How do I use an optional properties file in a Spring configuration file?

I am using an XML configuration file that is loaded into my Java application using ApplicationContext.

The XML configuration file resolves its properties by reading from several property files using PropertyPlaceholderConfigurer.

I want to make each property file optional. I thought that this is done by setting ignoreUnresolsvablePlaceholders to true, however I am getting the following exception when I run the application (db-default.properties exists but db.properties does not):

Exception in thread "main" org.springframework.beans.factory.BeanInitializationException: Could not load properties; nested exception is java.io.FileNotFoundException: class path resource [db.properties] cannot be opened because it does not exist

This is what my XML configuration looks like:

<?xml version="1.0" encoding="UTF-8"?>

<beans xmlns="http://www.springframework.org/schema/beans"
xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
xmlns:context="http://www.springframework.org/schema/context"
xsi:schemaLocation="http://www.springframework.org/schema/beans
    http://www.springframework.org/schema/beans/spring-beans-3.0.xsd
    http://www.springframework.org/schema/context
    http://www.springframework.org/schema/context/spring-context-3.0.xsd">

    <bean id="placeholder-configurer-1" class="org.springframework.beans.factory.config.PropertyPlaceholderConfigurer">
        <property name="order" value="1"/>
        <property name="ignoreUnresolvablePlaceholders" value="true"/>
    </bean>   

    <bean id="placeholder-configurer-2" class="org.springframework.beans.factory.config.PropertyPlaceholderConfigurer">
        <property name="order" value="2"/>
        <property name="ignoreUnresolvablePlaceholders" value="true"/>
        <property name="location" value="classpath:/db-default.properties"/>
    </bean>   

    <bean id="placeholder-configu