Spring源碼閱讀（2）- bean解析初體驗

Spring源碼閱讀（2）- bean解析初體驗

從一個簡單例子開始

通過一個簡單的bean加載例子來熱熱身，雖然我們平時不使用這裏的XmlBeanFactory,而是用ApplicationContext,但是後面我們看到二者還是有共通之處。

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>

從資源文件得到DOM對象

那麼就開始吧！先看看XmlBeanFactory所處的地位。

XmlBeanFactory擴展了DefaultListableBeanFactory，使用XmlBeanDefinitionReader從XML配置文件中讀取bean的定義。忽略其他的細節，我們先來看看這個配置文件（是一種Resource）是如何被加載的。跟蹤進去，進入XmlBeanDefinitionReader#loadBeanDefinitions方法，然後撲面而來的是下面這個重要的方法。

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();
            }
        }
    }

成員變量resourcesCurrentlyBeingLoaded是一個ThreadLocal，所以資源文件多線程加載是安全的，同時使用HashSet判斷資源文件的循環加載，接下來就是讀取我們的資源文件了， 其中InputSource表徵一個XML實體的輸入源，包裝了幾個字段。

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

從輸入源InputSource中獲得表示該XML文件的Document對象，後續就是操縱這個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對象裏的bean

然後就是根據Spring的”spring-beans” DTD（就是我們在配置文件開始制定的命名空間和location），註冊該DOM對象裏面的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);
    }

後續的工作就是解析這裏的DOM對象，從根節點開始，處理每個node，每個元素可以是默認命名空間有的(如”import”, “alias”, “bean”)，也可以是自定義的。

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);
        }
    }

我們重點是看bean的定義是如何實現的，其他的先不關注。

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標籤

接下來就是具體處理一個bean定義的過程，想象現在正在解析<bean id="foo" class="com.vonzhou.learn.spring.beanloading.Foo"/>這個元素。

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));
        }
    }

接下來委派給BeanDefinitionParserDelegate來專門解析XML裏的bean定義，如果深究下去太多了，暫時先略過其中的細節，解析成功後返回一個
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;
    }

我們來看看BeanDefinitionHolder的結構，其中包含BeanDefinition,bean name 以及bean的別名，通過對本例debug，我們可以大概得到一些信息，有助於理解。

註冊bean definition

當我們bean definition解析之後，就要進行註冊，註冊到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);
            }
        }
    }

這項工作由實現了BeanDefinitionRegistry接口的DefaultListableBeanFactory類實現的。

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的定義存儲在一個Map中,同時對於重複定義的bean也會報錯，同時也把 bean definition name 有序的存在beanDefinitionNames數組中。

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

小結

本文就跟了下 bean 的加載過程，到最後你會發現就是解析XML文件，把需要的信息存儲到內存中，後續就可以取出bean的定義信息，根據反射操縱對應的方法。