【Spring】- IOC容器初始化过程

IOC容器案例：XmlBeanFactory

XmlBeanFactory容器初始化过程：

XmlBeanFactory beanFactory = new XmlBeanFactory(new ClassPathResource("com/zhiwei/ioc/applicationContext.xml"));

步骤1：XmlBeanFactory 使用 ClassPathResource 映射Spring XML配置文件

new ClassPathResource("com/zhiwei/ioc/applicationContext.xml"）

步骤2：初始化空白的IOC容器，而后使用XmlBeanDefinitionReader解析Spring XML配置文件，并注入到IOC容器

public XmlBeanFactory(Resource resource, BeanFactory parentBeanFactory) throws        BeansException {
    //传递父BeanFactory初始化父类
    super(parentBeanFactory);

    //BeanDefinition解析
    this.reader.loadBeanDefinitions(resource);
}

步骤3： 查看XmlBeanDefinitionReader.loadBeanDefinitions() 方法

public int loadBeanDefinitions(EncodedResource encodedResource) throws BeanDefinitionStoreException {
    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();
        }
    }
}

方法解读： resourcesCurrentlyBeingLoaded：经过ThreadLocal绑定EncodedResource,XmlBeanDefinitionReader就持有配置资源映射，方便后续DOM XML解析，将配置资源封装为 inputSource对象，而后传递给doLoadBeanDefinitions方法

步骤4：查看XmlBeanDefinitionReader.doLoadBeanDefinitions方法 源码：

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

解析： doLoadDocument方法底层经过DOM解析的方法解析Spring XML配置文件，并返回Document对象，接着调用registerBeanDefinitions进行Bean实际注册工做

步骤5：查看XmlBeanDefinitionReader.registerBeanDefinitions方法实现

public int registerBeanDefinitions(Document doc, Resource resource) throws BeanDefinitionStoreException {
    BeanDefinitionDocumentReader documentReader = createBeanDefinitionDocumentReader();
    int countBefore = getRegistry().getBeanDefinitionCount();
    documentReader.registerBeanDefinitions(doc, createReaderContext(resource));
    return getRegistry().getBeanDefinitionCount() - countBefore;
}

解析： getRegistry()：可用于获取IOC容器，缘由XmlBeanFactory实例化： private final XmlBeanDefinitionReader reader = new XmlBeanDefinitionReader(this); 将XML的Bean定义阅读器与IOC容器绑定在一块儿 registerBeanDefinitions内部调用registerBeanDefinitions注册Bean，注意createReaderContext方法会建立XmlReaderContext，该对象会与XmlBeanDefinitionReader 绑定，从而实现与IOC容器的间接绑定，这一点对于后续的Bean实际IOC容器注册很是重要

步骤6：查看BeanDefinitionDocumentReader.registerBeanDefinitions方法

注意：BeanDefinitionDocumentReader主要做用就是将Spring XMl配置文件元素映射为IOC容器中的BeanDefinition数据结构，实现真正意义上的解析、注册Bean的功能

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

解析：将readerContext做为BeanDefinitionDocumentReader的内置组件，而后获取XML文件的Root节点，接着调用doRegisterBeanDefinitions方法处理

步骤7： BeanDefinitionDocumentReader.doRegisterBeanDefinitions方法代码

protected void doRegisterBeanDefinitions(Element root) {
		
    BeanDefinitionParserDelegate parent = this.delegate;
    this.delegate = createDelegate(getReaderContext(), root, parent);

    if (this.delegate.isDefaultNamespace(root)) {
        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;
}

解析：实际解析XMl文件的时BeanDefinitionParserDelegate对象，查看createDelegate方法：

protected BeanDefinitionParserDelegate createDelegate(
        XmlReaderContext readerContext, Element root, BeanDefinitionParserDelegate parentDelegate) {

    BeanDefinitionParserDelegate delegate = new BeanDefinitionParserDelegate(readerContext);
    delegate.initDefaults(root, parentDelegate);
    return delegate;
}

该方法执行时不单单返回BeanDefinitionParserDelegate 对象，还会在建立后进行Beans标签的属性信息的初始化工做，该方法核心代码：

preProcessXml(root);   //自定义标签前置处理
parseBeanDefinitions(root, this.delegate); //Spring原始标签处理
postProcessXml(root); //自定义标签后置处理

步骤8：查看BeanDefinitionDocumentReader.parseBeanDefinitions方法代码

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);
    }
}

解析：该方法遍历XML文件映射的DOM树的节点，delegate.isDefaultNamespace(root)主要用于XML文件命名空间校验，delegate.isDefaultNamespace(ele)用于区别配置文件中的元素是否时Spring自带的，如果Spring自身元素则parseDefaultElement方法处理，不然parseCustomElement处理（例如自定义标签）

步骤9：查看BeanDefinitionDocumentReader.parseDefaultElement方法代码

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);
    }
    else if (delegate.nodeNameEquals(ele, NESTED_BEANS_ELEMENT)) {
        // recurse
        doRegisterBeanDefinitions(ele);
    }
}

解析：该方法做为一个中转接口，根据标签元素的不一样选择相应的方法处理，这里主要探索Bean标签的处理方法processBeanDefinition(ele, delegate);

步骤10：查看BeanDefinitionDocumentReader.processBeanDefinition方法代码：

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));
    }
}

解析：

BeanDefinitionHolder bdHolder = delegate.parseBeanDefinitionElement(ele);

主要时根据Root元素节点遍历，而后将数据封装到 BeanDefinitionHolder 这个数据结构，真正IOC容器的Bean注册在 BeanDefinitionReaderUtils.registerBeanDefinition(bdHolder, getReaderContext().getRegistry()); 方法

步骤11：查看BeanDefinitionReaderUtils.registerBeanDefinition方法代码

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);
        }
    }
}

解析：这里Bean注册分为Bean注册和Bean别名注册，主要看registry.registerBeanDefinition方法

步骤12：查看BeanDefinitionRegistry.registerBeanDefinition（这里使用DefaultListableBeanFactory基础IOC容器）

@Override
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 oldBeanDefinition;
    oldBeanDefinition = this.beanDefinitionMap.get(beanName);
    if (oldBeanDefinition != null) {
        if (!isAllowBeanDefinitionOverriding()) {
            throw new BeanDefinitionStoreException(beanDefinition.getResourceDescription(), beanName,
                    "Cannot register bean definition [" + beanDefinition + "] for bean '" + beanName +
                    "': There is already [" + oldBeanDefinition + "] bound.");
        }
        else if (oldBeanDefinition.getRole() < beanDefinition.getRole()) {
            // e.g. was ROLE_APPLICATION, now overriding with ROLE_SUPPORT or ROLE_INFRASTRUCTURE
            if (this.logger.isWarnEnabled()) {
                this.logger.warn("Overriding user-defined bean definition for bean '" + beanName +
                        "' with a framework-generated bean definition: replacing [" +
                        oldBeanDefinition + "] with [" + beanDefinition + "]");
            }
        }
        else if (!beanDefinition.equals(oldBeanDefinition)) {
            if (this.logger.isInfoEnabled()) {
                this.logger.info("Overriding bean definition for bean '" + beanName +
                        "' with a different definition: replacing [" + oldBeanDefinition +
                        "] with [" + beanDefinition + "]");
            }
        }
        else {
            if (this.logger.isDebugEnabled()) {
                this.logger.debug("Overriding bean definition for bean '" + beanName +
                        "' with an equivalent definition: replacing [" + oldBeanDefinition +
                        "] 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 (oldBeanDefinition != null || containsSingleton(beanName)) {
        resetBeanDefinition(beanName);
    }
}

解析： 首先校验生成的BeanDefinition类型，接着校验新建立的Bean是否在IOC容器中注册过，若是注册过则按照Bean的相应配置策略进行处理，例如重写，接着根据hasBeanCreationStarted方法判断IOC容器是否启动过，若是未启动则beanDefinitionMap Map记录BeanName与BeanDefinition对象的关系，beanDefinitionNames记录注册的Bean名称，这里Map的key为BeanName这就时IOC容器中BeanName惟一的缘由，若是IOC容器实现启动则beanDefinitionMap直接存放，manualSingletonNames须要已有的BeanName进行校验，最终等于beanDefinitionNames数据，至此整个XMl解析与IOC容器的初始化工做所有完成。