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：

所有的标签和属性都需要Schema来定义。（schema本身由w3c来定义）。
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。
namespace值由 targetNamespace属性来指定
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。
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。
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

声明默认的名称空间（xmlns="http://www.springframework.org/schema/beans"）
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
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.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

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