【十三】图解 Spring 核心数据结构：BeanDefinition 其二

图解 Spring 核心数据结构：BeanDefinition 其二

概述

前面写过一篇相关文章作为开篇介绍了一下BeanDefinition，本篇将深入细节来向读者展示BeanDefinition的设计，让我们一起来揭开日常开发中使用的bean的神秘面纱，深入细节透彻理解spring bean的概念。

一、BeanDefinition的加载过程

首先我们来复习一下spring 容器的类的继承体系：

可以看到这是一个庞大的类继承体系，上面都是实现的接口，第一个实现类是AbstractApplicationContext，这里面有一个关键的方法fefresh()，该方法为 spring 启动入口，本文重点不是梳理这个方法，如果有需要可以留言后续出文章进行详细分析。

 @Override
    public void refresh() throws BeansException, IllegalStateException {
        synchronized (this.startupShutdownMonitor) {
            // Prepare this context for refreshing.
            // 初始化前的准备工作，比如系统属性、环境变量的准备及验证
            prepareRefresh();

            // Tell the subclass to refresh the internal bean factory.
            // 初始化BeanFactory,并进行XML文件解析
            ConfigurableListableBeanFactory beanFactory = obtainFreshBeanFactory();

            // Prepare the bean factory for use in this context.
            // 准备bean和非bean
            // BeanFactory各种功能的填充，比如对@Qualifier和@Autowired注解的支持
            prepareBeanFactory(beanFactory);

            try {
                // Allows post-processing of the bean factory in context subclasses.
                // 扩展点，具体功能由子类实现
                postProcessBeanFactory(beanFactory);

                // Invoke factory processors registered as beans in the context.
                // 激活各种BeanFactory处理器
                invokeBeanFactoryPostProcessors(beanFactory);

                // Register bean processors that intercept bean creation.
                // 注册拦截Bean创建的后处理器，这里只是注册，真正的调用在getBean的时候
                registerBeanPostProcessors(beanFactory);

                // Initialize message source for this context.
                // 国际化处理
                // 为上下文初始化Message源，即不同语音的消息体
                initMessageSource();

                // Initialize event multicaster for this context.
                // 初始化应用消息广播器，并初始化"applicationEventMulticaster"bean
                initApplicationEventMulticaster();

                // Initialize other special beans in specific context subclasses.
                // 模版方法，由子类扩展
                onRefresh();

                // Check for listener beans and register them.
                // 在所有注册的bean中查找Listener bean,注册到消息广播器中
                registerListeners();

                // Instantiate all remaining (non-lazy-init) singletons.
                // 对非惰性的单例进行初始化
                // 一般情况下单例都会在这里就初始化了，除非指定了惰性加载
                finishBeanFactoryInitialization(beanFactory);

                // Last step: publish corresponding event.
                // 完成刷新过程，通知生命周期处理器lifecycleProcessor刷新过程，
                // 同时发出ContextRefreshedEvent时间通知别人
                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();
            }
        }
    }

这里我们主要关注obtainFreshBeanFactory()方法，这个方法是初始化BeanFactory的，在初始化的过程中会解析BeanDefinition，为了继续分析BeanDefinition的解析，这时候我们需要忽略其他的一些实现逻辑，我们在AbstractRefreshableApplicationContext中找到一个抽象方法：

	/**
	 * 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;

看注释就知道这个方法是加载BeanDefinition的，我们找到了方法实现类：AnnotationConfigWebApplicationContext和XmlBeanDefinitionReader，可以明显的看出来这两个实现类一个是解析注解方式的，一个是xml配置方式的，限于篇幅这里分析一下经典的xml方式。

如下是XmlBeanDefinitionReader中的实现：

	/**
	 * 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(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);
	}

这里首先定义了一个xml文件读取类XmlBeanDefinitionReader,，之后继续跟进源码：

/**
	 * Load the bean definitions with the given XmlBeanDefinitionReader.
	 * <p>The lifecycle of the bean factory is handled by the refreshBeanFactory method;
	 * therefore this method is just supposed to load and/or register bean definitions.
	 * <p>Delegates to a ResourcePatternResolver for resolving location patterns
	 * into Resource instances.
	 * @throws IOException if the required XML document isn't found
	 * @see #refreshBeanFactory
	 * @see #getConfigLocations
	 * @see #getResources
	 * @see #getResourcePatternResolver
	 */
	protected void loadBeanDefinitions(XmlBeanDefinitionReader reader) throws IOException {
		String[] configLocations = getConfigLocations();
		if (configLocations != null) {
			for (String configLocation : configLocations) {
				reader.loadBeanDefinitions(configLocation);
			}
		}
	}

可以看到这里是循环遍历从配置的目录中加载BeanDefinition，继续跟进loadBeanDefinitions()方法

，我们找到了实际加载BeanDefinition的方法：

/**
	 * Actually load bean definitions from the specified XML file.
	 * @param inputSource the SAX InputSource to read from
	 * @param resource the resource descriptor for the XML file
	 * @return the number of bean definitions found
	 * @throws BeanDefinitionStoreException in case of loading or parsing errors
	 * @see #doLoadDocument
	 * @see #registerBeanDefinitions
	 */
	protected int doLoadBeanDefinitions(InputSource inputSource, Resource resource)
			throws BeanDefinitionStoreException {

		try {
			Document doc = doLoadDocument(inputSource, resource);
			int count = registerBeanDefinitions(doc, resource);
			if (logger.isDebugEnabled()) {
				logger.debug("Loaded " + count + " bean definitions from " + resource);
			}
			return count;
		}
		catch (BeanDefinitionStoreException ex) {
			throw ex;
		}
		catch (SAXParseException ex) {
			throw new XmlBeanDefinitionStoreException(resource.getDescription(),
					"Line " + ex.getLineNumber() + " in XML document from " + resource + " is invalid", ex);
		}
		catch (SAXException ex) {
			throw new XmlBeanDefinitionStoreException(resource.getDescription(),
					"XML document from " + resource + " is invalid", ex);
		}
		catch (ParserConfigurationException ex) {
			throw new BeanDefinitionStoreException(resource.getDescription(),
					"Parser configuration exception parsing XML from " + resource, ex);
		}
		catch (IOException ex) {
			throw new BeanDefinitionStoreException(resource.getDescription(),
					"IOException parsing XML document from " + resource, ex);
		}
		catch (Throwable ex) {
			throw new BeanDefinitionStoreException(resource.getDescription(),
					"Unexpected exception parsing XML document from " + resource, ex);
		}
	}

继续往下跟进我们到了Bean注册的方法中：

/**
	 * Register each bean definition within the given root {@code <beans/>} element.
	 */
	@SuppressWarnings("deprecation")  // for Environment.acceptsProfiles(String...)
	protected void doRegisterBeanDefinitions(Element root) {
		// Any nested <beans> elements will cause recursion in this method. In
		// order to propagate and preserve <beans> default-* attributes correctly,
		// keep track of the current (parent) delegate, which may be null. Create
		// the new (child) delegate with a reference to the parent for fallback purposes,
		// then ultimately reset this.delegate back to its original (parent) reference.
		// this behavior emulates a stack of delegates without actually necessitating one.
		BeanDefinitionParserDelegate parent = this.delegate;
		BeanDefinitionParserDelegate current = createDelegate(getReaderContext(), root, parent);
		this.delegate = current;

		if (current.isDefaultNamespace(root)) {
			String profileSpec = root.getAttribute(PROFILE_ATTRIBUTE);
			if (StringUtils.hasText(profileSpec)) {
				String[] specifiedProfiles = StringUtils.tokenizeToStringArray(
						profileSpec, BeanDefinitionParserDelegate.MULTI_VALUE_ATTRIBUTE_DELIMITERS);
				// We cannot use Profiles.of(...) since profile expressions are not supported
				// in XML config. See SPR-12458 for details.
				if (!getReaderContext().getEnvironment().acceptsProfiles(specifiedProfiles)) {
					if (logger.isDebugEnabled()) {
						logger.debug("Skipped XML bean definition file due to specified profiles [" + profileSpec +
								"] not matching: " + getReaderContext().getResource());
					}
					return;
				}
			}
		}

		preProcessXml(root);
		parseBeanDefinitions(root, current);
		postProcessXml(root);

		this.delegate = parent;
	}

这里我们关注一下这个方法：

	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);
		}
	}

这里就是根据xml中配置的标签元素来进行处理的，我们可以打开BeanDefinitionParserDelegate

类，发现我们找到了xml中标签元素的定义了。

到这里主流程基本讲完了细节不再拖沓了，直接进入DefaultListableBeanFactory类中，到此我们发现这样一行代码：

this.beanDefinitionMap.put(beanName, beanDefinition);

原来我们的bean最终都是存放在map中的：

/** Map of bean definition objects, keyed by bean name. */
	private final Map<String, BeanDefinition> beanDefinitionMap = new ConcurrentHashMap<>(256);