spring源码解析之IOC容器（二）------加载和注册

　　上一篇跟踪了IOC容器对配置文件的定位，现在我们继续跟踪代码，看看IOC容器是怎么加载和注册配置文件中的信息的。开始之前，首先我们先来了解一下IOC容器所使用的数据结构-------BeanDefinition，它是一个上层接口，有很多实现类，分别对应不同的数据载体。我们平时开发的时候，也会定义很多pojo类，来作为获取数据的载体。最常见的就是，从数据库中获取数据之后，使用一个定义的pojo来装载，然后我们就可以在程序中使用这个pojo类来编写各种业务逻辑。同样，IOC容器首先会读取配置的XML中各个节点，即各个标签元素，然后根据不同的标签元素，使用不同的数据结构来装载该元素中的各种属性的值。比如我们最熟悉的<bean>标签，就是使用AbstractBeanDefinition这个数据结构，接下来的分析中我们可以看到。

　　先回到上篇资源的定位那里，代码如下：

 public int loadBeanDefinitions(String location, Set<Resource> actualResources) throws BeanDefinitionStoreException {
         ResourceLoader resourceLoader = getResourceLoader();
         if (resourceLoader == null) {
             throw new BeanDefinitionStoreException(
                     "Cannot import bean definitions from location [" + location + "]: no ResourceLoader available");
         }

         if (resourceLoader instanceof ResourcePatternResolver) {
             // Resource pattern matching available.
             try {
                 Resource[] resources = ((ResourcePatternResolver) resourceLoader).getResources(location);
                 int loadCount = loadBeanDefinitions(resources);
                 if (actualResources != null) {
                     for (Resource resource : resources) {
                         actualResources.add(resource);
                     }
                 }
                 if (logger.isDebugEnabled()) {
                     logger.debug("Loaded " + loadCount + " bean definitions from location pattern [" + location + "]");
                 }
                 return loadCount;
             }
             catch (IOException ex) {
                 throw new BeanDefinitionStoreException(
                         "Could not resolve bean definition resource pattern [" + location + "]", ex);
             }
         }
         else {
             // 定位到资源之后，封装成一个resource对象
             Resource resource = resourceLoader.getResource(location);
             int loadCount = loadBeanDefinitions(resource);
             if (actualResources != null) {
                 actualResources.add(resource);
             }
             if (logger.isDebugEnabled()) {
                 logger.debug("Loaded " + loadCount + " bean definitions from location [" + location + "]");
             }
             return loadCount;
         }
     }

　　进入loadBeanDefinitions(resource)方法，正式开始加载源码的跟踪：

         @Override
     public int loadBeanDefinitions(Resource resource) throws BeanDefinitionStoreException {
         return loadBeanDefinitions(new EncodedResource(resource));
     }    

 public int loadBeanDefinitions(EncodedResource encodedResource) throws BeanDefinitionStoreException {
         Assert.notNull(encodedResource, "EncodedResource must not be null");
         if (logger.isInfoEnabled()) {
             logger.info("Loading XML bean definitions from " + encodedResource);
         }

         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 {
             InputStream inputStream = encodedResource.getResource().getInputStream();
             try {
                 InputSource inputSource = new InputSource(inputStream);
                 if (encodedResource.getEncoding() != null) {
                     inputSource.setEncoding(encodedResource.getEncoding());
                 }
                 return 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();
             }
         }
     }

　　进入doLoadBeanDefinitions(inputSource, encodedResource.getResource())方法：

 protected int doLoadBeanDefinitions(InputSource inputSource, Resource resource)
             throws BeanDefinitionStoreException {
         try {
             Document doc = doLoadDocument(inputSource, resource);
             return registerBeanDefinitions(doc, resource);
         }
         catch (BeanDefinitionStoreException ex) {
             throw ex;
         }
         catch (SAXParseException ex) {
             throw new XmlBeanDefinitionStoreException(resource.getDescription(),
                     "Line " + ex.getLineNumber() + " in XML document from " + resource + " is invalid", ex);
         }
         catch (SAXException ex) {
             throw new XmlBeanDefinitionStoreException(resource.getDescription(),
                     "XML document from " + resource + " is invalid", ex);
         }
         catch (ParserConfigurationException ex) {
             throw new BeanDefinitionStoreException(resource.getDescription(),
                     "Parser configuration exception parsing XML from " + resource, ex);
         }
         catch (IOException ex) {
             throw new BeanDefinitionStoreException(resource.getDescription(),
                     "IOException parsing XML document from " + resource, ex);
         }
         catch (Throwable ex) {
             throw new BeanDefinitionStoreException(resource.getDescription(),
                     "Unexpected exception parsing XML document from " + resource, ex);
         }
     }

　　继续进入registerBeanDefinitions(doc, resource)方法：

 public int registerBeanDefinitions(Document doc, Resource resource) throws BeanDefinitionStoreException {
         //此时documentReader已经是DefaultBeanDefinitionDocumentReader类了
         BeanDefinitionDocumentReader documentReader = createBeanDefinitionDocumentReader();
         int countBefore = getRegistry().getBeanDefinitionCount();
         documentReader.registerBeanDefinitions(doc, createReaderContext(resource));
         //返回当前注册的beanDefinition的个数
         return getRegistry().getBeanDefinitionCount() - countBefore;
     }

　　进入registerBeanDefinitions(doc, createReaderContext(resource))方法：

 public void registerBeanDefinitions(Document doc, XmlReaderContext readerContext) {
         this.readerContext = readerContext;
         logger.debug("Loading bean definitions");
         Element root = doc.getDocumentElement();
         doRegisterBeanDefinitions(root);
     }

　　进入doRegisterBeanDefinitions(root)方法：

 protected void doRegisterBeanDefinitions(Element root) {
         // Any nested <beans> elements will cause recursion in this method. In
         // order to propagate and preserve <beans> default-* attributes correctly,
         // keep track of the current (parent) delegate, which may be null. Create
         // the new (child) delegate with a reference to the parent for fallback purposes,
         // then ultimately reset this.delegate back to its original (parent) reference.
         // this behavior emulates a stack of delegates without actually necessitating one.
         BeanDefinitionParserDelegate parent = this.delegate;
         this.delegate = createDelegate(getReaderContext(), root, parent);

         if (this.delegate.isDefaultNamespace(root)) {
             //profile属性平时使用非常少，该属性可以用于配置数据库的切换（常用），使用时，需要在web.xml中配置context-parm
             //<context-parm>
             //    <parm-name>Spring.profiles.active</parm-name>
             //    <parm-value>dev(在applicationContext.xml中配置的profile属性的beans的profile属性值)</parm-name>
             //</context-parm>
             //在applicationContext.xml中的配置
             //<beans profile="dev">    </beans>
             //<beans profile="produce">   </beans>
             String profileSpec = root.getAttribute(PROFILE_ATTRIBUTE);
             if (StringUtils.hasText(profileSpec)) {
                 String[] specifiedProfiles = StringUtils.tokenizeToStringArray(
                         profileSpec, BeanDefinitionParserDelegate.MULTI_VALUE_ATTRIBUTE_DELIMITERS);
                 if (!getReaderContext().getEnvironment().acceptsProfiles(specifiedProfiles)) {
                     if (logger.isInfoEnabled()) {
                         logger.info("Skipped XML bean definition file due to specified profiles [" + profileSpec +
                                 "] not matching: " + getReaderContext().getResource());
                     }
                     return;
                 }
             }
         }

         preProcessXml(root);
         parseBeanDefinitions(root, this.delegate);
         postProcessXml(root);

         this.delegate = parent;
     }

　　这里也用到了模板方法，preProcessXml(root)和postProcessXml(root)这两个方法都是空实现，是留给客户来实现自己的逻辑的。重点研究一下parseBeanDefinitions(root, this.delegate)方法：

 protected void parseBeanDefinitions(Element root, BeanDefinitionParserDelegate delegate) {
         if (delegate.isDefaultNamespace(root)) {
             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)) {
                         parseDefaultElement(ele, delegate);
                     }
                     else {
                         delegate.parseCustomElement(ele);
                     }
                 }
             }
         }
         else {
             delegate.parseCustomElement(root);
         }
     }

　　parseCustomElement(root)方法不需要怎么研究，我们平时几乎不会用到自定义的标签，所以只跟踪parseDefaultElement(ele, delegate)里面的代码：

private void parseDefaultElement(Element ele, BeanDefinitionParserDelegate delegate) {
    //import标签
        if (delegate.nodeNameEquals(ele, IMPORT_ELEMENT)) {
            importBeanDefinitionResource(ele);
        }
    //alias标签
        else if (delegate.nodeNameEquals(ele, ALIAS_ELEMENT)) {
            processAliasRegistration(ele);
        }
   //bean标签
        else if (delegate.nodeNameEquals(ele, BEAN_ELEMENT)) {
            processBeanDefinition(ele, delegate);
        }
  //beans标签
        else if (delegate.nodeNameEquals(ele, NESTED_BEANS_ELEMENT)) {
            // recurse
            doRegisterBeanDefinitions(ele);
        }
    }

　　可以看到，对于不同的标签，spring采用不同的策略进行处理，重点跟踪一下处理bean标签的方法processBeanDefinition(ele, delegate)：

 protected void processBeanDefinition(Element ele, BeanDefinitionParserDelegate delegate) {
         //委托给delegate去进行各种标签的解析，parseBeanDefinitionElement方法中包含了各种标签元素的解析，
         //并将解析好的内容封装成BeanDefinitionHolder对象
         BeanDefinitionHolder bdHolder = delegate.parseBeanDefinitionElement(ele);
         if (bdHolder != null) {
             bdHolder = delegate.decorateBeanDefinitionIfRequired(ele, bdHolder);
             try {
                 // Register the final decorated instance.
                 BeanDefinitionReaderUtils.registerBeanDefinition(bdHolder, getReaderContext().getRegistry());
             }
             catch (BeanDefinitionStoreException ex) {
                 getReaderContext().error("Failed to register bean definition with name '" +
                         bdHolder.getBeanName() + "'", ele, ex);
             }
             // Send registration event.
             getReaderContext().fireComponentRegistered(new BeanComponentDefinition(bdHolder));
         }
     }

　　在这个方法中，delegate.parseBeanDefinitionElement(ele)是解析bean元素中各种属性的方法，registerBeanDefinition(bdHolder, getReaderContext().getRegistry())是将封装好的数据进行存储的方法。先看一下解析的方法：

 public BeanDefinitionHolder parseBeanDefinitionElement(Element ele, BeanDefinition containingBean) {
         //获取bean标签的id属性的值
         String id = ele.getAttribute(ID_ATTRIBUTE);
         //获取bean标签上name属性的值
         String nameAttr = ele.getAttribute(NAME_ATTRIBUTE);

         List<String> aliases = new ArrayList<String>();
         if (StringUtils.hasLength(nameAttr)) {
             //将name的值进行分割，并将它们当作别名存到aliases中
             String[] nameArr = StringUtils.tokenizeToStringArray(nameAttr, MULTI_VALUE_ATTRIBUTE_DELIMITERS);
             aliases.addAll(Arrays.asList(nameArr));
         }

         String beanName = id;
         //如果bean标签的id没有值，但是name属性有值，则将name属性的第一个值当作id的值，并从aliases中将第一个别名移除掉
         if (!StringUtils.hasText(beanName) && !aliases.isEmpty()) {
             beanName = aliases.remove(0);
             if (logger.isDebugEnabled()) {
                 logger.debug("No XML 'id' specified - using '" + beanName +
                         "' as bean name and " + aliases + " as aliases");
             }
         }

         if (containingBean == null) {
             //检查bean的唯一性
             checkNameUniqueness(beanName, aliases, ele);
         }

         //这里已经是将XML中bean元素中的所有属性都封装到beanDefinition对象中了
         AbstractBeanDefinition beanDefinition = parseBeanDefinitionElement(ele, beanName, containingBean);
         if (beanDefinition != null) {
             if (!StringUtils.hasText(beanName)) {
                 try {
                     if (containingBean != null) {
                         beanName = BeanDefinitionReaderUtils.generateBeanName(
                                 beanDefinition, this.readerContext.getRegistry(), true);
                     }
                     else {
                         beanName = this.readerContext.generateBeanName(beanDefinition);
                         // Register an alias for the plain bean class name, if still possible,
                         // if the generator returned the class name plus a suffix.
                         // This is expected for Spring 1.2/2.0 backwards compatibility.
                         String beanClassName = beanDefinition.getBeanClassName();
                         if (beanClassName != null &&
                                 beanName.startsWith(beanClassName) && beanName.length() > beanClassName.length() &&
                                 !this.readerContext.getRegistry().isBeanNameInUse(beanClassName)) {
                             aliases.add(beanClassName);
                         }
                     }
                     if (logger.isDebugEnabled()) {
                         logger.debug("Neither XML 'id' nor 'name' specified - " +
                                 "using generated bean name [" + beanName + "]");
                     }
                 }
                 catch (Exception ex) {
                     error(ex.getMessage(), ele);
                     return null;
                 }
             }
             String[] aliasesArray = StringUtils.toStringArray(aliases);
             //最后将封装好的beanDefinition、它的id、以及它的别名一起封装成BeanDefinitionHolder对象返回
             return new BeanDefinitionHolder(beanDefinition, beanName, aliasesArray);
         }

         return null;
     }

　　我们可以得到如下信息：

　　1、获取bean标签的id属性和name属性的值；

　　2、name属性是可以都有多个值的，以逗号或者分号分割；

　　3、如果id没有赋值，则取name的第一个值作为id的值。所以，我们一般都会给id赋值，这样效率高一些；

　　4、检查以这个id标识的bean是不是唯一的；

　　5、进行其他属性的解析，并最终封装测AbstractBeanDefinition对象，也就是我们前文中提到的数据结构；

　　6、最后封装成BeanDefinitionHolder对象之后返回。

　　进入parseBeanDefinitionElement(ele, beanName, containingBean)方法，看一下其他元素的解析过程：

 public AbstractBeanDefinition parseBeanDefinitionElement(
             Element ele, String beanName, BeanDefinition containingBean) {

         this.parseState.push(new BeanEntry(beanName));

         String className = null;
         if (ele.hasAttribute(CLASS_ATTRIBUTE)) {
             className = ele.getAttribute(CLASS_ATTRIBUTE).trim();
         }

         try {
             String parent = null;
             if (ele.hasAttribute(PARENT_ATTRIBUTE)) {
                 parent = ele.getAttribute(PARENT_ATTRIBUTE);
             }
             AbstractBeanDefinition bd = createBeanDefinition(className, parent);

             parseBeanDefinitionAttributes(ele, beanName, containingBean, bd);
             bd.setDescription(DomUtils.getChildElementValueByTagName(ele, DESCRIPTION_ELEMENT));

             parseMetaElements(ele, bd);
             parseLookupOverrideSubElements(ele, bd.getMethodOverrides());
             parseReplacedMethodSubElements(ele, bd.getMethodOverrides());

             parseConstructorArgElements(ele, bd);
             parsePropertyElements(ele, bd);
             parseQualifierElements(ele, bd);

             bd.setResource(this.readerContext.getResource());
             bd.setSource(extractSource(ele));

             return bd;
         }
         catch (ClassNotFoundException ex) {
             error("Bean class [" + className + "] not found", ele, ex);
         }
         catch (NoClassDefFoundError err) {
             error("Class that bean class [" + className + "] depends on not found", ele, err);
         }
         catch (Throwable ex) {
             error("Unexpected failure during bean definition parsing", ele, ex);
         }
         finally {
             this.parseState.pop();
         }

         return null;
     }

　　解析封装成BeanDefinitionHolder对象之后，就可以进行注册了，先回到之前的processBeanDefinition(ele, delegate)：

 protected void processBeanDefinition(Element ele, BeanDefinitionParserDelegate delegate) {
         //委托给delegate去进行各种标签的解析，parseBeanDefinitionElement方法中包含了各种标签元素的解析，
         //并将解析好的内容封装成BeanDefinitionHolder对象
         BeanDefinitionHolder bdHolder = delegate.parseBeanDefinitionElement(ele);
         if (bdHolder != null) {
             bdHolder = delegate.decorateBeanDefinitionIfRequired(ele, bdHolder);
             try {
                 // Register the final decorated instance.
                 BeanDefinitionReaderUtils.registerBeanDefinition(bdHolder, getReaderContext().getRegistry());
             }
             catch (BeanDefinitionStoreException ex) {
                 getReaderContext().error("Failed to register bean definition with name '" +
                         bdHolder.getBeanName() + "'", ele, ex);
             }
             // Send registration event.
             getReaderContext().fireComponentRegistered(new BeanComponentDefinition(bdHolder));
         }
     }

　　现在进入BeanDefinitionReaderUtils.registerBeanDefinition(bdHolder, getReaderContext().getRegistry())方法进行分析：

 public static void registerBeanDefinition(
             BeanDefinitionHolder definitionHolder, BeanDefinitionRegistry registry)
             throws BeanDefinitionStoreException {

         // Register bean definition under primary name.
         String beanName = definitionHolder.getBeanName();
         registry.registerBeanDefinition(beanName, definitionHolder.getBeanDefinition());

         // Register aliases for bean name, if any.
         String[] aliases = definitionHolder.getAliases();
         if (aliases != null) {
             for (String alias : aliases) {
                 registry.registerAlias(beanName, alias);
             }
         }
     }

　　这里的beanName就是之前封装好的bean的id。这个方法中分别以id和别名作为key来注册bean，其实就是存储在map中。

　　进入registerBeanDefinition(beanName, definitionHolder.getBeanDefinition())，在其子类DefaultListableBeanFactory中有实现：

 public void registerBeanDefinition(String beanName, BeanDefinition beanDefinition)
             throws BeanDefinitionStoreException {

         Assert.hasText(beanName, "Bean name must not be empty");
         Assert.notNull(beanDefinition, "BeanDefinition must not be null");

         if (beanDefinition instanceof AbstractBeanDefinition) {
             try {
                 ((AbstractBeanDefinition) beanDefinition).validate();
             }
             catch (BeanDefinitionValidationException ex) {
                 throw new BeanDefinitionStoreException(beanDefinition.getResourceDescription(), beanName,
                         "Validation of bean definition failed", ex);
             }
         }

         BeanDefinition existingDefinition = this.beanDefinitionMap.get(beanName);
         if (existingDefinition != null) {
             if (!isAllowBeanDefinitionOverriding()) {
                 throw new BeanDefinitionStoreException(beanDefinition.getResourceDescription(), beanName,
                         "Cannot register bean definition [" + beanDefinition + "] for bean '" + beanName +
                         "': There is already [" + existingDefinition + "] bound.");
             }
             else if (existingDefinition.getRole() < beanDefinition.getRole()) {
                 // e.g. was ROLE_APPLICATION, now overriding with ROLE_SUPPORT or ROLE_INFRASTRUCTURE
                 if (logger.isWarnEnabled()) {
                     logger.warn("Overriding user-defined bean definition for bean '" + beanName +
                             "' with a framework-generated bean definition: replacing [" +
                             existingDefinition + "] with [" + beanDefinition + "]");
                 }
             }
             else if (!beanDefinition.equals(existingDefinition)) {
                 if (logger.isInfoEnabled()) {
                     logger.info("Overriding bean definition for bean '" + beanName +
                             "' with a different definition: replacing [" + existingDefinition +
                             "] with [" + beanDefinition + "]");
                 }
             }
             else {
                 if (logger.isDebugEnabled()) {
                     logger.debug("Overriding bean definition for bean '" + beanName +
                             "' with an equivalent definition: replacing [" + existingDefinition +
                             "] with [" + beanDefinition + "]");
                 }
             }
             this.beanDefinitionMap.put(beanName, beanDefinition);
         }
         else {
             if (hasBeanCreationStarted()) {
                 // Cannot modify startup-time collection elements anymore (for stable iteration)
                 synchronized (this.beanDefinitionMap) {
                     this.beanDefinitionMap.put(beanName, beanDefinition);
                     List<String> updatedDefinitions = new ArrayList<String>(this.beanDefinitionNames.size() + 1);
                     updatedDefinitions.addAll(this.beanDefinitionNames);
                     updatedDefinitions.add(beanName);
                     this.beanDefinitionNames = updatedDefinitions;
                     if (this.manualSingletonNames.contains(beanName)) {
                         Set<String> updatedSingletons = new LinkedHashSet<String>(this.manualSingletonNames);
                         updatedSingletons.remove(beanName);
                         this.manualSingletonNames = updatedSingletons;
                     }
                 }
             }
             else {
                 // Still in startup registration phase
                 this.beanDefinitionMap.put(beanName, beanDefinition);
                 this.beanDefinitionNames.add(beanName);
                 this.manualSingletonNames.remove(beanName);
             }
             this.frozenBeanDefinitionNames = null;
         }

         if (existingDefinition != null || containsSingleton(beanName)) {
             resetBeanDefinition(beanName);
         }
     }

　　我们可以看到：这个beanDefinitionMap就是用来存储解析好的bean的，以id作为key。至此，就将所有的bean标签解析好之后封装成BeanDefinition注册到了IOC容器中。但是，到目前为止，IOC容器并没有为我们将这些解析好的数据生成一个一个bean实例，我们仍然不能就这样直接使用。下一篇接着跟踪。