Spring BeanDefinition
定义
/** * A BeanDefinition describes a bean instance, which has property values, * constructor argument values, and further information supplied by * concrete implementations. * * <p>This is just a minimal interface: The main intention is to allow a * {@link BeanFactoryPostProcessor} to introspect and modify property values * and other bean metadata. * * @author Juergen Hoeller * @author Rob Harrop * @since 19.03.2004 * @see ConfigurableListableBeanFactory#getBeanDefinition * @see org.springframework.beans.factory.support.RootBeanDefinition * @see org.springframework.beans.factory.support.ChildBeanDefinition */ public interface BeanDefinition extends AttributeAccessor, BeanMetadataElement {
解读：
BeanDefinition相当于一个数据结构，Spring容器对bean的管理是通过BeanDefinition来实现的。
方法
BeanDefinition中的方法如下图所示：
类图
解读：
从上述类图可以看到BeanDefinition有哪些实现类。
AnnotatedBeanDefinition
AnnotatedBeanDefinition的定义如下
/** * Extended {@link org.springframework.beans.factory.config.BeanDefinition} * interface that exposes {@link org.springframework.core.type.AnnotationMetadata} * about its bean class - without requiring the class to be loaded yet. * * @author Juergen Hoeller * @since 2.5 * @see AnnotatedGenericBeanDefinition * @see org.springframework.core.type.AnnotationMetadata */ public interface AnnotatedBeanDefinition extends BeanDefinition { /** * Obtain the annotation metadata (as well as basic class metadata) * for this bean definition's bean class. * @return the annotation metadata object (never {@code null}) */ AnnotationMetadata getMetadata(); /** * Obtain metadata for this bean definition's factory method, if any. * @return the factory method metadata, or {@code null} if none * @since 4.1.1 */ @Nullable MethodMetadata getFactoryMethodMetadata(); }
该类有如下几个子类
调用链路
本部分准备探究一下Spring中解析bean的调用链路，涉及到几个层次，譬如：ApplicationContext -> BeanDefinitionReader -> BeanDefinitionDocumentReader。
可以带着问题阅读：
- 这几个层次之间是怎么关联起来的？
- 最终是如何解析bean的？
ApplicationContext
AbstractApplicationContext
@Override public void refresh() throws BeansException, IllegalStateException { synchronized (this.startupShutdownMonitor) { StartupStep contextRefresh = this.applicationStartup.start("spring.context.refresh"); // Prepare this context for refreshing. prepareRefresh(); // Tell the subclass to refresh the internal bean factory. 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. postProcessBeanFactory(beanFactory); StartupStep beanPostProcess = this.applicationStartup.start("spring.context.beans.post-process"); // Invoke factory processors registered as beans in the context. invokeBeanFactoryPostProcessors(beanFactory); // Register bean processors that intercept bean creation. registerBeanPostProcessors(beanFactory); beanPostProcess.end(); // Initialize message source for this context. initMessageSource(); // Initialize event multicaster for this context. initApplicationEventMulticaster(); // Initialize other special beans in specific context subclasses. onRefresh(); // Check for listener beans and register them. registerListeners(); // Instantiate all remaining (non-lazy-init) singletons. finishBeanFactoryInitialization(beanFactory); // Last step: publish corresponding event. finishRefresh(); } catch (BeansException ex) { if (logger.isWarnEnabled()) { logger.warn("Exception encountered during context initialization - " + "cancelling refresh attempt: " + ex); } // Destroy already created singletons to avoid dangling resources. destroyBeans(); // Reset 'active' flag. cancelRefresh(ex); // Propagate exception to caller. throw ex; } finally { // Reset common introspection caches in Spring's core, since we // might not ever need metadata for singleton beans anymore... resetCommonCaches(); contextRefresh.end(); } } }
解读：
上述AbstractApplicationContext中的refresh方法调用的obtainFreshBeanFactory方法是整个调用链路的入口。
/** * Tell the subclass to refresh the internal bean factory. * @return the fresh BeanFactory instance * @see #refreshBeanFactory() * @see #getBeanFactory() */ protected ConfigurableListableBeanFactory obtainFreshBeanFactory() { refreshBeanFactory(); return getBeanFactory(); }
/** * Subclasses must implement this method to perform the actual configuration load. * The method is invoked by {@link #refresh()} before any other initialization work. * <p>A subclass will either create a new bean factory and hold a reference to it, * or return a single BeanFactory instance that it holds. In the latter case, it will * usually throw an IllegalStateException if refreshing the context more than once. * @throws BeansException if initialization of the bean factory failed * @throws IllegalStateException if already initialized and multiple refresh * attempts are not supported */ protected abstract void refreshBeanFactory() throws BeansException, IllegalStateException;
解读：
此处refreshBeanFactory方法是抽象方法，由如下两个子类实现：
（1）GenericApplicationContext
说明：
在GenericApplicationContext中，refreshBeanFactory方法仅允许被调用一次（由AtomicBoolean类型变量refreshed控制）
（2）AbstractRefreshableApplicationContext
/** * This implementation performs an actual refresh of this context's underlying * bean factory, shutting down the previous bean factory (if any) and * initializing a fresh bean factory for the next phase of the context's lifecycle. */ @Override protected final void refreshBeanFactory() throws BeansException { if (hasBeanFactory()) { destroyBeans(); closeBeanFactory(); } try { DefaultListableBeanFactory beanFactory = createBeanFactory(); beanFactory.setSerializationId(getId()); customizeBeanFactory(beanFactory); loadBeanDefinitions(beanFactory); this.beanFactory = beanFactory; } catch (IOException ex) { throw new ApplicationContextException("I/O error parsing bean definition source for " + getDisplayName(), ex); } }
/** * 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 */ protected abstract void loadBeanDefinitions(DefaultListableBeanFactory beanFactory) throws BeansException, IOException;
解读：
此处loadBeanDefinitions方法是抽象方法，有如下几个子类实现了该方法：
补充：
AbstractRefreshableConfigApplicationContext是AbstractRefreshableApplicationContext的子类，该类有如下几个子类
loadBeanDefinitions方法
挑选AbstractXmlApplicationContext以查看loadBeanDefinitions方法的实现细节，代码如下：
/** * 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); // 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); }
/** * Load the bean definitions with the given XmlBeanDefinitionReader. * <p>The lifecycle of the bean factory is handled by the {@link #refreshBeanFactory} * method; hence this method is just supposed to load and/or register bean definitions. * @param reader the XmlBeanDefinitionReader to use * @throws BeansException in case of bean registration errors * @throws IOException if the required XML document isn't found * @see #refreshBeanFactory * @see #getConfigLocations * @see #getResources * @see #getResourcePatternResolver */ 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); } }
解读：
上述loadBeanDefinitions方法最终是调用了AbstractBeanDefinitionReader中的loadBeanDefinitions方法，参数是数组类型。
问题：此处loadBeanDefinitions方法的返回值为空，而reader是new出来的变量，reader做什么事情看似跟ApplicationContext没啥关系啊。
答案：实则不然，构造reader时传递了beanFactory，在最终解析bean时用到了该参数，所以说beanfactory承担了关联解析bean的几个层次的作用。
