以编程的方式使用DefaultListableBeanFactory时，首先定义一个Resource来定位容器使用的BeanDefinition。这是使用的是ClassPathResource，意味着Spring会在类路径中去寻找以文件形式存在的BeanDefinition的信息。
ClassPathResource res =new ClassPathResource('beans.xml'); 这里定义的Resource不能由DefaultListableBeanFactory直接使用，Spring通过BeanDefinitionReader来对这些信息进行处理。在这里，我们也可以看到使用ApplicationContext相对于直接使用DefaultListableBeanFactory的好处。在ApplicationContext中，提供了一系列加载不同Resource的读取器的实现，而DefaultListableBeanFactory只是一个纯粹的IOC容器，需要为它配置特定的读取器来完成功能。但是使用DefaultListableBeanFactory这种底层的容器，能提高IOC容器的灵活性。
我们经常使用的ApplicationContext，比如FileSystemXmlApplicationContext、ClassPathXmlSystemXmlApplicationContext以及XmlWebApplicationContext。从类的名字可以看出它们提供哪些不同的Resource读入功能，依次比如为从文件系统，从class path，从web容器载入Resource等。
我们以FileSystemXmlApplicationContext为例，继承关系如下图：
因为基类是DefaultResourceLoader（它实现了ResourceLoader接口），这个FileSystemXmlApplicationContext已经具备ResourceLoader的读入功能。
先看下FileSystemXmlApplicationContext的实现：
public class FileSystemXmlApplicationContext extends AbstractXmlApplicationContext {/*** Create a new FileSystemXmlApplicationContext for bean-style configuration.* @see #setConfigLocation* @see #setConfigLocations* @see #afterPropertiesSet()*/public FileSystemXmlApplicationContext() {}/*** Create a new FileSystemXmlApplicationContext for bean-style configuration.* @param parent the parent context* @see #setConfigLocation* @see #setConfigLocations* @see #afterPropertiesSet()*/public FileSystemXmlApplicationContext(ApplicationContext parent) {super(parent);}/*** Create a new FileSystemXmlApplicationContext, loading the definitions* from the given XML file and automatically refreshing the context.* @param configLocation file path* @throws BeansException if context creation failed*///这个构造函数的configLocation包含的是BeanDefinition所在的文件路径public FileSystemXmlApplicationContext(String configLocation) throws BeansException {this(new String[] {configLocation}, true, null);}/*** Create a new FileSystemXmlApplicationContext, loading the definitions* from the given XML files and automatically refreshing the context.* @param configLocations array of file paths* @throws BeansException if context creation failed*///这个构造函数的configLocation包含多个BeanDefinition的文件路径public FileSystemXmlApplicationContext(String... configLocations) throws BeansException {this(configLocations, true, null);}/*** Create a new FileSystemXmlApplicationContext with the given parent,* loading the definitions from the given XML files and automatically* refreshing the context.* @param configLocations array of file paths* @param parent the parent context* @throws BeansException if context creation failed*///这个构造函数的configLocation包含的多个BeanDefinition文件路径的同时，还允许指定自己的双亲IOC容器public FileSystemXmlApplicationContext(String[] configLocations, ApplicationContext parent) throws BeansException {this(configLocations, true, parent);}/*** Create a new FileSystemXmlApplicationContext, loading the definitions* from the given XML files.* @param configLocations array of file paths* @param refresh whether to automatically refresh the context,* loading all bean definitions and creating all singletons.* Alternatively, call refresh manually after further configuring the context.* @throws BeansException if context creation failed* @see #refresh()*/public FileSystemXmlApplicationContext(String[] configLocations, boolean refresh) throws BeansException {this(configLocations, refresh, null);}/*** Create a new FileSystemXmlApplicationContext with the given parent,* loading the definitions from the given XML files.* @param configLocations array of file paths* @param refresh whether to automatically refresh the context,* loading all bean definitions and creating all singletons.* Alternatively, call refresh manually after further configuring the context.* @param parent the parent context* @throws BeansException if context creation failed* @see #refresh()*///在对象的初始化过程中，调用refresh方法载入BeanDefinition，这个refresh方法启动了BeanDefinition的载入过程，待会看refresh详细分析public FileSystemXmlApplicationContext(String[] configLocations, boolean refresh, ApplicationContext parent)throws BeansException {super(parent);setConfigLocations(configLocations);if (refresh) {refresh();}}/*** Resolve resource paths as file system paths.* <p>Note: Even if a given path starts with a slash, it will get* interpreted as relative to the current VM working directory.* This is consistent with the semantics in a Servlet container.* @param path path to the resource* @return Resource handle* @see org.springframework.web.context.support.XmlWebApplicationContext#getResourceByPath*/@Override//这是应用于文件系统中的Resource的实现，通过构造一个FileSystemResource来得到一个文件在//系统中定位的BeanDefinition//这个getResourceByPath是在BeanDefinitionReader的loadBeanDefinition中被调用//loadBeanDefinition采用模板模式，具体的定位实现是由各个子类来完成protected Resource getResourceByPath(String path) {if (path != null && path.startsWith("/")) {path = path.substring(1);}return new FileSystemResource(path);}}
refresh方法非常重要，容器初始化过程的一个重要入口。refresh()在AbstractApplicationContext实现，看下部分代码：
public void refresh() throws BeansException, IllegalStateException {synchronized (this.startupShutdownMonitor) {// Prepare this context for refreshing. prepareRefresh();// Tell the subclass to refresh the internal bean factory.//子类中启动refreshBeanFactory的地方ConfigurableListableBeanFactory beanFactory = obtainFreshBeanFactory();// Prepare the bean factory for use in this context. prepareBeanFactory(beanFactory);try {// Allows post-processing of the bean factory in context subclasses.//设置beanFactory的后置处理 postProcessBeanFactory(beanFactory);// Invoke factory processors registered as beans in the context.//调用beanFactory的后置处理器，这些后处理器在bean中向容器注册的 invokeBeanFactoryPostProcessors(beanFactory);// Register bean processors that intercept bean creation.//注册bean的后处理器，在bean创建过程中调用 registerBeanPostProcessors(beanFactory);// Initialize message source for this context.//对上下文中的消息源进行初始化 initMessageSource();// Initialize event multicaster for this context.//初始化上下文中的事件机制 initApplicationEventMulticaster();// Initialize other special beans in specific context subclasses.//初始化其中的特殊bean onRefresh();// Check for listener beans and register them.//检查监听bean并且将这些bean向容器注册 registerListeners();// Instantiate all remaining (non-lazy-init) singletons.//实例化所有non-lazy-init单件 finishBeanFactoryInitialization(beanFactory);// Last step: publish corresponding event.//发布容器事件，结束refresh过程 finishRefresh();}catch (BeansException ex) {// Destroy already created singletons to avoid dangling resources.//为防止bean资源占用，在异常处理中，销毁已经在前面过程中生成的单件bean destroyBeans();// Reset 'active' flag.//重置active标志 cancelRefresh(ex);// Propagate exception to caller.throw ex;}}}
protected ConfigurableListableBeanFactory obtainFreshBeanFactory() {refreshBeanFactory();ConfigurableListableBeanFactory beanFactory = getBeanFactory();if (logger.isDebugEnabled()) {logger.debug("Bean factory for " + getDisplayName() + ": " + beanFactory); } return beanFactory; }
在IOC容器的初始化过程中，BeanDefinition的定位，读入和载入过程是分开进行的，这是解耦的一个体现。关于读入器的配置，要先看下FileSystemXmlApplicationContext的基类AbstractRefreshableApplicationContext实现。需要重点看你下AbstractRefreshableApplicationContext的refreshBeanFactory方法，这个refreshBeanFactory被FileSystemXmlApplicationContext构造函数中的refresh方法调用(refresh方法调用了obtainFreshBeanFactory()上面代码可以看出)。
下面看下AbstractRefreshableApplicationContext对容器初始化的代码清单
protected final void refreshBeanFactory() throws BeansException {if (hasBeanFactory()) {destroyBeans();closeBeanFactory();}try {//创建ioc容器,DefaultListableBeanFactoryDefaultListableBeanFactory beanFactory = createBeanFactory();beanFactory.setSerializationId(getId());customizeBeanFactory(beanFactory);//启动对BeanDefinition的载入 loadBeanDefinitions(beanFactory);synchronized (this.beanFactoryMonitor) {this.beanFactory = beanFactory;}}catch (IOException ex) {throw new ApplicationContextException("I/O error parsing bean definition source for " + getDisplayName(), ex);}}
在这个方法中，通过createBeanFactory方法构建了一个IOC容器供Application使用。这个IOC容器就是前面提到的DefaultListableBeanFactory，同时它启动了loadBeanDefinitions来载入BeanDefinition。
//这就是在上下文中创建DefaultListableBeanFactory的地方，getInternalParentBeanFactory()的具体实现在//AbstractApplicationContext，会根据已有的双亲IOC容器信息来完成DefaultListableBeanFactory的双亲IOC容器protected DefaultListableBeanFactory createBeanFactory() {return new DefaultListableBeanFactory(getInternalParentBeanFactory());}/*** * protected BeanFactory getInternalParentBeanFactory() {return (getParent() instanceof ConfigurableApplicationContext) ?((ConfigurableApplicationContext) getParent()).getBeanFactory() : getParent();}*/
/*** Load bean definitions into the given bean factory, typically through* delegating to one or more bean definition readers.* @param beanFactory the bean factory to load bean definitions into* @throws BeansException if parsing of the bean definitions failed* @throws IOException if loading of bean definition files failed* @see org.springframework.beans.factory.support.PropertiesBeanDefinitionReader* @see org.springframework.beans.factory.xml.XmlBeanDefinitionReader*///这里是使用 BeanDefinitionReader载入Bean定义的地方，因为允许有多种载入方式，虽然用的最多的是XML定义的形式，//这里通过一个抽象函数把具体的实现委托给子类完成protected abstract void loadBeanDefinitions(DefaultListableBeanFactory beanFactory)throws BeansException, IOException;
public int loadBeanDefinitions(String location, Set<Resource> actualResources) throws BeanDefinitionStoreException {//取得ResourceLoader使用的是DefaultResourceLoaderResourceLoader resourceLoader = getResourceLoader();if (resourceLoader == null) {throw new BeanDefinitionStoreException("Cannot import bean definitions from location [" + location + "]: no ResourceLoader available");}//对Resource路径解析，Resource集合可以是多个文件if (resourceLoader instanceof ResourcePatternResolver) {// Resource pattern matching available.try {//取得具体的Resource定位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;}}
对于取得Resource的过程，看下DefaultResourceLoader是怎么完成的
public Resource getResource(String location) {Assert.notNull(location, "Location must not be null");if (location.startsWith(CLASSPATH_URL_PREFIX)) { // public static final String CLASSPATH_URL_PREFIX = "classpath:";return new ClassPathResource(location.substring(CLASSPATH_URL_PREFIX.length()), getClassLoader());}else {try {// Try to parse the location as a URL...//这里处理URL标识的Resource定位URL url = new URL(location);return new UrlResource(url);}catch (MalformedURLException ex) {// No URL -> resolve as resource path.//如果既不是classpath，也不是URL标识的Resource定位，则把getResource交给getResourceByPath，//这个方法是一个protected方法，默认实现是得到一个ClassPathContextResource，这个方法会用子类实现return getResourceByPath(location);}}}
protected Resource getResourceByPath(String path) {return new ClassPathContextResource(path, getClassLoader());}
对于上面的FileSystemXmlApplicationContext对getResourceByPath方法的实现就是下面方法
@Override//这是应用于文件系统中的Resource的实现，通过构造一个FileSystemResource来得到一个文件在//系统中定位的BeanDefinition//这个getResourceByPath是在BeanDefinitionReader的loadBeanDefinition中被调用//loadBeanDefinition采用模板模式，具体的定位实现是由各个子类来完成protected Resource getResourceByPath(String path) {if (path != null && path.startsWith("/")) {path = path.substring(1);}return new FileSystemResource(path);}
返回的FileSystemResource对象，Spring可以进行相关的I/O操作，完成BeanDefinition的定位。
如果是其他的ApplicationContext，那么会对应生成其他种类的Resource。比如ClassPathResource、ServletContextResource等。关于Resource种类，可以看下继承关系图
作为接口的Resource定义了许多与I/O相关的操作。
通过前面的实现原理的分析，我们以FileSystemXmlApplicationContext的实现原理为例，了解了Resource的定位问题，既是FileSystem方式存在的Resource的定位实现。在在BeanDefinition定位完成的基础上，就可以通过返回的Resource对象来进行载入了。在定位过程完成以后，为BeanDefinition的载入创造了I/O操作的条件，但是具体的数据还没有开始读入。