Spring学习----- Spring配置文件xml文档的schema约束

1、配置文件示例。

    <?xml version="1.0" encoding="UTF-8"?>
<beans xmlns="http://www.springframework.org/schema/beans"
xmlns:mvc="http://www.springframework.org/schema/mvc"
xmlns:tx="http://www.springframework.org/schema/tx"
xmlns:aop="http://www.springframework.org/schema/aop"
xmlns:context="http://www.springframework.org/schema/context"
xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
xsi:schemaLocation="
http://www.springframework.org/schema/beans
http://www.springframework.org/schema/beans/spring-beans.xsd
http://www.springframework.org/schema/context
http://www.springframework.org/schema/context/spring-context.xsd
http://www.springframework.org/schema/mvc
http://www.springframework.org/schema/mvc/spring-mvc.xsd
http://www.springframework.org/schema/tx
http://www.springframework.org/schema/tx/spring-tx.xsd
http://www.springframework.org/schema/aop
http://www.springframework.org/schema/aop/spring-aop.xsd "
default-autowire="byName">
</beans>

2、xml的schema约束

先来说说xml文档的schema约束,它定义了xml文档的结构,内容和语法,包括元素和属性、关系的结构以及数据类型等等。有以下几点需要遵循:

  1. 所有的标签和属性都需要Schema来定义。(schema本身由w3c来定义)。
  2. 所有的schema文件都需要一个ID,这里我们称之为 namespace,其值是一个url,通常是这个xml的.xsd文件的地址。
  3. namespace值由 targetNamespace属性来指定
  4. 引入一个schema约束,使用属性xmlns,属性值即为对应schema文件的命名空间 nameSpace。
  5. 如果引入的schema非w3c组织定义的,必须指定schema文件的位置,schema文件的位置由 schemaLocation来指定。
  6. 引入多个schema文件需要使用 别名。别名形式如下: xmlns:alias。

3、对于上述配置文件的详细解读

  1. 声明默认的名称空间(xmlns="http://www.springframework.org/schema/beans")
  2. 声明XML Schema实例名称空间(http://www.w3.org/2001/XMLSchema-instance),并将xsi前缀与该名称空间绑定,这样模式处理器就可以识别xsi:schemaLocation属性。XML Schema实例名称空间的前缀通常使用xsi
  3. 使用xsi:schemaLocation属性指定名称空间(http://www.springframework.org/schema/beans)和模式位置(http://www.springframework.org/schema/beans/spring-beans.xsd)相关。XML Schema推荐标准中指出,xsi:schemaLocation属性可以在实例中的任何元素上使用,而不一定是根元素,不过,xsi:schemaLocation属性必须出现在它要验证的任何元素和属性之前。

使用别名引入多个schema文件。如上述 xmlns:mvc 、xmlns:tx 、xmlns:context等等。

4、使用带版本号的xsd文件有何弊端?

首先来看看Spring是如何验证xml文档的。Spring默认在启动时是要加载XSD文件来验证xml文件的,所以如果有的时候断网了,或者一些开源软件切换域名,那么就很容易碰到应用启动不了,为了防止这种情况,Spring提供了一种机制,默认从本地加载XSD文件。打开spring-context-4.1.2.RELEASE.jar,可以看到里面有两个特别的文件:

Spring学习----- Spring配置文件xml文档的schema约束

spring.handlers如下:

    http\://www.springframework.org/schema/context=org.springframework.context.config.ContextNamespaceHandler
http\://www.springframework.org/schema/jee=org.springframework.ejb.config.JeeNamespaceHandler
http\://www.springframework.org/schema/lang=org.springframework.scripting.config.LangNamespaceHandler
http\://www.springframework.org/schema/task=org.springframework.scheduling.config.TaskNamespaceHandler
http\://www.springframework.org/schema/cache=org.springframework.cache.config.CacheNamespaceHandler

spring.schemas如下:

    http\://www.springframework.org/schema/context/spring-context-2.5.xsd=org/springframework/context/config/spring-context-2.5.xsd
http\://www.springframework.org/schema/context/spring-context-3.0.xsd=org/springframework/context/config/spring-context-3.0.xsd
http\://www.springframework.org/schema/context/spring-context-3.1.xsd=org/springframework/context/config/spring-context-3.1.xsd
http\://www.springframework.org/schema/context/spring-context-3.2.xsd=org/springframework/context/config/spring-context-3.2.xsd
http\://www.springframework.org/schema/context/spring-context-4.0.xsd=org/springframework/context/config/spring-context-4.0.xsd
http\://www.springframework.org/schema/context/spring-context-4.1.xsd=org/springframework/context/config/spring-context-4.1.xsd
http\://www.springframework.org/schema/context/spring-context.xsd=org/springframework/context/config/spring-context-4.1.xsd
http\://www.springframework.org/schema/jee/spring-jee-2.0.xsd=org/springframework/ejb/config/spring-jee-2.0.xsd
http\://www.springframework.org/schema/jee/spring-jee-2.5.xsd=org/springframework/ejb/config/spring-jee-2.5.xsd
http\://www.springframework.org/schema/jee/spring-jee-3.0.xsd=org/springframework/ejb/config/spring-jee-3.0.xsd
http\://www.springframework.org/schema/jee/spring-jee-3.1.xsd=org/springframework/ejb/config/spring-jee-3.1.xsd
http\://www.springframework.org/schema/jee/spring-jee-3.2.xsd=org/springframework/ejb/config/spring-jee-3.2.xsd
http\://www.springframework.org/schema/jee/spring-jee-4.0.xsd=org/springframework/ejb/config/spring-jee-4.0.xsd
http\://www.springframework.org/schema/jee/spring-jee-4.1.xsd=org/springframework/ejb/config/spring-jee-4.1.xsd
http\://www.springframework.org/schema/jee/spring-jee.xsd=org/springframework/ejb/config/spring-jee-4.1.xsd
http\://www.springframework.org/schema/lang/spring-lang-2.0.xsd=org/springframework/scripting/config/spring-lang-2.0.xsd
http\://www.springframework.org/schema/lang/spring-lang-2.5.xsd=org/springframework/scripting/config/spring-lang-2.5.xsd
http\://www.springframework.org/schema/lang/spring-lang-3.0.xsd=org/springframework/scripting/config/spring-lang-3.0.xsd
http\://www.springframework.org/schema/lang/spring-lang-3.1.xsd=org/springframework/scripting/config/spring-lang-3.1.xsd
http\://www.springframework.org/schema/lang/spring-lang-3.2.xsd=org/springframework/scripting/config/spring-lang-3.2.xsd
http\://www.springframework.org/schema/lang/spring-lang-4.0.xsd=org/springframework/scripting/config/spring-lang-4.0.xsd
http\://www.springframework.org/schema/lang/spring-lang-4.1.xsd=org/springframework/scripting/config/spring-lang-4.1.xsd
http\://www.springframework.org/schema/lang/spring-lang.xsd=org/springframework/scripting/config/spring-lang-4.1.xsd
http\://www.springframework.org/schema/task/spring-task-3.0.xsd=org/springframework/scheduling/config/spring-task-3.0.xsd
http\://www.springframework.org/schema/task/spring-task-3.1.xsd=org/springframework/scheduling/config/spring-task-3.1.xsd
http\://www.springframework.org/schema/task/spring-task-3.2.xsd=org/springframework/scheduling/config/spring-task-3.2.xsd
http\://www.springframework.org/schema/task/spring-task-4.0.xsd=org/springframework/scheduling/config/spring-task-4.0.xsd
http\://www.springframework.org/schema/task/spring-task-4.1.xsd=org/springframework/scheduling/config/spring-task-4.1.xsd
http\://www.springframework.org/schema/task/spring-task.xsd=org/springframework/scheduling/config/spring-task-4.1.xsd
http\://www.springframework.org/schema/cache/spring-cache-3.1.xsd=org/springframework/cache/config/spring-cache-3.1.xsd
http\://www.springframework.org/schema/cache/spring-cache-3.2.xsd=org/springframework/cache/config/spring-cache-3.2.xsd
http\://www.springframework.org/schema/cache/spring-cache-4.0.xsd=org/springframework/cache/config/spring-cache-4.0.xsd
http\://www.springframework.org/schema/cache/spring-cache-4.1.xsd=org/springframework/cache/config/spring-cache-4.1.xsd
http\://www.springframework.org/schema/cache/spring-cache.xsd=org/springframework/cache/config/spring-cache-4.1.xsd

再打开jar包里的org/springframework/context/config/ 目录,可以看到下面有

Spring学习----- Spring配置文件xml文档的schema约束
很明显,可以想到Spring是把XSD文件放到本地了,再在spring.schemas里做了一个映射,优先从本地里加载XSD文件。并且Spring很贴心,把旧版本的XSD文件也全放了。这样可以防止升级了Spring版本,而配置文件里用的还是旧版本的XSD文件,然后断网了,应用启动不了。

从上述xml文档中可以看出,当没有配置版本号时,使用的即当前版本的xml schema文档。

    http\://www.springframework.org/schema/context/spring-context.xsd=org/springframework/context/config/spring-context-4.1.xsd  
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