IOC容器的初始化-一

1、定位：

定位，就是获取到配置文件从而获取到资源，即通过资源文件路径定位到具体的资源；Srping内部用Resource接口来定位资源，用户输入资源路径，然后通过ResourceLoader根据输入的资源路径返回对应的Resource

2、加载：

加载，就是将资源文件加载到内存中，形成一个具体的对象，最初是通过reader形成一个Document对象，然后再逐渐深入解析最终成为BeanDefinition（这个就是最终描述xml配置文件的对象）

3、注册：

IOC容器中的beanefition加载到map的过程中，就是注册的过程

接下来跟随代码逐步解析IOC容器初始化过程：

 ApplicationContext context = new ClassPathXmlApplicationContext("classpath*:applicationContext.xml");

调用ClassPathXmlApplicationContext的构造方法：

public ClassPathXmlApplicationContext(
            String[] configLocations, boolean refresh, @Nullable ApplicationContext parent)
            throws BeansException {
        /**
         *调用父类构造方法，获取资源位置解析器Resolver==》PathMatchingResourcePatternResolver
         */
        super(parent);
        /**
         * 设置资源路径
         */
        setConfigLocations(configLocations);
        /**
         * 刷新容器
         */
        if (refresh) {
            refresh();
        }
}

重点关注refresh()方法，这里是IOC初始化的入口

由AbstractApplicationContext的refresh()完成,部分代码用伪代码做处理：

@Override
public void refresh() throws BeansException, IllegalStateException {
    //进行容器初始化前的一些准备
    
    ConfigurableListableBeanFactory beanFactory = obtainFreshBeanFactory();
    
    //容器初始化后的一些准备

}
    
/**
 * 这里refreshBeanFactory委托给子类，于是跳到子类的refreshBeanFactory
 * @return the fresh BeanFactory instance
 * @see #refreshBeanFactory()
 * @see #getBeanFactory()
 */
protected ConfigurableListableBeanFactory obtainFreshBeanFactory() {
    refreshBeanFactory();
    ConfigurableListableBeanFactory beanFactory = getBeanFactory();
    if (logger.isDebugEnabled()) {
        logger.debug("Bean factory for " + getDisplayName() + ": " + beanFactory);
    }
    return beanFactory;
}

于是，实际上刷新容器的任务委托到了AbstractRefreshableApplicationContext的refreshBeanFactory()

@Override
protected final void refreshBeanFactory() throws BeansException {
    //将原来bean销毁并关闭原有的beanFactory
    if (hasBeanFactory()) {
        destroyBeans();
        closeBeanFactory();
    }
    try {
        DefaultListableBeanFactory beanFactory = createBeanFactory();//由这里可得，DefaultListableBeanFactory才是真正干活的bean工厂
        beanFactory.setSerializationId(getId());
        customizeBeanFactory(beanFactory);
        loadBeanDefinitions(beanFactory);//bean加载入口，这里也是使用了委托模式，由子类实现
        synchronized (this.beanFactoryMonitor) {
            this.beanFactory = beanFactory;
        }
    }
    catch (IOException ex) {
        throw new ApplicationContextException("I/O error parsing bean definition source for " +         getDisplayName(), ex);
}

于是，加载的任务委托给了AbstractXmlApplicationContext的loadBeanDefinitions

/**
 * Loads the bean definitions via an XmlBeanDefinitionReader.
 * @see org.springframework.beans.factory.xml.XmlBeanDefinitionReader
 * @see #initBeanDefinitionReader
 * @see #loadBeanDefinitions
 */
@Override
protected void loadBeanDefinitions(DefaultListableBeanFactory beanFactory) throws BeansException, IOException {
   // Create a new XmlBeanDefinitionReader for the given BeanFactory.
   XmlBeanDefinitionReader beanDefinitionReader = new XmlBeanDefinitionReader(beanFactory);//这里可以看出，XmlBeanDefinitionReader负责bean配置的加载

   // Configure the bean definition reader with this context's
   // resource loading environment.
   beanDefinitionReader.setEnvironment(this.getEnvironment());
   beanDefinitionReader.setResourceLoader(this);
   beanDefinitionReader.setEntityResolver(new ResourceEntityResolver(this));

   // Allow a subclass to provide custom initialization of the reader,
   // then proceed with actually loading the bean definitions.
   initBeanDefinitionReader(beanDefinitionReader);
   loadBeanDefinitions(beanDefinitionReader);
}

protected void loadBeanDefinitions(XmlBeanDefinitionReader reader) throws BeansException, IOException {
        Resource[] configResources = getConfigResources();
        if (configResources != null) {
            reader.loadBeanDefinitions(configResources);
        }
        String[] configLocations = getConfigLocations();
        if (configLocations != null) {
            reader.loadBeanDefinitions(configLocations);//从这里可以看出，加载的任务交由XmlBeanDefinitionReader完成
        }
}

这里XmlBeanDefinitionReader的loadBeanDefinitions继续委托给了AbstractBeanDefinitionReader.loadBeanDefinitions

public int loadBeanDefinitions(String location, @Nullable Set<Resource> actualResources) throws BeanDefinitionStoreException {
   ResourceLoader resourceLoader = getResourceLoader();//获取ResourceLoader，将资源文件路径转化为Resource
   //进行相应的判断，省略了部分代码。。。
   Resource resource = resourceLoader.getResource(location);
   int loadCount = loadBeanDefinitions(resource);
   return loadCount;
 }

将loadBeanDefinitions继续委托给XmlBeanDefinitionReader

public int loadBeanDefinitions(EncodedResource encodedResource) throws BeanDefinitionStoreException {
    //关键代码
    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();
    }
}

在spring的设计中，do开头的方法一般就是真正做事的方法，进行doLoadBeanDefinitions

protected int doLoadBeanDefinitions(InputSource inputSource, Resource resource)
      throws BeanDefinitionStoreException {
    //关键代码
     Document doc = doLoadDocument(inputSource, resource);//将resource转为dom对象
     return registerBeanDefinitions(doc, resource);
}

public int registerBeanDefinitions(Document doc, Resource resource) throws BeanDefinitionStoreException {
    BeanDefinitionDocumentReader documentReader = createBeanDefinitionDocumentReader();
    int countBefore = getRegistry().getBeanDefinitionCount();
    documentReader.registerBeanDefinitions(doc, createReaderContext(resource));//BeanDefinitionDocumentReader对dom进行解析加载，该类将解析加载的任务委托给DefaultBeanDefinitionDocumentReader
    return getRegistry().getBeanDefinitionCount() - countBefore;
}

因此，见DefaultBeanDefinitionDocumentReader的registerBeanDefinitions

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

/**
 * Register each bean definition within the given root {@code <beans/>} element.
 */
protected void doRegisterBeanDefinitions(Element root) {
    //关键代码
    BeanDefinitionParserDelegate parent = this.delegate;
    this.delegate = createDelegate(getReaderContext(), root, parent);
    preProcessXml(root);
    parseBeanDefinitions(root, this.delegate);
    postProcessXml(root);

    this.delegate = parent;
}

/**
 * Parse the elements at the root level in the document:
 * "import", "alias", "bean".
 * @param root the DOM root element of the document
 */
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);//循环遍历解析dom对象
                }
                else {
                    delegate.parseCustomElement(ele);
                }
            }
        }
    }
    else {
        delegate.parseCustomElement(root);
    }
}

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

拿bean解析来看，解析之后由BeanDefinitionReaderUtils.registerBeanDefinition进行注册

/**
 * Process the given bean element, parsing the bean definition
 * and registering it with the registry.
 */
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));
   }
}

BeanDefinitionReaderUtils.registerBeanDefinition

/**
 * Register the given bean definition with the given bean factory.
 * @param definitionHolder the bean definition including name and aliases
 * @param registry the bean factory to register with
 * @throws BeanDefinitionStoreException if registration failed
 */
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);
      }
   }
}

由DefaultListableBeanFactory来进行注册

@Override
public void registerBeanDefinition(String beanName, BeanDefinition beanDefinition)
      throws BeanDefinitionStoreException {
    //关键代码
    this.beanDefinitionMap.put(beanName, beanDefinition);//注册，由beneDefinitionMap做容器进行存储
}