From now on, if you try to set the incompatible improvements to greater than the current FreeMarker version, or less than 2.3.0, an IllegalArgumentException will be thrown. Thus, new Configuration(new Version(2, 3, 21)) will not only active the fixes up to 2.3.21, but will ensure that the application is not deployed in an environment where an older FreeMarker version is installed. After all, on an older FreeMarker version the fixes that you have requested aren't implemented yet.