Rename (or remove, since in the example is not used) the getJSON method.
public String getJSON()
{
return execute();
}
JSON plugin may be calling all your methods that start with "get" in an attempt to serialize them for output.
See: Struts2 Action being called twice if result type is json
Edit:
You can also use the includeProperties or excludeProperties params to avoid the default behavior of call all the methods that start with "get", see: Problem with Json plugin in Struts 2