Often times, most Spring developers just put the entire deployment details (database details, log file path) in XML bean configuration file as following :

<beans xmlns="http://www.springframework.org/schema/beans"
	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-2.5.xsd">
 
	<bean id="customerDAO" class="com.mkyong.customer.dao.impl.JdbcCustomerDAO">
 
		<property name="dataSource" ref="dataSource" />
	</bean>
 
	<bean id="customerSimpleDAO" class="com.mkyong.customer.dao.impl.SimpleJdbcCustomerDAO">
 
		<property name="dataSource" ref="dataSource" />
	</bean>
 
	<bean id="dataSource"
		class="org.springframework.jdbc.datasource.DriverManagerDataSource">
 
		<property name="driverClassName" value="com.mysql.jdbc.Driver" />
		<property name="url" value="jdbc:mysql://localhost:3306/mkyongjava" />
		<property name="username" value="root" />
		<property name="password" value="password" />
	</bean>
 
</beans>

But, in a corporate environment, deployment detail is usually only can ‘touch’ by your system or database administrator, they just refuse to access your bean configuration file directly, and they will request a separate file for deployment configuration, for example, a simple properties, with deployment detail only.

PropertyPlaceholderConfigurer example

To fix it, you can use PropertyPlaceholderConfigurer class to externalize the deployment details into a properties file, and access from bean configuration file via a special format – ${variable}.

Create a properties file (database.properties), include your database details, put it into your project class path.

	jdbc.driverClassName=com.mysql.jdbc.Driver
	jdbc.url=jdbc:mysql://localhost:3306/mkyongjava
	jdbc.username=root
	jdbc.password=password

Declare a PropertyPlaceholderConfigurer in bean configuration file and map to the ‘database.properties‘ properties file you created just now.

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

Full example

<beans xmlns="http://www.springframework.org/schema/beans"
	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-2.5.xsd">
 
	<bean
		class="org.springframework.beans.factory.config.PropertyPlaceholderConfigurer">
 
		<property name="location">
			<value>database.properties</value>
		</property>
	</bean>
 
	<bean id="customerDAO" class="com.mkyong.customer.dao.impl.JdbcCustomerDAO">
 
		<property name="dataSource" ref="dataSource" />
	</bean>
 
	<bean id="customerSimpleDAO" 
                class="com.mkyong.customer.dao.impl.SimpleJdbcCustomerDAO">
 
		<property name="dataSource" ref="dataSource" />
	</bean>
 
	<bean id="dataSource"
		class="org.springframework.jdbc.datasource.DriverManagerDataSource">
 
		<property name="driverClassName" value="${jdbc.driverClassName}" />
		<property name="url" value="${jdbc.url}" />
		<property name="username" value="${jdbc.username}" />
		<property name="password" value="${jdbc.password}" />
	</bean>
 
</beans>
Alternative usage
You also can use PropertyPlaceholderConfigurer to share some constant variables to all other beans. For example, define your log file location in a properties file, and access the properties value from different beans configuration files via ${log.filepath}.

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