SpringBoot 注解原理，自动装配原理，图文并茂，万字长文！
首先，先看SpringBoot的主配置类：
@SpringBootApplicationpublic class StartEurekaApplication{public static void main(String[] args){SpringApplication.run(StartEurekaApplication.class, args);}}
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 {}
我们先来看
@SpringBootConfiguration：
@Target({ElementType.TYPE})@Retention(RetentionPolicy.RUNTIME)@Documented@Configurationpublic @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，它让我们能够去注册一些额外的Bean，并且导入一些额外的配置。那@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@Componentpublic @interface Configuration {@AliasFor(annotation = Component.class)String value() default "";}
继续来看下一个
@EnableAutoConfiguration
这个注解是开启自动配置的功能。
@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，我们先说
@AutoConfigurationPackage
他是说：让包中的类以及子包中的类能够被自动扫描到spring容器中。
@Target({ElementType.TYPE})@Retention(RetentionPolicy.RUNTIME)@Documented@Inherited@Import({Registrar.class})public @interface AutoConfigurationPackage {}
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DemosbApplication，也就是我们的主配置类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。因此我们要把DemoApplication放在项目的最高级中（最外层目录）。
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());}}
会给容器中导入非常多的自动配置类（xxxAutoConfiguration）；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，免去了我们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.AutoConfigurationEntrygetAutoConfigurationEntry(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);}}
我们可以看到getCandidateConfigurations()这个方法，他的作用就是引入系统已经加载好的一些类，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。以前我们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。
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@interfaceSpringBootApplication{}
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。
接下来继续看run方法：
publicstaticvoid 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，我们点进run：
publicConfigurableApplicationContext run(String... args) {//计时器StopWatch stopWatch = newStopWatch();stopWatch.start();ConfigurableApplicationContext context = null;Collection<SpringBootExceptionReporter> exceptionReporters = newArrayList();this.configureHeadlessProperty();//监听器SpringApplicationRunListeners listeners = this.getRunListeners(args);listeners.starting();Collection exceptionReporters;try{ApplicationArguments applicationArguments = newDefaultApplicationArguments(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,newClass[]{ConfigurableApplicationContext.class}, context);//预刷新contextthis.prepareContext(context, environment, listeners, applicationArguments, printedBanner);//刷新contextthis.refreshContext(context);//刷新之后的contextthis.afterRefresh(context, applicationArguments);stopWatch.stop();if(this.logStartupInfo) {(newStartupInfoLogger(this.mainApplicationClass)).logStarted(this.getApplicationLog(), stopWatch);}listeners.started(context);this.callRunners(context, applicationArguments);} catch(Throwable var10) {this.handleRunFailure(context, var10, exceptionReporters, listeners);thrownewIllegalStateException(var10);}try{listeners.running(context);return context;} catch(Throwable var9) {this.handleRunFailure(context, var9, exceptionReporters, (SpringApplicationRunListeners)null);thrownewIllegalStateException(var9);}}
那我们关注的就是 refreshContext(context); 刷新context，我们点进来看。
privatevoid refreshContext(ConfigurableApplicationContext context) {refresh(context);if(this.registerShutdownHook) {try{context.registerShutdownHook();}catch(AccessControlException ex) {// Not allowed in some environments.}}}
我们继续点进refresh(context);
protectedvoid refresh(ApplicationContext applicationContext) {Assert.isInstanceOf(AbstractApplicationContext.class, applicationContext);((AbstractApplicationContext) applicationContext).refresh();}
他会调用 ((AbstractApplicationContext) applicationContext).refresh();方法，我们点进来看：
publicvoid refresh() throwsBeansException, 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()：
protectedvoid onRefresh() throwsBeansException{// For subclasses: do nothing by default.}
他在这里并没有直接实现，但是我们找他的具体实现：
比如Tomcat跟web有关，我们可以看到有个ServletWebServerApplicationContext：
@Overrideprotectedvoid onRefresh() {super.onRefresh();try{createWebServer();}catch(Throwable ex) {thrownewApplicationContextException("Unable to start web server", ex);}}
可以看到有一个createWebServer();方法他是创建web容器的，而Tomcat不就是web容器，那是如何创建的呢，我们继续看：
privatevoid createWebServer() {WebServer webServer = this.webServer;ServletContext servletContext = getServletContext();if(webServer == null&& servletContext == null) {ServletWebServerFactory factory = getWebServerFactory();this.webServer = factory.getWebServer(getSelfInitializer());}elseif(servletContext != null) {try{getSelfInitializer().onStartup(servletContext);}catch(ServletException ex) {thrownewApplicationContextException("Cannot initialize servlet context",ex);}}initPropertySources();}
factory.getWebServer(getSelfInitializer());他是通过工厂的方式创建的。
publicinterfaceServletWebServerFactory{WebServer getWebServer(ServletContextInitializer... initializers);}
可以看到 它是一个接口，为什么会是接口。因为我们不止是Tomcat一种web容器。
我们看到还有Jetty，那我们来看TomcatServletWebServerFactory：
@OverridepublicWebServer getWebServer(ServletContextInitializer... initializers) {Tomcat tomcat = newTomcat();File baseDir = (this.baseDirectory != null) ? this.baseDirectory: createTempDir("tomcat");tomcat.setBaseDir(baseDir.getAbsolutePath());Connector connector = newConnector(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);}
那这块代码，就是我们要寻找的内置Tomcat，在这个过程当中，我们可以看到创建Tomcat的一个流程。
如果不明白的话， 我们在用另一种方式来理解下，大家要应该都知道stater举点例子。
<dependency><groupId>org.springframework.boot</groupId><artifactId>spring-boot-starter-data-redis</artifactId></dependency><dependency><groupId>org.springframework.boot</groupId><artifactId>spring-boot-starter-freemarker</artifactId></dependency>
首先自定义一个stater。
<parent><groupId>org.springframework.boot</groupId><artifactId>spring-boot-starter-parent</artifactId><version>2.1.4.RELEASE</version><relativePath/></parent><groupId>com.zgw</groupId><artifactId>gw-spring-boot-starter</artifactId><version>1.0-SNAPSHOT</version><dependencies><dependency><groupId>org.springframework.boot</groupId><artifactId>spring-boot-autoconfigure</artifactId></dependency></dependencies>
我们先来看maven配置写入版本号，如果自定义一个stater的话必须依赖spring-boot-autoconfigure这个包,我们先看下项目目录。
publicclassGwServiceImpl implementsGwService{@AutowiredGwProperties properties;@OverridepublicvoidHello(){String name=properties.getName();System.out.println(name+"说:你们好啊");}}
我们做的就是通过配置文件来定制name这个是具体实现。
@Component@ConfigurationProperties(prefix = "spring.gwname")publicclassGwProperties{String name="zgw";publicString getName() {return name;}publicvoid setName(String name) {this.name = name;}}
这个类可以通过@ConfigurationProperties读取配置文件。
@Configuration@ConditionalOnClass(GwService.class) //扫描类@EnableConfigurationProperties(GwProperties.class) //让配置类生效publicclassGwAutoConfiguration{/*** 功能描述 托管给spring* @author zgw* @return*/@Bean@ConditionalOnMissingBeanpublicGwService gwService(){returnnewGwServiceImpl();}}
这个为配置类，为什么这么写因为，spring-boot的stater都是这么写的，我们可以参照他仿写stater，以达到自动配置的目的，然后我们在通过spring.factories也来进行配置。
org.springframework.boot.autoconfigure.EnableAutoConfiguration=com.gw.GwAutoConfiguration
然后这样一个简单的stater就完成了，然后可以进行maven的打包，在其他项目引入就可以使用。
作者：stackOverFlow stackOverFlow
cnblogs.com/jing99/p/11504113.html
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