org.springframework.beans.factory.config.PropertyPlaceholderConfigurer

可以将上下文（配置文件）中的属性值放在另一个单独的标准java Properties文件中去。在XML文件中用${key}替换指定的properties文件中的值。这样的话，只需要对properties文件进行修改，而不用对xml配置文件进行修改。

从上图中，我们看到PropertyPlaceholderConfigurer实现了三个bean生命周期的接口：BeanFactoryAware & BeanNameAware & BeanFactoryPostProcessor。关于spring bean的生命周期，可以参考这里http://blog.csdn.net/gjb724332682/article/details/46767463。

PropertyResourceConfigurer.postProcessBeanFactory()将properties文件中的属性进行merge，convert，最后调用PropertyPlaceholderConfigurer.processProperties()完成遍历bean定义替换属性占位符。

例子：

<bean id="propertyConfigurer" class="org.springframework.beans.factory.config.PropertyPlaceholderConfigurer">
<property name="location">
<span style="white-space:pre">	</span><value>WEB-INF/conf/xx.properties</value>
</property>
<property name="fileEncoding">
<span style="white-space:pre">	</span><value>UTF-8</value>
</property>
</bean>

<!--当然也可以引入多个属性文件，如： -->

<bean id="propertyConfigurer" class="org.springframework.beans.factory.config.PropertyPlaceholderConfigurer">
<property name="locations">
<span style="white-space:pre">	</span><list>
<span style="white-space:pre">		</span><value>/WEB-INF/mail.properties</value>
<span style="white-space:pre">		</span><value>classpath:conf/sqlmap/jdbc.properties</value>
<span style="white-space:pre">	</span></list>
</property>
</bean>

<bean id="econsoleDS" class="com.mchange.v2.c3p0.ComboPooledDataSource" destroy-method="close">
<property name="driverClass">
<span style="white-space:pre">	</span><value>${jdbc.driverClassName}</value>
</property>
<property name="jdbcUrl">
<span style="white-space:pre">	</span><value>${jdbc.url}</value>
</property>
<property name="user">
<span style="white-space:pre">	</span><value>${jdbc.username}</value>
</property>
<property name="password">
<span style="white-space:pre">	</span><value>${jdbc.password}</value>
</property>
</bean>

除此之外，我们还可以扩展自这个类，用来诸如加解密配置信息等操作。如下：

import java.util.Properties;

import org.springframework.beans.BeansException;
import org.springframework.beans.factory.BeanInitializationException;
import org.springframework.beans.factory.config.ConfigurableListableBeanFactory;
import org.springframework.beans.factory.config.PropertyPlaceholderConfigurer;

import com.xxx.util.AESUtils;

public class DecryptPropertyPlaceholderConfigurer extends
		PropertyPlaceholderConfigurer {
	private String key = "xxxxxx";

	@Override
	protected void processProperties(
			ConfigurableListableBeanFactory beanFactory, Properties props)
			throws BeansException {
		try {
			String driverClassName = props.getProperty("driverClassName");
			if (driverClassName != null) {
				props.setProperty("driverClassName",
						AESUtils.aesDecrypt(driverClassName, key));
			}

			String url = props.getProperty("url");
			if (url != null) {
				props.setProperty("url", AESUtils.aesDecrypt(url, key));
			}

			String username = props.getProperty("username");
			if (username != null) {
				props.setProperty("username",
						AESUtils.aesDecrypt(username, key));
			}

			String password = props.getProperty("password");
			if (password != null) {
				props.setProperty("password",
						AESUtils.aesDecrypt(password, key));
			}
			super.processProperties(beanFactory, props);
		} catch (Exception e) {
			e.printStackTrace();
			throw new BeanInitializationException(e.getMessage());
		}
	}
}

重写processProperties方法就可以。