Spring源码阅读（2）- bean解析初体验

从一个简单例子开始

通过一个简单的bean加载例子来热热身，虽然我们平时不使用这里的XmlBeanFactory,而是用ApplicationContext,但是后面我们看到二者还是有共通之处。

public class Foo {    public void execute(){        System.out.println("Foo execute...");    }}public class TestFoo {    @Test    public void testExecute(){        BeanFactory factory = new XmlBeanFactory(new ClassPathResource("service-context.xml"));        Foo bean = (Foo) factory.getBean("foo");        bean.execute();    }}

配置文件：

<beans xmlns="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 id="foo" class="com.vonzhou.learn.spring.beanloading.Foo"/></beans>

从资源文件得到DOM对象

那么就开始吧！先看看XmlBeanFactory所处的地位。

XmlBeanFactory扩展了DefaultListableBeanFactory，使用XmlBeanDefinitionReader从XML配置文件中读取bean的定义。忽略其他的细节，我们先来看看这个配置文件（是一种Resource）是如何被加载的。跟踪进去，进入XmlBeanDefinitionReader#loadBeanDefinitions方法，然后扑面而来的是下面这个重要的方法。

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();            }        }    }

成员变量resourcesCurrentlyBeingLoaded是一个ThreadLocal，所以资源文件多线程加载是安全的，同时使用HashSet判断资源文件的循环加载，接下来就是读取我们的资源文件了， 其中InputSource表征一个XML实体的输入源，包装了几个字段。

    private String publicId;    private String systemId;    private InputStream byteStream;    private String encoding;    private Reader characterStream;

从输入源InputSource中获得表示该XML文件的Document对象，后续就是操纵这个document。

protected int doLoadBeanDefinitions(InputSource inputSource, Resource resource)            throws BeanDefinitionStoreException {        try {            int validationMode = getValidationModeForResource(resource);            Document doc = this.documentLoader.loadDocument(                    inputSource, getEntityResolver(), this.errorHandler, validationMode, isNamespaceAware());            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);        }    }

解析DOM对象里的bean

然后就是根据Spring的”spring-beans” DTD（就是我们在配置文件开始制定的命名空间和location），注册该DOM对象里面的bean。

public int registerBeanDefinitions(Document doc, Resource resource) throws BeanDefinitionStoreException {        // Read document based on new BeanDefinitionDocumentReader SPI.        BeanDefinitionDocumentReader documentReader = createBeanDefinitionDocumentReader();        int countBefore = getRegistry().getBeanDefinitionCount();        documentReader.registerBeanDefinitions(doc, createReaderContext(resource));        return getRegistry().getBeanDefinitionCount() - countBefore;    }public void registerBeanDefinitions(Document doc, XmlReaderContext readerContext) {        this.readerContext = readerContext;        logger.debug("Loading bean definitions");        Element root = doc.getDocumentElement();        BeanDefinitionParserDelegate delegate = createHelper(readerContext, root);        preProcessXml(root);        parseBeanDefinitions(root, delegate);        postProcessXml(root);    }

后续的工作就是解析这里的DOM对象，从根节点开始，处理每个node，每个元素可以是默认命名空间有的(如”import”, “alias”, “bean”)，也可以是自定义的。

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);        }    }

我们重点是看bean的定义是如何实现的，其他的先不关注。

private void parseDefaultElement(Element ele, BeanDefinitionParserDelegate delegate) {        if (delegate.nodeNameEquals(ele, IMPORT_ELEMENT)) {            importBeanDefinitionResource(ele);        }        else if (delegate.nodeNameEquals(ele, ALIAS_ELEMENT)) {            processAliasRegistration(ele);        }        else if (delegate.nodeNameEquals(ele, BEAN_ELEMENT)) {            processBeanDefinition(ele, delegate);        }    }

解析bean标签

接下来就是具体处理一个bean定义的过程，想象现在正在解析<bean id="foo" class="com.vonzhou.learn.spring.beanloading.Foo"/>这个元素。

protected void processBeanDefinition(Element ele, BeanDefinitionParserDelegate delegate) {        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));        }    }

接下来委派给BeanDefinitionParserDelegate来专门解析XML里的bean定义，如果深究下去太多了，暂时先略过其中的细节，解析成功后返回一个
BeanDefinitionHolder.

    public BeanDefinitionHolder parseBeanDefinitionElement(Element ele, BeanDefinition containingBean) {        String id = ele.getAttribute(ID_ATTRIBUTE);        String nameAttr = ele.getAttribute(NAME_ATTRIBUTE);        List<String> aliases = new ArrayList<String>();        if (StringUtils.hasLength(nameAttr)) {            String[] nameArr = StringUtils.tokenizeToStringArray(nameAttr, BEAN_NAME_DELIMITERS);            aliases.addAll(Arrays.asList(nameArr));        }        String beanName = id;        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) {            checkNameUniqueness(beanName, aliases, ele);        }        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);            return new BeanDefinitionHolder(beanDefinition, beanName, aliasesArray);        }        return null;    }

我们来看看BeanDefinitionHolder的结构，其中包含BeanDefinition,bean name 以及bean的别名，通过对本例debug，我们可以大概得到一些信息，有助于理解。

注册bean definition

当我们bean definition解析之后，就要进行注册，注册到bean factory中。

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 aliase : aliases) {                registry.registerAlias(beanName, aliase);            }        }    }

这项工作由实现了BeanDefinitionRegistry接口的DefaultListableBeanFactory类实现的。

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);            }        }        synchronized (this.beanDefinitionMap) {            Object oldBeanDefinition = this.beanDefinitionMap.get(beanName);            if (oldBeanDefinition != null) {                if (!this.allowBeanDefinitionOverriding) {                    throw new BeanDefinitionStoreException(beanDefinition.getResourceDescription(), beanName,                            "Cannot register bean definition [" + beanDefinition + "] for bean '" + beanName +                            "': There is already [" + oldBeanDefinition + "] bound.");                }                else {                    if (this.logger.isInfoEnabled()) {                        this.logger.info("Overriding bean definition for bean '" + beanName +                                "': replacing [" + oldBeanDefinition + "] with [" + beanDefinition + "]");                    }                }            }            else {                this.beanDefinitionNames.add(beanName);                this.frozenBeanDefinitionNames = null;            }            this.beanDefinitionMap.put(beanName, beanDefinition);            resetBeanDefinition(beanName);        }    }

可以看到就是把bean的定义存储在一个Map中,同时对于重复定义的bean也会报错，同时也把 bean definition name 有序的存在beanDefinitionNames数组中。

/** Map of bean definition objects, keyed by bean name */private final Map<String, BeanDefinition> beanDefinitionMap = new ConcurrentHashMap<String, BeanDefinition>();

小结

本文就跟了下 bean 的加载过程，到最后你会发现就是解析XML文件，把需要的信息存储到内存中，后续就可以取出bean的定义信息，根据反射操纵对应的方法。