Maven filter resources

Maven can also apply a filter to the resources that allows you to replace tokens within resource file. Just like variables are referenced in a POM using ${...} notation, you can reference variables in your project's resources using the same syntax. Coupled with build profiles, such a facility can be used to produce build artifacts which target different deployment platforms. This is something that is common in environments which need to produce output for development, testing, staging, and production platforms from the same project.

To illustrate resource filtering, assume that you have a project with an XML file in src/main/resources/META-INF/service.xml. You want to externalize some configuration variables to a properties file. In other words, you might want to reference a JDBC URL, username, and password for your database, and you don't want to put these values directly into the service.xml file. Instead, you would like to use a properties file to capture all of the configuration points for your program. Doing this will allow you to consolidate all configuration into a single properties file and make it easier to change configuration values when you need to target a new deployment environment. First, take a look at the contents of service.xml in src/main/resources/META-INF.

Example 10.4. Using Properties in Project Resources

<service>
  <!-- This URL was set by project version ${project.version} -->
  <url>${jdbc.url}</url>
  <user>${jdbc.username}</user>
  <password>${jdbc.password}</password>
</service>

This XML file uses the same property reference syntax you can use in the POM. In fact, the first variable referenced is the project variable which is also an implicit variable made available in the POM. The project variable provides access to POM information. The next three variable references are jdbc.url, jdbc.username, and jdbc.password. These custom variables are defined in a properties file src/main/filters/default.properties.

Example 10.5. default.properties in src/main/filters

jdbc.url=jdbc:hsqldb:mem:mydb
jdbc.username=sa
jdbc.password=

To configure resource filtering with this default.properties file, we need to specify two things in a project's POM: a list of properties files in the filters element of the build configuration, and a flag to Maven that the resources directory is to be filtered. The default Maven behavior is to skip filtering and just copy the resources to the output directory; you'll need to explicitly configure resource filter, or Maven will skip the step altogether. This default ensures that Maven's resource filtering feature doesn't surprise you out of nowhere and clobbering any ${...} references you didn't want it to replace.

Example 10.6. Filter Resources (Replacing Properties)

<build>
  <filters>
    <filter>src/main/filters/default.properties</filter>
  </filters>
  <resources>
    <resource>
      <directory>src/main/resources</directory>
      <filtering>true</filtering>
    </resource>
  </resources>
</build>