Spring @ModelAttribute interface

Question

How can I have an interface as a ModelAttribute as in the below scenario?

@GetMapping("/{id}")
public String get(@PathVariable String id, ModelMap map) {
  map.put("entity", service.getById(id));
  return "view";
}

@PostMapping("/{id}")
public String update(@ModelAttribute("entity") Entity entity) {
  service.store(entity);
  return "view";
}

Above snippet gives the follow errors

BeanInstantiationException: Failed to instantiate [foo.Entity]: Specified class is an interface

I don't want spring to instantiate entity for me, I want to use the existing instance provided by map.put("entity", ..).

ok that is correct , and make sure your property names in that entity class is the same name with th:field — Parth Solanki
– Parth Solanki, Commented Oct 14, 2016 at 12:22
@JohanSjöberg Is the entity stored in the session? Otherwise it wouldn't survive a request/response cycle. — Kayaman
– Kayaman, Commented Oct 14, 2016 at 12:29

Johan Sjöberg · Accepted Answer · 2016-10-14 14:06:15Z

4

As been pointed out in comments, the Entity instance does not survive between the get and post requests.

The solution is this

@ModelAttribute("entity")
public Entity entity(@PathVariable String id) {
    return service.getById(id);
}

@GetMapping("/{id}")
public String get() {
   return "view";
}

@PostMapping("/{id})
public String update(@ModelAttribute("entity") Entity entity) {
  service.store(entity);
  return "view";
}

What happens here is that the Entity in update binds to the Entity created from the @ModelAttribute annotated entity method. Spring then applies the form-values to the existing object.

edited Oct 14, 2016 at 14:06

answered Oct 14, 2016 at 13:41

Johan Sjöberg

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