Hibernate/JPA/Spring Problems with persisting (sorted) collection

Originally I wanted to write this as a question, but upon writing this post, I found a solution. If someone here got a better one, since mine is a bit ugly, feel free to post it. Otherwise I will just archive my solution here so that other people facing a similar problem will know. (Solution can be found at the bottom.)

I have the following demands which need to be met:

• Display all "modules" with Checkboxes on the webpage
• Sort the modules by two subsets of each other:
  1. Subset to be displayed at the top: All the modules that are selected and thus held inside the "modules" collection on the Information-Entity
  2. Subset to be displayed beneath: All the modules that haven't been selected yet (with ID>0) and are retrieved from the database directly
• Sort each of these subsets alphabetically (case insensitive)

The result should look like this:

[x] aaaModule
[x] AbbModule
[x] aBcModule
[ ] aabModule
[ ] acbModule

Note that the sorting needs to be case insensitive, which is why @OrderBy doesn't cut it. Instead I went with implementing the Comparable interface to the Entity-Class to be sorted and used the @SortNatural-Annotation of Hibernate on the @ManyToMany mapping.

My current solution (simplified of course) looks as follows:

Inside the Information.java Entity:

@ManyToMany
@JoinTable(
    name = "bla_modules", 
    schema = "blaSchema",
    joinColumns = { @JoinColumn(name = "bla_info_id") }, 
    inverseJoinColumns = { @JoinColumn(name = "bla_module_id") }
)
@SortNatural
private Set<Module> modules = new TreeSet<>();

Inside the Controller.java (in the GET-Mapping):

Set<Module> modulesToDisplay = new LinkedHashSet<>();
modulesToDisplay.addAll(currentInformation.getModules());
modulesToDisplay.addAll(moduleRepository.findByIdGreaterThanOrderByLabelAsc(0));
model.addAttribute("modulesToDisplay", modulesToDisplay);

Inside the information.jsp:

<form:form method="POST" action="/informations.bla/editInformation" modelAttribute="currentInformation">
	<ul style="list-style-type:none">
		<c:forEach items="${modulesToDisplay}" var="module">
			<li>
				<div class="inline">
					<form:checkbox path="modules" value="${module}" label="[${module.shortLabel}] ${module.label}"/>
				</div>
			</li>
		</c:forEach>
	</ul>
	<input type="submit" value="Save">
</form:form>

Inside the Controller.java (the POST-Mapping):

public String editInformation(@ModelAttribute Information submittedInformation) {
	Information informationAfterSaving = informationRepository.save(submittedInformation);

It all worked great prior to me having the @SortNatural annotation on the collection, but in that case JPA didn't sort the TreeSet when loading it from the database.

It also still works great, if I load an existing Information-Entity from the database, display that, change attributes, and then save it back.

However, when I load a new Information-Entity, which just got created in the Controller via "new Information()", when the GET-Mapping doesn't receive a valid ID to load, then I suddenly get the error java.lang.ClassCastException: class java.util.LinkedHashSet cannot be cast to class java.util.SortedSet when the Information informationAfterSaving = informationRepository.save(submittedInformation); inside the POST-Mapping of the Controller is executed.

The solution can be found in my own answer below. I hope it might help someone someday.

Bye

SOLUTION

Inside the POST-Mapping of the Controller prior to calling the .save method of the Repository, just do the following:

submittedInformation.setModules(new TreeSet<Module>(submittedInformation.getModules()));

I feel a bit stupid now that I didn't think of this obvious solution earlier, not gonna lie.

The solution is still a bit ugly though, so if someone knows how to tell JPA/Hibernate to properly convert the received collection from Spring back into a TreeSet, or to tell Spring to attach the Checkbox-Collection in the form of a TreeSet, then please tell me.

Alternative

Not a suitable solution for me, but @MartinFrey also gave a possible solution: Just skip the sorting on the backend side and instead sort on the GUI side.