You can use the spring injection with any file you want .
Usually i prefer to use a properties file (e.g. test.properties) under the "Shared" lib directory where i put some reference to the system files i need; For example a property for retrieve a pdf file: "myFile=/usr/local/src/myDocument.pdf" so i just update the value of the property without touching the java server code.
Then i inject the properties file "/opt/alfresco-community/tomcat/shared/lib/er/ere/test.properties" on my "spring-context.xml" on "/opt/alfresco-community/tomcat/shared/classes/alfresco/extension/spring-context.xml" (or you can put that even on your java project).
Then i usually use :
<bean id="properties" class="org.springframework.beans.factory.config.PropertyPlaceholderConfigurer"><propertyname="locations"><list><value>classpath:er/ere/test.properties</value></list></property><!-- Default values for backwards compatibility --><propertyname="properties"><props><propkey="name">value</prop></props></property></bean>