The action.xml utilizes the Open Symphony XWork framework, and so it also utilizes the XWork Document Type Definition (DTD) file.
An Example configuration file
action.xml
Error formatting macro: snippet: java.lang.NullPointerException
For more about configuration details, see Action Configuration Elements
The XWork Document Type Definition (DTD)
xwork-1.1.1.dtd
Error formatting macro: snippet: java.lang.NullPointerException