入口

@SpringBootApplication
public class DemoApplication {

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

}

以上是一个最简单的Springboot程序（2.0.3版本）示例，也是我们最通用的写法，但其中其实封装这一系列复杂的功能操作，让我们开始逐步进行分析。

首先这里最重要的必然是注解@SpringBootApplication

@SpringBootApplication注解

@Target(ElementType.TYPE)
@Retention(RetentionPolicy.RUNTIME)
@Documented
@Inherited
@SpringBootConfiguration
@EnableAutoConfiguration
@ComponentScan(excludeFilters = { @Filter(type = FilterType.CUSTOM, classes = TypeExcludeFilter.class),
        @Filter(type = FilterType.CUSTOM, classes = AutoConfigurationExcludeFilter.class) })
public @interface SpringBootApplication {

    @AliasFor(annotation = EnableAutoConfiguration.class)
    Class<?>[] exclude() default {};

    @AliasFor(annotation = EnableAutoConfiguration.class)
    String[] excludeName() default {};

    @AliasFor(annotation = ComponentScan.class, attribute = "basePackages")
    String[] scanBasePackages() default {};


    @AliasFor(annotation = ComponentScan.class, attribute = "basePackageClasses")
    Class<?>[] scanBasePackageClasses() default {};

}

@SpringBootApplication注解由几个注解复合组成，其中最主要的就是@SpringBootConfiguration、@EnableAutoConfiguration和@ComponentScan这三个。

@SpringBootConfiguration

@Target(ElementType.TYPE)
@Retention(RetentionPolicy.RUNTIME)
@Documented
@Configuration
public @interface SpringBootConfiguration {

}

其中的@ComponentScan是spring的原生注解，@SpringBootConfiguration虽然是springboot中的注解，但其实质就是包装后的@Configuration，仍然是spring中的注解，用于代替xml的方式管理配置bean

@EnableAutoConfiguration

@Target(ElementType.TYPE)
@Retention(RetentionPolicy.RUNTIME)
@Documented
@Inherited
@AutoConfigurationPackage
@Import(AutoConfigurationImportSelector.class)
public @interface EnableAutoConfiguration {

    String ENABLED_OVERRIDE_PROPERTY = "spring.boot.enableautoconfiguration";

    /**
     * Exclude specific auto-configuration classes such that they will never be applied.
     * @return the classes to exclude
     */
    Class<?>[] exclude() default {};

    /**
     * Exclude specific auto-configuration class names such that they will never be
     * applied.
     * @return the class names to exclude
     * @since 1.3.0
     */
    String[] excludeName() default {};

}

@EnableAutoConfiguration的定义如上，这里最重要的注解是@Import（@AutoConfigurationPackage注解的实现也是基于@Import），借助@Import的帮助，将所有符合自动配置条件的bean定义加载到IoC容器中。关于@EnableAutoConfiguration注解后续涉及到时会再详细说明。这里我们先回到启动类的run方法从头分析初始化流程。

run方法

    public static ConfigurableApplicationContext run(Class<?> primarySource, String... args) {
        return run(new Class<?>[] { primarySource }, args);
    }

    public static ConfigurableApplicationContext run(Class<?>[] primarySources, String[] args) {
        return new SpringApplication(primarySources).run(args);
    }

可以看到'run'方法最终调用的是new SpringApplication(primarySources).run(args)，这里首先创建了SpringApplication对象，然后调用其run方法

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

public SpringApplication(ResourceLoader resourceLoader, Class<?>... primarySources) {
        this.resourceLoader = resourceLoader;
        Assert.notNull(primarySources, "PrimarySources must not be null");
        this.primarySources = new LinkedHashSet<>(Arrays.asList(primarySources));
        this.webApplicationType = deduceWebApplicationType();
        setInitializers((Collection) getSpringFactoriesInstances(
                ApplicationContextInitializer.class));
        setListeners((Collection) getSpringFactoriesInstances(ApplicationListener.class));
        this.mainApplicationClass = deduceMainApplicationClass();
    }

这里主要是为SpringApplication对象进行初始化，这里要专门提一下的是webApplicationType和getSpringFactoriesInstances。

webApplicationType

它用来标识我们的应用是什么类型的应用，来看一下deduceWebApplicationType()方法的实现

    private WebApplicationType deduceWebApplicationType() {
        if (ClassUtils.isPresent(REACTIVE_WEB_ENVIRONMENT_CLASS, null)
                && !ClassUtils.isPresent(MVC_WEB_ENVIRONMENT_CLASS, null)) {
            return WebApplicationType.REACTIVE;
        }
        for (String className : WEB_ENVIRONMENT_CLASSES) {
            if (!ClassUtils.isPresent(className, null)) {
                return WebApplicationType.NONE;
            }
        }
        return WebApplicationType.SERVLET;
    }

其返回值是WebApplicationType类型的枚举类，其值有NONE、SERVLET、REACTIVE三种，分别对应非WEB应用，基于servlet的WEB应用和基于reactive的WEB应用。

getSpringFactoriesInstances

    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;
    }

这里的核心是SpringFactoriesLoader.loadFactoryNames(type, classLoader)方法，来看一下

    public static List<String> loadFactoryNames(Class<?> factoryClass, @Nullable ClassLoader classLoader) {
        String factoryClassName = factoryClass.getName();
        return loadSpringFactories(classLoader).getOrDefault(factoryClassName, Collections.emptyList());
    }

重点关注一下loadSpringFactories(classLoader)做了什么

    private static Map<String, List<String>> loadSpringFactories(@Nullable ClassLoader classLoader) {
        MultiValueMap<String, String> result = cache.get(classLoader);
        if (result != null) {
            return result;
        }

        try {
            Enumeration<URL> urls = (classLoader != null ?
                    classLoader.getResources(FACTORIES_RESOURCE_LOCATION) :
                    ClassLoader.getSystemResources(FACTORIES_RESOURCE_LOCATION));
            result = new LinkedMultiValueMap<>();
            while (urls.hasMoreElements()) {
                URL url = urls.nextElement();
                UrlResource resource = new UrlResource(url);
                Properties properties = PropertiesLoaderUtils.loadProperties(resource);
                for (Map.Entry<?, ?> entry : properties.entrySet()) {
                    List<String> factoryClassNames = Arrays.asList(
                            StringUtils.commaDelimitedListToStringArray((String) entry.getValue()));
                    result.addAll((String) entry.getKey(), factoryClassNames);
                }
            }
            cache.put(classLoader, result);
            return result;
        }
        catch (IOException ex) {
            throw new IllegalArgumentException("Unable to load factories from location [" +
                    FACTORIES_RESOURCE_LOCATION + "]", ex);
        }
    }

这里的FACTORIES_RESOURCE_LOCATION定义为META-INF/spring.factories，因此该方法会扫描所有包下的该文件，将其解析成map对象并缓存到cache中以避免重复加载，springboot包下该文件的部分片段如下

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

# Application Listeners
org.springframework.context.ApplicationListener=\
org.springframework.boot.ClearCachesApplicationListener,\
org.springframework.boot.builder.ParentContextCloserApplicationListener,\
org.springframework.boot.context.FileEncodingApplicationListener,\
org.springframework.boot.context.config.AnsiOutputApplicationListener,\
org.springframework.boot.context.config.ConfigFileApplicationListener,\
org.springframework.boot.context.config.DelegatingApplicationListener,\
org.springframework.boot.context.logging.ClasspathLoggingApplicationListener,\
org.springframework.boot.context.logging.LoggingApplicationListener,\
org.springframework.boot.liquibase.LiquibaseServiceLocatorApplicationListener

从这里可以看出，setInitializers((Collection) getSpringFactoriesInstances( ApplicationContextInitializer.class))和setListeners((Collection) getSpringFactoriesInstances(ApplicationListener.class));分别对应设置的是上述这些类。

解析完成后调用createSpringFactoriesInstances(type, parameterTypes, classLoader, args, names)处理解析结果，生成对应的实例，源码如下

    @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;
    }

这里的核心是通过ClassUtils.forName(name, classLoader)方法，以反射的方式生成类实例instanceClass。由此可以看出SpringFactoriesLoader.loadFactoryNames(type, classLoader)的作用就是将META-INF/spring.factories中配置的内容进行实例化的工厂方法类，具备很强的扩展性，与SPI机制有异曲同工
的效果。

看完SpringApplication的初始化，接着跳回run方法继续分析

    public ConfigurableApplicationContext run(String... args) {
        StopWatch stopWatch = new StopWatch();
        stopWatch.start();
        ConfigurableApplicationContext context = null;
        Collection<SpringBootExceptionReporter> exceptionReporters = new ArrayList<>();
        configureHeadlessProperty();
        SpringApplicationRunListeners listeners = getRunListeners(args);//获取并启动监听器
        listeners.starting();
        try {
            ApplicationArguments applicationArguments = new DefaultApplicationArguments(
                    args);
            ConfigurableEnvironment environment = prepareEnvironment(listeners,
                    applicationArguments);//构造容器环境
            configureIgnoreBeanInfo(environment);
            Banner printedBanner = printBanner(environment);
            context = createApplicationContext();//创建容器
            exceptionReporters = getSpringFactoriesInstances(
                    SpringBootExceptionReporter.class,
                    new Class[] { ConfigurableApplicationContext.class }, context);
            prepareContext(context, environment, listeners, applicationArguments,
                    printedBanner);//准备容器
            refreshContext(context);//刷新容器
            afterRefresh(context, applicationArguments);
            stopWatch.stop();
            if (this.logStartupInfo) {
                new StartupInfoLogger(this.mainApplicationClass)
                        .logStarted(getApplicationLog(), stopWatch);
            }
            listeners.started(context);
            callRunners(context, applicationArguments);
        }
        catch (Throwable ex) {
            handleRunFailure(context, ex, exceptionReporters, listeners);
            throw new IllegalStateException(ex);
        }

        try {
            listeners.running(context);
        }
        catch (Throwable ex) {
            handleRunFailure(context, ex, exceptionReporters, null);
            throw new IllegalStateException(ex);
        }
        return context;
    }

这里挑其中比较重要的几个方法进行分析

1. 创建ConfigurableEnvironment对象

    private ConfigurableEnvironment prepareEnvironment(
            SpringApplicationRunListeners listeners,
            ApplicationArguments applicationArguments) {
        // Create and configure the environment
        ConfigurableEnvironment environment = getOrCreateEnvironment();//初始化environment
        configureEnvironment(environment, applicationArguments.getSourceArgs());//加载默认配置
        listeners.environmentPrepared(environment);//通知环境监听器，加载项目中的配置文件
        bindToSpringApplication(environment);
        if (this.webApplicationType == WebApplicationType.NONE) {
            environment = new EnvironmentConverter(getClassLoader())
                    .convertToStandardEnvironmentIfNecessary(environment);
        }
        ConfigurationPropertySources.attach(environment);
        return environment;
    }

通过getOrCreateEnvironment()方法创建容器环境

    private ConfigurableEnvironment getOrCreateEnvironment() {
        if (this.environment != null) {
            return this.environment;
        }
        if (this.webApplicationType == WebApplicationType.SERVLET) {
            return new StandardServletEnvironment();
        }
        return new StandardEnvironment();
    }

可以看到environment存在则不会重复创建，当应用类型为servlet时创建的是StandardServletEnvironment对象，否则创建StandardEnvironment对象。

接着来看configureEnvironment(environment, applicationArguments.getSourceArgs())

    protected void configureEnvironment(ConfigurableEnvironment environment,
            String[] args) {
        configurePropertySources(environment, args);//加载启动命令行配置属性
        configureProfiles(environment, args);//设置active属性
    }

configurePropertySources(environment, args)加载启动命令行的配置属性，来看一下实现

    protected void configurePropertySources(ConfigurableEnvironment environment,
            String[] args) {
        MutablePropertySources sources = environment.getPropertySources();
        if (this.defaultProperties != null && !this.defaultProperties.isEmpty()) {
            sources.addLast(
                    new MapPropertySource("defaultProperties", this.defaultProperties));
        }
        //加载命令行配置
        if (this.addCommandLineProperties && args.length > 0) {
            String name = CommandLinePropertySource.COMMAND_LINE_PROPERTY_SOURCE_NAME;
            if (sources.contains(name)) {
                PropertySource<?> source = sources.get(name);
                CompositePropertySource composite = new CompositePropertySource(name);
                composite.addPropertySource(new SimpleCommandLinePropertySource(
                        "springApplicationCommandLineArgs", args));
                composite.addPropertySource(source);
                sources.replace(name, composite);
            }
            else {
                sources.addFirst(new SimpleCommandLinePropertySource(args));
            }
        }
    }

这里的MutablePropertySources对象用于存储配置集合，其内部维护了一个CopyOnWriteArrayList类型的list对象，当默认配置存在时，会向该list的尾部插入一个new MapPropertySource("defaultProperties", this.defaultProperties)对象。

接着来看configureProfiles(environment, args)

    protected void configureProfiles(ConfigurableEnvironment environment, String[] args) {
        environment.getActiveProfiles(); // ensure they are initialized
        // But these ones should go first (last wins in a property key clash)
        Set<String> profiles = new LinkedHashSet<>(this.additionalProfiles);
        profiles.addAll(Arrays.asList(environment.getActiveProfiles()));
        environment.setActiveProfiles(StringUtils.toStringArray(profiles));
    }

这里主要做的事情就是获取environment.getActiveProfiles()的参数设置到environment中，即spring.profiles.active对应的环境变量。

最后来看一下listeners.environmentPrepared(environment)

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

这里的listeners就是之前通过META-INF/spring.factories注册的所有listeners，后面我们先以其中最重要的ConfigFileApplicationListener做为例子进行分析，接着来看listener.environmentPrepared(environment)

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

可以看到这里创建了一个ApplicationEnvironmentPreparedEvent类型的事件，并且调用了multicastEvent方法，通过该方法最终会调用到listener的onApplicationEvent方法，触发事件监听器的执行。

接下来具体看一下ConfigFileApplicationListener的onApplicationEvent方法做了什么

    @Override
    public void onApplicationEvent(ApplicationEvent event) {
        if (event instanceof ApplicationEnvironmentPreparedEvent) {
            onApplicationEnvironmentPreparedEvent(
                    (ApplicationEnvironmentPreparedEvent) event);
        }
        if (event instanceof ApplicationPreparedEvent) {
            onApplicationPreparedEvent(event);
        }
    }

可以看到当监听到ApplicationEnvironmentPreparedEvent类型的事件时，调用onApplicationEnvironmentPreparedEvent( (ApplicationEnvironmentPreparedEvent) event)方法

    private void onApplicationEnvironmentPreparedEvent(
            ApplicationEnvironmentPreparedEvent event) {
        List<EnvironmentPostProcessor> postProcessors = loadPostProcessors();
        postProcessors.add(this);
        AnnotationAwareOrderComparator.sort(postProcessors);
        for (EnvironmentPostProcessor postProcessor : postProcessors) {
            postProcessor.postProcessEnvironment(event.getEnvironment(),
                    event.getSpringApplication());
        }
    }

可以看到这里通过loadPostProcessors()方法加载了META-INF/spring.factories中的所有EnvironmentPostProcessor类到list中，同时把ConfigFileApplicationListener自己也添加进去了。接着遍历list中所有对象，并执行postProcessEnvironment方法，于是接着来看该方法

    @Override
    public void postProcessEnvironment(ConfigurableEnvironment environment,
            SpringApplication application) {
        addPropertySources(environment, application.getResourceLoader());
    }

    protected void addPropertySources(ConfigurableEnvironment environment,
            ResourceLoader resourceLoader) {
        RandomValuePropertySource.addToEnvironment(environment);
        new Loader(environment, resourceLoader).load();
    }

这里的核心是new Loader(environment, resourceLoader).load()，这里的Loader是一个内部类，用于处理配置文件的加载，首先看一下其构造方法

        Loader(ConfigurableEnvironment environment, ResourceLoader resourceLoader) {
            this.environment = environment;
            this.resourceLoader = (resourceLoader != null ? resourceLoader
                    : new DefaultResourceLoader());
            this.propertySourceLoaders = SpringFactoriesLoader.loadFactories(
                    PropertySourceLoader.class, getClass().getClassLoader());
        }

可以看到这里的resourceLoader又是通过SpringFactoriesLoader进行加载，那么来看看META-INF/spring.factories中定义了哪些resourceLoader

org.springframework.boot.env.PropertySourceLoader=\
org.springframework.boot.env.PropertiesPropertySourceLoader,\
org.springframework.boot.env.YamlPropertySourceLoader

从名字就可以看出来，PropertiesPropertySourceLoader和YamlPropertySourceLoader分别用于处理.properties和.yml类型的配置文件。

接着来看看load()方法做了什么

        public void load() {
            this.profiles = new LinkedList<>();
            this.processedProfiles = new LinkedList<>();
            this.activatedProfiles = false;
            this.loaded = new LinkedHashMap<>();
            initializeProfiles();//初始化
            while (!this.profiles.isEmpty()) {//定位解析资源文件
                Profile profile = this.profiles.poll();
                if (profile != null && !profile.isDefaultProfile()) {
                    addProfileToEnvironment(profile.getName());
                }
                load(profile, this::getPositiveProfileFilter,
                        addToLoaded(MutablePropertySources::addLast, false));
                this.processedProfiles.add(profile);
            }
            load(null, this::getNegativeProfileFilter,
                    addToLoaded(MutablePropertySources::addFirst, true));//对加载过的配置文件进行排序
            addLoadedPropertySources();
        }

initializeProfiles()进行了profiles的初始化，默认会添加null和default到profiles中，null对应配置文件application.properties和application.yml，default对应配置文件application-default.yml和application-default.properties，这里的null会被优先处理，由于后处理的会覆盖先处理的，因此其优先级最低。

接着来看load(profile, this::getPositiveProfileFilter, addToLoaded(MutablePropertySources::addLast, false))方法

        private void load(Profile profile, DocumentFilterFactory filterFactory,
                DocumentConsumer consumer) {
            getSearchLocations().forEach((location) -> {
                boolean isFolder = location.endsWith("/");
                Set<String> names = (isFolder ? getSearchNames() : NO_SEARCH_NAMES);
                names.forEach(
                        (name) -> load(location, name, profile, filterFactory, consumer));
            });
        }

这里重点是通过getSearchLocations()获取配置文件的路径，默认会获得4个路径

• file:./config/
• file:./
• classpath:/config/
• classpath:/

接着会遍历这些路径，拼接配置文件名称，选择合适的yml或者properties解析器进行解析，最后将结果添加到environment的propertySources中。

2. 通过createApplicationContext()创建run方法的返回值对象context

    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);
    }

可以看到这里也是根据webApplicationType的取值，分别创建不同的返回类型。

3. 通过prepareContext(context, environment, listeners, applicationArguments,printedBanner)方法将listeners、environment、applicationArguments、printedBanner等重要组件与上下文对象context进行关联

    private void prepareContext(ConfigurableApplicationContext context,
            ConfigurableEnvironment environment, SpringApplicationRunListeners listeners,
            ApplicationArguments applicationArguments, Banner printedBanner) {
        context.setEnvironment(environment);//设置容器环境，包括各种变量
        postProcessApplicationContext(context);//执行容器后置处理，为自定义预留扩展
        applyInitializers(context);
        listeners.contextPrepared(context);
        if (this.logStartupInfo) {
            logStartupInfo(context.getParent() == null);
            logStartupProfileInfo(context);
        }

        // Add boot specific singleton beans
        context.getBeanFactory().registerSingleton("springApplicationArguments",
                applicationArguments);
        if (printedBanner != null) {
            context.getBeanFactory().registerSingleton("springBootBanner", printedBanner);
        }

        // Load the sources
        Set<Object> sources = getAllSources();//获取启动类
        Assert.notEmpty(sources, "Sources must not be empty");
        load(context, sources.toArray(new Object[0]));//加载我们的启动类，将启动类注入容器
        listeners.contextLoaded(context);//发布容器已加载事件。
    }

这里的sources装的就是我们的启动类，然后通过load(context, sources.toArray(new Object[0]))方法进行加载

    protected void load(ApplicationContext context, Object[] sources) {
        if (logger.isDebugEnabled()) {
            logger.debug(
                    "Loading source " + StringUtils.arrayToCommaDelimitedString(sources));
        }
        BeanDefinitionLoader loader = createBeanDefinitionLoader(
                getBeanDefinitionRegistry(context), sources);
        if (this.beanNameGenerator != null) {
            loader.setBeanNameGenerator(this.beanNameGenerator);
        }
        if (this.resourceLoader != null) {
            loader.setResourceLoader(this.resourceLoader);
        }
        if (this.environment != null) {
            loader.setEnvironment(this.environment);
        }
        loader.load();
    }

来看一下loader是如何被加载的

    public int load() {
        int count = 0;
        for (Object source : this.sources) {
            count += load(source);
        }
        return count;
    }

    private int load(Object source) {
        Assert.notNull(source, "Source must not be null");
        if (source instanceof Class<?>) {
            return load((Class<?>) source);
        }
        if (source instanceof Resource) {
            return load((Resource) source);
        }
        if (source instanceof Package) {
            return load((Package) source);
        }
        if (source instanceof CharSequence) {
            return load((CharSequence) source);
        }
        throw new IllegalArgumentException("Invalid source type " + source.getClass());
    }

    private int load(Class<?> source) {
        if (isGroovyPresent()
                && GroovyBeanDefinitionSource.class.isAssignableFrom(source)) {
            // Any GroovyLoaders added in beans{} DSL can contribute beans here
            GroovyBeanDefinitionSource loader = BeanUtils.instantiateClass(source,
                    GroovyBeanDefinitionSource.class);
            load(loader);
        }
        if (isComponent(source)) {
            this.annotatedReader.register(source);
            return 1;
        }
        return 0;
    }

经过一系列调用之后最终由load(Class<?> source)方法执行，这里比较有趣的是当Groovy存在时居然是优先调用Groovy的方式进行加载，否则才走this.annotatedReader.register(source)方法将启动类注册到beanDefinitionMap中。

4. refreshContext(context)刷新容器

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

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

    @Override
    public void refresh() throws BeansException, IllegalStateException {
        synchronized (this.startupShutdownMonitor) {
            // Prepare this context for refreshing.
            prepareRefresh();

            // Tell the subclass to refresh the internal bean factory.
            ConfigurableListableBeanFactory beanFactory = obtainFreshBeanFactory();

            // Prepare the bean factory for use in this context.
            prepareBeanFactory(beanFactory);

            try {
                // Allows post-processing of the bean factory in context subclasses.
                postProcessBeanFactory(beanFactory);

                // Invoke factory processors registered as beans in the context.
                invokeBeanFactoryPostProcessors(beanFactory);

                // Register bean processors that intercept bean creation.
                registerBeanPostProcessors(beanFactory);

                // Initialize message source for this context.
                initMessageSource();

                // Initialize event multicaster for this context.
                initApplicationEventMulticaster();

                // Initialize other special beans in specific context subclasses.
                onRefresh();

                // Check for listener beans and register them.
                registerListeners();

                // Instantiate all remaining (non-lazy-init) singletons.
                finishBeanFactoryInitialization(beanFactory);

                // Last step: publish corresponding event.
                finishRefresh();
            }

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

                // Destroy already created singletons to avoid dangling resources.
                destroyBeans();

                // Reset 'active' flag.
                cancelRefresh(ex);

                // Propagate exception to caller.
                throw ex;
            }

            finally {
                // Reset common introspection caches in Spring's core, since we
                // might not ever need metadata for singleton beans anymore...
                resetCommonCaches();
            }
        }
    }

这个refresh()方法相当重要，尤其是invokeBeanFactoryPostProcessors(beanFactory)，这是实现spring-boot-starter-*(mybatis、redis等)自动化配置的关键部分，后续再详细讲解。

总结

至此Springboot的启动流程已经大体分析完了，也了解了配置文件和启动类分别是是如何被加载的，但仍有两个问题待解，一是Springboot的核心思想约定大于配置是如何做到的，二是Springboot的各种spring-boot-starter-*是如何发挥作用的，这两个问题留待后续文章继续分析。