IOC容器初始化——BeanDefinition的Resource定位
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,DefaultListableBeanFactory DefaultListableBeanFactory 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DefaultResourceLoader ResourceLoader 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。
