Spring Property Placeholder Configurer - A few not so obvious options

Spring's PropertySourcesPlaceholderConfigurer is used for externalizing properties from the Spring bean definitions defined in XML or using Java Config. There are a few options that PlaceholderConfigurer supports that are not obvious from the documentation but are interesting and could be useful.

To start with, an example from Spring's documentation, consider a properties file with information to configure a datasource:

jdbc.driverClassName=org.hsqldb.jdbcDriver
jdbc.url=jdbc:hsqldb:hsql://production:9002
jdbc.username=sa
jdbc.password=root

The PropertySourcesPlaceholderConfigurer is configured using a custom namespace:

<context:property-placeholder location="database.properties"/>

A datasource bean making use of these properties can be defined using XML based bean definition this way:

<bean id="dataSource" destroy-method="close"
    class="org.apache.commons.dbcp.BasicDataSource">
  <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>

and using Java based configuration this way:

@Value("${jdbc.driverClassName}") private String driverClassName;
@Value("${jdbc.url}")             private String dbUrl;
@Value("${jdbc.username}")        private String dbUserName;
@Value("${jdbc.password}")        private String dbPassword;

@Bean
public BasicDataSource dataSource() {
 BasicDataSource dataSource = new BasicDataSource();
 dataSource.setDriverClassName(driverClassName);
 dataSource.setUrl(dbUrl);
 dataSource.setUsername(dbUserName);
 dataSource.setPassword(dbPassword);
 return dataSource;
}

The not so obvious options are:

First is the support for default values. Say for eg, if "sa" is to be provided as default for the jdbc user name, the way to do it is this way(using a ${propertyName:default} syntax) :

<property name="username" value="${jdbc.username:sa}"/>

or with Java Config:

..
..
@Value("${jdbc.username:sa}") private String dbUserName;

@Bean
public BasicDataSource dataSource() {
..
}

Second is the support for nested property resolution, for eg consider the following properties:
phase.properties file -

phase=qa
jdbc.username.qa=qasa
jdbc.username.dev=devsa

and using the "phase" property as part of another property in XML bean definition in this nested way:

<property name="username" value="${jdbc.username.${phase}}"/>

These options could be very useful for place holder based configuration.