详解Spring如何解析占位符详解Spring如何解析占位符

什么是Spring的占位符？
Spring什么时候去解析并占位符

什么是Spring的占位符？
在以前的Spring Xml配置中我们可能会有如下配置：

在上面的配置中jdbc这个Bean的url属性值${jdbc.url}就代表占位符，占位符的真实值就存放在上述配置中的自定义元素的location属性所代表的配置文件jdbc.properties中,这个配置文件里就一行内容：

jdbc.url=127.0.0.1

那问题就来了，Spring又是在什么阶段去解析并且把占位符替换为实际值的呢？

Spring什么时候去解析并占位符
【详解Spring如何解析占位符】从我们就可以知道它是一个自定义xml标签，那Spring势必要解析它，我就直接黏贴Spring解析这个自定义元素的入口代码给各位看官。该代码就在BeanDefinitionParserDelegate这个类中：

/*** Parse the elements at the root level in the document:* "import", "alias", "bean".* @param root the DOM root element of the document*///todo doRegisterBeanDefinitions ->parseBeanDefinitions -> parseDefaultElementprotected 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; //如果属于beans命名空间if (delegate.isDefaultNamespace(ele)) {//处理默认标签parseDefaultElement(ele, delegate); }else {//自定义标签//用到了parser//todo parser内部去注册BeanDefinition 2021-3-15delegate.parseCustomElement(ele); }}}}else {delegate.parseCustomElement(root); }}

主要关注点：delegate.parseCustomElement(ele);

@Nullablepublic BeanDefinition parseCustomElement(Element ele) {return parseCustomElement(ele, null); }//todo property 子元素也有可能解析自定义元素 parsePropertySubElement@Nullablepublic BeanDefinition parseCustomElement(Element ele, @Nullable BeanDefinition containingBd) {String namespaceUri = getNamespaceURI(ele); if (namespaceUri == null) {return null; }//resolve里有初始化过程//根据命名空间uri获取 NamespaceHandler//todo 获取命名空间下的自定义handler 比如 ContextNamespaceHandlerNamespaceHandler handler = this.readerContext.getNamespaceHandlerResolver().resolve(namespaceUri); if (handler == null) {//todo 如果在spring.handlers配置文件里没有定义这个命令空间的handler就会报这个错 2020-09-14error("Unable to locate Spring NamespaceHandler for XML schema namespace [" + namespaceUri + "]", ele); return null; }//调用parse方法//这里ParserContext注入registry//readerContext里 reader->XmlBeanDefinitionReader 里包含了 registry//TODO 会初始化一个ParserContext进去return handler.parse(ele, new ParserContext(this.readerContext, this, containingBd)); }

到这里我们又要关注这个NamespaceHandler是怎么获取到的，从上面代码可知，Spring会从当前readerContext获取到NamespaceHandlerResolver后通过其resolve方法并根据传入的当前namespaceUri就可以获得当前适合的NamespaceHandler。

/*** Create the {@link XmlReaderContext} to pass over to the document reader.*/public XmlReaderContext createReaderContext(Resource resource) {//把当前reader放进去 ,reader存放了 beanRegistry//beanRegistry 里定义了 registerBeanDefinition 这个重要的方法return new XmlReaderContext(resource, this.problemReporter, this.eventListener,this.sourceExtractor, this, getNamespaceHandlerResolver()); }/*** Lazily create a default NamespaceHandlerResolver, if not set before.* 解析自定义标签时用到* @see #createDefaultNamespaceHandlerResolver()*/public NamespaceHandlerResolver getNamespaceHandlerResolver() {if (this.namespaceHandlerResolver == null) {this.namespaceHandlerResolver = createDefaultNamespaceHandlerResolver(); }return this.namespaceHandlerResolver; }/*** Create the default implementation of {@link NamespaceHandlerResolver} used if none is specified.* The default implementation returns an instance of {@link DefaultNamespaceHandlerResolver}.* @see DefaultNamespaceHandlerResolver#DefaultNamespaceHandlerResolver(ClassLoader)*/protected NamespaceHandlerResolver createDefaultNamespaceHandlerResolver() {ClassLoader cl = (getResourceLoader() != null ? getResourceLoader().getClassLoader() : getBeanClassLoader()); return new DefaultNamespaceHandlerResolver(cl); }//返回默认的public DefaultNamespaceHandlerResolver(@Nullable ClassLoader classLoader) {this(classLoader, DEFAULT_HANDLER_MAPPINGS_LOCATION); }

从上面的代码中可知Spring使用的是默认的DefaultNamespaceHandlerResolver,它当然也给开发者留了自定义NamespaceHandlerResolver的机会。那我们现在就可以看看DefaultNamespaceHandlerResolver如何根据namespaceUri解析到对应的NamespaceHandler的。
首先获取到的就是context命名空间，完整路径为http\://www.springframework.org/schema/context。我们从DefaultNamespaceHandlerResolver类中可以看到它是如何解析这个命名空间的。

/*** Locate the {@link NamespaceHandler} for the supplied namespace URI* from the configured mappings.* @param namespaceUri the relevant namespace URI* @return the located {@link NamespaceHandler}, or {@code null} if none found*/@Override@Nullablepublic NamespaceHandler resolve(String namespaceUri) {Map handlerMappings = getHandlerMappings(); Object handlerOrClassName = handlerMappings.get(namespaceUri); if (handlerOrClassName == null) {return null; }else if (handlerOrClassName instanceof NamespaceHandler) {return (NamespaceHandler) handlerOrClassName; }else {String className = (String) handlerOrClassName; try {Class handlerClass = ClassUtils.forName(className, this.classLoader); if (!NamespaceHandler.class.isAssignableFrom(handlerClass)) {throw new FatalBeanException("Class [" + className + "] for namespace [" + namespaceUri +"] does not implement the [" + NamespaceHandler.class.getName() + "] interface"); }NamespaceHandler namespaceHandler = (NamespaceHandler) BeanUtils.instantiateClass(handlerClass); //todo命名空间处理器调用初始化过程 2020-09-04namespaceHandler.init(); handlerMappings.put(namespaceUri, namespaceHandler); return namespaceHandler; }catch (ClassNotFoundException ex) {throw new FatalBeanException("Could not find NamespaceHandler class [" + className +"] for namespace [" + namespaceUri + "]", ex); }catch (LinkageError err) {throw new FatalBeanException("Unresolvable class definition for NamespaceHandler class [" +className + "] for namespace [" + namespaceUri + "]", err); }}}/*** Load the specified NamespaceHandler mappings lazily.*/private Map getHandlerMappings() {Map handlerMappings = this.handlerMappings; if (handlerMappings == null) {synchronized (this) {handlerMappings = this.handlerMappings; if (handlerMappings == null) {if (logger.isTraceEnabled()) {logger.trace("Loading NamespaceHandler mappings from [" + this.handlerMappingsLocation + "]"); }try {//todo handlerMappings为空才去获取所有属性映射 2020-09-04//spring-aop spring-beans spring-contextProperties mappings =PropertiesLoaderUtils.loadAllProperties(this.handlerMappingsLocation, this.classLoader); if (logger.isTraceEnabled()) {logger.trace("Loaded NamespaceHandler mappings: " + mappings); }handlerMappings = new ConcurrentHashMap<>(mappings.size()); CollectionUtils.mergePropertiesIntoMap(mappings, handlerMappings); this.handlerMappings = handlerMappings; }catch (IOException ex) {throw new IllegalStateException("Unable to load NamespaceHandler mappings from location [" + this.handlerMappingsLocation + "]", ex); }}}}return handlerMappings; }

上面代码中的handlerMappingsLocation一般就是Spring默认的路径：

//指定了默认的handler路径，可以传入指定路径改变/*** The location to look for the mapping files. Can be present in multiple JAR files.*/public static final String DEFAULT_HANDLER_MAPPINGS_LOCATION = "META-INF/spring.handlers";

我们就回到spring-context项目工程下的resoures/META-INF文件夹下的spring.handlers文件里定义了如下规则：

http\://www.springframework.org/schema/context=org.springframework.context.config.ContextNamespaceHandlerhttp\://www.springframework.org/schema/jee=org.springframework.ejb.config.JeeNamespaceHandlerhttp\://www.springframework.org/schema/lang=org.springframework.scripting.config.LangNamespaceHandlerhttp\://www.springframework.org/schema/task=org.springframework.scheduling.config.TaskNamespaceHandlerhttp\://www.springframework.org/schema/cache=org.springframework.cache.config.CacheNamespaceHandler

可以看到context自定义命名空间就是对应的ContextNamespaceHandler。我们打开这个类瞧一瞧：

public class ContextNamespaceHandler extends NamespaceHandlerSupport {@Overridepublic void init() {//调用抽象类NamespaceHandlerSupport的注册解析器方法registerBeanDefinitionParser("property-placeholder", new PropertyPlaceholderBeanDefinitionParser()); registerBeanDefinitionParser("property-override", new PropertyOverrideBeanDefinitionParser()); registerBeanDefinitionParser("annotation-config", new AnnotationConfigBeanDefinitionParser()); registerBeanDefinitionParser("component-scan", new ComponentScanBeanDefinitionParser()); registerBeanDefinitionParser("load-time-weaver", new LoadTimeWeaverBeanDefinitionParser()); registerBeanDefinitionParser("spring-configured", new SpringConfiguredBeanDefinitionParser()); registerBeanDefinitionParser("mbean-export", new MBeanExportBeanDefinitionParser()); registerBeanDefinitionParser("mbean-server", new MBeanServerBeanDefinitionParser()); }}

发现它只定义了一个init方法，顾名思义就是初始化的意思，那想当然的就是它啥时候会执行初始化呢？我们回到DefaultNamespaceHandler的resolve方法，发现它内部有一处namespaceHandler.init(); ，这里就执行了对应命名空间处理器的初始化方法。接下来我们又要看看初始化了做了些啥，我们发现它调用了同一个方法registerBeanDefinitionParser也就是注册
Bean定义解析器,到这里我们先按下暂停键，再捋下上面的整体流程：