#400: Add support for GraphQL union type

[craig-day]

I eagerly started with defining the spec, but also wanted to get an implementation done.

This adds support for creating GraphQL union types using an annotation @Union on a Java interface. I chose an empty Java interface so there would be an explicit layer of abstraction to represent the union itself.

Example:

@Union 
interface MyUnion {
}

class Foo implements MyUnion {
  String getName();
}

class Bar implements MyUnion {
  int getCount();
}

would create

type Foo {
  name: String
}

type Bar {
  count: Int
}

union MyUnion = Foo | Bar

An alternative could have been to directly annotate a type with the name of the union(s) it should be a member of, then generate the union from there.

Expand for alternative Java example

  @Union("MyUnion")
  class Foo {
    String getName();
  }
  
  @Union("MyUnion")
  class Bar {
    int getCount();
  }

Event thought this is a little less code, I didn't like it because the @Union annotation no longer describes an object directly, rather a relationship between objects. This relationship now only exists in the world of strings, and has no concrete representation in code.

For lots more thoughts, see the PR description here: eclipse/microprofile-graphql#459

Resolves #400

[phillip-kruger]

@t1 @andymc12 @jmartisk @WhiteCat22 @kaqqao any thoughts on this ?

[phillip-kruger]

@craig-day the current main in SmallRye is using jakarta and are not yet release. Maybe you can also open a PR for the 1.6.x (javax) branch ?

[craig-day]

@phillip-kruger done! #1470

[t1]

I like the idea of using an interface. My first thought had been to use a wrapper type where exactly one field can be set, but the interface is better. Thanks @craig-day !

[phillip-kruger]

The failure of Quarkus build is expected with the Jakarta branch

[phillip-kruger]

Thanks !