Spring - best way to deal with binding to a list of beans in a simpleformcontroller - spring

Question

Without thinking about it too much, I've been doing something like the following:
1) building a list of SomeBean objects based on the results of a database call
2) putting that list in my command object
3) building a form based on that command object where users can modify attributes of the SomeBeans
4) extracting data out of the post-submit command object and writing the updated data to my database
My code looks something like this:
public class UpdateThingsinListController extends SimpleFormController {
protected Object formBackingObject(final HttpServletRequest request)
throws Exception {
List<SomeBean> beans = database.getBeans();
Command comamnd = new UpdateThingsCommand()
command.setList(beans);
return command;
}
protected ModelAndView onSubmit(final HttpServletRequest request,
final HttpServletResponse response, final Object commandArg,
final BindException errors) throws Exception {
database.setBeans(commandArg.getList());
}
}
my jsp looks somthething like:
<form:form>
<c:forEach var="bean" items="${beans}" varStatus="status">
<form:checkbox path="beans[${status.index}].someBooleanProperty" />${bean.name} <br>
</c:forEach>
</form:form>
The code works fine, but it's just dawned on me that my "beans" list is getting created twice (sessionform must be false in my case) -- once when displaying the form, once when binding. If anything changes on the second creation (a bean is missing, the results are in a different order), my binding will get messed up, and I'll get fired. I'm beginning to think that any biding scheme where a command object needs to be merged with a form submission is very risky.
So, my question is -- how do folks ensure that form submissions get bound to lists correctly? Is there another way to do it besides list index? Object ids maybe?
thanks,
-Morgan

Answer 1

If the sessionForm is set to false then during the submit the command is recreated in formBackingObject method. Order of the list in the onSubmit method will be exactly like order of the binded objects on page.

Answer 2

If you can't depend on the List always being the same whenever you retrieve it, then you have to use a different collection - a map.
Another approach would be to cache your List somewhere. Perhaps some AOP around your database.getBeans method.

Answer 3

The quick way is to first delete all existing SomeBeans from the database and create the ones bound on submit.
A more elegant way might be to put the unique identifier of the SomeBean in a hidden form field.