本文说点儿简单的。
如果你想研究基于 XML 配置的 spring mvc 的话，可以简单扫一眼本文。
在基于 XML 配置的 spring mvc 开发中，我们主要就是通过 spring 提供的各种标签来配置。
但是，大家是不是都有个疑问，spring 到底给我们提供了多少标签？都有哪些？子元素有哪些？属性有哪些？
当然，如果你对 XML 非常熟悉，那简单，直接看对应模块的 schema 文件就行了。
比如在 sping-webmvc 模块的 META-INF/spring.schemas 文件中就定义了如下这些 schema
http\://www.springframework.org/schema/mvc/spring-mvc-3.0.xsd=org/springframework/web/servlet/config/spring-mvc.xsd
http\://www.springframework.org/schema/mvc/spring-mvc-3.1.xsd=org/springframework/web/servlet/config/spring-mvc.xsd
http\://www.springframework.org/schema/mvc/spring-mvc-3.2.xsd=org/springframework/web/servlet/config/spring-mvc.xsd
http\://www.springframework.org/schema/mvc/spring-mvc-4.0.xsd=org/springframework/web/servlet/config/spring-mvc.xsd
http\://www.springframework.org/schema/mvc/spring-mvc-4.1.xsd=org/springframework/web/servlet/config/spring-mvc.xsd
http\://www.springframework.org/schema/mvc/spring-mvc-4.2.xsd=org/springframework/web/servlet/config/spring-mvc.xsd
http\://www.springframework.org/schema/mvc/spring-mvc-4.3.xsd=org/springframework/web/servlet/config/spring-mvc.xsd
http\://www.springframework.org/schema/mvc/spring-mvc.xsd=org/springframework/web/servlet/config/spring-mvc.xsd
你点开这些链接，研究其中的内容就行了，比如 http://www.springframework.org/schema/mvc/spring-mvc-4.3.xsd 这个文件内容如下（只是部分）
This XML file does not appear to have any style information associated with it. The document tree is shown below.
<xsd:schema xmlns="http://www.springframework.org/schema/mvc" xmlns:xsd="http://www.w3.org/2001/XMLSchema" xmlns:beans="http://www.springframework.org/schema/beans" xmlns:tool="http://www.springframework.org/schema/tool" targetNamespace="http://www.springframework.org/schema/mvc" elementFormDefault="qualified" attributeFormDefault="unqualified">
<xsd:import namespace="http://www.springframework.org/schema/beans" schemaLocation="https://www.springframework.org/schema/beans/spring-beans-4.3.xsd"/>
<xsd:import namespace="http://www.springframework.org/schema/tool" schemaLocation="https://www.springframework.org/schema/tool/spring-tool-4.3.xsd"/>
<xsd:element name="annotation-driven">
<xsd:annotation>
<xsd:documentation source="java:org.springframework.web.servlet.mvc.method.annotation.RequestMappingHandlerAdapter">
<![CDATA[ Configures the annotation-driven Spring MVC Controller programming model. Note that this tag works in Web MVC only, not in Portlet MVC! See org.springframework.web.servlet.config.annotation.EnableWebMvc javadoc for details on code-based alternatives to enabling annotation-driven Spring MVC support. ]]>
</xsd:documentation>
</xsd:annotation>
<xsd:complexType>
<xsd:all minOccurs="0">
<xsd:element name="path-matching" minOccurs="0">
<xsd:annotation>
<xsd:documentation>
<![CDATA[ Configures the path matching part of the Spring MVC Controller programming model. Like annotation-driven, code-based alternatives are also documented in EnableWebMvc javadoc. ]]>
</xsd:documentation>
</xsd:annotation>
<xsd:complexType>
<xsd:attribute name="suffix-pattern" type="xsd:boolean">
<xsd:annotation>
<xsd:documentation>
<![CDATA[ Whether to use suffix pattern match (".*") when matching patterns to requests. If enabled a method mapped to "/users" also matches to "/users.*". The default value is true. ]]>
</xsd:documentation>
</xsd:annotation>
</xsd:attribute>
<xsd:attribute name="trailing-slash" type="xsd:boolean">
<xsd:annotation>
<xsd:documentation>
<![CDATA[ Whether to match to URLs irrespective of the presence of a trailing slash. If enabled a method mapped to "/users" also matches to "/users/". The default value is true. ]]>
</xsd:documentation>
</xsd:annotation>
</xsd:attribute>
<xsd:attribute name="registered-suffixes-only" type="xsd:boolean">
<xsd:annotation>
<xsd:documentation>
<![CDATA[ Whether suffix pattern matching should work only against path extensions explicitly registered when you configure content negotiation. This is generally recommended to reduce ambiguity and to avoid issues such as when a "." appears in the path for other reasons. The default value is false. ]]>
</xsd:documentation>
</xsd:annotation>
</xsd:attribute>
<xsd:attribute name="path-helper" type="xsd:string">
<xsd:annotation>
<xsd:documentation>
<![CDATA[ The bean name of the UrlPathHelper to use for resolution of lookup paths. Use this to override the default UrlPathHelper with a custom subclass, or to share common UrlPathHelper settings across multiple HandlerMappings and MethodNameResolvers. ]]>
</xsd:documentation>
</xsd:annotation>
</xsd:attribute>
<xsd:attribute name="path-matcher" type="xsd:string">
<xsd:annotation>
<xsd:documentation>
<![CDATA[ The bean name of the PathMatcher implementation to use for matching URL paths against registered URL patterns. Default is AntPathMatcher. ]]>
</xsd:documentation>
</xsd:annotation>
</xsd:attribute>
</xsd:complexType>
</xsd:element>
<xsd:element name="message-converters" minOccurs="0">
<xsd:annotation>
<xsd:documentation>
<![CDATA[ Configures one or more HttpMessageConverter types to use for converting @RequestBody method parameters and @ResponseBody method return values. Using this configuration element is optional. HttpMessageConverter registrations provided here will take precedence over HttpMessageConverter types registered by default. Also see the register-defaults attribute if you want to turn off default registrations entirely. ]]>
</xsd:documentation>
</xsd:annotation>
<xsd:complexType>
<xsd:sequence>
<xsd:choice maxOccurs="unbounded">
<xsd:element ref="beans:bean">
<xsd:annotation>
<xsd:documentation>
<![CDATA[ An HttpMessageConverter bean definition. ]]>
</xsd:documentation>
</xsd:annotation>
</xsd:element>
<xsd:element ref="beans:ref">
<xsd:annotation>
<xsd:documentation>
<![CDATA[ A reference to an HttpMessageConverter bean. ]]>
</xsd:documentation>
</xsd:annotation>
</xsd:element>
</xsd:choice>
</xsd:sequence>
<xsd:attribute name="register-defaults" type="xsd:boolean" default="true">
<xsd:annotation>
<xsd:documentation>
<![CDATA[ Whether or not default HttpMessageConverter registrations should be added in addition to the ones provided within this element. ]]>
</xsd:documentation>
</xsd:annotation>
</xsd:attribute>
</xsd:complexType>
</xsd:element>
<xsd:element name="argument-resolvers" minOccurs="0">
<xsd:annotation>
<xsd:documentation>
<![CDATA[ Configures HandlerMethodArgumentResolver types to support custom controller method argument types. Using this option does not override the built-in support for resolving handler method arguments. To customize the built-in support for argument resolution configure RequestMappingHandlerAdapter directly. ]]>
</xsd:documentation>
</xsd:annotation>
<xsd:complexType>
<xsd:choice minOccurs="1" maxOccurs="unbounded">
<xsd:element ref="beans:bean" minOccurs="0" maxOccurs="unbounded">
<xsd:annotation>
<xsd:documentation>
<![CDATA[ The HandlerMethodArgumentResolver (or WebArgumentResolver for backwards compatibility) bean definition. ]]>
</xsd:documentation>
</xsd:annotation>
</xsd:element>
<xsd:element ref="beans:ref" minOccurs="0" maxOccurs="unbounded">
<xsd:annotation>
<xsd:documentation>
<![CDATA[ A reference to a HandlerMethodArgumentResolver bean definition. ]]>
</xsd:documentation>
<xsd:appinfo>
<tool:annotation kind="ref">
<tool:expected-type type="java:org.springframework.web.method.support.HandlerMethodArgumentResolver"/>
</tool:annotation>
</xsd:appinfo>
</xsd:annotation>
</xsd:element>
</xsd:choice>
</xsd:complexType>
</xsd:element>
<xsd:element name="return-value-handlers" minOccurs="0">
<xsd:annotation>
<xsd:documentation>
<![CDATA[ Configures HandlerMethodReturnValueHandler types to support custom controller method return value handling. Using this option does not override the built-in support for handling return values. To customize the built-in support for handling return values configure RequestMappingHandlerAdapter directly. ]]>
</xsd:documentation>
</xsd:annotation>
<xsd:complexType>
<xsd:choice minOccurs="1" maxOccurs="unbounded">
<xsd:element ref="beans:bean" minOccurs="0" maxOccurs="unbounded">
<xsd:annotation>
<xsd:documentation>
<![CDATA[ The HandlerMethodReturnValueHandler bean definition. ]]>
</xsd:documentation>
</xsd:annotation>
</xsd:element>
<xsd:element ref="beans:ref" minOccurs="0" maxOccurs="unbounded">
<xsd:annotation>
<xsd:documentation>
<![CDATA[ A reference to a HandlerMethodReturnValueHandler bean definition. ]]>
</xsd:documentation>
<xsd:appinfo>
<tool:annotation kind="ref">
<tool:expected-type type="java:org.springframework.web.method.support.HandlerMethodReturnValueHandler"/>
</tool:annotation>
</xsd:appinfo>
</xsd:annotation>
</xsd:element>
</xsd:choice>
</xsd:complexType>
</xsd:element>
<xsd:element name="async-support" minOccurs="0">
<xsd:annotation>
<xsd:documentation>
<![CDATA[ Configure options for asynchronous request processing. ]]>
</xsd:documentation>
</xsd:annotation>
<xsd:complexType>
<xsd:all minOccurs="0">
<xsd:element name="callable-interceptors" minOccurs="0">
<xsd:annotation>
<xsd:documentation>
<![CDATA[ The ordered set of interceptors that intercept the lifecycle of concurrently executed requests, which start after a controller returns a java.util.concurrent.Callable. ]]>
</xsd:documentation>
</xsd:annotation>
<xsd:complexType>
<xsd:sequence>
<xsd:element ref="beans:bean" minOccurs="1" maxOccurs="unbounded">
<xsd:annotation>
<xsd:documentation>
<![CDATA[ Registers a CallableProcessingInterceptor. ]]>
</xsd:documentation>
</xsd:annotation>
</xsd:element>
</xsd:sequence>
</xsd:complexType>
</xsd:element>
<xsd:element name="deferred-result-interceptors" minOccurs="0">
<xsd:annotation>
<xsd:documentation>
<![CDATA[ The ordered set of interceptors that intercept the lifecycle of concurrently executed requests, which start after a controller returns a DeferredResult. ]]>
</xsd:documentation>
</xsd:annotation>
<xsd:complexType>
<xsd:sequence>
<xsd:element ref="beans:bean" minOccurs="1" maxOccurs="unbounded">
<xsd:annotation>
<xsd:documentation>
<![CDATA[ Registers a DeferredResultProcessingInterceptor. ]]>
</xsd:documentation>
</xsd:annotation>
</xsd:element>
</xsd:sequence>
</xsd:complexType>
</xsd:element>
</xsd:all>
<xsd:attribute name="task-executor" type="xsd:string">
<xsd:annotation>
<xsd:documentation source="java:org.springframework.core.task.AsyncTaskExecutor">
<![CDATA[ The bean name of a default AsyncTaskExecutor to use when a controller method returns a {@link Callable}. Controller methods can override this default on a per-request basis by returning an AsyncTask. By default, a SimpleAsyncTaskExecutor is used which does not re-use threads and is not recommended for production. ]]>
</xsd:documentation>
<xsd:appinfo>
<tool:annotation kind="ref">
<tool:expected-type type="java:org.springframework.core.task.AsyncTaskExecutor"/>
</tool:annotation>
</xsd:appinfo>
</xsd:annotation>
</xsd:attribute>
<xsd:attribute name="default-timeout" type="xsd:long">
<xsd:annotation>
<xsd:documentation>
<![CDATA[ Specify the amount of time, in milliseconds, before asynchronous request handling times out. In Servlet 3, the timeout begins after the main request processing thread has exited and ends when the request is dispatched again for further processing of the concurrently produced result. If this value is not set, the default timeout of the underlying implementation is used, e.g. 10 seconds on Tomcat with Servlet 3. ]]>
</xsd:documentation>
</xsd:annotation>
</xsd:attribute>
</xsd:complexType>
</xsd:element>
</xsd:all>
<xsd:attribute name="conversion-service" type="xsd:string">
<xsd:annotation>
<xsd:element ref="beans:ref">
<xsd:annotation>
<xsd:documentation source="org.springframework.web.servlet.resource.ResourceResolver">
<![CDATA[ A reference to a ResourceResolver bean. ]]>
</xsd:documentation>
</xsd:annotation>
</xsd:element>
</xsd:choice>
</xsd:sequence>
</xsd:complexType>
<xsd:complexType name="resource-transformers">
<xsd:annotation>
<xsd:documentation source="org.springframework.web.servlet.resource.ResourceTransformer">
<![CDATA[ A list of ResourceTransformer beans definition and references. A ResourceTransformer provides mechanisms for transforming the content of a resource. ]]>
</xsd:documentation>
</xsd:annotation>
<xsd:sequence>
<xsd:choice maxOccurs="unbounded">
<xsd:element ref="beans:bean">
<xsd:annotation>
<xsd:documentation source="org.springframework.web.servlet.resource.ResourceTransformer">
<![CDATA[ A ResourceTransformer bean definition. ]]>
</xsd:documentation>
</xsd:annotation>
</xsd:element>
<xsd:element ref="beans:ref">
<xsd:annotation>
<xsd:documentation source="org.springframework.web.servlet.resource.ResourceTransformer">
<![CDATA[ A reference to a ResourceTransformer bean. ]]>
</xsd:documentation>
</xsd:annotation>
</xsd:element>
</xsd:choice>
</xsd:sequence>
</xsd:complexType>
<xsd:complexType name="resource-chain">
<xsd:annotation>
</xsd:attribute>
<xsd:attribute name="content-type" type="xsd:string">
<xsd:annotation>
<xsd:documentation>
<![CDATA[ Set the content type to use for the response (text/html by default). ]]>
</xsd:documentation>
</xsd:annotation>
</xsd:attribute>
<xsd:attribute name="charset" type="xsd:string">
<xsd:annotation>
<xsd:documentation>
<![CDATA[ Set the charset used to read script and template files (UTF-8 by default). ]]>
</xsd:documentation>
</xsd:annotation>
</xsd:attribute>
<xsd:attribute name="resource-loader-path" type="xsd:string">
<xsd:annotation>
<xsd:documentation>
<![CDATA[ The script engine resource loader path via a Spring resource location. ]]>
</xsd:documentation>
</xsd:annotation>
</xsd:attribute>
<xsd:attribute name="shared-engine" type="xsd:boolean">
<xsd:annotation>
<xsd:documentation>
<![CDATA[ When set to false, use thread-local ScriptEngine instances instead of one single shared instance. This flag should be set to false for those using non thread-safe script engines with templating libraries not designed for concurrency, like Handlebars or React running on Nashorn for example. In this case, Java 8u60 or greater is required due to this bug: https://bugs.openjdk.java.net/browse/JDK-8076099. ]]>
</xsd:documentation>
</xsd:annotation>
</xsd:attribute>
</xsd:complexType>
</xsd:element>
<xsd:element name="cors">
<xsd:annotation>
<xsd:documentation>
<![CDATA[ Configure cross origin requests processing. ]]>
</xsd:documentation>
</xsd:annotation>
<xsd:complexType>
<xsd:sequence>
<xsd:element name="mapping" minOccurs="1" maxOccurs="unbounded">
<xsd:annotation>
<xsd:documentation>
<![CDATA[ Enable cross origin requests processing on the specified path pattern. By default, all origins, GET HEAD POST methods, all headers and credentials are allowed and max age is set to 30 minutes. ]]>
</xsd:documentation>
</xsd:annotation>
<xsd:complexType>
<xsd:attribute name="path" type="xsd:string" use="required">
<xsd:annotation>
<xsd:documentation>
<![CDATA[ A path into the application that should handle CORS requests. Exact path mapping URIs (such as "/admin") are supported as well as Ant-stype path patterns (such as /admin/**). ]]>
</xsd:documentation>
</xsd:annotation>
</xsd:attribute>
<xsd:attribute name="allowed-origins" type="xsd:string">
<xsd:annotation>
<xsd:documentation>
<![CDATA[ Comma-separated list of origins to allow, e.g. "https://domain1.com, https://domain2.com". The special value "*" allows all domains (default). Note that CORS checks use values from "Forwarded" (RFC 7239), "X-Forwarded-Host", "X-Forwarded-Port", and "X-Forwarded-Proto" headers, if present, in order to reflect the client-originated address. Consider using the ForwardedHeaderFilter in order to choose from a central place whether to extract and use such headers, or whether to discard them. See the Spring Framework reference for more on this filter. ]]>
</xsd:documentation>
</xsd:annotation>
</xsd:attribute>
<xsd:attribute name="allowed-methods" type="xsd:string">
<xsd:annotation>
<xsd:documentation>
<![CDATA[ Comma-separated list of HTTP methods to allow, e.g. "GET, POST". The special value "*" allows all method. By default GET, HEAD and POST methods are allowed. ]]>
</xsd:documentation>
</xsd:annotation>
</xsd:attribute>
<xsd:attribute name="allowed-headers" type="xsd:string">
<xsd:annotation>
<xsd:documentation>
<![CDATA[ Comma-separated list of headers that a pre-flight request can list as allowed for use during an actual request. The special value of "*" allows actual requests to send any header (default). ]]>
</xsd:documentation>
</xsd:annotation>
</xsd:attribute>
<xsd:attribute name="exposed-headers" type="xsd:string">
<xsd:annotation>
<xsd:documentation>
<![CDATA[ Comma-separated list of response headers other than simple headers (i.e. Cache-Control, Content-Language, Content-Type, Expires, Last-Modified, Pragma) that an actual response might have and can be exposed. Empty by default. ]]>
</xsd:documentation>
</xsd:annotation>
</xsd:attribute>
<xsd:attribute name="allow-credentials" type="xsd:boolean">
<xsd:annotation>
<xsd:documentation>
<![CDATA[ Whether user credentials are supported (true by default). ]]>
</xsd:documentation>
</xsd:annotation>
</xsd:attribute>
<xsd:attribute name="max-age" type="xsd:long">
<xsd:annotation>
<xsd:documentation>
<![CDATA[ How long, in seconds, the response from a pre-flight request can be cached by clients. 1800 seconds (30 minutes) by default. ]]>
</xsd:documentation>
</xsd:annotation>
</xsd:attribute>
</xsd:complexType>
</xsd:element>
</xsd:sequence>
</xsd:complexType>
</xsd:element>
</xsd:schema>
从这个角度入手，是没问题的。但是我感觉大多数人看了这种都头疼（除非你精通 XML）。
接下来，可以看另外一个文件 META-INF/spring.handlers 。比如，spring-webmvc 模块中，这个文件内容如下
http\://www.springframework.org/schema/mvc=org.springframework.web.servlet.config.MvcNamespaceHandler
MvcNamespaceHandler 这个类的内容如下
/** Copyright 2002-2016 the original author or authors.** Licensed under the Apache License, Version 2.0 (the "License");* you may not use this file except in compliance with the License.* You may obtain a copy of the License at** http://www.apache.org/licenses/LICENSE-2.0** Unless required by applicable law or agreed to in writing, software* distributed under the License is distributed on an "AS IS" BASIS,* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.* See the License for the specific language governing permissions and* limitations under the License.*/package org.springframework.web.servlet.config;import org.springframework.beans.factory.xml.NamespaceHandler;
import org.springframework.beans.factory.xml.NamespaceHandlerSupport;/*** {@link NamespaceHandler} for Spring MVC configuration namespace.** @author Keith Donald* @author Jeremy Grelle* @author Sebastien Deleuze* @since 3.0*/
public class MvcNamespaceHandler extends NamespaceHandlerSupport {@Overridepublic void init() {registerBeanDefinitionParser("annotation-driven", new AnnotationDrivenBeanDefinitionParser());registerBeanDefinitionParser("default-servlet-handler", new DefaultServletHandlerBeanDefinitionParser());registerBeanDefinitionParser("interceptors", new InterceptorsBeanDefinitionParser());registerBeanDefinitionParser("resources", new ResourcesBeanDefinitionParser());registerBeanDefinitionParser("view-controller", new ViewControllerBeanDefinitionParser());registerBeanDefinitionParser("redirect-view-controller", new ViewControllerBeanDefinitionParser());registerBeanDefinitionParser("status-controller", new ViewControllerBeanDefinitionParser());registerBeanDefinitionParser("view-resolvers", new ViewResolversBeanDefinitionParser());registerBeanDefinitionParser("tiles-configurer", new TilesConfigurerBeanDefinitionParser());registerBeanDefinitionParser("freemarker-configurer", new FreeMarkerConfigurerBeanDefinitionParser());registerBeanDefinitionParser("groovy-configurer", new GroovyMarkupConfigurerBeanDefinitionParser());registerBeanDefinitionParser("script-template-configurer", new ScriptTemplateConfigurerBeanDefinitionParser());registerBeanDefinitionParser("cors", new CorsBeanDefinitionParser());}}看到这是不是就清晰了。
<mvc:annotation-driven/> 这个标签就是被类 AnnotationDrivenBeanDefinitionParser 解析的。以此类推。其他标签也是由对应的类解析的。
这里是以 spring-webmvc 模块为例来说的，其他模块如 spring-beans 、spring-context 都有对应 META-INF/spring.handlers 文件。分别用于解析对应的命名空间下的标签。
解释下命名空间的意思，就拿 <mvc:annotation-driven/> 来说，mvc 就是命名空间，再比如 <context:component-scan/> 这个标签，context 就是命名空间。
