启动流程

图如下：

以上流程图源文件（可导入https://www.processon.com）：https://github.com/Mysakura/DataFiles

相关Event（org.springframework.boot.context.event.SpringApplicationEvent的子类），这些Event是很好的标志，告诉我们程序执行到哪一步了，如下

ApplicationStartingEvent：在Environment和ApplicationContext可用之前 & 在ApplicationListener注册之后发布。
ApplicationContextInitializedEvent：在bean定义加载之前 & ApplicationContextInitializers被调用之后 & ApplicationContext开始准备之后发布
ApplicationPreparedEvent：在ApplicationContext完全准备好并且没有刷新之前发布，此时bean定义即将加载，Environment已经准备好被使用。
ApplicationStartedEvent：在ApplicationContext刷新之后，调用ApplicationRunner和CommandLineRunner之前发布
ApplicationReadyEvent：应用已经准备好接受请求时发布。

第一步，先进入启动类

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

第二步，执行run方法，实际上执行的是【org.springframework.boot.SpringApplication#run(java.lang.Class<?>[], java.lang.String[])】

public static ConfigurableApplicationContext run(Class<?> primarySource, String... args) {
    return run(new Class<?>[] { primarySource }, args);
}
 
public static ConfigurableApplicationContext run(Class<?>[] primarySources, String[] args) {
    // 初始化SpringApplication，然后执行run方法
    return new SpringApplication(primarySources).run(args);
}

1. new SpringApplication【org.springframework.boot.SpringApplication#SpringApplication(org.springframework.core.io.ResourceLoader, java.lang.Class<?>...)】

public SpringApplication(Class<?>... primarySources) {
    this(null, primarySources);
}
 
// 初始化一些成员变量：
public SpringApplication(ResourceLoader resourceLoader, Class<?>... primarySources) {
    this.resourceLoader = resourceLoader;
    Assert.notNull(primarySources, "PrimarySources must not be null");
    // 初始化primarySources
    this.primarySources = new LinkedHashSet<>(Arrays.asList(primarySources));
    // 判断当前应用是一个基于Servlet或者reactive的web应用还是一个非web应用
    this.webApplicationType = WebApplicationType.deduceFromClasspath();
    // 初始化一系列ApplicationContextInitializer
    setInitializers((Collection) getSpringFactoriesInstances(ApplicationContextInitializer.class));
    // 初始化一系列ApplicationListener，这些是针对SpringBoot事件的监听器
    setListeners((Collection) getSpringFactoriesInstances(ApplicationListener.class));
    // 配置主要应用启动类
    this.mainApplicationClass = deduceMainApplicationClass();
}

2. run【org.springframework.boot.SpringApplication#run(java.lang.String...)】

public ConfigurableApplicationContext run(String... args) {
    // 一个计算耗时小工具：A
    StopWatch stopWatch = new StopWatch();
    stopWatch.start();
    ConfigurableApplicationContext context = null;
    Collection<SpringBootExceptionReporter> exceptionReporters = new ArrayList<>();
    configureHeadlessProperty();
    // 获取监听器，SpringApplicationRunListener的作用是发布SpringBoot运行期间的事件：B
    SpringApplicationRunListeners listeners = getRunListeners(args);
    // 监听器发布事件：C
    listeners.starting();
    try {
        // 创建默认应用参数
        ApplicationArguments applicationArguments = new DefaultApplicationArguments(args);
        // 准备环境：D
        ConfigurableEnvironment environment = prepareEnvironment(listeners, applicationArguments);
        // 处理要忽略的bean信息
        configureIgnoreBeanInfo(environment);
        // 打印Banner
        Banner printedBanner = printBanner(environment);
        // 获取应用上下文：E
        context = createApplicationContext();
        // 获取异常报告实例列表
        exceptionReporters = getSpringFactoriesInstances(SpringBootExceptionReporter.class,
                new Class[] { ConfigurableApplicationContext.class }, context);
        // 准备上下文：F
        prepareContext(context, environment, listeners, applicationArguments, printedBanner);
        // 刷新上下文：G
        refreshContext(context);
        // 留给子类实现，在刷新之后做一下额外的处理
        afterRefresh(context, applicationArguments);
        // 计时器计时完毕
        stopWatch.stop();
        if (this.logStartupInfo) {
            new StartupInfoLogger(this.mainApplicationClass).logStarted(getApplicationLog(), stopWatch);
        }
        // 发布ApplicationStartedEvent事件：H
        listeners.started(context);
        // 调用Runners：I
        callRunners(context, applicationArguments);
    }
    catch (Throwable ex) {
        handleRunFailure(context, ex, exceptionReporters, listeners);
        throw new IllegalStateException(ex);
    }
 
    try {
        // 发布ApplicationReadyEvent事件：J
        listeners.running(context);
    }
    catch (Throwable ex) {
        handleRunFailure(context, ex, exceptionReporters, null);
        throw new IllegalStateException(ex);
    }
    return context;
}

A：计算耗时小工具（运行如下测试代码）

StopWatch stopWatch = new StopWatch("Test ABC");
stopWatch.start("Task A");
for (int i = 0; i < 10; i++){ }
stopWatch.stop();
stopWatch.start("Task B");
for (int i = 0; i < 5; i++){ }
stopWatch.stop();
stopWatch.start("Task C");
for (int i = 0; i < 15; i++){ }
stopWatch.stop();
System.out.println(stopWatch.prettyPrint());

输出：

B：获取监听器*

private SpringApplicationRunListeners getRunListeners(String[] args) {
    Class<?>[] types = new Class<?>[] { SpringApplication.class, String[].class };
    // 返回一个SpringApplicationRunListener的集合包装类
    return new SpringApplicationRunListeners(logger,
            getSpringFactoriesInstances(SpringApplicationRunListener.class, types, this, args));
}
 
private <T> Collection<T> getSpringFactoriesInstances(Class<T> type, Class<?>[] parameterTypes, Object... args) {
    ClassLoader classLoader = getClassLoader();
    // 使用Set集合确保name是唯一的；调用SpringFactoriesLoader.loadFactoryNames，使用给定的类加载器加载给定类型工厂实现的完全限定类名
    Set<String> names = new LinkedHashSet<>(SpringFactoriesLoader.loadFactoryNames(type, classLoader));
    // 根据给的Class、类加载器、完全限定类型列表已经相关参数，根据反射创建实例列表
    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;
}

C：发布【ApplicationStartingEvent】事件

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

D：准备环境*

private ConfigurableEnvironment prepareEnvironment(SpringApplicationRunListeners listeners,
        ApplicationArguments applicationArguments) {
    // 根据webApplicationType，决定创建StandardServletEnvironment还是StandardReactiveWebEnvironment还是StandardEnvironment
    ConfigurableEnvironment environment = getOrCreateEnvironment();
    // 配置属性文件和激活的环境（spring.profiles.active所配）
    configureEnvironment(environment, applicationArguments.getSourceArgs());
    // 将ConfigurationPropertySource附加到指定环境下
    ConfigurationPropertySources.attach(environment);
    // 发布ApplicationEnvironmentPreparedEvent事件
    listeners.environmentPrepared(environment);
    // 绑定环境到应用
    bindToSpringApplication(environment);
    if (!this.isCustomEnvironment) {
        environment = new EnvironmentConverter(getClassLoader()).convertEnvironmentIfNecessary(environment,
                deduceEnvironmentClass());
    }
    ConfigurationPropertySources.attach(environment);
    return environment;
}

E：获取应用上下文

// 根据应用类型创建不同的上下文
protected ConfigurableApplicationContext createApplicationContext() {
    Class<?> contextClass = this.applicationContextClass;
    if (contextClass == null) {
        try {
            switch (this.webApplicationType) {
            case SERVLET:
                contextClass = Class.forName(DEFAULT_SERVLET_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);
}

F：准备上下文*

private void prepareContext(ConfigurableApplicationContext context, ConfigurableEnvironment environment,
        SpringApplicationRunListeners listeners, ApplicationArguments applicationArguments, Banner printedBanner) {
    // 设置environment
    context.setEnvironment(environment);
    // 相关额外的处理
    postProcessApplicationContext(context);
    applyInitializers(context);
    // 派发ApplicationContextInitializedEvent事件，表示应用正在启动，上下文正在准备并且一系列Initializer已经调用
    listeners.contextPrepared(context);
    if (this.logStartupInfo) {
        logStartupInfo(context.getParent() == null);
        logStartupProfileInfo(context);
    }
    // Add boot specific singleton beans
    ConfigurableListableBeanFactory beanFactory = context.getBeanFactory();
    beanFactory.registerSingleton("springApplicationArguments", applicationArguments);
    if (printedBanner != null) {
        beanFactory.registerSingleton("springBootBanner", printedBanner);
    }
    if (beanFactory instanceof DefaultListableBeanFactory) {
        ((DefaultListableBeanFactory) beanFactory)
                .setAllowBeanDefinitionOverriding(this.allowBeanDefinitionOverriding);
    }
    if (this.lazyInitialization) {
        context.addBeanFactoryPostProcessor(new LazyInitializationBeanFactoryPostProcessor());
    }
    // Load the sources
    Set<Object> sources = getAllSources();
    Assert.notEmpty(sources, "Sources must not be empty");
    load(context, sources.toArray(new Object[0]));
    // 监听上下文，派发ApplicationPreparedEvent事件，表示应用正在启动，上下文已经准备好刷新了
    listeners.contextLoaded(context);
}
 
protected void postProcessApplicationContext(ConfigurableApplicationContext context) {
    if (this.beanNameGenerator != null) {
        // 注册注解工具类
        context.getBeanFactory().registerSingleton(AnnotationConfigUtils.CONFIGURATION_BEAN_NAME_GENERATOR,
                this.beanNameGenerator);
    }
    if (this.resourceLoader != null) {
        if (context instanceof GenericApplicationContext) {
            // 设置resourceLoader
            ((GenericApplicationContext) context).setResourceLoader(this.resourceLoader);
        }
        if (context instanceof DefaultResourceLoader) {
            ((DefaultResourceLoader) context).setClassLoader(this.resourceLoader.getClassLoader());
        }
    }
    if (this.addConversionService) {
        // 设置ConversionService
        context.getBeanFactory().setConversionService(ApplicationConversionService.getSharedInstance());
    }
}

G：刷新上下文（容器的初始化）【org.springframework.context.support.AbstractApplicationContext#refresh】*

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();
}
 
public void refresh() throws BeansException, IllegalStateException {
    synchronized (this.startupShutdownMonitor) {
        // Prepare this context for refreshing.
        // 为上下文刷新做准备
        prepareRefresh();
 
        // Tell the subclass to refresh the internal bean factory.
        // 告诉子类刷新内部bean工厂
        ConfigurableListableBeanFactory beanFactory = obtainFreshBeanFactory();
 
        // Prepare the bean factory for use in this context.
        // 准备在这个上下文要用的bean工厂
        prepareBeanFactory(beanFactory);
 
        try {
            // Allows post-processing of the bean factory in context subclasses.
            // 允许在上下文子类中对bean工厂进行后续处理
            postProcessBeanFactory(beanFactory);
 
            // Invoke factory processors registered as beans in the context.
            // 执行上下文中已注册的bean工厂后置处理器
            invokeBeanFactoryPostProcessors(beanFactory);
 
            // Register bean processors that intercept bean creation.
            // 注册拦截bean创建过程的后置处理器
            registerBeanPostProcessors(beanFactory);
 
            // Initialize message source for this context.
            // 初始化此上下文的消息源
            initMessageSource();
 
            // Initialize event multicaster for this context.
            // 初始化事件派发器
            initApplicationEventMulticaster();
 
            // Initialize other special beans in specific context subclasses.
            // 初始化子类实现的其它特殊bean
            onRefresh();
 
            // Check for listener beans and register them.
            // 检查并注册侦听器bean
            registerListeners();
 
            // Instantiate all remaining (non-lazy-init) singletons.
            // 实例化所有剩余的（非延迟初始化）单实例bean
            finishBeanFactoryInitialization(beanFactory);
 
            // Last step: publish corresponding event.
            // 最后，发布对应的事件
            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();
        }
    }
}

H：发布ApplicationStartedEvent事件（listeners.started(context);）【org.springframework.boot.context.event.EventPublishingRunListener#started】

public void started(ConfigurableApplicationContext context) {
    context.publishEvent(new ApplicationStartedEvent(this.application, this.args, context));
}

I：调用Runners

private void callRunners(ApplicationContext context, ApplicationArguments args) {
    List<Object> runners = new ArrayList<>();
    // 获取所有实现ApplicationRunner接口的子类
    runners.addAll(context.getBeansOfType(ApplicationRunner.class).values());
    // 获取所有实现CommandLineRunner接口的子类
    runners.addAll(context.getBeansOfType(CommandLineRunner.class).values());
    AnnotationAwareOrderComparator.sort(runners);
    // 循环遍历
    for (Object runner : new LinkedHashSet<>(runners)) {
        // 如果是ApplicationRunner的实现类
        if (runner instanceof ApplicationRunner) {
            callRunner((ApplicationRunner) runner, args);
        }
        // 如果是CommandLineRunner的实现类
        if (runner instanceof CommandLineRunner) {
            callRunner((CommandLineRunner) runner, args);
        }
    }
}
 
private void callRunner(ApplicationRunner runner, ApplicationArguments args) {
    try {
        (runner).run(args);
    }
    catch (Exception ex) {
        throw new IllegalStateException("Failed to execute ApplicationRunner", ex);
    }
}
 
private void callRunner(CommandLineRunner runner, ApplicationArguments args) {
    try {
        (runner).run(args.getSourceArgs());
    }
    catch (Exception ex) {
        throw new IllegalStateException("Failed to execute CommandLineRunner", ex);
    }
}

那么这两个类有啥作用呢

首先，SpringBoot并没有默认的实现类，那么肯定是给我们实现的；然后这个方法的执行时机是在spring容器启动完毕的时候，那么作用肯定是在容器启动之后做一些其它自定义的处理。

这两个接口都只有一个方法，就是run方法。类注解上写的说明：

当SpringApplication中包含了该接口的实现类，在callRunners的时候调用其中的run方法；可以在同一个应用上下文定义多个bean，也可以通过实现Order接口或者使用@Order注解来排序。

这两个类的唯一区别就是run方法的参数类型不同：

org.springframework.boot.CommandLineRunner#run(String... args)

org.springframework.boot.ApplicationRunner#run(ApplicationArguments args)

J：发布ApplicationReadyEvent事件（listeners.running(context);）【org.springframework.boot.context.event.EventPublishingRunListener#running】

public void running(ConfigurableApplicationContext context) {
    context.publishEvent(new ApplicationReadyEvent(this.application, this.args, context));
}

到此一个大概的流程已经走完，而我还想看一下具体的东西。

--------------------------------------------------------------------------------------------------------------------------------------------------------

进一步深入

问题0：我看有好几个地方都用到了同一个方法：org.springframework.boot.SpringApplication#getSpringFactoriesInstances(java.lang.Class<T>)

private <T> Collection<T> getSpringFactoriesInstances(Class<T> type, Class<?>[] parameterTypes, Object... args) {
    ClassLoader classLoader = getClassLoader();
    // 加载工程名称列表，通过Set集合保证唯一①
    Set<String> names = new LinkedHashSet<>(SpringFactoriesLoader.loadFactoryNames(type, classLoader));
    // 根据名称，创建相关实例②
    List<T> instances = createSpringFactoriesInstances(type, parameterTypes, classLoader, args, names);
    // 排序后返回
    AnnotationAwareOrderComparator.sort(instances);
    return instances;
}

来看①

public static List<String> loadFactoryNames(Class<?> factoryType, @Nullable ClassLoader classLoader) {
    String factoryTypeName = factoryType.getName();
    return loadSpringFactories(classLoader).getOrDefault(factoryTypeName, Collections.emptyList());
}
 
private static Map<String, List<String>> loadSpringFactories(@Nullable ClassLoader classLoader) {
    // 查看缓存有么有
    MultiValueMap<String, String> result = cache.get(classLoader);
    if (result != null) {
        return result;
    }
 
    try {
        // 获取spring.factories资源路径
        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);
            // 加载资源
            Properties properties = PropertiesLoaderUtils.loadProperties(resource);
            for (Map.Entry<?, ?> entry : properties.entrySet()) {
                String factoryTypeName = ((String) entry.getKey()).trim();
                for (String factoryImplementationName : StringUtils.commaDelimitedListToStringArray((String) entry.getValue())) {
                    // key为工厂类型接口名称，值为一系列子类实现
                    result.add(factoryTypeName, factoryImplementationName.trim());
                }
            }
        }
        // 放入缓存并返回
        cache.put(classLoader, result);
        return result;
    }
    catch (IOException ex) {
        throw new IllegalArgumentException("Unable to load factories from location [" +
                FACTORIES_RESOURCE_LOCATION + "]", ex);
    }
}

具体来看SpringBoot的spring.factories里面的内容：是一系列的组件以及具体的实现列表

通过把这些配置事先读取并放进缓存，我们就可以随时根据工厂接口类型获取对应的子类名称列表。

再来看②

private <T> List<T> createSpringFactoriesInstances(Class<T> type, Class<?>[] parameterTypes,
        ClassLoader classLoader, Object[] args, Set<String> names) {
    List<T> instances = new ArrayList<>(names.size());
    // 遍历子类名称，通过反射进行实例化
    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;
}

一句话作用：这个方法的作用就是找到spring.factories读取一系列组件的名称，通过传入的Class类型获取对应的子类名称列表进行实例化并返回。

问题1：那一系列Initializer都有哪些，有什么作用【setInitializers((Collection) getSpringFactoriesInstances(ApplicationContextInitializer.class));】

0：报告常见的错误配置警告

1：配置ApplicationContext ID

2：执行环境属性【context.initializer.classes】指定的初始化器，也就是给用户一个机会去指定自己实现的ApplicationContextInitializer从而对应用程序做一些初始化工作

3：将RSocketServer实际监听的端口写入Environment环境属性中。这样属性local.rsocket.server.port就可以直接通过@Value注入到测试中，或者通过Environment获取。

4：将WebServer实际监听的端口写入Environment环境属性中。这样属性local.server.port就可以直接通过@Value注入到测试中，或者通过Environment获取。

5：创建一个ConfigurationClassPostProcessor 和 Spring Boot共用的CachingMetadataReaderFactory对象

6：将ConditionEvaluationReport写入日志。

问题2：那一系列Listeners有哪些，有什么作用【setListeners((Collection) getSpringFactoriesInstances(ApplicationListener.class));】

0：一旦装载了上下文，就清空缓存。

1：在其父应用程序上下文关闭时关闭该应用程序上下文

2：如果系统文件编码与environment里设置的不一致，则停止启动应用程序。默认情况下么有影响，别手贱乱设置。

3：根据属性{spring.output.ansi.enabled}的值配置{AnsiOutput}

4：通过读取已知位置属性文件里的属性来配置上下文的environment，默认加载application.properties或者application.yml里的配置。

5：执行配置在context.listener.classes环境变量里的Listener

6：通过在{DEBUG}级别记录线程上下文类加载器(TCCL)的类路径来对ApplicationEnvironmentPreparedEvent和ApplicationFailedEvent事件做出反应

7：一个配置{LoggingSystem}的ApplicationListener。如果环境包含{logging.config}属性，它将用于引导日志系统，否则将使用默认配置。无论如何，如果环境包含{logging.level}，那么日志级别将被定制。

8：将liquibase {ServiceLocator}替换成和SpringBoot可执行归档一起工作的版本。

9：在耗时任务的后台线程中触发早期初始化

问题3：SpringApplicationRunListeners listeners = getRunListeners(args);怎么又出来一个listeners？

private SpringApplicationRunListeners getRunListeners(String[] args) {
    Class<?>[] types = new Class<?>[] { SpringApplication.class, String[].class };
    // 可以看出，通过getSpringFactoriesInstances读取对应的names并实例化返回，然后封装在SpringApplicationRunListeners
    return new SpringApplicationRunListeners(logger,
            getSpringFactoriesInstances(SpringApplicationRunListener.class, types, this, args));
}

在这里实例化的是：

问题4：listeners.starting();是启动监听器吗？

void starting() {
    // 遍历内部的listener，执行starting方法
    for (SpringApplicationRunListener listener : this.listeners) {
        listener.starting();
    }
}
 
// starting方法，实际上是广播ApplicationStartingEvent事件
public void starting() {
    this.initialMulticaster.multicastEvent(new ApplicationStartingEvent(this.application, this.args));
}

广播事件，有哪些收听者呢？

public void multicastEvent(ApplicationEvent event) {
    multicastEvent(event, resolveDefaultEventType(event));
}
 
@Override
public void multicastEvent(final ApplicationEvent event, @Nullable ResolvableType eventType) {
    ResolvableType type = (eventType != null ? eventType : resolveDefaultEventType(event));
    Executor executor = getTaskExecutor();
    // 典型的观察者模式
    for (ApplicationListener<?> listener : getApplicationListeners(event, type)) {
        if (executor != null) {
            executor.execute(() -> invokeListener(listener, event));
        }
        else {
            invokeListener(listener, event);
        }
    }
}

收听者就是我们之前第一步实例化的listener

问题5：Bean在哪里实例化的

先创建bean定义

进入org.springframework.boot.SpringApplication#run(java.lang.String...)
进入refreshContext(context);方法
最终进入org.springframework.context.support.AbstractApplicationContext#refresh
进入invokeBeanFactoryPostProcessors(beanFactory);
进入PostProcessorRegistrationDelegate.invokeBeanFactoryPostProcessors(beanFactory, getBeanFactoryPostProcessors());
进入invokeBeanDefinitionRegistryPostProcessors(currentRegistryProcessors, registry);
里面是一个循环，进入postProcessor.postProcessBeanDefinitionRegistry(registry);
进入processConfigBeanDefinitions(registry);
进入parser.parse(candidates);
里面是一个遍历BeanDefinitionHolder的过程，获取BeanDefinition。根据BeanDefinition实现的接口执行不同的方法，但是最终都进入org.springframework.context.annotation.ConfigurationClassParser#processConfigurationClass
进入sourceClass = doProcessConfigurationClass(configClass, sourceClass);

到此慢慢接近了真相

protected final SourceClass doProcessConfigurationClass(ConfigurationClass configClass, SourceClass sourceClass)
        throws IOException {
 
    if (configClass.getMetadata().isAnnotated(Component.class.getName())) {
        // Recursively process any member (nested) classes first
        processMemberClasses(configClass, sourceClass);
    }
 
    // 处理@PropertySource标记的类
    for (AnnotationAttributes propertySource : AnnotationConfigUtils.attributesForRepeatable(
            sourceClass.getMetadata(), PropertySources.class,
            org.springframework.context.annotation.PropertySource.class)) {
        if (this.environment instanceof ConfigurableEnvironment) {
            processPropertySource(propertySource);
        }
        else {
            logger.info("Ignoring @PropertySource annotation on [" + sourceClass.getMetadata().getClassName() +
                    "]. Reason: Environment must implement ConfigurableEnvironment");
        }
    }
 
    // 处理@ComponentScan标记的类
    Set<AnnotationAttributes> componentScans = AnnotationConfigUtils.attributesForRepeatable(
            sourceClass.getMetadata(), ComponentScans.class, ComponentScan.class);
    if (!componentScans.isEmpty() &&
            !this.conditionEvaluator.shouldSkip(sourceClass.getMetadata(), ConfigurationPhase.REGISTER_BEAN)) {
        for (AnnotationAttributes componentScan : componentScans) {
            // 立即执行扫描 A
            Set<BeanDefinitionHolder> scannedBeanDefinitions =
                    this.componentScanParser.parse(componentScan, sourceClass.getMetadata().getClassName());
            // 检查已扫描的定义集合以获得更多的配置类，并在需要时进行递归解析
            for (BeanDefinitionHolder holder : scannedBeanDefinitions) {
                BeanDefinition bdCand = holder.getBeanDefinition().getOriginatingBeanDefinition();
                if (bdCand == null) {
                    bdCand = holder.getBeanDefinition();
                }
                if (ConfigurationClassUtils.checkConfigurationClassCandidate(bdCand, this.metadataReaderFactory)) {
                    parse(bdCand.getBeanClassName(), holder.getBeanName());
                }
            }
        }
    }
 
    // 处理@Import标记的类
    processImports(configClass, sourceClass, getImports(sourceClass), true);
 
    // 处理@ImportResource标记的类
    AnnotationAttributes importResource =
            AnnotationConfigUtils.attributesFor(sourceClass.getMetadata(), ImportResource.class);
    if (importResource != null) {
        String[] resources = importResource.getStringArray("locations");
        Class<? extends BeanDefinitionReader> readerClass = importResource.getClass("reader");
        for (String resource : resources) {
            String resolvedResource = this.environment.resolveRequiredPlaceholders(resource);
            configClass.addImportedResource(resolvedResource, readerClass);
        }
    }
 
    // 处理单个@Bean标记的方法
    Set<MethodMetadata> beanMethods = retrieveBeanMethodMetadata(sourceClass);
    for (MethodMetadata methodMetadata : beanMethods) {
        configClass.addBeanMethod(new BeanMethod(methodMetadata, configClass));
    }
 
    // 处理接口里的默认方法
    processInterfaces(configClass, sourceClass);
 
    // 处理父类, 如果有的话
    if (sourceClass.getMetadata().hasSuperClass()) {
        String superclass = sourceClass.getMetadata().getSuperClassName();
        if (superclass != null && !superclass.startsWith("java") &&
                !this.knownSuperclasses.containsKey(superclass)) {
            this.knownSuperclasses.put(superclass, configClass);
            // Superclass found, return its annotation metadata and recurse
            return sourceClass.getSuperClass();
        }
    }
 
    // 没有父类，处理完成
    return null;
}

我们来看一下扫描过程，进入注释A的那段代码：this.componentScanParser.parse(componentScan, sourceClass.getMetadata().getClassName());

这个方法主要读取相关的属性，最后的方法是处理过程。其中一段代码，判断basePackages是否为空，如果为空，则默认扫描启动类所在的包

现在进入最后的doScan方法。

protected Set<BeanDefinitionHolder> doScan(String... basePackages) {
    Assert.notEmpty(basePackages, "At least one base package must be specified");
    Set<BeanDefinitionHolder> beanDefinitions = new LinkedHashSet<>();
    for (String basePackage : basePackages) {
        // 具体扫描包，组装成BeanDefinition返回
        Set<BeanDefinition> candidates = findCandidateComponents(basePackage);
        for (BeanDefinition candidate : candidates) {
            ScopeMetadata scopeMetadata = this.scopeMetadataResolver.resolveScopeMetadata(candidate);
            candidate.setScope(scopeMetadata.getScopeName());
            String beanName = this.beanNameGenerator.generateBeanName(candidate, this.registry);
            if (candidate instanceof AbstractBeanDefinition) {
                postProcessBeanDefinition((AbstractBeanDefinition) candidate, beanName);
            }
            if (candidate instanceof AnnotatedBeanDefinition) {
                AnnotationConfigUtils.processCommonDefinitionAnnotations((AnnotatedBeanDefinition) candidate);
            }
            if (checkCandidate(beanName, candidate)) {
                BeanDefinitionHolder definitionHolder = new BeanDefinitionHolder(candidate, beanName);
                definitionHolder =
                        AnnotationConfigUtils.applyScopedProxyMode(scopeMetadata, definitionHolder, this.registry);
                beanDefinitions.add(definitionHolder);
                // 注册bean定义，也就是注册到beanFactory里（this.registry的类型是org.springframework.beans.factory.support.DefaultListableBeanFactory）
                registerBeanDefinition(definitionHolder, this.registry);
            }
        }
    }
    return beanDefinitions;
}

进入注册bean的方法（最终进入org.springframework.beans.factory.support.BeanDefinitionReaderUtils#registerBeanDefinition）

public static void registerBeanDefinition(
        BeanDefinitionHolder definitionHolder, BeanDefinitionRegistry registry)
        throws BeanDefinitionStoreException {
 
    // 在主名称下注册bean定义
    String beanName = definitionHolder.getBeanName();
    // 具体注册方法
    registry.registerBeanDefinition(beanName, definitionHolder.getBeanDefinition());
 
    // 如果有的话，为bean名称注册别名
    String[] aliases = definitionHolder.getAliases();
    if (aliases != null) {
        for (String alias : aliases) {
            registry.registerAlias(beanName, alias);
        }
    }
}

进入具体注册方法（org.springframework.beans.factory.support.DefaultListableBeanFactory#registerBeanDefinition）

实际上是把bean定义放进了一个Map里面。到此为止，bean定义已经注册到beanFactory里了。但是这个时候还没有实例化，仅仅是拿到了bean定义。

实例化

刚才我们走的是invokeBeanFactoryPostProcessors(beanFactory);，现在走 finishBeanFactoryInitialization(beanFactory);

protected void finishBeanFactoryInitialization(ConfigurableListableBeanFactory beanFactory) {
    // 为上下文初始化conversion service
    if (beanFactory.containsBean(CONVERSION_SERVICE_BEAN_NAME) &&
            beanFactory.isTypeMatch(CONVERSION_SERVICE_BEAN_NAME, ConversionService.class)) {
        beanFactory.setConversionService(
                beanFactory.getBean(CONVERSION_SERVICE_BEAN_NAME, ConversionService.class));
    }
 
    // 如果之前没有任何bean后置处理器(例如PropertyPlaceholderConfigurer bean)注册，则注册一个默认的嵌入式值解析器
    // 此时，主要用于解析注解属性值。
    if (!beanFactory.hasEmbeddedValueResolver()) {
        beanFactory.addEmbeddedValueResolver(strVal -> getEnvironment().resolvePlaceholders(strVal));
    }
 
    // 尽早初始化LoadTimeWeaverAware bean，以便尽早注册它们的转换器
    String[] weaverAwareNames = beanFactory.getBeanNamesForType(LoadTimeWeaverAware.class, false, false);
    for (String weaverAwareName : weaverAwareNames) {
        getBean(weaverAwareName);
    }
 
    // 停止使用临时类加载器进行类型匹配。
    beanFactory.setTempClassLoader(null);
 
    // 允许缓存所有的bean定义元数据，不希望有进一步的更改。
    beanFactory.freezeConfiguration();
 
    // 实例化所有剩余的(非懒加载)单例。
    beanFactory.preInstantiateSingletons();
}

进入最后一行

public void preInstantiateSingletons() throws BeansException {
    if (logger.isTraceEnabled()) {
        logger.trace("Pre-instantiating singletons in " + this);
    }
 
    // 遍历一个副本以允许init方法，而init方法反过来注册新的bean定义。
    // 虽然这可能不是常规的工厂引导的一部分，但它在其他方面也可以正常工作。
    List<String> beanNames = new ArrayList<>(this.beanDefinitionNames);
 
    // 触发所有非懒加载bean的初始化
    for (String beanName : beanNames) {
        RootBeanDefinition bd = getMergedLocalBeanDefinition(beanName);
        if (!bd.isAbstract() && bd.isSingleton() && !bd.isLazyInit()) {
            if (isFactoryBean(beanName)) {
                Object bean = getBean(FACTORY_BEAN_PREFIX + beanName);
                if (bean instanceof FactoryBean) {
                    final FactoryBean<?> factory = (FactoryBean<?>) bean;
                    boolean isEagerInit;
                    if (System.getSecurityManager() != null && factory instanceof SmartFactoryBean) {
                        isEagerInit = AccessController.doPrivileged((PrivilegedAction<Boolean>)
                                        ((SmartFactoryBean<?>) factory)::isEagerInit,
                                getAccessControlContext());
                    }
                    else {
                        isEagerInit = (factory instanceof SmartFactoryBean &&
                                ((SmartFactoryBean<?>) factory).isEagerInit());
                    }
                    if (isEagerInit) {
                        getBean(beanName);
                    }
                }
            }
            else {
                // 实例化bean，实际进入的是org.springframework.beans.factory.support.AbstractBeanFactory#doGetBean方法，里面包含了具体实例化的过程
                getBean(beanName);
            }
        }
    }
 
    // 为所有适用的bean触发初始化后回调
    for (String beanName : beanNames) {
        Object singletonInstance = getSingleton(beanName);
        if (singletonInstance instanceof SmartInitializingSingleton) {
            final SmartInitializingSingleton smartSingleton = (SmartInitializingSingleton) singletonInstance;
            if (System.getSecurityManager() != null) {
                AccessController.doPrivileged((PrivilegedAction<Object>) () -> {
                    smartSingleton.afterSingletonsInstantiated();
                    return null;
                }, getAccessControlContext());
            }
            else {
                smartSingleton.afterSingletonsInstantiated();
            }
        }
    }
}

最后看一下具体实例化的过程【org.springframework.beans.factory.support.AbstractBeanFactory#doGetBean】

// Create bean instance.
if (mbd.isSingleton()) {
    sharedInstance = getSingleton(beanName, () -> {
        try {
            return createBean(beanName, mbd, args);
        }
        catch (BeansException ex) {
            // Explicitly remove instance from singleton cache: It might have been put there
            // eagerly by the creation process, to allow for circular reference resolution.
            // Also remove any beans that received a temporary reference to the bean.
            destroySingleton(beanName);
            throw ex;
        }
    });
    bean = getObjectForBeanInstance(sharedInstance, name, beanName, mbd);
}

进入createBean【org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory#createBean(java.lang.String, org.springframework.beans.factory.support.RootBeanDefinition, java.lang.Object[])】

try {
    Object beanInstance = doCreateBean(beanName, mbdToUse, args);
    if (logger.isTraceEnabled()) {
        logger.trace("Finished creating instance of bean '" + beanName + "'");
    }
    return beanInstance;
}
catch (BeanCreationException | ImplicitlyAppearedSingletonException ex) {
    // A previously detected exception with proper bean creation context already,
    // or illegal singleton state to be communicated up to DefaultSingletonBeanRegistry.
    throw ex;
}
catch (Throwable ex) {
    throw new BeanCreationException(
            mbdToUse.getResourceDescription(), beanName, "Unexpected exception during bean creation", ex);
}

进入doCreateBean【org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory#doCreateBean】

// spring把实例化的类都放在了包装类里
BeanWrapper instanceWrapper = null;
if (mbd.isSingleton()) {
    instanceWrapper = this.factoryBeanInstanceCache.remove(beanName);
}
if (instanceWrapper == null) {
    instanceWrapper = createBeanInstance(beanName, mbd, args);
}
final Object bean = instanceWrapper.getWrappedInstance();
Class<?> beanType = instanceWrapper.getWrappedClass();
if (beanType != NullBean.class) {
    mbd.resolvedTargetType = beanType;
}

进入createBeanInstance【org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory#createBeanInstance】

protected BeanWrapper createBeanInstance(String beanName, RootBeanDefinition mbd, @Nullable Object[] args) {
    // 确保此时bean类已经被解析。
    Class<?> beanClass = resolveBeanClass(mbd, beanName);
 
    if (beanClass != null && !Modifier.isPublic(beanClass.getModifiers()) && !mbd.isNonPublicAccessAllowed()) {
        throw new BeanCreationException(mbd.getResourceDescription(), beanName,
                "Bean class isn't public, and non-public access not allowed: " + beanClass.getName());
    }
 
    Supplier<?> instanceSupplier = mbd.getInstanceSupplier();
    if (instanceSupplier != null) {
        return obtainFromSupplier(instanceSupplier, beanName);
    }
 
    if (mbd.getFactoryMethodName() != null) {
        return instantiateUsingFactoryMethod(beanName, mbd, args);
    }
 
    // Shortcut when re-creating the same bean...
    boolean resolved = false;
    boolean autowireNecessary = false;
    if (args == null) {
        synchronized (mbd.constructorArgumentLock) {
            if (mbd.resolvedConstructorOrFactoryMethod != null) {
                resolved = true;
                autowireNecessary = mbd.constructorArgumentsResolved;
            }
        }
    }
    if (resolved) {
        if (autowireNecessary) {
            return autowireConstructor(beanName, mbd, null, null);
        }
        else {
            return instantiateBean(beanName, mbd);
        }
    }
 
    // 为自动装配准备的候选构造函数
    Constructor<?>[] ctors = determineConstructorsFromBeanPostProcessors(beanClass, beanName);
    if (ctors != null || mbd.getResolvedAutowireMode() == AUTOWIRE_CONSTRUCTOR ||
            mbd.hasConstructorArgumentValues() || !ObjectUtils.isEmpty(args)) {
        return autowireConstructor(beanName, mbd, ctors, args);
    }
 
    // 为默认构造函数准备的首选构造函数
    ctors = mbd.getPreferredConstructors();
    if (ctors != null) {
        return autowireConstructor(beanName, mbd, ctors, null);
    }
 
    // 不需要特殊处理:只需使用无参构造函数。
    return instantiateBean(beanName, mbd);
}

因为本例只有一个Controller，所以运行到了最后一行：

进入instantiateBean【org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory#instantiateBean】

protected BeanWrapper instantiateBean(final String beanName, final RootBeanDefinition mbd) {
    try {
        Object beanInstance;
        final BeanFactory parent = this;
        if (System.getSecurityManager() != null) {
            beanInstance = AccessController.doPrivileged((PrivilegedAction<Object>) () ->
                    getInstantiationStrategy().instantiate(mbd, beanName, parent),
                    getAccessControlContext());
        }
        else {
            // 获取instantiationStrategy策略，并调用instanttiate方法进行实例化
            beanInstance = getInstantiationStrategy().instantiate(mbd, beanName, parent);
        }
        // 包装
        BeanWrapper bw = new BeanWrapperImpl(beanInstance);
        initBeanWrapper(bw);
        return bw;
    }
    catch (Throwable ex) {
        throw new BeanCreationException(
                mbd.getResourceDescription(), beanName, "Instantiation of bean failed", ex);
    }
}

进入instantiate【org.springframework.beans.factory.support.SimpleInstantiationStrategy#instantiate(org.springframework.beans.factory.support.RootBeanDefinition, java.lang.String, org.springframework.beans.factory.BeanFactory)】

public Object instantiate(RootBeanDefinition bd, @Nullable String beanName, BeanFactory owner) {
    // Don't override the class with CGLIB if no overrides.
    if (!bd.hasMethodOverrides()) {
        Constructor<?> constructorToUse;
        synchronized (bd.constructorArgumentLock) {
            constructorToUse = (Constructor<?>) bd.resolvedConstructorOrFactoryMethod;
            if (constructorToUse == null) {
                // 获取Class
                final Class<?> clazz = bd.getBeanClass();
                if (clazz.isInterface()) {
                    throw new BeanInstantiationException(clazz, "Specified class is an interface");
                }
                try {
                    if (System.getSecurityManager() != null) {
                        constructorToUse = AccessController.doPrivileged(
                                (PrivilegedExceptionAction<Constructor<?>>) clazz::getDeclaredConstructor);
                    }
                    else {
                        // 获取已声明的构造方法
                        constructorToUse = clazz.getDeclaredConstructor();
                    }
                    bd.resolvedConstructorOrFactoryMethod = constructorToUse;
                }
                catch (Throwable ex) {
                    throw new BeanInstantiationException(clazz, "No default constructor found", ex);
                }
            }
        }
        // 通过工具类实例化
        return BeanUtils.instantiateClass(constructorToUse);
    }
    else {
        // Must generate CGLIB subclass.
        return instantiateWithMethodInjection(bd, beanName, owner);
    }
}

最后一步，看下如何实例化的【org.springframework.beans.BeanUtils#instantiateClass(java.lang.reflect.Constructor<T>, java.lang.Object...)】

public static <T> T instantiateClass(Constructor<T> ctor, Object... args) throws BeanInstantiationException {
    Assert.notNull(ctor, "Constructor must not be null");
    try {
        // 反射工具类：使构造方法可访问
        ReflectionUtils.makeAccessible(ctor);
        if (KotlinDetector.isKotlinReflectPresent() && KotlinDetector.isKotlinType(ctor.getDeclaringClass())) {
            return KotlinDelegate.instantiateClass(ctor, args);
        }
        else {
            // 获取参数类型列表
            Class<?>[] parameterTypes = ctor.getParameterTypes();
            Assert.isTrue(args.length <= parameterTypes.length, "Can't specify more arguments than constructor parameters");
            Object[] argsWithDefaultValues = new Object[args.length];
            for (int i = 0 ; i < args.length; i++) {
                if (args[i] == null) {
                    Class<?> parameterType = parameterTypes[i];
                    argsWithDefaultValues[i] = (parameterType.isPrimitive() ? DEFAULT_TYPE_VALUES.get(parameterType) : null);
                }
                else {
                    argsWithDefaultValues[i] = args[i];
                }
            }
            // 很熟悉的反射代码【java.lang.reflect.Constructor#newInstance】
            return ctor.newInstance(argsWithDefaultValues);
        }
    }
    catch (InstantiationException ex) {
        throw new BeanInstantiationException(ctor, "Is it an abstract class?", ex);
    }
    catch (IllegalAccessException ex) {
        throw new BeanInstantiationException(ctor, "Is the constructor accessible?", ex);
    }
    catch (IllegalArgumentException ex) {
        throw new BeanInstantiationException(ctor, "Illegal arguments for constructor", ex);
    }
    catch (InvocationTargetException ex) {
        throw new BeanInstantiationException(ctor, "Constructor threw exception", ex.getTargetException());
    }
}

总结：

// 在刷新上下文方法里
org.springframework.context.support.AbstractApplicationContext#refresh
// 获取并注册bean定义（包含了注解扫描等过程）
postProcessBeanFactory(beanFactory);
// 实例化所有非懒加载的bean（本质用了反射实现了实例化bean）
finishBeanFactoryInitialization(beanFactory);

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