@Override
  public String resolveReplaceString(String replaceString) {
    String replacement = null;

    // Get System property first.
    replacement = System.getProperty(replaceString);

    // Override system property if replacement is null or we want to override system property.
    if ((replacement == null || getPropertyOverridden() == SYSTEM_PROPERTIES_OVERRIDE)
        && props != null) {
      String userDefined = props.getProperty(replaceString);
      if (userDefined != null) {
        replacement = userDefined;
      }
    }
    return replacement;
  }
Resolves the given property string either from system properties or given properties. and
returns the replacement of the property found in available properties. If no string replacement
is found then {@link IllegalArgumentException} would be thrown based on
<code>ignoreUnresolvedProperties</code> flag being set by {@link CacheXmlParser}.
geode-core/src/main/java/org/apache/geode/internal/cache/xmlcache/CacheXmlPropertyResolver.java