@Configuration簡介

用於標識一個類為配置類，與xml配置效果類似

用法簡介

public class TestApplication {

    public static void main(String args[]) {
        AnnotationConfigApplicationContext ac = new AnnotationConfigApplicationContext(AppConfig.class);

    }
}

@Configuration
public class AppConfig {

    @Bean
    public A a(){
        return new A();
    }
    @Bean
    public B b(){
        return new B();
    }
} 




public class A {

    public A(){
        System.out.println("Call A constructor");
    }
}



public class B {

    public B(){
        System.out.println("Call B constructor");
    }
}
 

上面的例子應該是@Configuration最普遍一種使用場景了，在@Configuration class下面配置@Bean method，用於想Spring Ioc容器注入bean.但其實我們把AppConfig的@Configuration註解去掉，對應的Bean也是可以被注入到容器中去的。

那麼問題來了@Configuration到底有什麼作用？

我們給AppConfig改寫一下,如下：

@Configuration
@EnableAspectJAutoProxy
public class AppConfig {

   @Bean
   public A a(){
      b();
      return new A();
   }
   @Bean
   public B b(){
      return new B();
   }
 

再去執行TestApplication#main,那麼執行結果會是什麼樣呢？

Call A constructor
Call B constructor

嗯哼？按照Java的語法，B的建構函式應該是被呼叫了兩次啊？為什麼只有輸出一句Call B constructor?

這其實就是@Configuration再發揮作用啦，不信你去掉@Configuration，再去執行一下，就會發現B的建構函式被執行了兩次。

官方給出了這樣一段解釋對於被@Configuration註解的類

In common scenarios,@Beanmethods are to be declared within@Configurationclasses, ensuring that “full” mode is always used and that cross-method references therefore get redirected to the container’s lifecycle management.

在一般情況下，@Bean method 是被宣告在@Configuration類中的，以確保 full mode總是被使用，並且跨方法的引用會被重定向到容器生命週期管理。

怎麼理解呢？

原來Spring將被@Configuration註解的配置類定義為full configuration, 而將沒有被@Configuration註解的配置類定義為lite configuration。full configuration能重定向從跨方法的引用，從而保證上述程式碼中的b bean是一個單例.

原始碼中是如何實現@Configuration語意

public AnnotationConfigApplicationContext(Class<?>... componentClasses) {
   /**
    * 呼叫無參建構函式，例項化AnnotatedBeanDefinitionReader和ClassPathBeanDefinitionScanner
    * 同時會呼叫父類GenericApplicationContext無參建構函式例項化一個關鍵的工廠DefaultListableBeanFactory
    * 同時還會註冊一些開天闢地的後置處理器到beanDefinitionMap，這些後置處理器有bean工廠後置處理器;有bean後置處理器
    */
   this();
   //將componentClasses註冊到beanDefinitionMap集合中去
   register(componentClasses);
  
   refresh();
}

跟蹤refresh()

@Override
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.
         //供上下文(Context)子類繼承，允許在這裡後置處理bean factory
         postProcessBeanFactory(beanFactory);

         // Invoke factory processors registered as beans in the context.
         //按順序呼叫BeanFactoryPostProcessor,這裡的按順序僅實現了PriorityOrdered和Ordered的語意，未實現@Order註解的語意
         //通過呼叫ConfigurationConfigPostProcessor#postProcessBeanDefinitionRegistry
         //解析@Configuration配置類，將自定義的BeanFactoryPostProcessor、BeanPostProcessor註冊到beanDefinitionMap
         //接著例項化所有（包括開天闢地）的BeanFactoryPostProcessor，然後再呼叫BeanFactoryPostProcessor#postProcessBeanFactory
         invokeBeanFactoryPostProcessors(beanFactory);

         // Register bean processors that intercept bean creation.
         //按順序將BeanPostProcessor例項化成bean並註冊到beanFactory的beanPostProcessors,
         //這裡的按順序僅實現了PriorityOrdered和Ordered的語意，未實現@Order註解的語意
         //因為BeanPostProcessor要在普通bean初始化（）前後被呼叫，所以需要提前完成例項化並註冊到beanFactory的beanPostProcessors
         registerBeanPostProcessors(beanFactory);

         // Initialize message source for this context.
         //註冊國際化相關的Bean
         initMessageSource();

         // Initialize event multicaster for this context.
         //為上下文註冊應用事件廣播器（用於ApplicationEvent的廣播），如果有自定義則使用自定義的，如果沒有則內部例項化一個
         initApplicationEventMulticaster();

         // Initialize other special beans in specific context subclasses.
         onRefresh();

         // Check for listener beans and register them.
         //註冊所有（靜態、動態）的listener,並廣播earlyApplicationEvents
         registerListeners();

         // Instantiate all remaining (non-lazy-init) singletons.
         //例項化使用者自定義的普通單例Bean（非開天闢地的、非後置處理器）
         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();
      }
   }
}

跟蹤invokeBeanFactoryPostProcessors(beanFactory)

protected void invokeBeanFactoryPostProcessors(ConfigurableListableBeanFactory beanFactory) {
   PostProcessorRegistrationDelegate.invokeBeanFactoryPostProcessors(beanFactory, getBeanFactoryPostProcessors());

   // Detect a LoadTimeWeaver and prepare for weaving, if found in the meantime
   // (e.g. through an @Bean method registered by ConfigurationClassPostProcessor)
   if (beanFactory.getTempClassLoader() == null && beanFactory.containsBean(LOAD_TIME_WEAVER_BEAN_NAME)) {
      beanFactory.addBeanPostProcessor(new LoadTimeWeaverAwareProcessor(beanFactory));
      beanFactory.setTempClassLoader(new ContextTypeMatchClassLoader(beanFactory.getBeanClassLoader()));
   }
}

跟蹤invokeBeanFactoryPostProcessors(beanFactory, getBeanFactoryPostProcessors());

public static void invokeBeanFactoryPostProcessors(
            ConfigurableListableBeanFactory beanFactory, List<BeanFactoryPostProcessor> beanFactoryPostProcessors) {

        // Invoke BeanDefinitionRegistryPostProcessors first, if any.
        Set<String> processedBeans = new HashSet<>();

        if (beanFactory instanceof BeanDefinitionRegistry) {
            BeanDefinitionRegistry registry = (BeanDefinitionRegistry) beanFactory;
            List<BeanFactoryPostProcessor> regularPostProcessors = new ArrayList<>();
            List<BeanDefinitionRegistryPostProcessor> registryProcessors = new ArrayList<>();

            for (BeanFactoryPostProcessor postProcessor : beanFactoryPostProcessors) {
                if (postProcessor instanceof BeanDefinitionRegistryPostProcessor) {
                    BeanDefinitionRegistryPostProcessor registryProcessor =
                            (BeanDefinitionRegistryPostProcessor) postProcessor;
                    registryProcessor.postProcessBeanDefinitionRegistry(registry);
                    registryProcessors.add(registryProcessor);
                }
                else {
                    regularPostProcessors.add(postProcessor);
                }
            }

            // Do not initialize FactoryBeans here: We need to leave all regular beans
            // uninitialized to let the bean factory post-processors apply to them!
            // Separate between BeanDefinitionRegistryPostProcessors that implement
            // PriorityOrdered, Ordered, and the rest.
            List<BeanDefinitionRegistryPostProcessor> currentRegistryProcessors = new ArrayList<>();

            // First, invoke the BeanDefinitionRegistryPostProcessors that implement PriorityOrdered.
            String[] postProcessorNames =
                    beanFactory.getBeanNamesForType(BeanDefinitionRegistryPostProcessor.class, true, false);
            for (String ppName : postProcessorNames) {
                if (beanFactory.isTypeMatch(ppName, PriorityOrdered.class)) {
                    currentRegistryProcessors.add(beanFactory.getBean(ppName, BeanDefinitionRegistryPostProcessor.class));
                    processedBeans.add(ppName);
                }
            }
            sortPostProcessors(currentRegistryProcessors, beanFactory);
            registryProcessors.addAll(currentRegistryProcessors);
            //此處呼叫ConfigurationClassPostProcessor#postProcessBeanDefinitionRegistry，
            //解析配置類，為配置中的bean定義生成對應beanDefinition,並注入到registry的beanDefinitionMap
            invokeBeanDefinitionRegistryPostProcessors(currentRegistryProcessors, registry);
            currentRegistryProcessors.clear();

            // Next, invoke the BeanDefinitionRegistryPostProcessors that implement Ordered.
            postProcessorNames = beanFactory.getBeanNamesForType(BeanDefinitionRegistryPostProcessor.class, true, false);
            for (String ppName : postProcessorNames) {
                if (!processedBeans.contains(ppName) && beanFactory.isTypeMatch(ppName, Ordered.class)) {
                    currentRegistryProcessors.add(beanFactory.getBean(ppName, BeanDefinitionRegistryPostProcessor.class));
                    processedBeans.add(ppName);
                }
            }
            sortPostProcessors(currentRegistryProcessors, beanFactory);
            registryProcessors.addAll(currentRegistryProcessors);
            invokeBeanDefinitionRegistryPostProcessors(currentRegistryProcessors, registry);
            currentRegistryProcessors.clear();

            // Finally, invoke all other BeanDefinitionRegistryPostProcessors until no further ones appear.
            boolean reiterate = true;
            while (reiterate) {
                reiterate = false;
                postProcessorNames = beanFactory.getBeanNamesForType(BeanDefinitionRegistryPostProcessor.class, true, false);
                for (String ppName : postProcessorNames) {
                    if (!processedBeans.contains(ppName)) {
                        currentRegistryProcessors.add(beanFactory.getBean(ppName, BeanDefinitionRegistryPostProcessor.class));
                        processedBeans.add(ppName);
                        reiterate = true;
                    }
                }
                sortPostProcessors(currentRegistryProcessors, beanFactory);
                registryProcessors.addAll(currentRegistryProcessors);
                invokeBeanDefinitionRegistryPostProcessors(currentRegistryProcessors, registry);
                currentRegistryProcessors.clear();
            }

            // Now, invoke the postProcessBeanFactory callback of all processors handled so far.
            //呼叫ConfigurationClassPostProcessor#postProcessBeanFactory增強配置類(通過cglib生成增強類,load到jvm記憶體，
            //設定beanDefinition的beanClass為增強類)
            //為什麼要增強配置類？主要是為了讓@Bean生成的bean是單例,
            invokeBeanFactoryPostProcessors(registryProcessors, beanFactory);
            invokeBeanFactoryPostProcessors(regularPostProcessors, beanFactory);
        }

        else {
            // Invoke factory processors registered with the context instance.
            invokeBeanFactoryPostProcessors(beanFactoryPostProcessors, beanFactory);
        }

        // Do not initialize FactoryBeans here: We need to leave all regular beans
        // uninitialized to let the bean factory post-processors apply to them!
        String[] postProcessorNames =
                beanFactory.getBeanNamesForType(BeanFactoryPostProcessor.class, true, false);

        // Separate between BeanFactoryPostProcessors that implement PriorityOrdered,
        // Ordered, and the rest.
        List<BeanFactoryPostProcessor> priorityOrderedPostProcessors = new ArrayList<>();
        List<String> orderedPostProcessorNames = new ArrayList<>();
        List<String> nonOrderedPostProcessorNames = new ArrayList<>();
        for (String ppName : postProcessorNames) {
            if (processedBeans.contains(ppName)) {
                // skip - already processed in first phase above
            }
            else if (beanFactory.isTypeMatch(ppName, PriorityOrdered.class)) {
                priorityOrderedPostProcessors.add(beanFactory.getBean(ppName, BeanFactoryPostProcessor.class));
            }
            else if (beanFactory.isTypeMatch(ppName, Ordered.class)) {
                orderedPostProcessorNames.add(ppName);
            }
            else {
                nonOrderedPostProcessorNames.add(ppName);
            }
        }

        // First, invoke the BeanFactoryPostProcessors that implement PriorityOrdered.
        sortPostProcessors(priorityOrderedPostProcessors, beanFactory);
        invokeBeanFactoryPostProcessors(priorityOrderedPostProcessors, beanFactory);

        // Next, invoke the BeanFactoryPostProcessors that implement Ordered.
        List<BeanFactoryPostProcessor> orderedPostProcessors = new ArrayList<>(orderedPostProcessorNames.size());
        for (String postProcessorName : orderedPostProcessorNames) {
            orderedPostProcessors.add(beanFactory.getBean(postProcessorName, BeanFactoryPostProcessor.class));
        }
        sortPostProcessors(orderedPostProcessors, beanFactory);
        invokeBeanFactoryPostProcessors(orderedPostProcessors, beanFactory);

        // Finally, invoke all other BeanFactoryPostProcessors.
        List<BeanFactoryPostProcessor> nonOrderedPostProcessors = new ArrayList<>(nonOrderedPostProcessorNames.size());
        for (String postProcessorName : nonOrderedPostProcessorNames) {
            nonOrderedPostProcessors.add(beanFactory.getBean(postProcessorName, BeanFactoryPostProcessor.class));
        }
        invokeBeanFactoryPostProcessors(nonOrderedPostProcessors, beanFactory);

        // Clear cached merged bean definitions since the post-processors might have
        // modified the original metadata, e.g. replacing placeholders in values...
        beanFactory.clearMetadataCache();
    }

跟蹤invokeBeanFactoryPostProcessors(registryProcessors, beanFactory);

private static void invokeBeanFactoryPostProcessors(
      Collection<? extends BeanFactoryPostProcessor> postProcessors, ConfigurableListableBeanFactory beanFactory) {

   for (BeanFactoryPostProcessor postProcessor : postProcessors) {
      postProcessor.postProcessBeanFactory(beanFactory);
   }
}

跟蹤ConfigurationClassPostProcessor#postProcessBeanFactory

@Override
public void postProcessBeanFactory(ConfigurableListableBeanFactory beanFactory) {
   int factoryId = System.identityHashCode(beanFactory);
   if (this.factoriesPostProcessed.contains(factoryId)) {
      throw new IllegalStateException(
            "postProcessBeanFactory already called on this post-processor against " + beanFactory);
   }
   this.factoriesPostProcessed.add(factoryId);
   if (!this.registriesPostProcessed.contains(factoryId)) {
      // BeanDefinitionRegistryPostProcessor hook apparently not supported...
      // Simply call processConfigurationClasses lazily at this point then.
      processConfigBeanDefinitions((BeanDefinitionRegistry) beanFactory);
   }
   //為@Configuration註解的類生成增強類（如果有必要），並替換bd中的beanClass屬性，
   enhanceConfigurationClasses(beanFactory);
   beanFactory.addBeanPostProcessor(new ImportAwareBeanPostProcessor(beanFactory));
}

到了這一步謎底幾乎已經揭曉了，@Configuration class是通過增強來實現它的語義的。通過增強把跨方法的引用呼叫重定向到Spring生命週期管理.我們近一步探索下這個enhanceConfigurationClasses方法

public void enhanceConfigurationClasses(ConfigurableListableBeanFactory beanFactory) {
   Map<String, AbstractBeanDefinition> configBeanDefs = new LinkedHashMap<>();
   for (String beanName : beanFactory.getBeanDefinitionNames()) {
      BeanDefinition beanDef = beanFactory.getBeanDefinition(beanName);
      Object configClassAttr = beanDef.getAttribute(ConfigurationClassUtils.CONFIGURATION_CLASS_ATTRIBUTE);
      MethodMetadata methodMetadata = null;
      if (beanDef instanceof AnnotatedBeanDefinition) {
         methodMetadata = ((AnnotatedBeanDefinition) beanDef).getFactoryMethodMetadata();
      }
      if ((configClassAttr != null || methodMetadata != null) && beanDef instanceof AbstractBeanDefinition) {
         // Configuration class (full or lite) or a configuration-derived @Bean method
         // -> resolve bean class at this point...
         AbstractBeanDefinition abd = (AbstractBeanDefinition) beanDef;
         if (!abd.hasBeanClass()) {
            try {
               abd.resolveBeanClass(this.beanClassLoader);
            }
            catch (Throwable ex) {
               throw new IllegalStateException(
                     "Cannot load configuration class: " + beanDef.getBeanClassName(), ex);
            }
         }
      }
      //在ConfigurationClassUtils.checkConfigurationClassCandidate方法中會標記Configuration is full or lite
      if (ConfigurationClassUtils.CONFIGURATION_CLASS_FULL.equals(configClassAttr)) {
         if (!(beanDef instanceof AbstractBeanDefinition)) {
            throw new BeanDefinitionStoreException("Cannot enhance @Configuration bean definition '" +
                  beanName + "' since it is not stored in an AbstractBeanDefinition subclass");
         }
         else if (logger.isInfoEnabled() && beanFactory.containsSingleton(beanName)) {
            logger.info("Cannot enhance @Configuration bean definition '" + beanName +
                  "' since its singleton instance has been created too early. The typical cause " +
                  "is a non-static @Bean method with a BeanDefinitionRegistryPostProcessor " +
                  "return type: Consider declaring such methods as 'static'.");
         }
         configBeanDefs.put(beanName, (AbstractBeanDefinition) beanDef);
      }
   }
   if (configBeanDefs.isEmpty()) {
      // nothing to enhance -> return immediately
      return;
   }

   ConfigurationClassEnhancer enhancer = new ConfigurationClassEnhancer();
   for (Map.Entry<String, AbstractBeanDefinition> 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();
      //為@Configuration註解的類生成增強類
      Class<?> enhancedClass = enhancer.enhance(configClass, this.beanClassLoader);
      if (configClass != enhancedClass) {
         if (logger.isTraceEnabled()) {
            logger.trace(String.format("Replacing bean definition '%s' existing class '%s' with " +
                  "enhanced class '%s'", entry.getKey(), configClass.getName(), enhancedClass.getName()));
         }
         beanDef.setBeanClass(enhancedClass);
      }
   }
}

看到有那麼一句話Class<?> enhancedClass = enhancer.enhance(configClass, this.beanClassLoader);

很明顯了，使用cglib技術為config class生成一個enhancedClass，再通過beanDef.setBeanClass(enhancedClass);修改beanDefinition的BeanClass屬性，在bean例項化階段，會利用反射技術將beanClass屬性對應的類例項化出來，所以最終例項化出來的@Configuration bean是一個代理類的例項。這裡稍微提一下為什麼要使用cglib，而不是jdk動態代理，主要是因為jdk動態代理是基於介面的，而這裡AppConfig並沒有實現任何介面，所以必須用cglib技術。

總結

被@Configuration 註解的類，是 full configuration class，該類會被增強(通過cglib)，從而實現跨方法引用呼叫被重定向到Spring 生命週期管理，最終保證@Bean method生成的bean是一個單例