0044 spring框架的applicationContext.xml的命名空间

Spring框架中，创建bean，装配bean，事务控制等，可以用xml配置或者注解扫描的方法实现。如果用注解扫描，在xml配置中得加上

<context:component-scan base-package="XXX" />
<mvc:annotation-driven />
<tx:annotation-driven transaction-manager="XXX" />

但以上配置生效的前提，还要配置好相应的命名空间，比如：

<beans xmlns="http://www.springframework.org/schema/beans" 
    xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" 
    xmlns:util="http://www.springframework.org/schema/util" 
    xmlns:context="http://www.springframework.org/schema/context" 
    xmlns:mvc="http://www.springframework.org/schema/mvc"  
    xmlns:aop="http://www.springframework.org/schema/aop" 
    xmlns:tx="http://www.springframework.org/schema/tx" 
    xsi:schemaLocation="http://www.springframework.org/schema/beans http://www.springframework.org/schema/beans/spring-beans-4.0.xsd 
                        http://www.springframework.org/schema/context http://www.springframework.org/schema/context/spring-context-4.0.xsd 
                        http://www.springframework.org/schema/mvc  http://www.springframework.org/schema/mvc/spring-mvc-4.0.xsd  
                        http://www.springframework.org/schema/aop http://www.springframework.org/schema/aop/spring-aop-4.0.xsd 
                        http://www.springframework.org/schema/util http://www.springframework.org/schema/util/spring-util-4.0.xsd 
                        http://www.springframework.org/schema/tx http://www.springframework.org/schema/tx/spring-tx-4.0.xsd "> 
。。。
</beans>

那这些命名空间又是什么，以tx为例：

xmlns:tx="http://www.springframework.org/schema/tx"
xsi:schemaLocation="http://www.springframework.org/schema/tx http://www.springframework.org/schema/tx/spring-tx-4.0.xsd "

『xmlns:tx="http://www.springframework.org/schema/tx"』声明了一个命名空间，tx就代表了『http://www.springframework.org/schema/tx』，后者看起来像个url，其实只是个字符串，之所以写成url的样子，只是为了确保唯一，就类似于包名要用域名的反写+项目名+模块名一样

『xsi:schemaLocation="http://www.springframework.org/schema/tx【空格】http://www.springframework.org/schema/tx/spring-tx-4.0.xsd "』指定了用于解析和校验xml文件的xsd文件的位置，那如何找到这个xsd文件呢

把tx.jar解压缩后，在META-INF目录下有一个spring.schemas文件，打开后如下所示：

http\://www.springframework.org/schema/tx/spring-tx-2.0.xsd=org/springframework/transaction/config/spring-tx-2.0.xsd
http\://www.springframework.org/schema/tx/spring-tx-2.5.xsd=org/springframework/transaction/config/spring-tx-2.5.xsd
http\://www.springframework.org/schema/tx/spring-tx-3.0.xsd=org/springframework/transaction/config/spring-tx-3.0.xsd
http\://www.springframework.org/schema/tx/spring-tx-3.1.xsd=org/springframework/transaction/config/spring-tx-3.1.xsd
http\://www.springframework.org/schema/tx/spring-tx-3.2.xsd=org/springframework/transaction/config/spring-tx-3.2.xsd
http\://www.springframework.org/schema/tx/spring-tx-4.0.xsd=org/springframework/transaction/config/spring-tx-4.0.xsd
http\://www.springframework.org/schema/tx/spring-tx-4.1.xsd=org/springframework/transaction/config/spring-tx-4.1.xsd
http\://www.springframework.org/schema/tx/spring-tx-4.2.xsd=org/springframework/transaction/config/spring-tx-4.2.xsd
http\://www.springframework.org/schema/tx/spring-tx-4.3.xsd=org/springframework/transaction/config/spring-tx-4.3.xsd
http\://www.springframework.org/schema/tx/spring-tx.xsd=org/springframework/transaction/config/spring-tx-4.3.xsd

这里有xsd文件的位置的名称跟具体xsd文件所在目录的映射，到『org/springframework/transaction/config/』目录下就可以找到对应的xsd文件，打开后基本如下所示：

<?xml version="1.0" encoding="UTF-8" standalone="no"?>

<xsd:schema xmlns="http://www.springframework.org/schema/tx"    <!-- 与上面的xmlns:tx的值相同 --> 
        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/tx"
        elementFormDefault="qualified"
        attributeFormDefault="unqualified">

    <xsd:import namespace="http://www.springframework.org/schema/beans" schemaLocation="http://www.springframework.org/schema/beans/spring-beans-4.0.xsd"/>
    <xsd:import namespace="http://www.springframework.org/schema/tool" schemaLocation="http://www.springframework.org/schema/tool/spring-tool-4.0.xsd"/>

    <xsd:annotation>
        .................
    </xsd:annotation>

    <xsd:element name="advice">              <!-- 引入这个命名空间后，才能用<tx:advice><tx:annotation-driven>等 -->
        .................
    </xsd:element>

    <xsd:element name="annotation-driven">
        .................
    </xsd:element>

    <xsd:element name="jta-transaction-manager">
        .................
    </xsd:complexType>

</xsd:schema>

如何引入一个命名空间呢？
1. 声明：xmlns:tx="命名空间" 这里命名空间跟xsd文件的xmlns的值相同
2. 在xsi:schemaLocation指定xsd文件的位置：命名空间【空格】config目录下的.schemas的映射