SpringBoot源碼分析之---SpringBoot項目啓動類SpringApplication淺析

時間 2019-11-07

標籤 springboot 源碼分析項目啓動 springapplication 淺析欄目 Spring 简体版

原文原文鏈接

源碼版本說明

本文源碼採用版本爲SpringBoot 2.1.0BUILD,對應的SpringFramework 5.1.0.RC1java

注意：本文只是從總體上梳理流程，不作具體深刻分析react

SpringBoot入口類

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

　　這是咱們平常使用springboot開發見到次數最多的引導類了，完成這個類的編寫，就完成了一個springboot項目的框架，springboot就回自動爲咱們完成一些默認配置，並幫咱們初始化上下文容器，但細節咱們是不知道的，下面咱們就一塊兒探索下SpringApplication.run(DemoApplication .class, args);這行代碼背後的故事！git

SpringApplication初始化階段

public SpringApplication(Class<?>... primarySources) {
    this(null, primarySources);
}

// 初始化準備階段
@SuppressWarnings({ "unchecked", "rawtypes" })
public SpringApplication(ResourceLoader resourceLoader, Class<?>... primarySources) {
    this.resourceLoader = resourceLoader;
    Assert.notNull(primarySources, "PrimarySources must not be null");
    // 參數初始化
    this.primarySources = new LinkedHashSet<>(Arrays.asList(primarySources));
    // 推斷應用類型
    this.webApplicationType = deduceWebApplicationType();
    // 加載ApplicationContextInitializer系列初始化器（從spring.factories文件加載，並實例化和排序後存到this.initializers）
    setInitializers((Collection) getSpringFactoriesInstances(
            ApplicationContextInitializer.class));
    // 加載ApplicationListener系列監聽器（從spring.factories文件加載，並實例化和排序後存到this.listeners）
    setListeners((Collection) getSpringFactoriesInstances(ApplicationListener.class));
    // 推斷應用入口類（main函數所在類）
    this.mainApplicationClass = deduceMainApplicationClass();
}複製代碼

推斷應用類型

// 推斷應用類型
this.webApplicationType = deduceWebApplicationType();複製代碼

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;
}複製代碼

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" };複製代碼

根據當前應用ClassPath下是否存在相關類，來肯定應用類型。github

加載`ApplicationContextInitializer`系列初始化器

// 加載ApplicationContextInitializer系列初始化器（從spring.factories文件加載，並實例化和排序後存到this.initializers）
setInitializers((Collection) getSpringFactoriesInstances(
        ApplicationContextInitializer.class));複製代碼

public void setInitializers(
        Collection<? extends ApplicationContextInitializer<?>> initializers) {
    this.initializers = new ArrayList<>();
    this.initializers.addAll(initializers);
}複製代碼

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

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;
}複製代碼

@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;
}複製代碼

　　利用Spring工廠加載機制，實例化ApplicationContextInitializer接口的實現類，被加載的實現類都配置在MATE-INF/spring.factories文件中，getSpringFactoriesInstances(Class type,Class [] parameterTypes, Object... args)這個方法就負責加載配置類並實例化和排序後返回，後面監聽器、異常收集器和Runner等也是經過這個類實現實例化對應實現類的。下面是spring-boot-autoconfiguresrcmainresourcesMETA-INFspring.factories文件的配置內容。web

# 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
複製代碼

加載ApplicationListener系列監聽器

// 加載ApplicationListener系列監聽器（從spring.factories文件加載，並實例化和排序後存到this.listeners）
setListeners((Collection) getSpringFactoriesInstances(ApplicationListener.class));複製代碼

public void setListeners(Collection<? extends ApplicationListener<?>> listeners) {
    this.listeners = new ArrayList<>();
    this.listeners.addAll(listeners);
}複製代碼

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

　　如上面的初始化器，ApplicationListener系列監聽器也是經過`getSpringFactoriesInstances(Class type,Class [] parameterTypes, Object... args)`這個方法完成加載、排序及實例化的，然後存入到`this.listeners`中。 redis

推斷應用入口類

// 推斷應用入口類（main函數所在類）
this.mainApplicationClass = deduceMainApplicationClass();複製代碼

private Class<?> deduceMainApplicationClass() {
    try {
        // 經過new一個運行時異常獲取堆棧信息
        StackTraceElement[] stackTrace = new RuntimeException().getStackTrace();
        for (StackTraceElement stackTraceElement : stackTrace) {
            // 找到main函數所在的入口類
            if ("main".equals(stackTraceElement.getMethodName())) {
                return Class.forName(stackTraceElement.getClassName());
            }
        }
    }
    catch (ClassNotFoundException ex) {
        // Swallow and continue
    }
    return null;
}複製代碼

　　推斷應用入口類這部分比較有意思，他是經過new了一個運行時異常來拿到main線程的堆棧信息，遍歷全部方法找到main方法所在的類。spring

運行階段

// 運行階段
public ConfigurableApplicationContext run(String... args) {
    // 初始化容器啓動計時器
    StopWatch stopWatch = new StopWatch();
    // 開始計時
    stopWatch.start();
    // 初始化上下文ConfigurableApplicationContext
    ConfigurableApplicationContext context = null;
    // 初始化異常收集器
    Collection<SpringBootExceptionReporter> exceptionReporters = new ArrayList<>();
    // 配置系統參數"java.awt.headless"
    configureHeadlessProperty();
    // 獲取SpringApplicationRunListener系列監聽器（從spring.factories文件加載，並實例化和排序）
    SpringApplicationRunListeners listeners = getRunListeners(args);
    // 遍歷全部SpringApplicationRunListener系列監聽器，廣播ApplicationStartingEvent
    listeners.starting();
    try {
        // 處理args參數
        ApplicationArguments applicationArguments = new DefaultApplicationArguments(
                args);
        // 準備環境（建立、配置、綁定環境、廣播ApplicationEnvironmentPreparedEvent）
        ConfigurableEnvironment environment = prepareEnvironment(listeners,
                applicationArguments);
        configureIgnoreBeanInfo(environment);
        // 打印Banner
        Banner printedBanner = printBanner(environment);
        // 根據應用類型建立上下文
        context = createApplicationContext();
        // 獲取SpringBootExceptionReporter系列異常收集器（從spring.factories文件加載，並實例化和排序）
        exceptionReporters = getSpringFactoriesInstances(
                SpringBootExceptionReporter.class,
                new Class[] { ConfigurableApplicationContext.class }, context);
        // 上下文前置處理（執行ApplicationContextInitializer系列初始化器、加載資源、廣播ApplicationPreparedEvent）
        prepareContext(context, environment, listeners, applicationArguments,
                printedBanner);
        // 刷新上下文（）
        refreshContext(context);
        // 上下文後置處理(目前啥也沒幹)
        afterRefresh(context, applicationArguments);
        // 啓動完成，打印用時
        stopWatch.stop();
        if (this.logStartupInfo) {
            new StartupInfoLogger(this.mainApplicationClass)
                    .logStarted(getApplicationLog(), stopWatch);
        }
        // 遍歷前面設置的ConfigurableApplicationContext監聽器，發佈ApplicationStartedEvent
        listeners.started(context);
        // 按順序回調實現了ApplicationRunner或CommandLineRunner接口的Runners
        callRunners(context, applicationArguments);
    }
    catch (Throwable ex) {
        // 處理異常（發佈ExitCodeEvent和ApplicationFailedEvent事件、異常收集器處理異常）
        handleRunFailure(context, ex, exceptionReporters, listeners);
        throw new IllegalStateException(ex);
    }

    try {
        // 遍歷前面設置好的SpringApplicationRunListener監聽器，發佈ApplicationReadyEvent
        listeners.running(context);
    }
    catch (Throwable ex) {
        handleRunFailure(context, ex, exceptionReporters, null);
        throw new IllegalStateException(ex);
    }
    return context;
}複製代碼

獲取`SpringApplicationRunListener`系列監聽器

// 獲取SpringApplicationRunListener系列監聽器（從spring.factories文件加載，並實例化和排序）
SpringApplicationRunListeners listeners = getRunListeners(args);複製代碼

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

　　這裏咱們再一次見到getSpringFactoriesInstances(Class type,Class [] parameterTypes, Object... args)這個方法，功能同上。json

廣播`ApplicationStartingEvent`事件

// 遍歷全部SpringApplicationRunListener系列監聽器，廣播ApplicationStartingEvent
listeners.starting();複製代碼

public void starting() {
    for (SpringApplicationRunListener listener : this.listeners) {
        listener.starting();
    }
}複製代碼

public void starting() {
    this.initialMulticaster.multicastEvent(
            new ApplicationStartingEvent(this.application, this.args));
}複製代碼

　　這裏會遍歷上面拿到的排序好的全部SpringApplicationRunListener系列監聽器，廣播ApplicationStartingEvent事件，這表明Spring應用開始啓動，在這以前只進行了註冊化初始化器和監聽器。springboot

準備環境、廣播`ApplicationEnvironmentPreparedEvent`事件

// 準備環境（建立、配置、綁定環境、廣播ApplicationEnvironmentPreparedEvent）
ConfigurableEnvironment environment = prepareEnvironment(listeners,
        applicationArguments);複製代碼

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;
}複製代碼

public void environmentPrepared(ConfigurableEnvironment environment) {
    for (SpringApplicationRunListener listener : this.listeners) {
        listener.environmentPrepared(environment);
    }
}複製代碼

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

　　這裏開始建立、配置和綁定ConfigurableEnvironment環境，環境準備好以後開始遍歷SpringApplicationRunListener系列監聽器，廣播ApplicationEnvironmentPreparedEvent事件，表明環境已準備好。websocket

開始打印Banner

// 打印Banner
Banner printedBanner = printBanner(environment);複製代碼

private Banner printBanner(ConfigurableEnvironment environment) {
    // Mode.OFF：不打印banner
    if (this.bannerMode == Banner.Mode.OFF) {
        return null;
    }
    // 加載banner資源，若是自定義了banner樣式，在這裏加載，不然加載默認banner
    ResourceLoader resourceLoader = (this.resourceLoader != null)
            ? this.resourceLoader : new DefaultResourceLoader(getClassLoader());
    // 初始化bannerPrinter
    SpringApplicationBannerPrinter bannerPrinter = new SpringApplicationBannerPrinter(
            resourceLoader, this.banner);
    // Mode.LOG：經過日誌打印banner
    if (this.bannerMode == Mode.LOG) {
        return bannerPrinter.print(environment, this.mainApplicationClass, logger);
    }
    // 默認經過控制檯打印banner
    return bannerPrinter.print(environment, this.mainApplicationClass, System.out);
}複製代碼

　　關於Banner，SpringBoot支持關閉banner打印、打印到log日誌和打印到system日誌三種方式；同時支持自定義banner，自定義banner又有圖片和txt文本兩種（同時存在時先打印圖片banner，在打印文本banner），圖片banner又支持gif, jpg, png這三種類型的圖片格式banner（git優先於jpg優先於png），自定義banner很是簡單，只須要將banner文件放到classpath:下就行了（resources目錄下），若是存在多個banner文件，想指定某一個文件，只須要在application.properties文件加入以下配置就行了，很是方便。

spring.banner.image.location=banner.png
spring.banner.location=banner.txt複製代碼

根據應用類型建立上下文

// 根據應用類型建立上下文
context = createApplicationContext();複製代碼

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);
}複製代碼

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";複製代碼

　　開始建立ConfigurableApplicationContext上下文，其中Servlet類型的web應用會建立AnnotationConfigServletWebServerApplicationContext類型的上下文，Reactive類型的web應用會建立AnnotationConfigReactiveWebServerApplicationContext類型的上下文，非web應用會建立AnnotationConfigApplicationContext類型的上下文。

獲取`SpringBootExceptionReporter`系列異常收集器

// 獲取SpringBootExceptionReporter系列異常收集器（從spring.factories文件加載，並實例化和排序）
exceptionReporters = getSpringFactoriesInstances(
        SpringBootExceptionReporter.class,
        new Class[] { ConfigurableApplicationContext.class }, context);複製代碼

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;
}複製代碼

　　能夠看到，加載異常收集器與上面初始化器和監聽器一模一樣，不作過多闡述。

上下文前置處理、廣播`ApplicationPreparedEvent`事件

// 上下文前置處理（執行ApplicationContextInitializer系列初始化器、加載資源、廣播ApplicationPreparedEvent）
prepareContext(context, environment, listeners, applicationArguments,
            printedBanner);複製代碼

private void prepareContext(ConfigurableApplicationContext context,
        ConfigurableEnvironment environment, SpringApplicationRunListeners listeners,
        ApplicationArguments applicationArguments, Banner printedBanner) {
    // 關聯上下文和環境
    context.setEnvironment(environment);
    //
    postProcessApplicationContext(context);
    // 去重並排序前面獲取好的ApplicationContextInitializer初始化器，執行初始化
    applyInitializers(context);
    // 遍歷前面設置好的SpringApplicationRunListener，但並無發佈（目前什麼都沒作，貌似爲了之後擴展）
    listeners.contextPrepared(context);
    if (this.logStartupInfo) {
        logStartupInfo(context.getParent() == null);
        logStartupProfileInfo(context);
    }

    // Add boot specific singleton beans
    // 添加啓動特定的單例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
    // 加載sources資源
    Set<Object> sources = getAllSources();
    Assert.notEmpty(sources, "Sources must not be empty");

    load(context, sources.toArray(new Object[0]));
    // 廣播ApplicationPreparedEvent
    listeners.contextLoaded(context);
}複製代碼

　　在啓動上下文以前，會調用前面初始化好的ApplicationContextInitializer接口實現類，固然也包含咱們自定義的，因此咱們能夠自定義初始化器，在上下文啓動前作一些操做。以後會廣播ApplicationPreparedEvent事件，通知SpringApplicationRunListener監聽器ConfigurableApplicationContext上下文已準備好（框架中用listeners.contextLoaded(context);方法廣播了ApplicationPreparedEvent事件，而ApplicationLoadedEvent事件並無發佈，感受這裏之後還會變更。。。）。

刷新上下文

// 刷新上下文（）
refreshContext(context);複製代碼

private void refreshContext(ConfigurableApplicationContext context) {
    refresh(context);
    if (this.registerShutdownHook) {
        try {
            context.registerShutdownHook();
        }
        catch (AccessControlException ex) {
            // Not allowed in some environments.
        }
    }
}複製代碼

發佈`ApplicationStartedEvent`事件

// 遍歷前面設置的ConfigurableApplicationContext監聽器，發佈ApplicationStartedEvent
listeners.started(context);複製代碼

public void started(ConfigurableApplicationContext context) {
    for (SpringApplicationRunListener listener : this.listeners) {
        listener.started(context);
    }
}複製代碼

@Override
public void started(ConfigurableApplicationContext context) {
    context.publishEvent(
            new ApplicationStartedEvent(this.application, this.args, context));
}複製代碼

　　發佈ApplicationStartedEvent事件，通知　SpringApplicationRunListener系列監聽器ConfigurableApplicationContext上下文已啓動完成，Spring Bean 已初始化完成。

回調實現了`ApplicationRunner`或`CommandLineRunner`接口的`Runners`

// 按順序回調實現了ApplicationRunner或CommandLineRunner接口的Runners
callRunners(context, applicationArguments);複製代碼

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);
        }
    }
}複製代碼

　　這裏是有順序的，ApplicationRunner的實現類優先於CommandLineRunner的實現類被回調

發佈`ApplicationReadyEvent`事件

try {
    // 遍歷前面設置好的SpringApplicationRunListener監聽器，發佈ApplicationReadyEvent
    listeners.running(context);
}複製代碼

public void running(ConfigurableApplicationContext context) {
    for (SpringApplicationRunListener listener : this.listeners) {
        listener.running(context);
    }
}複製代碼

@Override
public void running(ConfigurableApplicationContext context) {
    context.publishEvent(
            new ApplicationReadyEvent(this.application, this.args, context));
}複製代碼

　　發佈ApplicationReadyEvent事件,通知SpringApplicationRunListener系列監聽器ConfigurableApplicationContext上下文已經在運行，整個容器已經準備好。

發佈`ExitCodeEvent`和`ApplicationFailedEvent`事件和異常收集器收集異常信息

catch (Throwable ex) {
    handleRunFailure(context, ex, exceptionReporters, listeners);
    throw new IllegalStateException(ex);
}複製代碼

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);
}複製代碼

　　若是啓動過程當中出現異常，springboot將會發布ExitCodeEvent事件通知上下文中止或重啓，併發布ApplicationFailedEvent事件通知SpringApplicationRunListener系列監聽器，最後SpringBootExceptionReporter異常收集器收集打印異常。

總結

SpringBoot啓動過程大體脈絡

準備階段
- 參數初始化
- 推斷應用類型
- 加載ApplicationContextInitializer系列初始化器
- 加載ApplicationListener系列監聽器
- 推斷應用入口類（main函數所在類）
運行階段
- 初始化容器啓動計時器，開始計時
- 初始化上下文ConfigurableApplicationContext、異常收集器
- 配置系統參數"java.awt.headless"
- 獲取SpringApplicationRunListener系列監聽器
- 遍歷全部SpringApplicationRunListener系列監聽器，廣播ApplicationStartingEvent
- 處理args參數
- 準備環境（建立、配置、綁定環境、廣播ApplicationEnvironmentPreparedEvent）
- 配置忽略Bean信息
- 打印Banner
- 根據應用類型建立上下文
- 獲取SpringBootExceptionReporter系列異常收集器
- 上下文前置處理（執行ApplicationContextInitializer系列初始化器、加載資源、廣播ApplicationPreparedEvent）
- 刷新上下文
- 啓動完成，打印用時
- 遍歷前面設置的ConfigurableApplicationContext監聽器，發佈ApplicationStartedEvent
- 回調實現了ApplicationRunner或CommandLineRunner接口的Runners
- 遍歷前面設置好的SpringApplicationRunListener監聽器，發佈ApplicationReadyEvent

　　至此，整個SpringBoot項目已經啓動完成，咱們能夠看到，整個過程當中Spring的事件驅動機制起着舉足輕重的做用，有了這個機制咱們能夠知曉容器的啓動過程，而且能夠監聽到某些事件，對容器中咱們關心的實例作進一步處理，咱們深刻理解事件驅動機制頗有必要，它將幫助咱們更好的理解和使用這個Spring框架體系。若是想要文中中文版SpringBoot註釋源碼，能夠在個人github下載，若是發現哪裏寫的不對，煩請留言通知我。