spring propertyplaceholderconfigurer

Spring的框架中为您提供了一个 BeanFactoryPostProcessor 的实作类别： org.springframework.beans.factory.config.PropertyPlaceholderConfigurer。藉由这个类别，您可以将一些组态设定，移出至.properties档案中，如此的安排可以让XML定义档负责系统相关设定，而.properties档可以作为客户根据需求，自定义一些相关的参数。
来看一个Bean定义档的实际例子：
beans-config.xml

Xml代码

<? xml version = "1.0" encoding = "UTF-8" ?>
<!DOCTYPE beans PUBLIC "-//SPRING/DTD BEAN/EN"
"http://www.springframework.org/dtd/spring-beans.dtd">
< beans >
< bean id = "configBean"
class = "org.springframework.beans.factory.config.PropertyPlaceholderConfigurer" > < property name = "location" > < value > hello.properties </ value > </ property > </ bean >
< bean id = "helloBean" class = "onlyfun.caterpillar.HelloBean" >
< property name = "helloWord" >
< value > ${onlyfun.caterpillar.helloWord} </ value >
</ property >
</ bean >
</ beans >

<?xml version="1.0" encoding="UTF-8"?> 
<!DOCTYPE beans PUBLIC "-//SPRING/DTD BEAN/EN" 
    "http://www.springframework.org/dtd/spring-beans.dtd"> 

<beans>  
      <bean id="configBean" 
 class="org.springframework.beans.factory.config.PropertyPlaceholderConfigurer">           <property name="location">               <value>hello.properties</value>           </property>       </bean> 

      <bean id="helloBean" class="onlyfun.caterpillar.HelloBean"> 
          <property name="helloWord"> 
              <value>${onlyfun.caterpillar.helloWord}</value> 
          </property> 
      </bean>
</beans>

假设在helloBean中有许多依赖注入的属性，这些都是比较不常变动的属性，而其中helloWord会经常变动，可以透过hello.properties来简单的设定，而这个资讯已设定在configBean的location属性中：
hello.properties

Java代码

onlyfun.caterpillar.helloWord=Welcome!

onlyfun.caterpillar.helloWord=Welcome!

在helloBean的helloWord属性中，${onlyfun.caterpillar.helloWord}将会被hello.properties的helloWord所取代。

如果有多个.properties档案，则可以透过locations属性来设定，例如：
beans-config.xml

Xml代码

<? xml version = "1.0" encoding = "UTF-8" ?>
<!DOCTYPE beans PUBLIC "-//SPRING/DTD BEAN/EN"
"http://www.springframework.org/dtd/spring-beans.dtd">
< beans >
< bean id = "configBean"
class = "org.springframework.beans.factory.config.PropertyPlaceholderConfigurer" > < property name = "locations" >
< list > < value > hello.properties </ value >
< value > welcome.properties </ value >
< value > other.properties </ value >
</ list >
</ property > </ bean >
< bean id = "helloBean" class = "onlyfun.caterpillar.HelloBean" >
< property name = "helloWord" >
< value > ${onlyfun.caterpillar.helloWord} </ value >
</ property >
...
</ bean >
</ beans >

<?xml version="1.0" encoding="UTF-8"?> 
<!DOCTYPE beans PUBLIC "-//SPRING/DTD BEAN/EN" 
    "http://www.springframework.org/dtd/spring-beans.dtd"> 

<beans>  
      <bean id="configBean" 
    class="org.springframework.beans.factory.config.PropertyPlaceholderConfigurer">           <property name="locations"> 
              <list>                  <value>hello.properties</value> 
                  <value>welcome.properties</value> 
                  <value>other.properties</value>     
              </list>
          </property>           </bean> 

      <bean id="helloBean" class="onlyfun.caterpillar.HelloBean"> 
          <property name="helloWord"> 
              <value>${onlyfun.caterpillar.helloWord}</value> 
          </property> 
          ...
      </bean>
</beans>

PropertyPlaceholderConfigurer类就是bean factory post-processor的一种，它的作用是一个资源属性的配置器，能够将BeanFactory的里定义的内容放在一个以.propertis后缀的文件中。
例如－－－spring-context.xml－－－－

Xml代码

< bean id = "propertyConfigurer" class = "org.springframework.beans.factory.config.PropertyPlaceholderConfigurer" >
< property name = "locations" >
< list >
< value > /WEB-INF/jdbc.properties </ value >
</ list >
</ property >
</ bean >
< bean id = "dataSource" class = "org.springframework.jdbc.datasource.DriverManagerDataSource" >
< property name = "driverClassName" > < value > ${driver} </ value > </ property >
< property name = "url" > < value > jdbc:${dbname} </ value > </ property >
</ bean >

<bean id="propertyConfigurer"   class="org.springframework.beans.factory.config.PropertyPlaceholderConfigurer">
         <property name="locations">
             <list>
                 <value>/WEB-INF/jdbc.properties</value>
             </list>
         </property>
 </bean>
<bean id="dataSource" class="org.springframework.jdbc.datasource.DriverManagerDataSource">
    <property name="driverClassName"><value>${driver}</value></property>
    <property name="url"><value>jdbc:${dbname}</value></property>
</bean>

而实际的jdbc.propertis文件是
jdbc.driverClassName=org.hsqldb.jdbcDriver
jdbc.url=jdbc:hsqldb:hsql://production:9002
jdbc.username=sa
jdbc.password=root
而jdbc.propertis是怎样引用的呢：将上边一段配置注册在web.xml中就可以了

Xml代码

< context-param >
< param-name > contextConfigLocation </ param-name >
< param-value > /WEB-INF/spring-context.xml </ param-value >
</ context-param > 当然，不要忘了spring的监听器注册
< listener >
< listener-class >
org.springframework.web.context.ContextLoaderListener
</ listener-class >
</ listener >

<context-param>
<param-name>contextConfigLocation</param-name>
<param-value>/WEB-INF/spring-context.xml</param-value>
</context-param> 当然，不要忘了spring的监听器注册
<listener>
<listener-class>
org.springframework.web.context.ContextLoaderListener
</listener-class>
</listener>

这样，一个简单的数据源就设置完毕了。
实际上，PropertyPlaceholderConfigurer起的作用就是将占位符指向的数据库配置信息放在bean中定义
的工具。