SpringBoot源碼分析之---SpringBoot項(xiàng)目啟動(dòng)類SpringApplication淺析

源碼版本說(shuō)明

本文源碼采用版本為SpringBoot 2.1.0BUILD,對(duì)應(yīng)的SpringFramework 5.1.0.RC1

注意：本文只是從整體上梳理流程，不做具體深入分析

SpringBoot入口類

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

這是我們?nèi)粘Ｊ褂胹pringboot開發(fā)見到次數(shù)最多的引導(dǎo)類了，完成這個(gè)類的編寫，就完成了一個(gè)springboot項(xiàng)目的框架，springboot就回自動(dòng)為我們完成一些默認(rèn)配置，并幫我們初始化上下文容器，但細(xì)節(jié)我們是不知道的，下面我們就一起探索下SpringApplication.run(DemoApplication .class, args);這行代碼背后的故事！

SpringApplication初始化階段

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

// 初始化準(zhǔn)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(shù)初始化
    this.primarySources = new LinkedHashSet<>(Arrays.asList(primarySources));
    // 推斷應(yīng)用類型
    this.webApplicationType = deduceWebApplicationType();
    // 加載ApplicationContextInitializer系列初始化器（從spring.factories文件加載，并實(shí)例化和排序后存到this.initializers）
    setInitializers((Collection) getSpringFactoriesInstances(
            ApplicationContextInitializer.class));
    // 加載ApplicationListener系列監(jiān)聽器（從spring.factories文件加載，并實(shí)例化和排序后存到this.listeners）
    setListeners((Collection) getSpringFactoriesInstances(ApplicationListener.class));
    // 推斷應(yīng)用入口類（main函數(shù)所在類）
    this.mainApplicationClass = deduceMainApplicationClass();
}

推斷應(yīng)用類型

// 推斷應(yīng)用類型
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" };

根據(jù)當(dāng)前應(yīng)用ClassPath下是否存在相關(guān)類，來(lái)確定應(yīng)用類型。

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，并實(shí)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工廠加載機(jī)制，實(shí)例化ApplicationContextInitializer接口的實(shí)現(xiàn)類，被加載的實(shí)現(xiàn)類都配置在MATE-INF/spring.factories文件中，getSpringFactoriesInstances(Class<T> type,Class<?>[] parameterTypes, Object... args)這個(gè)方法就負(fù)責(zé)加載配置類并實(shí)例化和排序后返回，后面監(jiān)聽器、異常收集器和Runner等也是通過(guò)這個(gè)類實(shí)現(xiàn)實(shí)例化對(duì)應(yīng)實(shí)現(xiàn)類的。下面是spring-boot-autoconfigure\src\main\resources\META-INF\spring.factories文件的配置內(nèi)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

加載ApplicationListener系列監(jiān)聽器

// 加載ApplicationListener系列監(jiān)聽器（從spring.factories文件加載，并實(shí)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系列監(jiān)聽器也是通過(guò)getSpringFactoriesInstances(Class<T> type, Class<?>[] parameterTypes, Object... args)這個(gè)方法完成加載、排序及實(shí)例化的，而后存入到this.listeners中。

推斷應(yīng)用入口類

// 推斷應(yīng)用入口類（main函數(shù)所在類）
this.mainApplicationClass = deduceMainApplicationClass();

private Class<?> deduceMainApplicationClass() {
    try {
        // 通過(guò)new一個(gè)運(yùn)行時(shí)異常獲取堆棧信息
        StackTraceElement[] stackTrace = new RuntimeException().getStackTrace();
        for (StackTraceElement stackTraceElement : stackTrace) {
            // 找到main函數(shù)所在的入口類
            if ("main".equals(stackTraceElement.getMethodName())) {
                return Class.forName(stackTraceElement.getClassName());
            }
        }
    }
    catch (ClassNotFoundException ex) {
        // Swallow and continue
    }
    return null;
}

推斷應(yīng)用入口類這部分比較有意思，他是通過(guò)new了一個(gè)運(yùn)行時(shí)異常來(lái)拿到main線程的堆棧信息，遍歷所有方法找到main方法所在的類。

運(yùn)行階段

// 運(yùn)行階段
public ConfigurableApplicationContext run(String... args) {
    // 初始化容器啟動(dòng)計(jì)時(shí)器
    StopWatch stopWatch = new StopWatch();
    // 開始計(jì)時(shí)
    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(tǒng)參數(shù)"java.awt.headless"
    configureHeadlessProperty();
    // 獲取SpringApplicationRunListener系列監(jiān)聽器（從spring.factories文件加載，并實(shí)例化和排序）
    SpringApplicationRunListeners listeners = getRunListeners(args);
    // 遍歷所有SpringApplicationRunListener系列監(jiān)聽器，廣播ApplicationStartingEvent
    listeners.starting();
    try {
        // 處理args參數(shù)
        ApplicationArguments applicationArguments = new DefaultApplicationArguments(
                args);
        // 準(zhǔn)備環(huán)境（創(chuàng)建、配置、綁定環(huán)境、廣播ApplicationEnvironmentPreparedEvent）
        ConfigurableEnvironment environment = prepareEnvironment(listeners,
                applicationArguments);
        configureIgnoreBeanInfo(environment);
        // 打印Banner
        Banner printedBanner = printBanner(environment);
        // 根據(jù)應(yīng)用類型創(chuàng)建上下文
        context = createApplicationContext();
        // 獲取SpringBootExceptionReporter系列異常收集器（從spring.factories文件加載，并實(shí)例化和排序）
        exceptionReporters = getSpringFactoriesInstances(
                SpringBootExceptionReporter.class,
                new Class[] { ConfigurableApplicationContext.class }, context);
        // 上下文前置處理（執(zhí)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);
        // 上下文后置處理(目前啥也沒(méi)干)
        afterRefresh(context, applicationArguments);
        // 啟動(dòng)完成，打印用時(shí)
        stopWatch.stop();
        if (this.logStartupInfo) {
            new StartupInfoLogger(this.mainApplicationClass)
                    .logStarted(getApplicationLog(), stopWatch);
        }
        // 遍歷前面設(shè)置的ConfigurableApplicationContext監(jiān)聽器，發(fā)布ApplicationStartedEvent
        listeners.started(context);
        // 按順序回調(diào)實(shí)現(xiàn)了ApplicationRunner或CommandLineRunner接口的Runners
        callRunners(context, applicationArguments);
    }
    catch (Throwable ex) {
        // 處理異常（發(fā)布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(shè)置好的SpringApplicationRunListener監(jiān)聽器，發(fā)布ApplicationReadyEvent
        listeners.running(context);
    }
    catch (Throwable ex) {
        handleRunFailure(context, ex, exceptionReporters, null);
        throw new IllegalStateException(ex);
    }
    return context;
}

獲取SpringApplicationRunListener系列監(jiān)聽器

// 獲取SpringApplicationRunListener系列監(jiān)聽器（從spring.factories文件加載，并實(shí)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));
}

這里我們?cè)僖淮我姷?code>getSpringFactoriesInstances(Class<T> type,Class<?>[] parameterTypes, Object... args)這個(gè)方法，功能同上。

廣播ApplicationStartingEvent事件

// 遍歷所有SpringApplicationRunListener系列監(jiān)聽器，廣播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));
}

這里會(huì)遍歷上面拿到的排序好的所有SpringApplicationRunListener系列監(jiān)聽器，廣播ApplicationStartingEvent事件，這代表Spring應(yīng)用開始啟動(dòng)，在這之前只進(jìn)行了注冊(cè)化初始化器和監(jiān)聽器。

準(zhǔn)備環(huán)境、廣播ApplicationEnvironmentPreparedEvent事件

// 準(zhǔn)備環(huán)境（創(chuàng)建、配置、綁定環(huán)境、廣播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));
}

這里開始創(chuàng)建、配置和綁定ConfigurableEnvironment環(huán)境，環(huán)境準(zhǔn)備好之后開始遍歷SpringApplicationRunListener系列監(jiān)聽器，廣播ApplicationEnvironmentPreparedEvent事件，代表環(huán)境已準(zhǔn)備好。

開始打印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;
    }
    // 加載banner資源，如果自定義了banner樣式，在這里加載，否則加載默認(rèn)banner
    ResourceLoader resourceLoader = (this.resourceLoader != null)
            ? this.resourceLoader : new DefaultResourceLoader(getClassLoader());
    // 初始化bannerPrinter
    SpringApplicationBannerPrinter bannerPrinter = new SpringApplicationBannerPrinter(
            resourceLoader, this.banner);
    // Mode.LOG：通過(guò)日志打印banner
    if (this.bannerMode == Mode.LOG) {
        return bannerPrinter.print(environment, this.mainApplicationClass, logger);
    }
    // 默認(rèn)通過(guò)控制臺(tái)打印banner
    return bannerPrinter.print(environment, this.mainApplicationClass, System.out);
}

關(guān)于Banner，SpringBoot支持關(guān)閉banner打印、打印到log日志和打印到system日志三種方式；同時(shí)支持自定義banner，自定義banner又有圖片和txt文本兩種（同時(shí)存在時(shí)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(yōu)先于jpg優(yōu)先于png），自定義banner非常簡(jiǎn)單，只需要將banner文件放到classpath:下就好了（resources目錄下），如果存在多個(gè)banner文件，想指定某一個(gè)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

根據(jù)應(yīng)用類型創(chuàng)建上下文

// 根據(jù)應(yīng)用類型創(chuàng)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";

開始創(chuàng)建ConfigurableApplicationContext上下文，其中Servlet類型的web應(yīng)用會(huì)創(chuàng)建AnnotationConfigServletWebServerApplicationContext類型的上下文，Reactive類型的web應(yīng)用會(huì)創(chuàng)建AnnotationConfigReactiveWebServerApplicationContext類型的上下文，非web應(yīng)用會(huì)創(chuàng)建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，并實(shí)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，加載異常收集器與上面初始化器和監(jiān)聽器如出一轍，不做過(guò)多闡述。

上下文前置處理、廣播ApplicationPreparedEvent事件

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

private void prepareContext(ConfigurableApplicationContext context,
        ConfigurableEnvironment environment, SpringApplicationRunListeners listeners,
        ApplicationArguments applicationArguments, Banner printedBanner) {
    // 關(guān)聯(lián)上下文和環(huán)境
    context.setEnvironment(environment);
    //
    postProcessApplicationContext(context);
    // 去重并排序前面獲取好的ApplicationContextInitializer初始化器，執(zhí)行初始化
    applyInitializers(context);
    // 遍歷前面設(shè)置好的SpringApplicationRunListener，但并沒(méi)有發(fā)布（目前什么都沒(méi)做，貌似為了以后擴(kuò)展）
    listeners.contextPrepared(context);
    if (this.logStartupInfo) {
        logStartupInfo(context.getParent() == null);
        logStartupProfileInfo(context);
    }

    // Add boot specific singleton beans
    // 添加啟動(dòng)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);
}

在啟動(dòng)上下文之前，會(huì)調(diào)用前面初始化好的ApplicationContextInitializer接口實(shí)現(xiàn)類，當(dāng)然也包含我們自定義的，所以我們可以自定義初始化器，在上下文啟動(dòng)前做一些操作。之后會(huì)廣播ApplicationPreparedEvent事件，通知SpringApplicationRunListener監(jiān)聽器ConfigurableApplicationContext上下文已準(zhǔn)備好（框架中用listeners.contextLoaded(context);方法廣播了ApplicationPreparedEvent事件，而ApplicationLoadedEvent事件并沒(méi)有發(fā)布，感覺(jué)這里以后還會(huì)變動(dòng)。。。）。

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.
        }
    }
}

發(fā)布ApplicationStartedEvent事件

// 遍歷前面設(shè)置的ConfigurableApplicationContext監(jiān)聽器，發(fā)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));
}

發(fā)布ApplicationStartedEvent事件，通知　SpringApplicationRunListener系列監(jiān)聽器ConfigurableApplicationContext上下文已啟動(dòng)完成，Spring Bean 已初始化完成。

回調(diào)實(shí)現(xiàn)了ApplicationRunner或CommandLineRunner接口的Runners 

// 按順序回調(diào)實(shí)現(xiàn)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);
        }
    }
}

這里是有順序的，ApplicationRunner的實(shí)現(xiàn)類優(yōu)先于CommandLineRunner的實(shí)現(xiàn)類被回調(diào)

發(fā)布ApplicationReadyEvent事件

try {
    // 遍歷前面設(shè)置好的SpringApplicationRunListener監(jiān)聽器，發(fā)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));
}

發(fā)布ApplicationReadyEvent事件,通知SpringApplicationRunListener系列監(jiān)聽器ConfigurableApplicationContext上下文已經(jīng)在運(yùn)行，整個(gè)容器已經(jīng)準(zhǔn)備好。

發(fā)布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);
}

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);
}

如果啟動(dòng)過(guò)程中出現(xiàn)異常，springboot將會(huì)發(fā)布ExitCodeEvent事件通知上下文停止或重啟，并發(fā)布ApplicationFailedEvent事件通知SpringApplicationRunListener系列監(jiān)聽器，最后SpringBootExceptionReporter異常收集器收集打印異常。

總結(jié)

SpringBoot啟動(dòng)過(guò)程大致脈絡(luò)

  • 準(zhǔn)備階段
    • 參數(shù)初始化
    • 推斷應(yīng)用類型
    • 加載ApplicationContextInitializer系列初始化器
    • 加載ApplicationListener系列監(jiān)聽器
    • 推斷應(yīng)用入口類（main函數(shù)所在類）
  • 運(yùn)行階段
    • 初始化容器啟動(dòng)計(jì)時(shí)器，開始計(jì)時(shí)
    • 初始化上下文ConfigurableApplicationContext、異常收集器
    • 配置系統(tǒng)參數(shù)"java.awt.headless"
    • 獲取SpringApplicationRunListener系列監(jiān)聽器
    • 遍歷所有SpringApplicationRunListener系列監(jiān)聽器，廣播ApplicationStartingEvent
    • 處理args參數(shù)
    • 準(zhǔn)備環(huán)境（創(chuàng)建、配置、綁定環(huán)境、廣播ApplicationEnvironmentPreparedEvent）
    • 配置忽略Bean信息
    • 打印Banner
    • 根據(jù)應(yīng)用類型創(chuàng)建上下文
    • 獲取SpringBootExceptionReporter系列異常收集器
    • 上下文前置處理（執(zhí)行ApplicationContextInitializer系列初始化器、加載資源、廣播ApplicationPreparedEvent）
    • 刷新上下文
    • 啟動(dòng)完成，打印用時(shí)
    • 遍歷前面設(shè)置的ConfigurableApplicationContext監(jiān)聽器，發(fā)布ApplicationStartedEvent
    • 回調(diào)實(shí)現(xiàn)了ApplicationRunner或CommandLineRunner接口的Runners
    • 遍歷前面設(shè)置好的SpringApplicationRunListener監(jiān)聽器，發(fā)布ApplicationReadyEvent

至此，整個(gè)SpringBoot項(xiàng)目已經(jīng)啟動(dòng)完成，我們可以看到，整個(gè)過(guò)程中Spring的事件驅(qū)動(dòng)機(jī)制起著舉足輕重的作用，有了這個(gè)機(jī)制我們可以知曉容器的啟動(dòng)過(guò)程，并且可以監(jiān)聽到某些事件，對(duì)容器中我們關(guān)心的實(shí)例做進(jìn)一步處理，我們深入理解事件驅(qū)動(dòng)機(jī)制很有必要，它將幫助我們更好的理解和使用這個(gè)Spring框架體系。如果想要文中中文版SpringBoot注釋源碼，可以在我的github下載，如果發(fā)現(xiàn)哪里寫的不對(duì)，煩請(qǐng)留言通知我。

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