一文帶你搞懂SpringBoot中自動裝配原理

更新時間：2025年01月20日 08:37:04 作者：seven97_top

這篇文章主要為大家詳細(xì)介紹了SpringBoot中自動裝配原理的相關(guān)知識,文中的示例代碼講解詳細(xì),具有一定的借鑒價值,有需要的小伙伴可以參考下

引入

先看SpringBoot的主配置類

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

@SpringBootApplication

點進(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 {

}

@SpringBootApplication 由 @Configuration、@EnableAutoConfiguration、@ComponentScan 注解的集合組成：

@Configuration：允許注冊額外的 bean 或?qū)肫渌渲妙?/li>
@EnableAutoConfiguration：啟用 SpringBoot 的自動配置機(jī)制
@ComponentScan：掃描被@Component (@Repository,@Service,@Controller)注解的 bean，注解默認(rèn)會掃描該類所在的包下所有的類。

@SpringBootConfiguration

@SpringBootConfiguration 注解源碼如下：

@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，也就是能夠去注冊一些額外的Bean，并且導(dǎo)入一些額外的配置。

@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 "";
}

@EnableAutoConfiguration

繼續(xù)看@EnableAutoConfiguration，這個注解是開啟自動配置的功能，源碼如下：

@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)這兩個而組成的，

@AutoConfigurationPackage

先看@AutoConfigurationPackage，這是為了讓包中的類以及子包中的類能夠被自動掃描到spring容器中。

源碼如下：

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

可以看到，這里使用@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DemoApplication，也就是主配置類Application：

其實就是將主配置類（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。因此要把DemoApplication放在項目的最高級中（最外層目錄）。

@Import(AutoConfigurationImportSelector.class)

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());
    }
}

這里會給容器中導(dǎo)入自動配置類（xxxAutoConfiguration），也就是給容器中導(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，就免去了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);
    }
}

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，那么到底是那些類呢：

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。

@ComponentScan

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。

小結(jié)

總的來說，SpringBoot的自動裝配原理就是通過@EnableAutoConfiguration注解在類路徑的META-INF/spring.factories文件中找到所有的對應(yīng)配置類，然后將這些自動配置類加載到spring容器中

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，點進(jìn)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(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(guān)注的就是 refreshContext(context); 刷新context，我們點進(jìn)來看。

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

繼續(xù)點進(jìn)refresh(context);

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

會調(diào)用 ((AbstractApplicationContext) applicationContext).refresh();方法，點進(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.
}

在這里并沒有直接實現(xiàn)，找他的具體實現(xiàn)：

比如Tomcat跟web有關(guān)，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，用于創(chuàng)建web容器，而Tomcat不就是web容器。

那是如何創(chuàng)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())，顯然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。因為不止是Tomcat一種web容器，可以看到還有Jetty

接下來看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);
}

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

也就是：

首先從main找到run()方法，在執(zhí)行run()方法之前new一個SpringApplication對象
進(jìn)入run()方法，創(chuàng)建應(yīng)用監(jiān)聽器SpringApplicationRunListeners開始監(jiān)聽
然后加載SpringBoot配置環(huán)境(ConfigurableEnvironment)，然后把配置環(huán)境(Environment)加入監(jiān)聽對象中
然后加載應(yīng)用上下文(ConfigurableApplicationContext)，當(dāng)做run方法的返回對象
最后創(chuàng)建Spring容器，refreshContext(context)，實現(xiàn)starter自動化配置和bean的實例化等工作。

到此這篇關(guān)于一文帶你搞懂SpringBoot中自動裝配原理的文章就介紹到這了,更多相關(guān)SpringBoot自動裝配原理內(nèi)容請搜索腳本之家以前的文章或繼續(xù)瀏覽下面的相關(guān)文章希望大家以后多多支持腳本之家！

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