Discover Subclasses of a sealed class automatically

src/main/kotlin/com/fasterxml/jackson/module/kotlin/KotlinAnnotationIntrospector.kt

@@ -3,11 +3,8 @@ package com.fasterxml.jackson.module.kotlin
 import com.fasterxml.jackson.annotation.JsonProperty
 import com.fasterxml.jackson.databind.DeserializationFeature
 import com.fasterxml.jackson.databind.Module
-import com.fasterxml.jackson.databind.introspect.AnnotatedField
-import com.fasterxml.jackson.databind.introspect.AnnotatedMember
-import com.fasterxml.jackson.databind.introspect.AnnotatedMethod
-import com.fasterxml.jackson.databind.introspect.AnnotatedParameter
-import com.fasterxml.jackson.databind.introspect.NopAnnotationIntrospector
+import com.fasterxml.jackson.databind.introspect.*
+import com.fasterxml.jackson.databind.jsontype.NamedType
 import java.lang.reflect.Constructor
 import java.lang.reflect.Field
 import java.lang.reflect.Method
@@ -41,6 +38,26 @@ internal class KotlinAnnotationIntrospector(private val context: Module.SetupCon
             }
         }
 
+    /**
+     * Subclasses can be detected automatically for sealed classes, since all possible subclasses are known
+     * at compile-time to Kotlin. This makes [com.fasterxml.jackson.annotation.JsonSubTypes] redundant.
+     */
+    override fun findSubtypes(a: Annotated): MutableList<NamedType>? {
+
+        val rawType = a.rawType
+        if (rawType.isKotlinClass()) {
+            val kClass = rawType.kotlin
+            if (kClass.isSealed) {
+                return kClass.sealedSubclasses
+                    .map { NamedType(it.java) }
+                    .toMutableList()
+            }
+        }
+
+        return null
+
+    }
+
     private fun AnnotatedField.hasRequiredMarker(): Boolean? {
         val byAnnotation = (member as Field).getAnnotationsByType(JsonProperty::class.java).firstOrNull()?.required
         val byNullability =  (member as Field).kotlinProperty?.returnType?.isRequired()

src/test/kotlin/com/fasterxml/jackson/module/kotlin/test/Github239.kt

@@ -0,0 +1,73 @@
+package com.fasterxml.jackson.module.kotlin.test
+
+import com.fasterxml.jackson.annotation.JsonSubTypes
+import com.fasterxml.jackson.annotation.JsonTypeInfo
+import com.fasterxml.jackson.annotation.JsonTypeName
+import com.fasterxml.jackson.databind.ObjectMapper
+import com.fasterxml.jackson.module.kotlin.KotlinModule
+import com.fasterxml.jackson.module.kotlin.readValue
+import org.junit.Test
+import kotlin.test.assertEquals
+
+@JsonTypeInfo(use = JsonTypeInfo.Id.NAME)
+sealed class Github239Either {
+
+    @JsonTypeName("a")
+    data class A(var field: String = "") : Github239Either()
+
+    @JsonTypeName("b")
+    data class B(var otherField: String = "") : Github239Either()
+
+}
+
+@JsonTypeInfo(use = JsonTypeInfo.Id.NAME)
+@JsonSubTypes(
+        JsonSubTypes.Type(Github239EitherCustomized.A::class, name = "a"),
+        JsonSubTypes.Type(Github239EitherCustomized.B::class, name = "b")
+)
+sealed class Github239EitherCustomized {
+
+    data class A(var field: String = "") : Github239EitherCustomized()
+
+    data class B(var otherField: String = "") : Github239EitherCustomized()
+
+}
+
+class TestGithub239 {
+    val json = """[
+        {
+            "@type": "a",
+            "field": "value"
+        },
+        {
+            "@type": "b",
+            "otherField": "1234"
+        }
+    ]"""
+
+    val mapper = ObjectMapper()
+            .registerModule(KotlinModule())
+
+    @Test
+    fun test_implicit_subclasses() {
+
+        val array = mapper.readValue<Array<Github239Either>>(json)
+
+        assertEquals(2, array.size)
+        assertEquals(Github239Either.A("value"), array[0])
+        assertEquals(Github239Either.B("1234"), array[1])
+
+    }
+
+    @Test
+    fun test_explicit_subclasses() {
+
+        val array = mapper.readValue<Array<Github239EitherCustomized>>(json)
+
+        assertEquals(2, array.size)
+        assertEquals(Github239EitherCustomized.A("value"), array[0])
+        assertEquals(Github239EitherCustomized.B("1234"), array[1])
+
+    }
+
+}