在使用springboot時(shí)，很多配置我們都沒有做，都是springboot在幫我們完成，這很大一部分歸功于springboot自動(dòng)裝配，那springboot的自動(dòng)裝配的原理是怎么實(shí)現(xiàn)的呢？

自動(dòng)裝配原理

springboot 版本：2.4.3

SpringBootApplication

springboot啟動(dòng)類必須要加@SpringBootApplication注解，那這個(gè)注解是什么意思呢？

@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 {

拋開元數(shù)據(jù)注解來說，SpringBootApplication注解主要由@SpringBootConfiguration、@EnableAutoConfiguration、@ComponentScan組成。這三個(gè)又有不同的作用如下：

• @SpringBootConfiguration:被@Configuration標(biāo)記，表示這是個(gè)springboot配置，支持JavaConfig的方式來進(jìn)行配置。
• @EnableAutoConfiguration：表示開啟自動(dòng)裝配(重點(diǎn)介紹)
• @ComponentScan：掃描注解，掃描basePackages包下的bean并將他們注入到IOC容器中，比如：@Service、@Controller、@Component等注解。

EnableAutoConfiguration

真正開啟自動(dòng)配置的還是@EnableAutoConfiguration注解，來看下EnableAutoConfiguration注解源碼：

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

@EnableAutoConfiguration 又是由@AutoConfigurationPackage和@Import注解組成。

• @AutoConfigurationPackage是一個(gè)復(fù)合注解的，它在內(nèi)部使用@Import(AutoConfigurationPackages.Registrar.class)注解，Registrar是AutoConfigurationPackages的一個(gè)內(nèi)部類，它的作用就是注冊(cè)一個(gè)springboot啟動(dòng)類所在的包名，這個(gè)包名可以供列如JPA的使用。
• AutoConfigurationImportSelector通過selectImports方法將配置類導(dǎo)入，從而完成bean的裝配

AutoConfigurationImportSelector

AutoConfigurationImportSelector實(shí)現(xiàn)了DeferredImportSelector接口，DeferredImportSelector是ImportSelector的變種，它是一個(gè)延遲選擇器。實(shí)現(xiàn)了DeferredImportSelector接口的子類如果重新了getImportGroup方法并返回DeferredImportSelector內(nèi)部接口Group的子類，DeferredImportSelector接口的子類的子類將不會(huì)調(diào)用selectImports而是調(diào)用Group的selectImports方法。

接下來看看AutoConfigurationImportSelector重寫了getImportGroup方法并返回一個(gè)內(nèi)部類AutoConfigurationGroup，AutoConfigurationGroup#selectImports方法只是對(duì)配置數(shù)組進(jìn)行排序篩選，真正處理自動(dòng)配置的流程的是process方法。 process方法源碼如下：

@Override
  public void process(AnnotationMetadata annotationMetadata, DeferredImportSelector deferredImportSelector) {
   Assert.state(deferredImportSelector instanceof AutoConfigurationImportSelector,
     () -> String.format("Only %s implementations are supported, got %s",
       AutoConfigurationImportSelector.class.getSimpleName(),
       deferredImportSelector.getClass().getName()));
   AutoConfigurationEntry autoConfigurationEntry = ((AutoConfigurationImportSelector) deferredImportSelector)
     .getAutoConfigurationEntry(annotationMetadata);
   this.autoConfigurationEntries.add(autoConfigurationEntry);
   for (String importClassName : autoConfigurationEntry.getConfigurations()) {
    this.entries.putIfAbsent(importClassName, annotationMetadata);
   }
  }

process方法就是對(duì)AutoConfigurationGroup一些屬性的填充，起作用的還是AutoConfigurationImportSelector.getAutoConfigurationEntry方法。 getAutoConfigurationEntry方法中經(jīng)過各種判斷過濾、去重等操作，最后返回AutoConfigurationEntry對(duì)象。源碼如下

  protected AutoConfigurationEntry getAutoConfigurationEntry(AnnotationMetadata annotationMetadata) {
  if (!isEnabled(annotationMetadata)) {
   return EMPTY_ENTRY;
  }
    //獲取EnableAutoConfiguration注解的exclude和excludeName屬性
  AnnotationAttributes attributes = getAttributes(annotationMetadata);
    //獲取所有配置類
  List<String> configurations = getCandidateConfigurations(annotationMetadata, attributes);
    //去除重復(fù)配置類
  configurations = removeDuplicates(configurations);
  Set<String> exclusions = getExclusions(annotationMetadata, attributes);
  checkExcludedClasses(configurations, exclusions);
    //移除 exclude的配置類
  configurations.removeAll(exclusions);
  configurations = getConfigurationClassFilter().filter(configurations);
  fireAutoConfigurationImportEvents(configurations, exclusions);
  return new AutoConfigurationEntry(configurations, exclusions);
 }

getAutoConfigurationEntry 篩選的自動(dòng)配置類：

image

getAutoConfigurationEntry方法中要重點(diǎn)分析的是getCandidateConfigurations方法。getCandidateConfigurations的作用是獲取所有自動(dòng)裝配的配置類的全限定名。

來看下getCandidateConfigurations方法源碼：

protected List<String> getCandidateConfigurations(AnnotationMetadata metadata, AnnotationAttributes attributes) {
  List<String> configurations = SpringFactoriesLoader.loadFactoryNames(getSpringFactoriesLoaderFactoryClass(),
    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;
 }
  
  protected Class<?> getSpringFactoriesLoaderFactoryClass() {
  return EnableAutoConfiguration.class;
 }

這里面用到了SpringFactoriesLoader是spring提供的一種加載配置的方式，它會(huì)將類從配置文件中讀取到，然后利用反射將bean加載到IOC容器中。

SpringFactoriesLoader.loadFactoryNames中會(huì)加載META-INF/spring.factories自動(dòng)配置類。這些配置類在spring.factories文件中是以key=value的形式存儲(chǔ)的，來看下部分自動(dòng)配置類：

# Auto Configure
org.springframework.boot.autoconfigure.EnableAutoConfiguration=\
org.springframework.boot.autoconfigure.admin.SpringApplicationAdminJmxAutoConfiguration,\
org.springframework.boot.autoconfigure.aop.AopAutoConfiguration,\
org.springframework.boot.autoconfigure.amqp.RabbitAutoConfiguration,\
org.springframework.boot.autoconfigure.batch.BatchAutoConfiguration,\
org.springframework.boot.autoconfigure.cache.CacheAutoConfiguration,\
org.springframework.boot.autoconfigure.cassandra.CassandraAutoConfiguration,\
org.springframework.boot.autoconfigure.cloud.CloudAutoConfiguration,\
org.springframework.boot.autoconfigure.context.ConfigurationPropertiesAutoConfiguration,\
org.springframework.boot.autoconfigure.context.MessageSourceAutoConfiguration,\
org.springframework.boot.autoconfigure.context.PropertyPlaceholderAutoConfiguration,\
org.springframework.boot.autoconfigure.couchbase.CouchbaseAutoConfiguration,\
org.springframework.boot.autoconfigure.dao.PersistenceExceptionTranslationAutoConfiguration,\
org.springframework.boot.autoconfigure.data.cassandra.CassandraDataAutoConfiguration,\
org.springframework.boot.autoconfigure.data.cassandra.CassandraReactiveDataAutoConfiguration,\

來看下SpringFactoriesLoader.loadFactoryNames源碼：

public static final String FACTORIES_RESOURCE_LOCATION = "META-INF/spring.factories";

public static List<String> loadFactoryNames(Class<?> factoryType, @Nullable ClassLoader classLoader) {
  ClassLoader classLoaderToUse = classLoader;
  if (classLoaderToUse == null) {
   classLoaderToUse = SpringFactoriesLoader.class.getClassLoader();
  }
  String factoryTypeName = factoryType.getName();
  return loadSpringFactories(classLoaderToUse).getOrDefault(factoryTypeName, Collections.emptyList());
 }

 private static Map<String, List<String>> loadSpringFactories(ClassLoader classLoader) {
  Map<String, List<String>> result = cache.get(classLoader);
  if (result != null) {
   return result;
  }

  result = new HashMap<>();
  try {
   Enumeration<URL> urls = classLoader.getResources(FACTORIES_RESOURCE_LOCATION);
   while (urls.hasMoreElements()) {
    URL url = urls.nextElement();
    UrlResource resource = new UrlResource(url);
    Properties properties = PropertiesLoaderUtils.loadProperties(resource);
    for (Map.Entry<?, ?> entry : properties.entrySet()) {
     String factoryTypeName = ((String) entry.getKey()).trim();
     String[] factoryImplementationNames =
       StringUtils.commaDelimitedListToStringArray((String) entry.getValue());
     for (String factoryImplementationName : factoryImplementationNames) {
      result.computeIfAbsent(factoryTypeName, key -> new ArrayList<>())
        .add(factoryImplementationName.trim());
     }
    }
   }

   // Replace all lists with unmodifiable lists containing unique elements
   result.replaceAll((factoryType, implementations) -> implementations.stream().distinct()
     .collect(Collectors.collectingAndThen(Collectors.toList(), Collections::unmodifiableList)));
   cache.put(classLoader, result);
  }
  catch (IOException ex) {
   throw new IllegalArgumentException("Unable to load factories from location [" +
     FACTORIES_RESOURCE_LOCATION + "]", ex);
  }
  return result;
 }

loadFactoryNames方法只是傳遞一個(gè)org.springframework.boot.autoconfigure.EnableAutoConfiguration作為key，然后取到對(duì)應(yīng)的自動(dòng)配置類列表。 最終調(diào)用的是loadSpringFactories方法，loadSpringFactories會(huì)從jar包中找到spring.factories文件然后將其中的自動(dòng)配置類存到一個(gè)map中，從下圖可以看到map中存在很多bean，loadFactoryNames方法在加載自動(dòng)配置類時(shí)只取了一個(gè)key。弱水三千，只取一瓢。

image

自動(dòng)裝配流程圖大致如下：

image

總結(jié)

• EnableAutoConfiguration注解開啟自動(dòng)裝配，其上的標(biāo)記的@Import(AutoConfigurationImportSelector.class)注解中導(dǎo)入配置類
• AutoConfigurationImportSelector實(shí)現(xiàn)DeferredImportSelector接口，并重寫了getImportGroup方法并返回AutoConfigurationImportSelector.AutoConfigurationGroup，AutoConfigurationImportSelector.AutoConfigurationGroup.process開始處理自動(dòng)配置流程。
• AutoConfigurationImportSelector.getCandidateConfigurations獲取所有配置類getAutoConfigurationEntry方法篩選，去重、移除不符合條件的自動(dòng)配置類。
• SpringFactoriesLoader.loadSpringFactories從jar包中找到所有META-INF/spring.factories文件并讀取自動(dòng)配置類，存放到map中， loadFactoryNames方法通過全限定名org.springframework.boot.autoconfigure.EnableAutoConfiguration找到自動(dòng)配置類。
• 最后經(jīng)過層層篩選，去重、移除不符合條件的bean，由ConfigurationClassPostProcessor#processConfigBeanDefinitions注冊(cè)所有的自動(dòng)配置類。

能力一般，水平有限，如有錯(cuò)誤，請(qǐng)多指出。 如果對(duì)你有用點(diǎn)個(gè)關(guān)注給個(gè)贊唄