引入
先看SpringBoot的主配置类
@SpringBootApplication public class DemoApplication{ public static void main(String[] args) { SpringApplication.run(StartEurekaApplication.class, args); } } @SpringBootApplication
点进@SpringBootApplication来看，发现@SpringBootApplication是一个组合注解。
@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 { } @SpringBootApplication 由 @Configuration、@EnableAutoConfiguration、@ComponentScan 注解的集合组成：
- @Configuration：允许注册额外的 bean 或导入其他配置类
- @EnableAutoConfiguration：启用 SpringBoot 的自动配置机制
- @ComponentScan：扫描被@Component (@Repository,@Service,@Controller)注解的 bean，注解默认会扫描该类所在的包下所有的类。
@SpringBootConfiguration
@SpringBootConfiguration 注解源码如下：
@Target({ElementType.TYPE}) @Retention(RetentionPolicy.RUNTIME) @Documented @Configuration public @interface SpringBootConfiguration { } 可以看到这个注解除了元注解以外，就只有一个@Configuration，那也就是说这个注解相当于@Configuration，所以这两个注解作用是一样的，也就是能够去注册一些额外的Bean，并且导入一些额外的配置。
@Configuration还有一个作用就是把该类变成一个配置类，不需要额外的XML进行配置。所以@SpringBootConfiguration就相当于@Configuration。
进入@Configuration，发现@Configuration核心是@Component，说明Spring的配置类也是Spring的一个组件。
@Target({ElementType.TYPE}) @Retention(RetentionPolicy.RUNTIME) @Documented @Component public @interface Configuration { @AliasFor( annotation = Component.class ) String value() default ""; } @EnableAutoConfiguration
继续看@EnableAutoConfiguration，这个注解是开启自动配置的功能，源码如下：
@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 {}; } 可以看到它是由 @AutoConfigurationPackage，@Import(EnableAutoConfigurationImportSelector.class)这两个而组成的，
@AutoConfigurationPackage
先看@AutoConfigurationPackage，这是为了让包中的类以及子包中的类能够被自动扫描到spring容器中。
源码如下：
@Target({ElementType.TYPE}) @Retention(RetentionPolicy.RUNTIME) @Documented @Inherited @Import({Registrar.class}) public @interface AutoConfigurationPackage { } 可以看到，这里使用@Import 来给Spring容器中导入一个组件 ，这里导入的是Registrar.class。来看下这个Registrar：
static class Registrar implements ImportBeanDefinitionRegistrar, DeterminableImports { Registrar() { } public void registerBeanDefinitions(AnnotationMetadata metadata, BeanDefinitionRegistry registry) { AutoConfigurationPackages.register(registry, (new AutoConfigurationPackages.PackageImport(metadata)).getPackageName()); } public Set<Object> determineImports(AnnotationMetadata metadata) { return Collections.singleton(new AutoConfigurationPackages.PackageImport(metadata)); } } 就是通过以上这个方法获取扫描的包路径，可以debug查看具体的值：
那metadata是什么呢，可以看到是标注在@SpringBootApplication注解上的DemoApplication，也就是主配置类Application：
其实就是将主配置类（即@SpringBootApplication标注的类）的所在包及子包里面所有组件扫描加载到Spring容器。因此要把DemoApplication放在项目的最高级中（最外层目录）。
@Import(AutoConfigurationImportSelector.class)
看看注解@Import(AutoConfigurationImportSelector.class)，@Import注解就是给Spring容器中导入一些组件，这里传入了一个组件的选择器:AutoConfigurationImportSelector。
可以从图中看出AutoConfigurationImportSelector 继承了 DeferredImportSelector 继承了 ImportSelector，ImportSelector有一个方法为：selectImports。将所有需要导入的组件以全类名的方式返回，这些组件就会被添加到容器中。
public String[] selectImports(AnnotationMetadata annotationMetadata) { if (!this.isEnabled(annotationMetadata)) { return NO_IMPORTS; } else { AutoConfigurationMetadata autoConfigurationMetadata = AutoConfigurationMetadataLoader.loadMetadata(this.beanClassLoader); AutoConfigurationImportSelector.AutoConfigurationEntry autoConfigurationEntry = this.getAutoConfigurationEntry(autoConfigurationMetadata, annotationMetadata); return StringUtils.toStringArray(autoConfigurationEntry.getConfigurations()); } } 这里会给容器中导入 自动配置类（xxxAutoConfiguration），也就是给容器中导入这个场景需要的所有组件，并配置好这些组件。
有了自动配置类，就免去了手动编写配置注入功能组件等的工作。
那是如何获取到这些配置类的呢，看看下面这个方法：
protected AutoConfigurationImportSelector.AutoConfigurationEntry getAutoConfigurationEntry(AutoConfigurationMetadata autoConfigurationMetadata, AnnotationMetadata annotationMetadata) { if (!this.isEnabled(annotationMetadata)) { return EMPTY_ENTRY; } else { AnnotationAttributes attributes = this.getAttributes(annotationMetadata); List<String> configurations = this.getCandidateConfigurations(annotationMetadata, attributes); configurations = this.removeDuplicates(configurations); Set<String> exclusions = this.getExclusions(annotationMetadata, attributes); this.checkExcludedClasses(configurations, exclusions); configurations.removeAll(exclusions); configurations = this.filter(configurations, autoConfigurationMetadata); this.fireAutoConfigurationImportEvents(configurations, exclusions); return new AutoConfigurationImportSelector.AutoConfigurationEntry(configurations, exclusions); } } 可以看到getCandidateConfigurations()这个方法，他的作用就是引入系统已经加载好的一些类，那么到底是那些类呢：
protected List<String> getCandidateConfigurations(AnnotationMetadata metadata, AnnotationAttributes attributes) { List<String> configurations = SpringFactoriesLoader.loadFactoryNames(this.getSpringFactoriesLoaderFactoryClass(), this.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; } public static List<String> loadFactoryNames(Class<?> factoryClass, @Nullable ClassLoader classLoader) { String factoryClassName = factoryClass.getName(); return (List)loadSpringFactories(classLoader).getOrDefault(factoryClassName, Collections.emptyList()); } 会从META-INF/spring.factories中获取资源，然后通过Properties加载资源：
private static Map<String, List<String>> loadSpringFactories(@Nullable ClassLoader classLoader) { MultiValueMap<String, String> result = (MultiValueMap)cache.get(classLoader); if (result != null) { return result; } else { try { Enumeration<URL> urls = classLoader != null ? classLoader.getResources("META-INF/spring.factories") : ClassLoader.getSystemResources("META-INF/spring.factories"); LinkedMultiValueMap result = new LinkedMultiValueMap(); while(urls.hasMoreElements()) { URL url = (URL)urls.nextElement(); UrlResource resource = new UrlResource(url); Properties properties = PropertiesLoaderUtils.loadProperties(resource); Iterator var6 = properties.entrySet().iterator(); while(var6.hasNext()) { Map.Entry<?, ?> entry = (Map.Entry)var6.next(); String factoryClassName = ((String)entry.getKey()).trim(); String[] var9 = StringUtils.commaDelimitedListToStringArray((String)entry.getValue()); int var10 = var9.length; for(int var11 = 0; var11 < var10; ++var11) { String factoryName = var9[var11]; result.add(factoryClassName, factoryName.trim()); } } } cache.put(classLoader, result); return result; } catch (IOException var13) { throw new IllegalArgumentException("Unable to load factories from location [META-INF/spring.factories]", var13); } } } 可以知道SpringBoot在启动的时候从类路径下的META-INF/spring.factories中获取EnableAutoConfiguration指定的值，将这些值作为自动配置类导入到容器中，自动配置类就生效，帮我们进行自动配置工作。以前需要自己配置的东西，自动配置类都帮我们完成了。
如下图可以发现Spring常见的一些类已经自动导入。
@ComponentScan
接下来看@ComponentScan注解，@ComponentScan(excludeFilters = { @Filter(type = FilterType.CUSTOM, classes = TypeExcludeFilter.class), @Filter(type = FilterType.CUSTOM, classes = AutoConfigurationExcludeFilter.class) })，这个注解就是扫描包，然后放入spring容器。
@ComponentScan(excludeFilters = { @Filter(type = FilterType.CUSTOM,classes = {TypeExcludeFilter.class}), @Filter(type = FilterType.CUSTOM,classes = {AutoConfigurationExcludeFilter.class})}) public @interface SpringBootApplication {} 总结下@SpringbootApplication：就是说，他已经把很多东西准备好，具体是否使用取决于我们的程序或者说配置。
小结
总的来说，SpringBoot的自动装配原理就是 通过@EnableAutoConfiguration注解在类路径的META-INF/spring.factories文件中找到所有的对应配置类，然后将这些自动配置类加载到spring容器中
run方法
public static void main(String[] args) { SpringApplication.run(Application.class, args); } 来看下在执行run方法到底有没有用到哪些自动配置的东西，点进run：
public ConfigurableApplicationContext run(String... args) { //计时器 StopWatch stopWatch = new StopWatch(); stopWatch.start(); ConfigurableApplicationContext context = null; Collection<SpringBootExceptionReporter> exceptionReporters = new ArrayList(); this.configureHeadlessProperty(); //监听器 SpringApplicationRunListeners listeners = this.getRunListeners(args); listeners.starting(); Collection exceptionReporters; try { ApplicationArguments applicationArguments = new DefaultApplicationArguments(args); ConfigurableEnvironment environment = this.prepareEnvironment(listeners, applicationArguments); this.configureIgnoreBeanInfo(environment); Banner printedBanner = this.printBanner(environment); //准备上下文 context = this.createApplicationContext(); exceptionReporters = this.getSpringFactoriesInstances(SpringBootExceptionReporter.class, new Class[]{ConfigurableApplicationContext.class}, context); //预刷新context this.prepareContext(context, environment, listeners, applicationArguments, printedBanner); //刷新context this.refreshContext(context); //刷新之后的context this.afterRefresh(context, applicationArguments); stopWatch.stop(); if (this.logStartupInfo) { (new StartupInfoLogger(this.mainApplicationClass)).logStarted(this.getApplicationLog(), stopWatch); } listeners.started(context); this.callRunners(context, applicationArguments); } catch (Throwable var10) { this.handleRunFailure(context, var10, exceptionReporters, listeners); throw new IllegalStateException(var10); } try { listeners.running(context); return context; } catch (Throwable var9) { this.handleRunFailure(context, var9, exceptionReporters, (SpringApplicationRunListeners)null); throw new IllegalStateException(var9); } } 那我们关注的就是 refreshContext(context); 刷新context，我们点进来看。
private void refreshContext(ConfigurableApplicationContext context) { refresh(context); if (this.registerShutdownHook) { try { context.registerShutdownHook(); } catch (AccessControlException ex) { // Not allowed in some environments. } } } 继续点进refresh(context);
protected void refresh(ApplicationContext applicationContext) { Assert.isInstanceOf(AbstractApplicationContext.class, applicationContext); ((AbstractApplicationContext) applicationContext).refresh(); } 会调用 ((AbstractApplicationContext) applicationContext).refresh();方法，点进来看：
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(); } } } 由此可知，就是一个spring的bean的加载过程。继续来看一个方法叫做 onRefresh()：
protected void onRefresh() throws BeansException { // For subclasses: do nothing by default. } 在这里并没有直接实现，找他的具体实现：
比如Tomcat跟web有关，可以看到有个ServletWebServerApplicationContext：
@Override protected void onRefresh() { super.onRefresh(); try { createWebServer(); } catch (Throwable ex) { throw new ApplicationContextException("Unable to start web server", ex); } } 可以看到有一个createWebServer()方法，用于创建web容器，而Tomcat不就是web容器。
那是如何创建的呢：
private void createWebServer() { WebServer webServer = this.webServer; ServletContext servletContext = getServletContext(); if (webServer == null && servletContext == null) { ServletWebServerFactory factory = getWebServerFactory(); this.webServer = factory.getWebServer(getSelfInitializer()); } else if (servletContext != null) { try { getSelfInitializer().onStartup(servletContext); } catch (ServletException ex) { throw new ApplicationContextException("Cannot initialize servlet context", ex); } } initPropertySources(); } factory.getWebServer(getSelfInitializer())，显然是通过工厂的方式创建的。
public interface ServletWebServerFactory { WebServer getWebServer(ServletContextInitializer... initializers); } 可以看到 它是一个接口，为什么会是接口。因为不止是Tomcat一种web容器，可以看到还有Jetty
接下来看TomcatServletWebServerFactory：
@Override public WebServer getWebServer(ServletContextInitializer... initializers) { Tomcat tomcat = new Tomcat(); File baseDir = (this.baseDirectory != null) ? this.baseDirectory : createTempDir("tomcat"); tomcat.setBaseDir(baseDir.getAbsolutePath()); Connector connector = new Connector(this.protocol); tomcat.getService().addConnector(connector); customizeConnector(connector); tomcat.setConnector(connector); tomcat.getHost().setAutoDeploy(false); configureEngine(tomcat.getEngine()); for (Connector additionalConnector : this.additionalTomcatConnectors) { tomcat.getService().addConnector(additionalConnector); } prepareContext(tomcat.getHost(), initializers); return getTomcatWebServer(tomcat); } 这块代码，就是要寻找的内置Tomcat，在这个过程当中，可以看到创建Tomcat的一个流程。
也就是：
- 首先从main找到run()方法，在执行run()方法之前new一个SpringApplication对象
- 进入run()方法，创建应用监听器SpringApplicationRunListeners开始监听
- 然后加载SpringBoot配置环境(ConfigurableEnvironment)，然后把配置环境(Environment)加入监听对象中
- 然后加载应用上下文(ConfigurableApplicationContext)，当做run方法的返回对象
- 最后创建Spring容器，refreshContext(context)，实现starter自动化配置和bean的实例化等工作。
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