obabichevjb opened a new pull request, #16490:
URL: https://github.com/apache/dubbo/pull/16490
## What is the purpose of the change?
Fixes #16489.
Kotlin's `object` (and `data object`) declares a singleton: the compiler
emits a final class with a
**private** constructor and a single `public static final INSTANCE`, and the
language guarantees no
other instance exists. The default bean deserializers do not honour that —
they allocate a fresh
instance, bypassing the private constructor, and populate its fields. Dubbo
therefore produces an
instance the language says cannot exist.
For a plain `object`, whose `equals` is inherited identity comparison,
losing identity also loses
equality. Kotlin's `when` compares with `==`, so dispatch silently changes:
```kotlin
sealed interface Reply
object NotFound : Reply
when (service.lookup(id)) {
NotFound -> handleMissing() // never runs after an RPC hop
else -> somethingElse() // runs instead
}
```
No exception and no log line — only the wrong code path.
Hessian2 already solves this for the Java equivalent: `EnumDeserializer`
writes the constant's name
and resolves the real JVM constant, which is why an enum survives a round
trip with `===` intact.
This change applies the same approach to Kotlin object declarations.
| Construct | before | after |
|---|---|---|
| `object` | `===` ✗, `==` ✗, **wrong `when` branch** | `===` ✓, `==` ✓,
correct branch |
| `data object` | `===` ✗, `==` ✓, branch ok | `===` ✓ |
| `enum` | `===` ✓ | unchanged |
| `data class` | `===` ✗, value semantics | **unchanged** |
The `data class` row not moving is deliberate: it has no singleton claim and
must keep value
semantics. A test asserts the Java-singleton case is likewise untouched.
### How
`KotlinObjectDeserializer` mirrors `EnumDeserializer`: it reads the
serialized field data so the
stream position stays correct, discards it, and returns the declared
`INSTANCE`. It is wired into
`Hessian2SerializerFactory#getDefaultDeserializer` beside the existing
special case for Java
records, so no change to `dubbo-hessian-lite` is required.
**Discarding transmitted state is deliberate.** An enum constant's fields
are not restored from the
wire either, and the alternative — populating the singleton's fields — would
mutate shared global
state on every call. The identity guarantee is the property being preserved.
**Detection is gated on the `kotlin.Metadata` annotation**, matched by
fully-qualified name through
`Class#getAnnotations`, so Dubbo needs no dependency on kotlin-stdlib in
main code. Requiring the
annotation rather than detecting the shape structurally (static final
`INSTANCE` of the class's own
type plus a private constructor) keeps the behaviour change scoped to
Kotlin: a hand-written Java
singleton of identical shape continues to deserialize to a new instance.
Dropping the gate would
silently change behaviour for existing Java users, so
`testJavaSingletonBehaviourIsUnchanged` pins
it down — removing the gate leaves 809 of 810 tests in the module green and
fails only that one.
### Tests
`KotlinObjectDeserializerTest` covers both directions:
- a Kotlin object declaration deserializes to its declared `INSTANCE`
- an ordinary Java singleton still deserializes to a new instance
The Kotlin fixture is written in **Java**, annotated `@kotlin.Metadata` and
shaped exactly as the
Kotlin compiler emits, so no Kotlin compiler enters the build; the detection
only inspects those
bytecode features. `kotlin-stdlib` is added at **test scope only**, purely
to supply that
annotation. If you would rather avoid the test dependency, the annotation
can be stubbed in test
sources instead — happy to switch.
### Verification
- the reproducer in the issue goes from `>>> FELL THROUGH TO else <<<` to
`matched NotFoundObject`
- the new test fails without the change (two distinct instances), so it is
not vacuous
- 810 tests in `dubbo-serialization-hessian2`, 806 in
`dubbo-serialization-fastjson2` (as a control
on the other serializer) and 155 in `dubbo-rpc-api` all pass
- full clean reactor build: 114 modules, 0 failures
- `mvn spotless:check` passes project-wide
## Checklist
- [x] Make sure there is a
[GitHub_issue](https://github.com/apache/dubbo/issues) field for the change. —
#16489
- [x] Write a pull request description that is detailed enough to understand
what the pull request does, how, and why.
- [x] Write necessary unit-test to verify your logic correction. If the new
feature or significant change is committed, please remember to add sample in
[dubbo samples](https://github.com/apache/dubbo-samples) project. —
`KotlinObjectDeserializerTest`, covering the fixed behaviour and the unchanged
Java-singleton behaviour.
- [x] Make sure gitHub actions can pass. [Why the workflow is failing and
how to fix it?](../CONTRIBUTING.md)
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