当返回值为null时，Optional不引发异常

Question

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

我希望在数据库中找不到该项时出现异常

Answer 1

optional.of需要纯值。您也可以在文档中找到相关信息,

/**
 * 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);
}

例如,

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)

如果您的值在运行时可以为null，则可以使用.ofnullable，

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

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]

@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
}

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"))
}