Why does Jersey 3.1.0 fail to send some fields of the response that earlier Jersey version do? - jersey

Question

We recently had to upgrade from Jersey 3.0.2 to Jersey 3.1.0 (due to an upgrade of Tomcat), and discovered that a REST method would sometimes fail to send all the appropriate fields of the response structure to the client, unlike the previous version of Jersey. That is, some fields would get sent, and others won't. How to handle this?
class Display {
int foo; public int getFoo() { ... }
}
class Display2 extends Display {
int bar; public int getBar() {...}
}
#POST
#Path("/move")
#Consumes(MediaType.APPLICATION_FORM_URLENCODED)
#Produces(MediaType.APPLICATION_JSON)
public Display move(....) {
Display x= new Display2(...);
return x;
}
One would of course expect the return JSON structure sent by /move to the client to include both x.foo and x.bar. That's what it did in Jersey 2.* and 3.0.2. But now in Jersey 3.1.0, only x.foo gets sent!

Answer 1

A bit of experimentation shows that Jersey has apparently switched to somewhat more static typing! With Jersey 3.1.0, if a method is declared as
Display move() { ... }
and it returns an object x of the type Display2 that's a subclass of Display, then only the fields of x that are declared in Display are returned, and not those that are declared in Display2.
Our solution was to change the signatures of our REST methods. That is, if the method is likely to return an object of the type Display2 rather than Display, then we need to explicitly declare the method as returning Display2... and then all is well!
Of course, if a method sometimes returns an object of the type Display and sometimes Display2, then we need to to add a "converting" method,
class Display2 {
static Display2 convertFrom(Display x) { ... }
}
which would basically create a Display2 object with the same fields as the input Display object was, plus some dummy values (nulls or 0s) in the other fields.
So this works for us, but I still have no idea why the Jersey people needed to make that change!