Spring中的Configuration注解你真的了解吗？

　　环境：Springboot2.3.12.RELEASE
　　关于使用@Configuration注解后为何 类内部使用@Bean注解方法声明的方法在多次调用后会是返回相同的实例？ 1 示例
　　如下：  @Configuration  public class BeanConfig {            @Bean      public Date date() {          CustomerDAO c1 = customer() ;          CustomerDAO c2 = customer() ;          System.out.println(＂c1 = ＂ + c1) ;          System.out.println(＂c2 = ＂ + c2) ;          return new Date() ;      }            @Bean      public CustomerDAO customer() {          return new CustomerDAO() ;      }        }
　　输出：  c1 = com.pack.config.CustomerDAO@1c0cf193  c2 = com.pack.config.CustomerDAO@1c0cf193
　　将上面的@Configuration注解换成@Component后再次运行：  c1 = com.pack.config.CustomerDAO@7b7e4b20  c2 = com.pack.config.CustomerDAO@5ac0bf84
　　多个实例。 2 原理
　　使用@Configuration注解的类会为何会保证多次方法的调用返回的是同一个实例？ 2.1 创建Spring上下文
　　创建上下文最核心的就是注册了一个 BeanFactory  处理器  public class SpringApplication {      public ConfigurableApplicationContext run(String... args) {          ConfigurableApplicationContext context = null;          context = createApplicationContext();      }      protected ConfigurableApplicationContext createApplicationContext() {          return this.applicationContextFactory.create(this.webApplicationType);      }  }
　　跟踪createApplicationContext方法  public class SpringApplication {      protected ConfigurableApplicationContext createApplicationContext() {          return this.applicationContextFactory.create(this.webApplicationType);      }  }
　　进入 ApplicationContextFactory   public interface ApplicationContextFactory {      ApplicationContextFactory DEFAULT = (webApplicationType) -> {          try {              switch (webApplicationType) {              case SERVLET:                  // 当前是基于SERVLET容器的，所以创建下面这个实例                  return new AnnotationConfigServletWebServerApplicationContext();              case REACTIVE:                  return new AnnotationConfigReactiveWebServerApplicationContext();              default:                  return new AnnotationConfigApplicationContext();              }          }      };  }
　　进入 AnnotationConfigServletWebServerApplicationContext   public class AnnotationConfigServletWebServerApplicationContext extends ServletWebServerApplicationContext          implements AnnotationConfigRegistry {      public AnnotationConfigServletWebServerApplicationContext() {          this.reader = new AnnotatedBeanDefinitionReader(this);          this.scanner = new ClassPathBeanDefinitionScanner(this);      }  }
　　进入 AnnotatedBeanDefinitionReader  构造方法  public class AnnotatedBeanDefinitionReader {      public AnnotatedBeanDefinitionReader(BeanDefinitionRegistry registry) {          this(registry, getOrCreateEnvironment(registry));      }      public AnnotatedBeanDefinitionReader(BeanDefinitionRegistry registry, Environment environment) {          this.registry = registry;          this.conditionEvaluator = new ConditionEvaluator(registry, environment, null);          // 核心方法，这里非常重要，会注册系统的一些处理器          AnnotationConfigUtils.registerAnnotationConfigProcessors(this.registry);      }  }
　　进入 AnnotationConfigUtils#registerAnnotationConfigProcessors  方法  public abstract class AnnotationConfigUtils {      public static void registerAnnotationConfigProcessors(BeanDefinitionRegistry registry) {          registerAnnotationConfigProcessors(registry, null);      }  }
　　进入 AnnotationConfiguUtils#registerAnnotationConfigProcessors  方法  public class AnnotationBeanDefinitionReader {      public static final String CONFIGURATION_ANNOTATION_PROCESSOR_BEAN_NAME =              ＂org.springframework.context.annotation.internalConfigurationAnnotationProcessor＂;      public static Set registerAnnotationConfigProcessors(              BeanDefinitionRegistry registry, @Nullable Object source) {          // ...          // 判断当前上下文中是否有internalConfigurationAnnotationProcessor          // 没有则注册专门用来处理@Configuration注解的          // ConfigurationClassPostProcessor处理器          if (!registry.containsBeanDefinition(CONFIGURATION_ANNOTATION_PROCESSOR_BEAN_NAME)) {              RootBeanDefinition def = new RootBeanDefinition(ConfigurationClassPostProcessor.class);              def.setSource(source);              beanDefs.add(registerPostProcessor(registry, def, CONFIGURATION_ANNOTATION_PROCESSOR_BEAN_NAME));          }          // ...          return beanDefs;      }  }
　　以上在创建Spring上下文对象的时候注册了一个非常核心的处理器 ConfigurationClassPostProcessor  该类会处理带有@Configuration注解的类。 2.2 执行处理器
　　这里就会执行上一步中注册的 BeanFactoryPostProcessor  。
　　上面注册的 ConfigurationClassPostProcessor  实现了 BeanFactoryPostProcessor  接口  public class SpringApplication {      public ConfigurableApplicationContext run(String... args) {          ConfigurableApplicationContext context = null;          context = createApplicationContext();          // 刷新上下文          refreshContext(context);      }  }
　　跟踪refreshContext方法，进入到如下代码执行  public abstract class AbstractApplicationContext extends DefaultResourceLoader          implements ConfigurableApplicationContext {      public void refresh() throws BeansException, IllegalStateException {          synchronized (this.startupShutdownMonitor) {              // ...              // 执行BeanFactoryPostProcessor中的回调方法              invokeBeanFactoryPostProcessors(beanFactory);              // ...          }      }  }
　　进入invokeBeanFactoryPostProcessors方法  public abstract class AbstractApplicationContext extends DefaultResourceLoader          implements ConfigurableApplicationContext {      protected void invokeBeanFactoryPostProcessors(ConfigurableListableBeanFactory beanFactory) {          PostProcessorRegistrationDelegate.invokeBeanFactoryPostProcessors(beanFactory, getBeanFactoryPostProcessors());      }  }
　　进入invokeBeanfactoryPostProcessors方法
　　注意ConfigurationClassPostProcessor实现了 BeanDefinitionRegistryPostProcessor  也实现了 BeanFactoryPostProcessor  接口，这里方法的执行是会先执行 BeanDefinitionRegistryPostProcessor  的回调方法，最后再执行 BeanFactoryPostProcessor  中的回调方法。
　　执行流程为： ConfigurationClassPostProcessor#postProcessBeanDefinitionRegistry  该方法主要左右通过方法名称也能猜到处理并注册当前CLASSPATH下的所有类中符合条件的Bean。 ConfigurationClassPostProcessor#postProcessBeanFactory  该方法的作用  final class PostProcessorRegistrationDelegate {      public static void invokeBeanFactoryPostProcessors(              ConfigurableListableBeanFactory beanFactory, List beanFactoryPostProcessors) {          // ...          // Now, invoke the postProcessBeanFactory callback of all processors handled so far.          // 执行ConfigurationClassPostProcessor#postProcessBeanFactory方法          invokeBeanFactoryPostProcessors(registryProcessors, beanFactory);          invokeBeanFactoryPostProcessors(regularPostProcessors, beanFactory);      }      private static void invokeBeanFactoryPostProcessors(              Collection<? extends BeanFactoryPostProcessor> postProcessors, ConfigurableListableBeanFactory beanFactory) {          // 执行ConfigurationClassPostProcessor中的回调          for (BeanFactoryPostProcessor postProcessor : postProcessors) {              postProcessor.postProcessBeanFactory(beanFactory);          }      }  }
　　进入 ConfigurationClassPostProcessor#postProcessBeanFactory  方法  public class ConfigurationClassPostProcessor implements BeanDefinitionRegistryPostProcessor,  PriorityOrdered, ResourceLoaderAware,   ApplicationStartupAware, BeanClassLoaderAware, EnvironmentAware {      public void postProcessBeanFactory(ConfigurableListableBeanFactory beanFactory) {          // 增强配置类          enhanceConfigurationClasses(beanFactory);          beanFactory.addBeanPostProcessor(new ImportAwareBeanPostProcessor(beanFactory));      }  }2.3 增强配置类
　　接着上面进入enhanceConfigurationClasses方法  public class ConfigurationClassPostProcessor {      public void enhanceConfigurationClasses(ConfigurableListableBeanFactory beanFactory) {          Map configBeanDefs = new LinkedHashMap<>();          // 遍历当前上下文中注册的所有BeanDefinition集合，找到所有的配置类          // 这里会就会找到两个，一个是启动类，一个是上面示例中的配置类BeanConfig          for (String beanName : beanFactory.getBeanDefinitionNames()) {              BeanDefinition beanDef = beanFactory.getBeanDefinition(beanName);              // ...              configBeanDefs.put(beanName, (AbstractBeanDefinition) beanDef);          }          if (configBeanDefs.isEmpty() || NativeDetector.inNativeImage()) {              // nothing to enhance -> return immediately              enhanceConfigClasses.end();              return;          }            ConfigurationClassEnhancer enhancer = new ConfigurationClassEnhancer();          // 遍历找到所有的@Configuration配置类          for (Map.Entry entry : configBeanDefs.entrySet()) {              AbstractBeanDefinition beanDef = entry.getValue();              // If a @Configuration class gets proxied, always proxy the target class              beanDef.setAttribute(AutoProxyUtils.PRESERVE_TARGET_CLASS_ATTRIBUTE, Boolean.TRUE);              // Set enhanced subclass of the user-specified bean class              Class<?> configClass = beanDef.getBeanClass();              // 这里就将BeanConfig配置类进行创建代理              Class<?> enhancedClass = enhancer.enhance(configClass, this.beanClassLoader);              if (configClass != enhancedClass) {                  beanDef.setBeanClass(enhancedClass);              }          }      }  }
　　进入 ConfigurationClassEnhancer#enhance  方法  class ConfigurationClassEnhancer {        // The callbacks to use. Note that these callbacks must be stateless.      private static final Callback[] CALLBACKS = new Callback[] {              new BeanMethodInterceptor(),              new BeanFactoryAwareMethodInterceptor(),              NoOp.INSTANCE      };      private static final ConditionalCallbackFilter CALLBACK_FILTER = new ConditionalCallbackFilter(CALLBACKS);      public Class<?> enhance(Class<?> configClass, @Nullable ClassLoader classLoader) {          if (EnhancedConfiguration.class.isAssignableFrom(configClass)) {              return configClass;          }          // 创建代理类          Class<?> enhancedClass = createClass(newEnhancer(configClass, classLoader));          return enhancedClass;      }      private Enhancer newEnhancer(Class<?> configSuperClass, @Nullable ClassLoader classLoader) {          Enhancer enhancer = new Enhancer();          enhancer.setSuperclass(configSuperClass);          enhancer.setInterfaces(new Class<?>[] {EnhancedConfiguration.class});          enhancer.setUseFactory(false);          enhancer.setNamingPolicy(SpringNamingPolicy.INSTANCE);          enhancer.setStrategy(new BeanFactoryAwareGeneratorStrategy(classLoader));          // 这里就是设置代理类执行的方法拦截器          // 最核心的就是上面的BeanMethodInterceptor类          enhancer.setCallbackFilter(CALLBACK_FILTER);          enhancer.setCallbackTypes(CALLBACK_FILTER.getCallbackTypes());          return enhancer;      }  }
　　进入 BeanMethodInterceptor  类  private static class BeanMethodInterceptor implements MethodInterceptor, ConditionalCallback {      public Object intercept(Object enhancedConfigInstance, Method beanMethod, Object[] beanMethodArgs,                              MethodProxy cglibMethodProxy) throws Throwable {            // ...           // 该方法中就是从Spring容器中获取当前的Bean          return resolveBeanReference(beanMethod, beanMethodArgs, beanFactory, beanName);      }  }
　　进入resolveBeanReference方法  class ConfigurationClassEnhancer {      private Object resolveBeanReference(Method beanMethod, Object[] beanMethodArgs,                  ConfigurableBeanFactory beanFactory, String beanName) {          Object beanInstance = (useArgs ? beanFactory.getBean(beanName, beanMethodArgs) :                          beanFactory.getBean(beanName));          return beanInstance ;      }  }
　　完毕！！！
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