Iterating over Map in Freemarker

Imagine you have a Map of car dealers and vehicles:

Map<String, List<Vehicle>> vehiclesByDealer = new TreeMap<String, List<Vehicle>>();

Turns out it can be tricky to iterate over such a Map in Freemarker. As a workaround, add the keySet to the Model as a separate String list:

mv.addObject("vehiclesByDealerName", vehiclesByDealer);
mv.addObject("dealerNames", vehiclesByDealer.keySet());

Then iterate like this:

<#list dealerNames as dealerName>
	<#assign vehicles = vehiclesByDealerName[dealerName] />
...
</#list>