上一篇博客說到，ApplicationContext將解析BeanDefinition的工作委托給BeanDefinitionReader組件，這篇就接著分析一下BeanDefinition的解析過程。

loadBeanDefinitions: 源碼閱讀

入口是loadBeanDefinitions方法

protected void loadBeanDefinitions(XmlBeanDefinitionReader reader) 
throws IOException {
        String[] configLocations = getConfigLocations();
        if (configLocations != null) {
            for (String configLocation : configLocations) {
                reader.loadBeanDefinitions(configLocation);
            }
        }
}

這是解析過程最外圍的代碼，首先要獲取到配置文件的路徑，這在之前已經(jīng)完成了。
然后將每個配置文件的路徑，作為參數(shù)傳給BeanDefinitionReader的loadBeanDefinitions方法里

public int loadBeanDefinitions(String location) throws BeanDefinitionStoreException {
        return loadBeanDefinitions(location, null);
}

這個方法又調(diào)用了重載方法

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 {
            // Can only load single resources by absolute URL.
            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;
        }
    }

首先getResourceLoader()的實(shí)現(xiàn)的前提條件是因?yàn)閄mlBeanDefinitionReader在實(shí)例化的時候已經(jīng)確定了創(chuàng)建了實(shí)例ResourceLoader實(shí)例, 代碼位于 AbstractBeanDefinitionReader

protected AbstractBeanDefinitionReader(BeanDefinitionRegistry registry) {   
     Assert.notNull(registry, "BeanDefinitionRegistry must not be null"); 
     this.registry = registry;   
     // Determine ResourceLoader to use.  
     if (this.registry instanceof ResourceLoader) {     
         this.resourceLoader = (ResourceLoader) this.registry;   
      }  else {      
         this.resourceLoader = new PathMatchingResourcePatternResolver();  
      }   
     // Inherit Environment if possible   
     if (this.registry instanceof EnvironmentCapable) {      
          this.environment = ((EnvironmentCapable)this.registry).getEnvironment();  
      }  else {      
          this.environment = new StandardEnvironment(); 
      }
}

這個方法比較長，BeanDefinitionReader不能直接加載配置文件，需要把配置文件封裝成Resource，然后才能調(diào)用重載方法loadBeanDefinitions()。所以這個方法其實(shí)就是2段，第一部分是委托ResourceLoader將配置文件封裝成Resource，第二部分是調(diào)用loadBeanDefinitions()，對Resource進(jìn)行解析

而這里的ResourceLoader，就是前面的XmlWebApplicationContext，因?yàn)锳pplicationContext接口，是繼承自ResourceLoader接口的

Resource也是一個接口體系，在web環(huán)境下，這里就是ServletContextResource

接下來進(jìn)入重載方法loadBeanDefinitions()

public int loadBeanDefinitions(Resource... resources) throws BeanDefinitionStoreException {
        Assert.notNull(resources, "Resource array must not be null");
        int counter = 0;
        for (Resource resource : resources) {
            counter += loadBeanDefinitions(resource);
        }
        return counter;
    }

這里就不用說了，就是把每一個Resource作為參數(shù)，繼續(xù)調(diào)用重載方法。讀spring源碼，會發(fā)現(xiàn)重載方法特別多。

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

還是重載方法，不過這里對傳進(jìn)來的Resource又進(jìn)行了一次封裝，變成了編碼后的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.getResource());
        }

        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();
            }
        }
    }

這個就是loadBeanDefinitions()的最后一個重載方法，比較長，可以拆看來看。

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

        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!");
        }

這第一部分，是處理線程相關(guān)的工作，把當(dāng)前正在解析的Resource，設(shè)置為當(dāng)前Resource。

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();
            }
        }

這里是第二部分，是核心，首先把Resource還原為InputStream，然后調(diào)用實(shí)際解析的方法doLoadBeanDefinitions()。可以看到，這種命名方式是很值得學(xué)習(xí)的，一種業(yè)務(wù)方法，比如parse()，可能需要做一些外圍的工作，然后實(shí)際解析的方法，可以命名為doParse()。這種doXXX()的命名方法，在很多開源框架中都有應(yīng)用，比如logback等。
接下來就看一下這個doLoadBeanDefinitions()方法

protected int doLoadBeanDefinitions(InputSource inputSource, Resource resource)
            throws BeanDefinitionStoreException {
        try {
            Document doc = doLoadDocument(inputSource, resource);return registerBeanDefinitions(doc, 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);
        }
    }

拋開異常處理：核心代碼如下：

 Document doc = doLoadDocument(inputSource, resource);
 return  registerBeanDefinitions(doc, resource);

doLoadDocument方法將InputStream讀取成標(biāo)準(zhǔn)的Document對象，然后調(diào)用registerBeanDefinitions()，進(jìn)行解析工作。

protected Document doLoadDocument(InputSource inputSource, Resource resource) throws Exception {   
    return this.documentLoader.loadDocument(inputSource,  
                                            getEntityResolver(), this.errorHandler,  
                                            getValidationModeForResource(resource),  
                                            isNamespaceAware());
}

接下來就看一下這個核心方法registerBeanDefinitions

public int registerBeanDefinitions(Document doc, Resource resource) throws BeanDefinitionStoreException {
        //創(chuàng)建的其實(shí)是DefaultBeanDefinitionDocumentReader 的實(shí)例，利用反射創(chuàng)建的。
        BeanDefinitionDocumentReader documentReader = createBeanDefinitionDocumentReader();
        documentReader.setEnvironment(this.getEnvironment());
        int countBefore = getRegistry().getBeanDefinitionCount();
        documentReader.registerBeanDefinitions(doc, createReaderContext(resource));
        return getRegistry().getBeanDefinitionCount() - countBefore;
}

這里注意兩點(diǎn) :

1.Document對象
首先這個Document對象，是W3C定義的標(biāo)準(zhǔn)XML對象，跟spring無關(guān)。其次這個registerBeanDefinitions方法，我覺得命名有點(diǎn)誤導(dǎo)性。因?yàn)檫@個時候?qū)嶋H上解析還沒有開始，怎么直接就注冊了呢。比較好的命名，我覺得可以是parseAndRegisterBeanDefinitions()。
2.documentReader的創(chuàng)建時使用反射創(chuàng)建的，代碼如下

protected BeanDefinitionDocumentReader    
 createBeanDefinitionDocumentReader() {   
          return BeanDefinitionDocumentReader.class.cast(BeanUtils.
            instantiateClass(this.documentReaderClass));
}

instantiateClass方法中傳入了一個Class類型的參數(shù)。追溯發(fā)現(xiàn)下述代碼：

private Class<?> documentReaderClass = 
DefaultBeanDefinitionDocumentReader.class;

所以創(chuàng)建的documentReaderClass是DefaultBeanDefinitionDocumentReader類的實(shí)例。
接下來就進(jìn)入BeanDefinitionDocumentReader 中定義的registerBeanDefinitions()方法看看

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

處理完外圍事務(wù)之后，進(jìn)入doRegisterBeanDefinitions()方法，這種命名規(guī)范，上文已經(jīng)介紹過了

protected void doRegisterBeanDefinitions(Element root) {
        String profileSpec = root.getAttribute(PROFILE_ATTRIBUTE);
        if (StringUtils.hasText(profileSpec)) {
            Assert.state(this.environment != null, "environment property must not be null");
            String[] specifiedProfiles = StringUtils.tokenizeToStringArray(profileSpec, BeanDefinitionParserDelegate.MULTI_VALUE_ATTRIBUTE_DELIMITERS);
            if (!this.environment.acceptsProfiles(specifiedProfiles)) {
                return;
            }
        }
        // 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 = createHelper(readerContext, root, parent);
        preProcessXml(root);
        parseBeanDefinitions(root, this.delegate);
        postProcessXml(root);
        this.delegate = parent;
}

這個方法也比較長，拆開來看

String profileSpec = root.getAttribute(PROFILE_ATTRIBUTE);
        if (StringUtils.hasText(profileSpec)) {
            Assert.state(this.environment != null, "environment property must not be null");
            String[] specifiedProfiles = StringUtils.tokenizeToStringArray(profileSpec, BeanDefinitionParserDelegate.MULTI_VALUE_ATTRIBUTE_DELIMITERS);
            if (!this.environment.acceptsProfiles(specifiedProfiles)) {
                return;
            }
}

如果配置文件中元素，配有profile屬性，就會進(jìn)入這一段，不過一般都是不會的

        BeanDefinitionParserDelegate parent = this.delegate;
        this.delegate = createHelper(readerContext, root, parent);
        preProcessXml(root);
        parseBeanDefinitions(root, this.delegate);
        postProcessXml(root);
        this.delegate = parent;

然后這里創(chuàng)建了BeanDefinitionParserDelegate對象，preProcessXml()和postProcessXml()都是空方法，核心就是parseBeanDefinitions()方法。這里又把BeanDefinition解析和注冊的工作，委托給了BeanDefinitionParserDelegate對象，在parseBeanDefinitions()方法中完成
總的來說，解析工作的委托鏈?zhǔn)沁@樣的：ClassPathXmlApplicationContext，XmlBeanDefinitionReader，DefaultBeanDefinitionDocumentReader，BeanDefinitionParserDelegate
ClassPathXmlApplicationContext作為最外圍的組件，發(fā)起解析的請求
XmlBeanDefinitionReader將配置文件路徑封裝為Resource，讀取出w3c定義的Document對象，然后委托給DefaultBeanDefinitionDocumentReader
DefaultBeanDefinitionDocumentReader就開始做實(shí)際的解析工作了，但是涉及到bean的具體解析，它還是會繼續(xù)委托給BeanDefinitionParserDelegate來做。
接下來在parseBeanDefinitions()方法中發(fā)生了什么，以及BeanDefinitionParserDelegate類完成的工作，在下一篇博客中繼續(xù)介紹。

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