How to bind Maps of Maps, or some other way to structure the data - spring

Question

I am not even sure how to go about asking this, but I will try to give it a shot.
I have an existing application that I support that consists of a dynamically generated form of a bunch of values. The form is broken into a matrix of rows and columns, and the JSP uses this information to construct the required form:inputs. To build the form I have some Java classes like such (mostly pseudo code just so you get an idea as to what I am doing):
class Form {
List<FormParts> parts;
}
class FormParts {
List<FormRow> rows;
List<FormHeader> headers;
}
class FormRow {
String name;
}
class FormHeader {
String name;
}
I use these classes to dynamically build the form in the JSP by looping through the FormParts, then using the FormRows and FormHeaders to build a table of form:inputs, using a counter to index the resulting answers List described here in the FormResponse:
class FormResponse {
List<FormAnswer> answers;
}
class FormAnswer {
int rowNumber;
int headerNumber;
String value;
}
The problem I am having is the List<FormAnswer> answers. Because it's a List I have to store all the Answers, even the empty ones (nulls), in order to be able to provide the ability to reload the FormResponses from the DB. This is creating a HUGE amount of junk rows in my table, and making the application slow. When I first wrote this about 4 months ago, however, I remember I struggled for some time trying to figure out how to make Spring MVC bind to a Map of Maps, which would be a MUCH better way to implement this since I could simply skip the answers I don't need. I remember the problem was that, internally in Spring MVC, those Lists and Maps become LazyList and LazyMap.
Does anyone have another possible solution to this?

Answer 1

Check my answer on dynamic forms with Spring here at Auto populating Set .
The key is to use Map<Integer, YourObject> instead of lists. With that you can have gaps in your indexes.
Using Map of Maps is not possible, because the generics will be erased. The map always need to be property of object. So your example should be OK.
Concept of LazyList / LazyMap is long dead. Spring is capable of manipulating any list and map type. If the Map property is null, an instance of LinkedHashMap is created by default.
Other possibility is to do your binding and validation manually and just send JSON via the wire (i.e. submit your form as JSON in <input type="hidden"> or directly via AJAX).