On Wed, Apr 1, 2020 at 5:24 PM maxxyme _ <[email protected]> wrote:
>
> Hello,
>
> I already posted a question on SO but unfortunately didn't get any relevant
> answer (just one, and the guy told he's not keen on @JsonCreator...)
> Everything's detailed there (Java classes & call code):
> https://stackoverflow.com/questions/60132067/cant-properly-deserialize-json-using-jsoncreator-as-described-in-the-javadoc
>
> If someone's willing to point me to what's wrong with my class definitions,
> esp. with the 2 distinct exceptions:
> - I don't understand the 1st one (why doesn't it works...?)
> - For what I understand from the 2nd, Jackson tries to deserialize the whole
> JSON as for the Inner object... but why?
Because there are 2 different possibilities of what might be expected.
POJO1: expect JSON Object with same properties as POJO:
public class Pojo1 {
public int x, y;
@JsonCreator(mode = JsonCreator.Mode.PROPERTIES)
public Pojo(@JsonProperty("x") int x0, @JsonProperty("y") int y0) {
x = x0;
y = y0;
}
}
JSON that can be read: {"x":1, "y":2}
POJO 2: expect JSON _value_ that is used to construct POJO, but is not
property-based (often not JSON Object, but may be)
public class Pojo2 {
int x, y;
@JsonCreator(mode = JsonCreator.Mode.DELEGATING)
public Pojo2(/* Note: NO property name */ int[] args) {
x = args[0];
y = args[1];
}
}
JSON that can be read: [1, 2]
-------------
So far so good? One thing to note is that "delegating" style is only
applicable with just one parameter for constructor (or factory
method); whereas "properties" style works for any number.
But this means that the special case of 1 argument constructor (or
factory method) can be ambiguous if `mode` is not specified. With
this:
class NameBean {
private String name;
@JsonCreator
public NameBean(String name) {
this.name = name;
}
}
which JSON should this map to/from:
{ "name" : "Bob" }
OR
"Bob"
?
There is no good way to figure out that I am aware of. Heuristics
fail; and as the answer at SO mentioned, parameter names for
constructors may or may not be included even on Java 8 (and were never
included before that; Jackson still only requires Java 7 for
databind).
This is why you need to either add the `mode` property OR add explicit
`@JsonProperty` for constructor parameter if you want to force
Properties-style Creator.
And if conversely you want delegating style, mode.
I hope this helps,
-+ Tatu +-
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