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-autoconfigure\src\main\resources\META-INF\spring.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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SpringBoot源码分析之---SpringBoot项目启动类SpringApplication浅析