Spring Ioc 源码分析(三)--loadBeanDefinitions-原创手记-慕课网

上一篇博客说到，ApplicationContext将解析BeanDefinition的工作委托给BeanDefinitionReader组件，这篇就接着分析一下BeanDefinition的解析过程。

loadBeanDefinitions: 源码阅读

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);
            }
        }
}

这是解析过程最外围的代码，首先要获取到配置文件的路径，这在之前已经完成了。
然后将每个配置文件的路径，作为参数传给BeanDefinitionReader的loadBeanDefinitions方法里

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

这个方法又调用了重载方法

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()的实现的前提条件是因为XmlBeanDefinitionReader在实例化的时候已经确定了创建了实例ResourceLoader实例, 代码位于 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(); 
      }
}

这个方法比较长，BeanDefinitionReader不能直接加载配置文件，需要把配置文件封装成Resource，然后才能调用重载方法loadBeanDefinitions()。所以这个方法其实就是2段，第一部分是委托ResourceLoader将配置文件封装成Resource，第二部分是调用loadBeanDefinitions()，对Resource进行解析

而这里的ResourceLoader，就是前面的XmlWebApplicationContext，因为ApplicationContext接口，是继承自ResourceLoader接口的

Resource也是一个接口体系，在web环境下，这里就是ServletContextResource

接下来进入重载方法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;
    }

这里就不用说了，就是把每一个Resource作为参数，继续调用重载方法。读spring源码，会发现重载方法特别多。

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

还是重载方法，不过这里对传进来的Resource又进行了一次封装，变成了编码后的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();
            }
        }
    }

这个就是loadBeanDefinitions()的最后一个重载方法，比较长，可以拆看来看。

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!");
        }

这第一部分，是处理线程相关的工作，把当前正在解析的Resource，设置为当前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();
            }
        }

这里是第二部分，是核心，首先把Resource还原为InputStream，然后调用实际解析的方法doLoadBeanDefinitions()。可以看到，这种命名方式是很值得学习的，一种业务方法，比如parse()，可能需要做一些外围的工作，然后实际解析的方法，可以命名为doParse()。这种doXXX()的命名方法，在很多开源框架中都有应用，比如logback等。
接下来就看一下这个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);
        }
    }

抛开异常处理：核心代码如下：

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

doLoadDocument方法将InputStream读取成标准的Document对象，然后调用registerBeanDefinitions()，进行解析工作。

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

接下来就看一下这个核心方法registerBeanDefinitions

public int registerBeanDefinitions(Document doc, Resource resource) throws BeanDefinitionStoreException {
        //创建的其实是DefaultBeanDefinitionDocumentReader 的实例，利用反射创建的。
        BeanDefinitionDocumentReader documentReader = createBeanDefinitionDocumentReader();
        documentReader.setEnvironment(this.getEnvironment());
        int countBefore = getRegistry().getBeanDefinitionCount();
        documentReader.registerBeanDefinitions(doc, createReaderContext(resource));
        return getRegistry().getBeanDefinitionCount() - countBefore;
}

这里注意两点 :

1.Document对象
首先这个Document对象，是W3C定义的标准XML对象，跟spring无关。其次这个registerBeanDefinitions方法，我觉得命名有点误导性。因为这个时候实际上解析还没有开始，怎么直接就注册了呢。比较好的命名，我觉得可以是parseAndRegisterBeanDefinitions()。
2.documentReader的创建时使用反射创建的，代码如下
protected BeanDefinitionDocumentReader    
createBeanDefinitionDocumentReader() {   
return BeanDefinitionDocumentReader.class.cast(BeanUtils.
instantiateClass(this.documentReaderClass));
}
instantiateClass方法中传入了一个Class类型的参数。追溯发现下述代码：
private Class<?> documentReaderClass = 
DefaultBeanDefinitionDocumentReader.class;
所以创建的documentReaderClass是DefaultBeanDefinitionDocumentReader类的实例。
接下来就进入BeanDefinitionDocumentReader 中定义的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);
}
处理完外围事务之后，进入doRegisterBeanDefinitions()方法，这种命名规范，上文已经介绍过了
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;
}
这个方法也比较长，拆开来看
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属性，就会进入这一段，不过一般都是不会的
BeanDefinitionParserDelegate parent = this.delegate;
this.delegate = createHelper(readerContext, root, parent);
preProcessXml(root);
parseBeanDefinitions(root, this.delegate);
postProcessXml(root);
this.delegate = parent;
然后这里创建了BeanDefinitionParserDelegate对象，preProcessXml()和postProcessXml()都是空方法，核心就是parseBeanDefinitions()方法。这里又把BeanDefinition解析和注册的工作，委托给了BeanDefinitionParserDelegate对象，在parseBeanDefinitions()方法中完成
总的来说，解析工作的委托链是这样的：ClassPathXmlApplicationContext，XmlBeanDefinitionReader，DefaultBeanDefinitionDocumentReader，BeanDefinitionParserDelegate
ClassPathXmlApplicationContext作为最外围的组件，发起解析的请求
XmlBeanDefinitionReader将配置文件路径封装为Resource，读取出w3c定义的Document对象，然后委托给DefaultBeanDefinitionDocumentReader
DefaultBeanDefinitionDocumentReader就开始做实际的解析工作了，但是涉及到bean的具体解析，它还是会继续委托给BeanDefinitionParserDelegate来做。
接下来在parseBeanDefinitions()方法中发生了什么，以及BeanDefinitionParserDelegate类完成的工作，在下一篇博客中继续介绍