1. SpringBoot自动加载原理

@SpringBootApplication
public class Application {
    

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

}

首先看@SpringBootApplication注解

@Target({
    ElementType.TYPE}) // 标记在类上
@Retention(RetentionPolicy.RUNTIME) // 作用于运行时
@Documented // 生成javadoc文档
@Inherited // 允许被继承
@SpringBootConfiguration
@EnableAutoConfiguration
@ComponentScan(
    excludeFilters = {
    @Filter(
    type = FilterType.CUSTOM,
    classes = {
    TypeExcludeFilter.class}
), @Filter(
    type = FilterType.CUSTOM,
    classes = {
    AutoConfigurationExcludeFilter.class}
)}
)
public @interface SpringBootApplication {
    
  // ...
}

其中包含了@SpringBootConfiguration和@EnableAutoConfiguration重要注解

@SpringBootConfiguration

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

该注解实际上起的还是@Configuration的作用，指定配置类

@EnableAutoConfiguration

@Target({
    ElementType.TYPE})
@Retention(RetentionPolicy.RUNTIME)
@Documented
@Inherited
@AutoConfigurationPackage
@Import({
    AutoConfigurationImportSelector.class})
public @interface EnableAutoConfiguration {
    
    String ENABLED_OVERRIDE_PROPERTY = "spring.boot.enableautoconfiguration";

    Class<?>[] exclude() default {
    };

    String[] excludeName() default {
    };
}

EnableAutoConfiguration中有两个注解，分别是

• @AutoConfigurationPackage
• @Import({AutoConfigurationImportSelector.class})

@AutoConfigurationPackage

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

这个注解的关键是@Import({Registrar.class})，@Import的作用是让Spring加载Registrar.class作为bean，那么看看Registrar中有什么

Registrar.class

static class Registrar implements ImportBeanDefinitionRegistrar, DeterminableImports {
    

		@Override
		public void registerBeanDefinitions(AnnotationMetadata metadata, BeanDefinitionRegistry registry) {
    
			register(registry, new PackageImport(metadata).getPackageName());
		}

		@Override
		public Set<Object> determineImports(AnnotationMetadata metadata) {
    
			return Collections.singleton(new PackageImport(metadata));
		}

	}

@Import加载的类如果实现了ImportBeanDefinitionRegistrar接口，那么就会调用该接口的方法并注册成bean，再看一下该方法做了什么

register(registry, new PackageImport(metadata).getPackageName());

在这段代码上打断点然后debug项目，可以看到getPackageName()最终返回的是整个项目启动类所在的包，即该类的作用就是扫描包下面的组件注册成bean

@Import({AutoConfigurationImportSelector.class})

public class AutoConfigurationImportSelector implements DeferredImportSelector, BeanClassLoaderAware,
		ResourceLoaderAware, BeanFactoryAware, EnvironmentAware, Ordered{
    
      // ...
    }

@Import导入的类如果实现了DeferredImportSelector或ImportSelector接口，则会执行selectImports方法，看一下selectImports方法的内容

@Override
	public String[] selectImports(AnnotationMetadata annotationMetadata) {
    
		if (!isEnabled(annotationMetadata)) {
    
			return NO_IMPORTS;
		}
		AutoConfigurationMetadata autoConfigurationMetadata = AutoConfigurationMetadataLoader
				.loadMetadata(this.beanClassLoader);
		// 跟到这一段里面去
		AutoConfigurationEntry autoConfigurationEntry = getAutoConfigurationEntry(autoConfigurationMetadata,
				annotationMetadata);
		return StringUtils.toStringArray(autoConfigurationEntry.getConfigurations());
	}

这里重点看AutoConfigurationEntry autoConfigurationEntry = getAutoConfigurationEntry(autoConfigurationMetadata,annotationMetadata);这段代码，跟到方法里面去

protected AutoConfigurationEntry getAutoConfigurationEntry(AutoConfigurationMetadata autoConfigurationMetadata,
			AnnotationMetadata annotationMetadata) {
    
		if (!isEnabled(annotationMetadata)) {
    
			return EMPTY_ENTRY;
		}
		AnnotationAttributes attributes = getAttributes(annotationMetadata);
    // 这里是重点
		List<String> configurations = getCandidateConfigurations(annotationMetadata, attributes);
		configurations = removeDuplicates(configurations);
		Set<String> exclusions = getExclusions(annotationMetadata, attributes);
		checkExcludedClasses(configurations, exclusions);
		configurations.removeAll(exclusions);
		configurations = filter(configurations, autoConfigurationMetadata);
		fireAutoConfigurationImportEvents(configurations, exclusions);
		return new AutoConfigurationEntry(configurations, exclusions);
	}

在跟到getCandidateConfigurations方法

protected List<String> getCandidateConfigurations(AnnotationMetadata metadata, AnnotationAttributes attributes) {
    
		List<String> configurations = SpringFactoriesLoader.loadFactoryNames(getSpringFactoriesLoaderFactoryClass(),
				getBeanClassLoader());
		Assert.notEmpty(configurations, "No auto configuration classes found in META-INF/spring.factories. If you "
				+ "are using a custom packaging, make sure that file is correct.");
		return configurations;
	}

此处有SpringFactoriesLoader.loadFactoryNames，这个方法比较关键了，跟进去

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可以看到自动配置实际上会加载jar包中的spring.factories文件

所以自动配置实际上分2种情况

1. 启动类同包下以及子包下的所有组件都会进行扫描并按需加载(如@ConditionXXXX)，通过@ComponentScan可以修改扫描位置
2. 读取resource目录下的META-INF/spring.factories文件中配置的类型

2.SpringApplication.run()

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

这一句代码包含两步骤

1. 创建SpringApplication的实例
2. 使用实例调用run方法

创建SpringApplication实例

public SpringApplication(ResourceLoader resourceLoader, Class<?>... primarySources) {
    
		this.resourceLoader = resourceLoader;
		Assert.notNull(primarySources, "PrimarySources must not be null");
		this.primarySources = new LinkedHashSet<>(Arrays.asList(primarySources));
  	// 推断web项目使用的mvc还是webflux，通过判断dispatcher的全限定名属于reactive包还是servlet包
		this.webApplicationType = WebApplicationType.deduceFromClasspath();
    // bootstrap层的初始化，这里我们知道配置问价加载顺序是bootstrap>application，所以此处先初始化bootstrap
		this.bootstrappers = new ArrayList<>(getSpringFactoriesInstances(Bootstrapper.class));
    // 初始化ApplicationContextInitializer，从spring.factories中加载
		setInitializers((Collection) getSpringFactoriesInstances(ApplicationContextInitializer.class));
    // 初始化ApplicationListener，从spring.factories中加载
		setListeners((Collection) getSpringFactoriesInstances(ApplicationListener.class));
    // 通过栈 判断调用main方法的class
		this.mainApplicationClass = deduceMainApplicationClass();
	}

在这个构造方法中，由于容器还没有进行refresh方法(bean的初始化方法)，所以可以通过实现ApplicationContextInitializer类来修改配置等上下文信息

run方法

public ConfigurableApplicationContext run(String... args) {
    
		StopWatch stopWatch = new StopWatch();
		stopWatch.start();
		DefaultBootstrapContext bootstrapContext = createBootstrapContext();
		ConfigurableApplicationContext context = null;
		configureHeadlessProperty();
  // 获取SpringApplicationRunListener类型的启动监听器并实例化，从spring.factories中获取
		SpringApplicationRunListeners listeners = getRunListeners(args);
  // 发送启动事件
		listeners.starting(bootstrapContext, this.mainApplicationClass);
		try {
    
      // 准备环境，包括了环境变量、java环境、spring环境、配置文件配置
			ApplicationArguments applicationArguments = new DefaultApplicationArguments(args);
			ConfigurableEnvironment environment = prepareEnvironment(listeners, bootstrapContext, applicationArguments);
			configureIgnoreBeanInfo(environment);
      // 打印banner
			Banner printedBanner = printBanner(environment);
			context = createApplicationContext();
			context.setApplicationStartup(this.applicationStartup);
      // 准备上下文
			prepareContext(bootstrapContext, context, environment, listeners, applicationArguments, printedBanner);
      // 刷新上下文
			refreshContext(context);
      // 预留刷新上下文后的扩展，默认没有实现
			afterRefresh(context, applicationArguments);
			stopWatch.stop();
			if (this.logStartupInfo) {
    
				new StartupInfoLogger(this.mainApplicationClass).logStarted(getApplicationLog(), stopWatch);
			}
      // 发送已启动事件
			listeners.started(context);
			callRunners(context, applicationArguments);
		}
		catch (Throwable ex) {
    
			handleRunFailure(context, ex, listeners);
			throw new IllegalStateException(ex);
		}

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

1. 获取并启动监听器

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

   这里实际上找的是SpringApplicationRunListener类型的监听器，用于监听应用的启用

   监听器实际上最终使用的是spring的事件机制实现

2. 构造应用上下文环境
   包括系统环境变量、java环境、spring环境、spring配置文件

3. 初始化应用上下文
   通过SpringApplication实例化的时候对webType进行的推断，当webType为MVC类型的时候，创建的上下文实际上是AnnotationConfigServletWebServerApplicationContext
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   AnnotationConfigServletWebServerApplicationContext实现了BeanFactory，具有IOC容器能力，其次这个类也是GenericApplicationContext的子类，GenericApplicationContext具备上下文能力，他使用一个BeanFactory字段保存了IOC容器，所以说，AnnotationConfigServletWebServerApplicationContext是上下文，并且持有了容器，对容器能力可以进行扩展

4. 刷新应用上下文前的准备阶段

   private void prepareContext(DefaultBootstrapContext bootstrapContext, ConfigurableApplicationContext context,
   			ConfigurableEnvironment environment, SpringApplicationRunListeners listeners,
   			ApplicationArguments applicationArguments, Banner printedBanner) {
         
     // 设置环境参数
   		context.setEnvironment(environment);
     // 上下文创建好的后置处理
   		postProcessApplicationContext(context);
     // springapplication初始化时找到的initializers做调用
   		applyInitializers(context);
   		listeners.contextPrepared(context);
   		bootstrapContext.close(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");
     // 这里是重点跟进去
   		load(context, sources.toArray(new Object[0]));
   		listeners.contextLoaded(context);
   	}
   

   上下文对象创建出来之后，给上下文设置环境参数，以及调用之前找到的initializer做调用，跟入load方法

   protected void load(ApplicationContext context, Object[] sources) {
         
   		if (logger.isDebugEnabled()) {
         
   			logger.debug("Loading source " + StringUtils.arrayToCommaDelimitedString(sources));
   		}
     // 创建bean定义的加载器，跟进去看如何创建的
   		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);
   		}
     // bean定义加载器执行加载
   		loader.load();
   	}
   

   BeanDefinitionLoader(BeanDefinitionRegistry registry, Object... sources) {
         
   		Assert.notNull(registry, "Registry must not be null");
   		Assert.notEmpty(sources, "Sources must not be empty");
   		this.sources = sources;
     // 注解读取器
   		this.annotatedReader = new AnnotatedBeanDefinitionReader(registry);
     // xml读取器
   		this.xmlReader = (XML_ENABLED ? new XmlBeanDefinitionReader(registry) : null);
   		this.groovyReader = (isGroovyPresent() ? new GroovyBeanDefinitionReader(registry) : null);
     // 包扫描读取器
   		this.scanner = new ClassPathBeanDefinitionScanner(registry);
     // 排除bean过滤器
   		this.scanner.addExcludeFilter(new ClassExcludeFilter(sources));
   	}
   

   说明会通过注解方式、包扫描方式和xml方式读取哪些地方配置了需要交给spring管理的bean，最终执行.load()方法，将这三种读取方式分别执行，拿到所有的bean定义，这里只是读取到这个项目一共有哪些bean定义，然后给保存起来，并不是说这些bean定义一定都要载入到上下文中去，至于为什么在瞎main的刷新上下文中讲解

   BeanDefinition其实就是bean的基本信息，比如这个bean是什么class类型的，bean是否是懒加载，这个bean是否是primary修饰的，这个bean是单例还是property(每次都创新对象)模式等等信息…

5. 刷新应用上下文

   public void refresh() throws BeansException, IllegalStateException {
         
   		synchronized (this.startupShutdownMonitor) {
         
   			StartupStep contextRefresh = this.applicationStartup.start("spring.context.refresh");
   
   			// Prepare this context for refreshing.
         // 刷新前的准备，主要校验了环境参数是否完整，比如某些必填配置没有写
   			prepareRefresh();
   
   			// Tell the subclass to refresh the internal bean factory.
         // 告诉子类刷新内部的beanFactory，其实里面没做啥事，主要就是刷新BeanFactory，获取BeanFactory，返回BeanFactory
   			ConfigurableListableBeanFactory beanFactory = obtainFreshBeanFactory();
   
   			// 准备beanFactory，添加spel表达式的解析器、设置后置处理器等设置一堆东西
   			prepareBeanFactory(beanFactory);
   
   			try {
         
   				// Allows post-processing of the bean factory in context subclasses.
           // 添加bean的后置处理器，bean的后置处理器执行时机是beanDefinition创建之后，bean实例化之前，这样就拥有修改beanDefinition信息的能力
   				postProcessBeanFactory(beanFactory);
   
   				StartupStep beanPostProcess = this.applicationStartup.start("spring.context.beans.post-process");
   				// Invoke factory processors registered as beans in the context.
           // 这里是springboot自动加载的入口，通过主类转换成的BeanDefinition，拿到basePackage，然后找到basePackage下的所有@Component，然后将beanDefinition加入到容器中
   				invokeBeanFactoryPostProcessors(beanFactory);
   
   				// Register bean processors that intercept bean creation.
   
   				registerBeanPostProcessors(beanFactory);
   				beanPostProcess.end();
   
   				// 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();
   				contextRefresh.end();
   			}
   		}
   	}
   

6. 刷新应用上下文后的扩展接口
   方便扩展