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首先，先看SpringBoot的主配置類：

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

點(diǎn)進(jìn)@SpringBootApplication來看，發(fā)現(xiàn)@SpringBootApplication是一個組合注解。

@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?{}

首先我們先來看 @SpringBootConfiguration：

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

可以看到這個注解除了元注解以外，就只有一個@Configuration，那也就是說這個注解相當(dāng)于@Configuration，所以這兩個注解作用是一樣的，它讓我們能夠去注冊一些額外的Bean，并且導(dǎo)入一些額外的配置。

那@Configuration還有一個作用就是把該類變成一個配置類，不需要額外的XML進(jìn)行配置。所以@SpringBootConfiguration就相當(dāng)于@Configuration。進(jìn)入@Configuration，發(fā)現(xiàn)@Configuration核心是@Component，說明Spring的配置類也是Spring的一個組件。

@Target({ElementType.TYPE}) @Retention(RetentionPolicy.RUNTIME) @Documented @Component public?@interface?Configuration?{@AliasFor(annotation?=?Component.class)String?value()?default?""; }

繼續(xù)來看下一個@EnableAutoConfiguration,這個注解是開啟自動配置的功能。

@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)這兩個而組成的，我們先說@AutoConfigurationPackage，他是說：讓包中的類以及子包中的類能夠被自動掃描到spring容器中。

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

使用@Import來給Spring容器中導(dǎo)入一個組件 ，這里導(dǎo)入的是Registrar.class。來看下這個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));}}

就是通過以上這個方法獲取掃描的包路徑，可以debug查看具體的值：

那metadata是什么呢，可以看到是標(biāo)注在@SpringBootApplication注解上的DemosbApplication，也就是我們的主配置類Application：

其實(shí)就是將主配置類（即@SpringBootApplication標(biāo)注的類）的所在包及子包里面所有組件掃描加載到Spring容器。因此我們要把DemoApplication放在項(xiàng)目的最高級中（最外層目錄）。

看看注解@Import(AutoConfigurationImportSelector.class)，@Import注解就是給Spring容器中導(dǎo)入一些組件，這里傳入了一個組件的選擇器:AutoConfigurationImportSelector。

可以從圖中看出AutoConfigurationImportSelector 繼承了 DeferredImportSelector 繼承了 ImportSelector，ImportSelector有一個方法為：selectImports。將所有需要導(dǎo)入的組件以全類名的方式返回，這些組件就會被添加到容器中。

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());} }

會給容器中導(dǎo)入非常多的自動配置類（xxxAutoConfiguration）；就是給容器中導(dǎo)入這個場景需要的所有組件，并配置好這些組件。

有了自動配置類，免去了我們手動編寫配置注入功能組件等的工作。那是如何獲取到這些配置類的呢，看看下面這個方法：

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);} }

我們可以看到getCandidateConfigurations()這個方法，他的作用就是引入系統(tǒng)已經(jīng)加載好的一些類，到底是那些類呢：

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()); }

會從META-INF/spring.factories中獲取資源，然后通過Properties加載資源：

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在啟動的時候從類路徑下的META-INF/spring.factories中獲取EnableAutoConfiguration指定的值，將這些值作為自動配置類導(dǎo)入到容器中，自動配置類就生效，幫我們進(jìn)行自動配置工作。以前我們需要自己配置的東西，自動配置類都幫我們完成了。如下圖可以發(fā)現(xiàn)Spring常見的一些類已經(jīng)自動導(dǎo)入。

接下來看@ComponentScan注解，@ComponentScan(excludeFilters = { @Filter(type = FilterType.CUSTOM, classes = TypeExcludeFilter.class), @Filter(type = FilterType.CUSTOM, classes = AutoConfigurationExcludeFilter.class) })，這個注解就是掃描包，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?{}

總結(jié)下@SpringbootApplication：就是說，他已經(jīng)把很多東西準(zhǔn)備好，具體是否使用取決于我們的程序或者說配置。

接下來繼續(xù)看run方法：

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

來看下在執(zhí)行run方法到底有沒有用到哪些自動配置的東西，我們點(diǎn)進(jìn)run：

public?ConfigurableApplicationContext?run(String...?args)?{//計(jì)時器StopWatch?stopWatch?=?new?StopWatch();stopWatch.start();ConfigurableApplicationContext?context?=?null;Collection<SpringBootExceptionReporter>?exceptionReporters?=?new?ArrayList();this.configureHeadlessProperty();//監(jiān)聽器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);//準(zhǔn)備上下文context?=?this.createApplicationContext();exceptionReporters?=?this.getSpringFactoriesInstances(SpringBootExceptionReporter.class,???????????????????????new?Class[]{ConfigurableApplicationContext.class},?context);//預(yù)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)?{(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);} }

那我們關(guān)注的就是 refreshContext(context); 刷新context，我們點(diǎn)進(jìn)來看。

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

我們繼續(xù)點(diǎn)進(jìn)refresh(context);

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

他會調(diào)用 ((AbstractApplicationContext) applicationContext).refresh();方法，我們點(diǎn)進(jìn)來看：

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，就是一個spring的bean的加載過程。繼續(xù)來看一個方法叫做 onRefresh()：

protected?void?onRefresh()?throws?BeansException?{//?For?subclasses:?do?nothing?by?default. }

他在這里并沒有直接實(shí)現(xiàn)，但是我們找他的具體實(shí)現(xiàn)：

比如Tomcat跟web有關(guān)，我們可以看到有個ServletWebServerApplicationContext：

@Override protected?void?onRefresh()?{super.onRefresh();try?{createWebServer();}catch?(Throwable?ex)?{throw?new?ApplicationContextException("Unable?to?start?web?server",?ex);} }

可以看到有一個createWebServer();方法他是創(chuàng)建web容器的，而Tomcat不就是web容器，那是如何創(chuàng)建的呢，我們繼續(xù)看：

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());他是通過工廠的方式創(chuàng)建的。

public?interface?ServletWebServerFactory?{WebServer?getWebServer(ServletContextInitializer...?initializers); }

可以看到 它是一個接口，為什么會是接口。因?yàn)槲覀儾恢故荰omcat一種web容器。

我們看到還有Jetty，那我們來看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); }

那這塊代碼，就是我們要尋找的內(nèi)置Tomcat，在這個過程當(dāng)中，我們可以看到創(chuàng)建Tomcat的一個流程。

如果不明白的話， 我們在用另一種方式來理解下，大家要應(yīng)該都知道stater舉點(diǎn)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>

首先自定義一個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>

我們先來看maven配置寫入版本號，如果自定義一個stater的話必須依賴spring-boot-autoconfigure這個包,我們先看下項(xiàng)目目錄。

public?class?GwServiceImpl??implements?GwService{@AutowiredGwProperties?properties;@Overridepublic?void?Hello(){String?name=properties.getName();System.out.println(name+"說:你們好啊");} }

我們做的就是通過配置文件來定制name這個是具體實(shí)現(xiàn)。

@Component @ConfigurationProperties(prefix?=?"spring.gwname") public?class?GwProperties?{String?name="zgw";public?String?getName()?{return?name;}public?void?setName(String?name)?{this.name?=?name;} }

這個類可以通過@ConfigurationProperties讀取配置文件。

@Configuration @ConditionalOnClass(GwService.class)??//掃描類 @EnableConfigurationProperties(GwProperties.class)?//讓配置類生效 public?class?GwAutoConfiguration?{/***?功能描述?托管給spring*?@author?zgw*?@return*/@Bean@ConditionalOnMissingBeanpublic?GwService?gwService(){return?new?GwServiceImpl();} }

這個為配置類，為什么這么寫因?yàn)?#xff0c;spring-boot的stater都是這么寫的，我們可以參照他仿寫stater，以達(dá)到自動配置的目的，然后我們在通過spring.factories也來進(jìn)行配置。

org.springframework.boot.autoconfigure.EnableAutoConfiguration=com.gw.GwAutoConfiguration

然后這樣一個簡單的stater就完成了，然后可以進(jìn)行maven的打包，在其他項(xiàng)目引入就可以使用。

來自：博客園

鏈接：cnblogs.com/cmt/p/14553189.html

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