SSH 之 Spring的源码(一)——Bean加载过程

淡淡的烟草味﹌ 2022-08-07 06:55 198阅读 0赞

## 看看Spring的源码，看看巨人的底层实现，拓展思路，为了更好的理解原理，看看源码，深入浅出吧。本文基于Spring 4.0.8版本。 ##

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## 首先Web项目使用Spring是通过在web.xml里面配置 ##

## `org.springframework.web.context.ContextLoaderListener`初始化IOC容器的 ##

## `<listener> <listener-class>org.springframework.web.context.ContextLoaderListener</listener-class> </listener>` 那就以此为切入点顺藤摸瓜 ##

## `public class ContextLoaderListener extends ContextLoader implements ServletContextListener ` `ContextLoaderListener`继承了`ContextLoader`，并且实现`ServletContextListener`接口。当Server容器（一般指tomcat）启动时，会收到事件初始化。 ##

## `@Override public void contextInitialized(ServletContextEvent event) { initWebApplicationContext(event.getServletContext()); }` `initWebApplicationContext`方法是在`org.springframework.web.context.ContextLoader`类里面。方法太长，分段读一下。 ##

## `if (servletContext.getAttribute(WebApplicationContext.ROOT_WEB_APPLICATION_CONTEXT_ATTRIBUTE) != null) { throw new IllegalStateException("Cannot initialize context because there is already a root application context present - " +"check whether you have multiple ContextLoader* definitions in your web.xml!"); } Log logger = LogFactory.getLog(ContextLoader.class); servletContext.log("Initializing Spring root WebApplicationContext"); if (logger.isInfoEnabled()) { logger.info("Root WebApplicationContext: initialization started"); } long startTime = System.currentTimeMillis();` 首先是判断`servletContext`中是否已经注册了`WebApplicationContext`，如果有则抛出异常，避免重复注册。然后就是启用log，启动计时。本方法的关键就在于try代码块里的内容 ##

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## `try { // Store context in local instance variable, to guarantee that // it is available on ServletContext shutdown. if (this.context == null) { this.context = createWebApplicationContext(servletContext); } if (this.context instanceof ConfigurableWebApplicationContext) { ConfigurableWebApplicationContext cwac = (ConfigurableWebApplicationContext) this.context; if (!cwac.isActive()) { // The context has not yet been refreshed -> provide services such as // setting the parent context, setting the application context id, etc if (cwac.getParent() == null) { // The context instance was injected without an explicit parent -> // determine parent for root web application context, if any. ApplicationContext parent = loadParentContext(servletContext); cwac.setParent(parent); } configureAndRefreshWebApplicationContext(cwac, servletContext); } } servletContext.setAttribute(WebApplicationContext.ROOT_WEB_APPLICATION_CONTEXT_ATTRIBUTE, this.context); ClassLoader ccl = Thread.currentThread().getContextClassLoader(); if (ccl == ContextLoader.class.getClassLoader()) { currentContext = this.context; } else if (ccl != null) { currentContextPerThread.put(ccl, this.context); } if (logger.isDebugEnabled()) { logger.debug("Published root WebApplicationContext as ServletContext attribute with name [" + WebApplicationContext.ROOT_WEB_APPLICATION_CONTEXT_ATTRIBUTE + "]"); } if (logger.isInfoEnabled()) { long elapsedTime = System.currentTimeMillis() - startTime; logger.info("Root WebApplicationContext: initialization completed in " + elapsedTime + " ms"); } return this.context; } 这里面有几个关键的方法。首先看一下createWebApplicationContext() protected WebApplicationContext createWebApplicationContext(ServletContext sc) { Class<?> contextClass = determineContextClass(sc); if (!ConfigurableWebApplicationContext.class.isAssignableFrom(contextClass)) { throw new ApplicationContextException("Custom context class [" + contextClass.getName() + "] is not of type [" + ConfigurableWebApplicationContext.class.getName() + "]"); } return (ConfigurableWebApplicationContext) BeanUtils.instantiateClass(contextClass); } ` 首先`determineContextClass()`方法查明具体的`Context`类，他会读取`servletContext`的初始化参数`contextClass`，此参数我们一半不配置，所以`Spring`就会读取跟`org.springframework.web.context.WebApplicationContext`同一个包下面的`ContextLoader.properties`文件读取默认设置，反射出`org.springframework.web.context.support.XmlWebApplicationContext`类来。接下来就是在`configureAndRefreshWebApplicationContext()`方法里将新创建的`XmlWebApplicationContext`进行初始化。首先会设置一个默认ID，即`org.springframework.web.context.WebApplicationContext:`\+你项目的`ContextPath`。 ##

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## `if (ObjectUtils.identityToString(wac).equals(wac.getId())) { // The application context id is still set to its original default // value // -> assign a more useful id based on available information String idParam = sc.getInitParameter(CONTEXT_ID_PARAM); if (idParam != null) { wac.setId(idParam); } else { // Generate default id... wac.setId(ConfigurableWebApplicationContext.APPLICATION_CONTEXT_ID_PREFIX + ObjectUtils.getDisplayString(sc.getContextPath())); } }` 紧接着就是将`ServletContext`设置成`XmlWebApplicationContext`的属性，这样`Spring`就能在上下文里轻松拿到`ServletContext`了。 ##

## `wac.setServletContext(sc);` 接下来就是读取`web.xml`文件中的`contextConfigLocation`参数。如果没有配置就会去读`WEB-INF下的applicationContext.xml`文件。 ##

## `<context-param> <param-name>contextConfigLocation</param-name> <param-value>classpath:beans.xml</param-value> </context-param>` 并将值设置(就是我们的Spring配置文件的路径)进`XmlWebApplicationContext`中。然后就会在指定的路径加载配置文件 ##

## `String configLocationParam = sc.getInitParameter(CONFIG_LOCATION_PARAM); if (configLocationParam != null) { wac.setConfigLocation(configLocationParam); }` 接下来就是`customizeContext(sc, wac)`方法，此方法会根据用户配置的`globalInitializerClasses`参数来初始化一些用户自定义的属性，一般我们不配置，所以这里什么也不做。 最后登场的就是最核心的方法了， `wac.refresh();` 在这个方法里，会完成资源文件的加载、配置文件解析、Bean定义的注册、组件的初始化等核心工作，我们一探究竟。 ##

## `@Override public void refresh() throws BeansException, IllegalStateException { synchronized (this.startupShutdownMonitor) { // 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); // Invoke factory processors registered as beans in the context. invokeBeanFactoryPostProcessors(beanFactory); // Register bean processors that intercept bean creation. registerBeanPostProcessors(beanFactory); // 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) { // Destroy already created singletons to avoid dangling resources. destroyBeans(); // Reset 'active' flag. cancelRefresh(ex); // Propagate exception to caller. throw ex; } } }` ##

## 次方法是同步的，避免重复刷新，每个步骤都放在单独的方法内，流程清晰，是值得学习的地方。这里面有个重要的方法是`finishBeanFactoryInitialization(beanFactory);`，里面的内容是Spring如何实例化bean，并注入依赖的，这个内容下一节讲，本节只说明Spring是如何加载class文件的。 首先就是`prepareRefresh()`方法。 `protected void prepareRefresh() { this.startupDate = System.currentTimeMillis(); synchronized (this.activeMonitor) { this.active = true; } if (logger.isInfoEnabled()) { logger.info("Refreshing " + this); } // Initialize any placeholder property sources in the context environment initPropertySources(); // Validate that all properties marked as required are resolvable // see ConfigurablePropertyResolver#setRequiredProperties getEnvironment().validateRequiredProperties(); }` 此方法做一些准备工作，如记录开始时间，输出日志，`initPropertySources();`和`getEnvironment().validateRequiredProperties();`一般没干什么事。 接下来就是初始化`BeanFactory`，是整个`refresh()`方法的核心，其中完成了配置文件的加载、解析、注册 `ConfigurableListableBeanFactory beanFactory = obtainFreshBeanFactory();` 看看它里面都做了些什么？ `protected ConfigurableListableBeanFactory obtainFreshBeanFactory() { refreshBeanFactory(); ConfigurableListableBeanFactory beanFactory = getBeanFactory(); if (logger.isDebugEnabled()) { logger.debug("Bean factory for " + getDisplayName() + ": " + beanFactory); } return beanFactory; }` 首先`refreshBeanFactory()`： ##

## `protected final void refreshBeanFactory() throws BeansException { if (hasBeanFactory()) { destroyBeans(); closeBeanFactory(); } try { DefaultListableBeanFactory beanFactory = createBeanFactory(); beanFactory.setSerializationId(getId()); customizeBeanFactory(beanFactory); loadBeanDefinitions(beanFactory); synchronized (this.beanFactoryMonitor) { this.beanFactory = beanFactory; } } catch (IOException ex) { throw new ApplicationContextException("I/O error parsing bean definition source for " + getDisplayName(), ex); } }` 我们看到会创建一个`DefaultListableBeanFactory`实例 `DefaultListableBeanFactory beanFactory = createBeanFactory();` 再设置一个ID `beanFactory.setSerializationId(getId());` 然后设置一些自定义参数： `customizeBeanFactory(beanFactory);` 这里面最重要的就是`loadBeanDefinitions(beanFactory);`方法了。 `@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); }` 此方法会通过`XmlBeanDefinitionReader`加载bean定义。具体的实现方法是在`org.springframework.beans.factory.xml.XmlBeanDefinitionReader.loadBeanDefinitions`方法中定义的。这里设计了层层调用，有好多重载方法，主要就是加载Spring所有的配置文件(可能会有多个)，以备后面解析，注册之用。我一路追踪到`org.springframework.beans.factory.xml.DefaultBeanDefinitionDocumentReader.doRegisterBeanDefinitions(Element root)` ##

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## `protected void doRegisterBeanDefinitions(Element root) { String profileSpec = root.getAttribute(PROFILE_ATTRIBUTE); if (StringUtils.hasText(profileSpec)) { Assert.state(this.environment != null, "Environment must be set for evaluating profiles"); String[] specifiedProfiles = StringUtils.tokenizeToStringArray( profileSpec, BeanDefinitionParserDelegate.MULTI_VALUE_ATTRIBUTE_DELIMITERS); if (!this.environment.acceptsProfiles(specifiedProfiles)) { return; } } BeanDefinitionParserDelegate parent = this.delegate; this.delegate = createDelegate(this.readerContext, root, parent); preProcessXml(root); parseBeanDefinitions(root, this.delegate); postProcessXml(root); this.delegate = parent; }` 这里创建了一个`BeanDefinitionParserDelegate`示例，解析XML的过程就是委托它完成的，我们不关心它是怎样解析XML的，我们只关心是怎么加载类的，所以就要看`parseBeanDefinitions(root, this.delegate)`方法了。 ##

## `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); } else { delegate.parseCustomElement(ele); } } } } else { delegate.parseCustomElement(root); } }` 我们看到最终解析XML元素的是`delegate.parseCustomElement(ele)`方法，最终会走到一下方法. ##

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## `public BeanDefinition parseCustomElement(Element ele, BeanDefinition containingBd) { String namespaceUri = getNamespaceURI(ele); NamespaceHandler handler = this.readerContext.getNamespaceHandlerResolver().resolve(namespaceUri); if (handler == null) { error("Unable to locate Spring NamespaceHandler for XML schema namespace [" + namespaceUri + "]", ele); return null; } return handler.parse(ele, new ParserContext(this.readerContext, this, containingBd)); }` 这里会根据不同的XML节点，会委托`NamespaceHandlerSupport`找出合适的`BeanDefinitionParser`，如果我们配置了 ##

## `<context:component-scan base-package="com.geeekr.service,com.geeekr.dao" />` 那么对应`BeanDefinitionParser`就是`org.springframework.context.annotation.ComponentScanBeanDefinitionParser`，来看看它的`parse`方法。 `@Override public BeanDefinition parse(Element element, ParserContext parserContext) { String[] basePackages = StringUtils.tokenizeToStringArray(element.getAttribute(BASE_PACKAGE_ATTRIBUTE), ConfigurableApplicationContext.CONFIG_LOCATION_DELIMITERS); // Actually scan for bean definitions and register them. ClassPathBeanDefinitionScanner scanner = configureScanner(parserContext, element); Set<BeanDefinitionHolder> beanDefinitions = scanner.doScan(basePackages); registerComponents(parserContext.getReaderContext(), beanDefinitions, element); return null; }` 不难看出这里定义了一个`ClassPathBeanDefinitionScanner`，通过它去扫描包中的类文件，***注意：这里是类文件而不是类，因为现在这些类还没有被加载，只是ClassLoader能找到这些class的路径而已。***到目前为止，感觉真想距离我们越来越近了。顺着继续往下摸。进入`doSacn`方法里，映入眼帘的又是一大坨代码，但是我们只关心观点的部分。 `rotected Set<BeanDefinitionHolder> doScan(String... basePackages) { Assert.notEmpty(basePackages, "At least one base package must be specified"); Set<BeanDefinitionHolder> beanDefinitions = new LinkedHashSet<BeanDefinitionHolder>(); for (String basePackage : basePackages) { Set<BeanDefinition> candidates = findCandidateComponents(basePackage); for (BeanDefinition candidate : candidates) { ScopeMetadata scopeMetadata = this.scopeMetadataResolver.resolveScopeMetadata(candidate); candidate.setScope(scopeMetadata.getScopeName()); String beanName = this.beanNameGenerator.generateBeanName(candidate, this.registry); if (candidate instanceof AbstractBeanDefinition) { postProcessBeanDefinition((AbstractBeanDefinition) candidate, beanName); } if (candidate instanceof AnnotatedBeanDefinition) { AnnotationConfigUtils.processCommonDefinitionAnnotations((AnnotatedBeanDefinition) candidate); } if (checkCandidate(beanName, candidate)) { BeanDefinitionHolder definitionHolder = new BeanDefinitionHolder(candidate, beanName); definitionHolder = AnnotationConfigUtils.applyScopedProxyMode(scopeMetadata, definitionHolder, this.registry); beanDefinitions.add(definitionHolder); registerBeanDefinition(definitionHolder, this.registry); } } } return beanDefinitions; }` 一眼就能看出是通过 ##

## `Set<BeanDefinition> candidates = findCandidateComponents(basePackage);` 有时候不得不佩服这些外国人起名字的功力，把扫描出来的类叫做candidates(候选人)；真是不服不行啊，这种名字真的很容易理解有不有？哈哈，貌似扯远了。继续往下看。这里只列出方法的主题部分。 `public Set<BeanDefinition> findCandidateComponents(String basePackage) { Set<BeanDefinition> candidates = new LinkedHashSet<BeanDefinition>(); try { String packageSearchPath = ResourcePatternResolver.CLASSPATH_ALL_URL_PREFIX + resolveBasePackage(basePackage) + "/" + this.resourcePattern; Resource[] resources = this.resourcePatternResolver.getResources(packageSearchPath); boolean traceEnabled = logger.isTraceEnabled(); boolean debugEnabled = logger.isDebugEnabled(); for (Resource resource : resources) { if (traceEnabled) { logger.trace("Scanning " + resource); } if (resource.isReadable()) { try { MetadataReader metadataReader = this.metadataReaderFactory.getMetadataReader(resource); if (isCandidateComponent(metadataReader)) { ScannedGenericBeanDefinition sbd = new ScannedGenericBeanDefinition(metadataReader); sbd.setResource(resource); sbd.setSource(resource);` 先看这两句： `String packageSearchPath = ResourcePatternResolver.CLASSPATH_ALL_URL_PREFIX + resolveBasePackage(basePack` 假设我们配置的需要扫描的包名为`com.geeekr.service`，那么`packageSearchPath`的值就是`classpath*:com.geeekr.service/**/*.class`，意思就是com.geeekr.service包(包括子包)下所有class文件；如果配置的是`*`，那么`packageSearchPath`的值就是`classpath*:*/**/*.class`。这里的表达式是Spring自己定义的。Spring会根据这种表达式找出相关的class文件。 `Resource[] resources = this.resourcePatternResolver.getResources(packageSearchPath);` 这一句就把相关class文件加载出来了，那我们就要看看，Spring究竟是如何把class文件找到的了。首先看看`resourcePatternResolver`的定义: `private ResourcePatternResolver resourcePatternResolver = new PathMatchingResourcePatternResolver();` 进入`getResources`方法 `@Override public Resource[] getResources(String locationPattern) throws IOException { Assert.notNull(locationPattern, "Location pattern must not be null"); if (locationPattern.startsWith(CLASSPATH_ALL_URL_PREFIX)) { // a class path resource (multiple resources for same name possible) if (getPathMatcher().isPattern(locationPattern.substring(CLASSPATH_ALL_URL_PREFIX.length()))) { // a class path resource pattern return findPathMatchingResources(locationPattern); } else { // all class path resources with the given name return findAllClassPathResources(locationPattern.substring(CLASSPATH_ALL_URL_PREFIX.length())); } } else { // Only look for a pattern after a prefix here // (to not get fooled by a pattern symbol in a strange prefix). int prefixEnd = locationPattern.indexOf(":") + 1; if (getPathMatcher().isPattern(locationPattern.substring(prefixEnd))) { // a file pattern return findPathMatchingResources(locationPattern); } else { // a single resource with the given name return new Resource[] {getResourceLoader().getResource(locationPattern)}; } } }` 这里会先判断表达式是否以`classpath*:`开头。前面我们看到Spring已经给我们添加了这个头，这里当然符合条件了。接着会进入`findPathMatchingResources`方法。在这里又把`**/*.class`去掉了，然后在调用`getResources`方法，然后在进入`findAllClassPathResources`方法。这里的参数只剩下包名了例如`com/geeekr/service/`。 `protected Resource[] findAllClassPathResources(String location) throws IOException { String path = location; if (path.startsWith("/")) { path = path.substring(1); } ClassLoader cl = getClassLoader(); Enumeration<URL> resourceUrls = (cl != null ? cl.getResources(path) : ClassLoader.getSystemResources(path)); Set<Resource> result = new LinkedHashSet<Resource>(16); while (resourceUrls.hasMoreElements()) { URL url = resourceUrls.nextElement(); result.add(convertClassLoaderURL(url)); } return result.toArray(new Resource[result.size()]); }` 真相大白了，Spring也是用的`ClassLoader`加载的class文件。一路追踪，原始的ClassLoader是`Thread.currentThread().getContextClassLoader();`。到此为止，就拿到class文件了。 Spring会将class信息封装成`BeanDefinition`，然后再放进`DefaultListableBeanFactory`的`beanDefinitionMap`中。 拿到了class文件后，就要看看Spring是如何装配bean的了，下一节，继续看。 ##