Spring PropertyPlaceholderConfig

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

<?xml version="1.0"?>
<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

<?xml version="1.0"?>
<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}.