Optional does not throw exception when the return value is null

Question

I am learning Spring Boot, I am trying to throw an exception when the service does not found an item on the database,therefore, I tried with optional but when I test it I only get a null response besides an exception

    @GetMapping(value = "/compras", produces = "application/json")
public Optional<Compras> retrieveAllCompras(@RequestParam String id) {
    return Optional.of(compraRepository.findById(id)).orElseThrow(RuntimeException::new);

I expect an exception when the item is not found in Database

try findById(id).get() – Mohamad Mirzadeh Feb 20 '23 at 11:43 — Mohamad Mirzadeh, Feb 20 '23 at 11:43

prayagupa · Answer 1 · 2019-07-22T01:06:01.330

Optional.of expects pure value. You can find the info in documentation as well,

/**
 * Constructs an instance with the described value.
 *
 * @param value the non-{@code null} value to describe
 * @throws NullPointerException if value is {@code null}
 */
private Optional(T value) {
    this.value = Objects.requireNonNull(value);
}

example,

jshell> Optional.of(100)
$2 ==> Optional[100]

jshell> Optional.of(null)
|  Exception java.lang.NullPointerException
|        at Objects.requireNonNull (Objects.java:221)
|        at Optional.<init> (Optional.java:107)
|        at Optional.of (Optional.java:120)
|        at (#1:1)

If your value could be null at runtime, you can use .ofNullable,

jshell> Optional.ofNullable(null)
$3 ==> Optional.empty

ALSO The idea of functional programming is to return a value for all the inputs, instead of throwing Exception which breaks the function composition.

jshell> Function<Integer, Optional<Integer>> f = x -> Optional.of(x + 1)
f ==> $Lambda$23/0x0000000801171c40@6996db8

jshell> Function<Integer, Optional<Integer>> g = x -> Optional.of(x * 2)
g ==> $Lambda$24/0x0000000801172840@7fbe847c

jshell> f.apply(5).flatMap(x -> g.apply(x))
$13 ==> Optional[12]

So in your example you can treat Optional.empty() as item not found, but Spring will consider that as 200 as well which is still better than throwing 500. You might want to send 404 to be accurate.

@GetMapping(
  value = "/compras", 
  produces = "application/json"
)
public Optional<Compras> retrieveAllCompras(@RequestParam String id) {
    return Optional.ofNullable(compraRepository.findById(id)); //will response as 200 even when no item found
}

You can use ResponseEntity<A> to set specific http status

The traditional way of responding 404 is defining specific exception.

import org.springframework.web.server.ResponseStatusException;
import org.springframework.http.HttpStatus;

@GetMapping(
  value = "/compras", 
  produces = "application/json"
)
public Compras retrieveAllCompras(@RequestParam String id) {
    return Optional.ofNullable(compraRepository.findById(id))
                   .orElseThrow(() -> new ResponseStatusException(HttpStatus.NOT_FOUND, "item not found"))
}

pretty nice, i should add optional.empty and if so return the new response by telling item not found right?, thanks a lot ! — Santiago molano perdomo, Jul 22 '19 at 00:46
Yes, you can set http response code to be `404`. Spring by default either let you send back `200` if everything goes well or `500` if you throw generic Exception. But as you know `500` represents Server Error. In your example if item is not found its ideally `404`. — prayagupa, Jul 22 '19 at 00:57
@Santiagomolanoperdomo I added an example that will send `404` when `Optional.empty` — prayagupa, Jul 22 '19 at 01:08
Thanks a Lot!, I did not know about ResponeStatusException, I did it by creating a new Class, thank you so much! — Santiago molano perdomo, Jul 22 '19 at 01:14
this is indeed a definition of an excellent answer! plus one! — developer_hatch, Jul 22 '19 at 03:50

score 1 · Answer 2 · answered Jul 22 '19 at 03:29

Think about the types here, if you want to return Optional, and you will do an .orElse, you have to wrap it again. It wold be a way to do:

 @GetMapping(value = "/compras", produces = "application/json")
public Optional<Compras> retrieveAllCompras(@RequestParam String id) {
    return Optional.ofNullable(compraRepository.findById(id)).
                     map(Optional::of).
                     orElseThrow(() -> new RuntimeException("Not found"));

I'll explain:

Optional.ofNullable() will return an Optional<Compras>, with the value Optional("some compras") or the value Optional.Empty
map(Optional::of) will wrap the optional again, so you will have Optional[Optional<Compras>] or (here the trick) Optional.Empty, because map doesn't wrap the Optional.Empty (that's his reason of being among us)
Finally, orElseThrow(() -> new RuntimeException("Not found")) will unpack your optional, in this case from Optional[Optional<Compras>] to Optional<Compras> or, in case of being Optional.Empty it will throw the exception

score 0 · Answer 3 · answered Jul 21 '19 at 22:04

0

You controller returns Optional<Compras>. This means there will not be #get method called. Your controller will return null if Optional value.

Change to: public Compras retrieveAllCompras... and you'll get your exception

answered Jul 21 '19 at 22:04

Artem Ptushkin

1,151
9
17

score 0 · Answer 4 · answered Jul 22 '19 at 07:21

Judging by the object name and invoked method (compraRepository.findById(id)), your service is actually a repository. If it's a Spring Data repository then you should leave creation of the Optional to Spring Data: Null Handling of Repository Methods. So you'll have

compraRepository.findById(id).orElseThrow(NotFoundException::new)

Optional does not throw exception when the return value is null

4 Answers4