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); } }
點進@SpringBootApplication來看，發現@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於@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，並且導入一些額外的配置。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進行配置。所以@SpringBootConfiguration就相當於@Configuration。進入@Configuration，發現@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容器中導入一個組件 ，這里導入的是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，可以看到是標注在@SpringBootApplication注解上的DemosbApplication，也就是我們的主配置類Application：
其實就是將主配置類（即@SpringBootApplication標注的類）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()); } }
會給容器中導入非常多的自動配置類（xxxAutoConfiguration）；就是給容器中導入這個場景需要的所有組件，並配置好這些組件。
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已經加載好的一些類，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入到容器中，自動配置類就生效，幫我們進行自動配置工作。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。如下圖可以發現Spring常見的一些類已經自動導入。
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 {}
總結下@SpringbootApplication：就是說，他已經把很多東西准備好，具體是否使用取決於我們的程序或者說配置。
接下來繼續看run方法：
public static void main(String[] args) { SpringApplication.run(Application.class, args); }
來看下在執行run方法到底有沒有用到哪些自動配置的東西，我們點進run：
public ConfigurableApplicationContext run(String... args) { //計時器 StopWatch stopWatch = new StopWatch(); stopWatch.start(); ConfigurableApplicationContext context = null; Collection<SpringBootExceptionReporter> exceptionReporters = new ArrayList(); this.configureHeadlessProperty(); //監聽器 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); //准備上下文 context = this.createApplicationContext(); exceptionReporters = this.getSpringFactoriesInstances(SpringBootExceptionReporter.class,
new Class[]{ConfigurableApplicationContext.class}, context); //預刷新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); } }
那我們關注的就是 refreshContext(context); 刷新context，我們點進來看。
private void refreshContext(ConfigurableApplicationContext context) { refresh(context); if (this.registerShutdownHook) { try { context.registerShutdownHook(); } catch (AccessControlException ex) { // Not allowed in some environments. } } }
我們繼續點進refresh(context);
protected void refresh(ApplicationContext applicationContext) { Assert.isInstanceOf(AbstractApplicationContext.class, applicationContext); ((AbstractApplicationContext) applicationContext).refresh(); }
他會調用 ((AbstractApplicationContext) applicationContext).refresh();方法，我們點進來看：
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。繼續來看一個方法叫做 onRefresh()：
protected void onRefresh() throws BeansException { // For subclasses: do nothing by default. }
他在這里並沒有直接實現，但是我們找他的具體實現：
比如Tomcat跟web有關，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();方法他是創建web容器的，而Tomcat不就是web容器，那是如何創建的呢，我們繼續看：
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。因為我們不止是Tomcat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置Tomcat，在這個過程當中，我們可以看到創建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，大家要應該都知道stater舉點例子。
<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,我們先看下項目目錄。
public class GwServiceImpl implements GwService{ @Autowired GwProperties properties; @Override public 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這個是具體實現。
@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 @ConditionalOnMissingBean public GwService gwService() { return new GwServiceImpl(); } }
這個為配置類，為什么這么寫因為，spring-boot的stater都是這么寫的，我們可以參照他仿寫stater，以達到自動配置的目的，然后我們在通過spring.factories也來進行配置。
org.springframework.boot.autoconfigure.EnableAutoConfiguration=com.gw.GwAutoConfiguration
然后這樣一個簡單的stater就完成了，然后可以進行maven的打包，在其他項目引入就可以使用。