详解Spring简单容器中的Bean基本加载过程

<?xml version="1.0" encoding="UTF-8"?>
<beansxmlns="http://www.springframework.org/schema/beans"
    xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
    xsi:schemaLocation="http://www.springframework.org/schema/beans http://www.springframework.org/schema/beans/spring-beans.xsd">

  <!-- bean的基本配置 -->
  <beanname="myBean"class="org.zhenchao.framework.MyBean"/>

</beans>

// 1. 定义资源
Resource resource = new ClassPathResource("spring-core.xml");
// 2. 利用XmlBeanFactory解析并注册bean定义
XmlBeanFactory beanFactory = new XmlBeanFactory(resource);
// 3. 从IOC容器加载获取bean
MyBean myBean = (MyBean) beanFactory.getBean("myBean");
// 4. 使用bean
myBean.sayHello();

Resource resource = new ClassPathResource("spring-core.xml");
DefaultListableBeanFactory beanFactory = new DefaultListableBeanFactory();
XmlBeanDefinitionReader reader = new XmlBeanDefinitionReader(beanFactory);
reader.loadBeanDefinitions(resource);
MyBean myBean = (MyBean) beanFactory.getBean("myBean");
myBean.sayHello();

publicXmlBeanFactory(Resource resource)throwsBeansException{
  this(resource, null);
}

publicXmlBeanFactory(Resource resource, BeanFactory parentBeanFactory)throwsBeansException{
  super(parentBeanFactory);
  // 加载xml资源
  this.reader.loadBeanDefinitions(resource);
}

publicintloadBeanDefinitions(EncodedResource encodedResource)throwsBeanDefinitionStoreException{
  Assert.notNull(encodedResource, "EncodedResource must not be null");
  if (logger.isInfoEnabled()) {
    logger.info("Loading XML bean definitions from " + encodedResource.getResource());
  }

  // 标记正在加载的资源，防止循环引用
  Set<EncodedResource> currentResources = this.resourcesCurrentlyBeingLoaded.get();
  if (currentResources == null) {
    currentResources = new HashSet<EncodedResource>(4);
    this.resourcesCurrentlyBeingLoaded.set(currentResources);
  }
  if (!currentResources.add(encodedResource)) {
    throw new BeanDefinitionStoreException("Detected cyclic loading of " + encodedResource + " - check your import definitions!");
  }

  try {
    // 获取资源的输入流
    InputStream inputStream = encodedResource.getResource().getInputStream();
    try {
      // 构造InputSource对象
      InputSource inputSource = new InputSource(inputStream);
      if (encodedResource.getEncoding() != null) {
        inputSource.setEncoding(encodedResource.getEncoding());
      }
      // 真正开始从XML文件中加载Bean定义
      return this.doLoadBeanDefinitions(inputSource, encodedResource.getResource());
    } finally {
      inputStream.close();
    }
  } catch (IOException ex) {
    throw new BeanDefinitionStoreException("IOException parsing XML document from " + encodedResource.getResource(), ex);
  } finally {
    currentResources.remove(encodedResource);
    if (currentResources.isEmpty()) {
      this.resourcesCurrentlyBeingLoaded.remove();
    }
  }
}

protectedintdoLoadBeanDefinitions(InputSource inputSource, Resource resource)throwsBeanDefinitionStoreException{
  try {

    // 1. 加载xml文件，获取到对应的Document（包含获取xml文件的实体解析器和验证模式）
    Document doc = this.doLoadDocument(inputSource, resource);

    // 2. 解析Document对象，并注册bean
    return this.registerBeanDefinitions(doc, resource);

  } catch (BeanDefinitionStoreException ex) {
    // 这里是连环catch，省略
  }
}

protectedDocumentdoLoadDocument(InputSource inputSource, Resource resource)throwsException{
  return this.documentLoader.loadDocument(
      inputSource,
      this.getEntityResolver(), // 获取实体解析器
      this.errorHandler,
      this.getValidationModeForResource(resource), // 获取验证模式
      this.isNamespaceAware());
}

protectedintgetValidationModeForResource(Resource resource){
  int validationModeToUse = this.getValidationMode();
  if (validationModeToUse != VALIDATION_AUTO) {
    // 手动指定了验证模式
    return validationModeToUse;
  }

  // 没有指定验证模式，则自动检测
  int detectedMode = this.detectValidationMode(resource);
  if (detectedMode != VALIDATION_AUTO) {
    return detectedMode;
  }

  // 检测验证模式失败，默认采用XSD验证
  return VALIDATION_XSD;
}

publicDocumentloadDocument(InputSource inputSource, EntityResolver entityResolver,
               ErrorHandler errorHandler, int validationMode, boolean namespaceAware) throws Exception {

  DocumentBuilderFactory factory = this.createDocumentBuilderFactory(validationMode, namespaceAware);
  if (logger.isDebugEnabled()) {
    logger.debug("Using JAXP provider [" + factory.getClass().getName() + "]");
  }
  DocumentBuilder builder = this.createDocumentBuilder(factory, entityResolver, errorHandler);
  return builder.parse(inputSource);
}

publicintregisterBeanDefinitions(Document doc, Resource resource)throwsBeanDefinitionStoreException{
  // 使用DefaultBeanDefinitionDocumentReader构造
  BeanDefinitionDocumentReader documentReader = this.createBeanDefinitionDocumentReader();

  // 记录之前已经注册的BeanDefinition个数
  int countBefore = this.getRegistry().getBeanDefinitionCount();

  // 加载并注册bean
  documentReader.registerBeanDefinitions(doc, createReaderContext(resource));

  // 返回本次加载的bean的数量
  return getRegistry().getBeanDefinitionCount() - countBefore;
}

publicvoidregisterBeanDefinitions(Document doc, XmlReaderContext readerContext){
  this.readerContext = readerContext;
  logger.debug("Loading bean definitions");

  // 获取文档的root结点
  Element root = doc.getDocumentElement();

  this.doRegisterBeanDefinitions(root);
}

protectedvoiddoRegisterBeanDefinitions(Element root){
  BeanDefinitionParserDelegate parent = this.delegate;
  this.delegate = this.createDelegate(getReaderContext(), root, parent);

  if (this.delegate.isDefaultNamespace(root)) {
    // 处理profile标签（其作用类比pom.xml中的profile）
    String profileSpec = root.getAttribute(PROFILE_ATTRIBUTE);
    if (StringUtils.hasText(profileSpec)) {
      String[] specifiedProfiles =
          StringUtils.tokenizeToStringArray(profileSpec, BeanDefinitionParserDelegate.MULTI_VALUE_ATTRIBUTE_DELIMITERS);
      if (!this.getReaderContext().getEnvironment().acceptsProfiles(specifiedProfiles)) {
        if (logger.isInfoEnabled()) {
          logger.info("Skipped XML bean definition file due to specified profiles [" + profileSpec + "] not matching: " + getReaderContext().getResource());
        }
        return;
      }
    }
  }

  // 解析预处理，留给子类实现
  this.preProcessXml(root);

  // 解析并注册BeanDefinition
  this.parseBeanDefinitions(root, this.delegate);

  // 解析后处理，留给子类实现
  this.postProcessXml(root);

  this.delegate = parent;
}

protectedvoidparseBeanDefinitions(Element root, BeanDefinitionParserDelegate delegate){
  if (delegate.isDefaultNamespace(root)) {
    // 解析默认标签
    NodeList nl = root.getChildNodes();
    for (int i = 0; i < nl.getLength(); i++) {
      Node node = nl.item(i);
      if (node instanceof Element) {
        Element ele = (Element) node;
        if (delegate.isDefaultNamespace(ele)) {
          // 解析默认标签
          this.parseDefaultElement(ele, delegate);
        } else {
          // 解析自定义标签
          delegate.parseCustomElement(ele);
        }
      }
    }
  } else {
    // 解析自定义标签
    delegate.parseCustomElement(root);
  }
}

publicObjectgetBean(String name)throwsBeansException{
  return this.doGetBean(name, null, null, false);
}

protected <T> TdoGetBean(
    final String name, final Class<T> requiredType, final Object[] args, boolean typeCheckOnly) throws BeansException {

  /*
   * 转化对应的beanName
   *
   * 传入的参数可能是alias，也可能是FactoryBean，所以需要进行解析，主要包含以下内容：
   * 1. 去除FactoryBean的修饰符“&”
   * 2. 取指定alias对应的最终的name
   */
  final String beanName = this.transformedBeanName(name);

  Object bean;

  /*
   * 检查缓存或者实例工厂中是否有对应的实例
   *
   * 为什么会一开始就进行检查？
   * 因为在创建单例bean的时候会存在依赖注入的情况，而在创建依赖的时候为了避免循环依赖
   * Spring创建bean的原则是不等bean创建完成就会将创建bean的ObjectFactory提前曝光，即将对应的ObjectFactory加入到缓存
   * 一旦下一个bean创建需要依赖上一个bean，则直接使用ObjectFactory
   */
  Object sharedInstance = this.getSingleton(beanName); // 获取单例
  if (sharedInstance != null && args == null) {
    // 实例已经存在
    if (logger.isDebugEnabled()) {
      if (this.isSingletonCurrentlyInCreation(beanName)) {
        logger.debug("Returning eagerly cached instance of singleton bean '" + beanName + "' that is not fully initialized yet - a consequence of a circular reference");
      } else {
        logger.debug("Returning cached instance of singleton bean '" + beanName + "'");
      }
    }
    // 返回对应的实例
    bean = this.getObjectForBeanInstance(sharedInstance, name, beanName, null);
  } else {
    // 单例实例不存在
    if (this.isPrototypeCurrentlyInCreation(beanName)) {
      /*
       * 只有在单例模式下才会尝试解决循环依赖问题
       * 对于原型模式，如果存在循环依赖，也就是满足this.isPrototypeCurrentlyInCreation(beanName)，抛出异常
       */
      throw new BeanCurrentlyInCreationException(beanName);
    }

    BeanFactory parentBeanFactory = this.getParentBeanFactory();
    if (parentBeanFactory != null && !this.containsBeanDefinition(beanName)) {
      // 如果在beanDefinitionMap中（即所有已经加载的类中）不包含目标bean，则尝试从parentBeanFactory中检测
      String nameToLookup = this.originalBeanName(name);
      if (args != null) {
        // 递归到BeanFactory中寻找
        return (T) parentBeanFactory.getBean(nameToLookup, args);
      } else {
        return parentBeanFactory.getBean(nameToLookup, requiredType);
      }
    }

    // 如果不仅仅是做类型检查，则创建bean
    if (!typeCheckOnly) {
      this.markBeanAsCreated(beanName);
    }

    try {
      /*
       * 将存储XML配置的GenericBeanDefinition转换成RootBeanDefinition
       * 如果指定了beanName是子bean的话，同时会合并父类的相关属性
       */
      final RootBeanDefinition mbd = this.getMergedLocalBeanDefinition(beanName);
      this.checkMergedBeanDefinition(mbd, beanName, args);

      // 获取当前bean依赖的bean
      String[] dependsOn = mbd.getDependsOn();
      if (dependsOn != null) {
        // 存在依赖，递归实例化依赖的bean
        for (String dep : dependsOn) {
          if (this.isDependent(beanName, dep)) {
            throw new BeanCreationException(mbd.getResourceDescription(), beanName, "Circular depends-on relationship between '" + beanName + "' and '" + dep + "'");
          }
          // 缓存依赖调用
          this.registerDependentBean(dep, beanName);
          this.getBean(dep);
        }
      }

      // 实例化依赖的bean后，实例化mbd自身
      if (mbd.isSingleton()) {
        // scope == singleton
        sharedInstance = this.getSingleton(beanName, new ObjectFactory<Object>() {
          @Override
          publicObjectgetObject()throwsBeansException{
            try {
              return createBean(beanName, mbd, args);
            } catch (BeansException ex) {
              // Explicitly remove instance from singleton cache: It might have been put there
              // eagerly by the creation process, to allow for circular reference resolution.
              // Also remove any beans that received a temporary reference to the bean.
              destroySingleton(beanName);
              throw ex;
            }
          }
        });
        bean = this.getObjectForBeanInstance(sharedInstance, name, beanName, mbd);
      } else if (mbd.isPrototype()) {
        // scope == prototype
        Object prototypeInstance;
        try {
          this.beforePrototypeCreation(beanName);
          prototypeInstance = this.createBean(beanName, mbd, args);
        } finally {
          this.afterPrototypeCreation(beanName);
        }
        // 返回对应的实例
        bean = this.getObjectForBeanInstance(prototypeInstance, name, beanName, mbd);
      } else {
        // 其它scope
        String scopeName = mbd.getScope();
        final Scope scope = this.scopes.get(scopeName);
        if (scope == null) {
          throw new IllegalStateException("No Scope registered for scope name '" + scopeName + "'");
        }
        try {
          Object scopedInstance = scope.get(beanName, new ObjectFactory<Object>() {
            @Override
            publicObjectgetObject()throwsBeansException{
              beforePrototypeCreation(beanName);
              try {
                return createBean(beanName, mbd, args);
              } finally {
                afterPrototypeCreation(beanName);
              }
            }
          });
          // 返回对应的实例
          bean = this.getObjectForBeanInstance(scopedInstance, name, beanName, mbd);
        } catch (IllegalStateException ex) {
          throw new BeanCreationException(beanName, "Scope '" + scopeName + "' is not active for the current thread; consider defining a scoped proxy for this bean if you intend to refer to it from a singleton", ex);
        }
      }
    } catch (BeansException ex) {
      cleanupAfterBeanCreationFailure(beanName);
      throw ex;
    }
  }

  // 检查需要的类型是否符合bean的实际类型，对应getBean时指定的requireType
  if (requiredType != null && bean != null && !requiredType.isAssignableFrom(bean.getClass())) {
    try {
      return this.getTypeConverter().convertIfNecessary(bean, requiredType);
    } catch (TypeMismatchException ex) {
      if (logger.isDebugEnabled()) {
        logger.debug("Failed to convert bean '" + name + "' to required type '" + ClassUtils.getQualifiedName(requiredType) + "'", ex);
      }
      throw new BeanNotOfRequiredTypeException(name, requiredType, bean.getClass());
    }
  }
  return (T) bean;
}