The PlaceholderProcessor was created as a result of this thread discussion: [!topic/wro4j/9d91oA4FhSs Handling expression language in CSS].

The main idea is to be able to handle expressions in web resources dynamically.


An expression is referenced with: ${} characters.

Here are some examples of web resource with experssions:

body {
  color: ${BODY_COLOR}


  function notify() {

There are multiple use-cases for this feature. Below you'll find how to implement it.

Creating PlaceholderProcessor

    //1. create properties object
    final Properties properties = new Properties();
    properties.setProperty("prop1", "value1");
    //2. Create properties Factory
    final ObjectFactory<Properties> propertiesFactory = new ObjectFactory<Properties>() {
      public Properties create() {
        return properties;
    //3. instantiate PlaceholderProcessor
    final ResourcePreProcessor processor = new PlaceholderProcessor().setPropertiesFactory(propertiesFactory).setIgnoreMissingVariables(false);

Notice the following in the above code: * Creates a Property object containing values to interpolate. This can be dynamically created from some properties file based on your environment configurations. * ObjectFactory allows PlaceholderProcessor to retrieve each time the resource is parsed the fresh instance of the Properties object by calling the create method. Using ObjectFactory is a flexible approach for dynamic interpolation. * The PlaceholderProcessor is created with propertiesFactory set, along with the flag indicating whether the missing variables should be ignored or not. If this value is set to false, a runtime exception will be thrown when a expression defined in web resource doesn't have a correspondent value in supplied Properties object.