> instances = new ArrayList<T>(names.size());
        for (String name : names) {
            try {
                Class<?> instanceClass = ClassUtils.forName(name, classLoader);// 利用反射获取类
                Assert.isAssignable(type, instanceClass);
                Constructor<?> constructor = instanceClass
                        .getDeclaredConstructor(parameterTypes);// 得到构造器
                T instance = (T) BeanUtils.instantiateClass(constructor, args);// 根据构造器和参数构造实例
                instances.add(instance);
            }
            catch (Throwable ex) {
                throw new IllegalArgumentException(
                        "Cannot instantiate " + type + " : " + name, ex);
            }
        }
        return instances;
    }

2.准备好环境

构造一个ConfigurableEnvironment,这里不多说。

3.准备上下文

private void prepareContext(ConfigurableApplicationContext context,
            ConfigurableEnvironment environment, SpringApplicationRunListeners listeners,
            ApplicationArguments applicationArguments, Banner printedBanner) {
        context.setEnvironment(environment);
        postProcessApplicationContext(context);//单例一个BeanNameGenerator，把ResourceLoader设置进应用上下文
        applyInitializers(context);//执行初始化器
        listeners.contextPrepared(context);// 监听器执行上下文"已准备好"方法
        if (this.logStartupInfo) {
            logStartupInfo(context.getParent() == null);
            logStartupProfileInfo(context);
        }

        // 添加spring boot特殊单例bean
        context.getBeanFactory().registerSingleton("springApplicationArguments",
                applicationArguments);
        if (printedBanner != null) {
            context.getBeanFactory().registerSingleton("springBootBanner", printedBanner);
        }

        // 载入资源
        Set<Object> sources = getSources();
        Assert.notEmpty(sources, "Sources must not be empty");
        load(context, sources.toArray(new Object[sources.size()]));
        listeners.contextLoaded(context);// 监听器执行"上下文已加载"方法
    }

4.刷新上下文

private void refreshContext(ConfigurableApplicationContext context) {
    refresh(context);//核心类
    if (this.registerShutdownHook) {
        try {
            context.registerShutdownHook();//注册关闭钩子，容器关闭时执行
        }
        catch (AccessControlException ex) {
            // Not allowed in some environments.
        }
    }
}

protected void refresh(ApplicationContext applicationContext) {
    Assert.isInstanceOf(AbstractApplicationContext.class, applicationContext);
    ((AbstractApplicationContext) applicationContext).refresh();
}

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

public void refresh() throws BeansException, IllegalStateException {
        synchronized (this.startupShutdownMonitor) {
            //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。
            prepareRefresh();

            //启动子类的refreshBeanFactory方法.解析xml
            ConfigurableListableBeanFactory beanFactory = obtainFreshBeanFactory();

            //为BeanFactory配置容器特性，例如类加载器、事件处理器等.
            prepareBeanFactory(beanFactory);

            try {
                //设置BeanFactory的后置处理. 空方法，留给子类拓展用。
                postProcessBeanFactory(beanFactory);

                //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.
                invokeBeanFactoryPostProcessors(beanFactory);

                //注册Bean的后处理器, 在Bean创建过程中调用.
                registerBeanPostProcessors(beanFactory);

                //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
                initMessageSource();

                //初始化ApplicationEventMulticaster bean,应用事件广播器
                initApplicationEventMulticaster();

                //初始化其它特殊的Bean， 空方法，留给子类拓展用。
                onRefresh();

                //检查并向容器注册监听器Bean
                registerListeners();

                //实例化所有剩余的(non-lazy-init) 单例Bean.
                finishBeanFactoryInitialization(beanFactory);

                //发布容器事件, 结束refresh过程.
                finishRefresh();
            }

            catch (BeansException ex) {
                if (logger.isWarnEnabled()) {
                    logger.warn("Exception encountered during context initialization - " +
                            "cancelling refresh attempt: " + ex);
                }

                //销毁已经创建的单例Bean, 以避免资源占用.
                destroyBeans();

                //取消refresh操作, 重置active标志.
                cancelRefresh(ex);

                // Propagate exception to caller.
                throw ex;
            }

            finally {
                //重置Spring的核心缓存
                resetCommonCaches();
            }
        }
    }

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。

private void callRunners(ApplicationContext context, ApplicationArguments args) {
        List<Object> runners = new ArrayList<Object>();
        runners.addAll(context.getBeansOfType(ApplicationRunner.class).values());//从上下文中获取ApplicationRunner类型的bean
        runners.addAll(context.getBeansOfType(CommandLineRunner.class).values());//从上下文中获取CommandLineRunner类型的bean
        AnnotationAwareOrderComparator.sort(runners);//排序
        for (Object runner : new LinkedHashSet<Object>(runners)) {
            if (runner instanceof ApplicationRunner) {
                callRunner((ApplicationRunner) runner, args);//执行
            }
            if (runner instanceof CommandLineRunner) {
                callRunner((CommandLineRunner) runner, args);
            }
        }
    }

两个区别在于入参不同，根据实际情况自己选择。

 public interface CommandLineRunner {
     void run(String... args) throws Exception;
 }

 public interface ApplicationRunner {
     void run(ApplicationArguments args) throws Exception;

 }

CommandLineRunner中执行参数是原始的java启动类main方法的String[] args字符串数组参数；ApplicationRunner中的参数经过处理提供一些方法例如:“

List<String> getOptionValues(String name);

根据名称获取值list,java 启动命令中 –foo=bar –foo=baz，则根据foo参数名返回list["bar", "baz"]

三、总结

20210406142558524.jpg

按照前面的分析，Spring-boot容器启动流程总体可划分为2部分：

1）执行注解：扫描指定范围下的bean、载入自动配置类对应的bean加载到IOC容器。

2）man方法中具体SpringAppliocation.run()，全流程贯穿SpringApplicationEvent,有6个子类：

ApplicationEnvironmentPreparedEvent

ApplicationFailedEvent.class

ApplicationPreparedEvent.class

ApplicationReadyEvent.class

ApplicationStartedEvent.class

ApplicationStartingEvent.class

这里用到了很经典的spring事件驱动模型，飞机票：Spring事件驱动模型和观察者模式

类图如下：

20210406142558597.png

如上图，就是一个经典spring 事件驱动模型，包含3种角色：事件发布者、事件、监听者。对应到spring-boot中就是：

1.EventPublishingRunListener这个类封装了事件发布，

2.SpringApplicationEvent是spring-boot中定义的事件（上面说的6种事件），继承自ApplicationEvent（spring中定义的）

3.ApplicationListener 是spring-boot针对上述6种事件的监听者，这里用户也可以自己实现监听者（上述6种事件）来注入spring boot容器启动流程，触发相应的事件。

例如：实现ApplicationListener这个接口，在容器启动完毕时最后一步listener.finished时，如果启动没有异常，就会执行！可以做一些数据初始化之类的操作。

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