Introduce kotlin #554 code.

from FasterXML/jackson-module-kotlin#554

Author: k163377

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

@@ -61,6 +61,11 @@ public inline fun <reified T> ObjectReader.readValuesTyped(jp: JsonParser): Iter
 public inline fun <reified T> ObjectReader.treeToValue(n: TreeNode): T? =
     readValue(this.treeAsTokens(n), jacksonTypeRef<T>())
 
+public inline fun <reified T, reified U> ObjectMapper.addMixIn(): ObjectMapper =
+    this.addMixIn(T::class.java, U::class.java)
+public inline fun <reified T, reified U> JsonMapper.Builder.addMixIn(): JsonMapper.Builder =
+    this.addMixIn(T::class.java, U::class.java)
+
 public operator fun ArrayNode.plus(element: Boolean) { add(element) }
 public operator fun ArrayNode.plus(element: Short) { add(element) }
 public operator fun ArrayNode.plus(element: Int) { add(element) }

src/test/kotlin/com/fasterxml/jackson/module/kotlin/_ported/test/ExtensionMethodsTests.kt

@@ -1,11 +1,15 @@
 package com.fasterxml.jackson.module.kotlin._ported.test
 
+import com.fasterxml.jackson.annotation.JsonIgnore
 import com.fasterxml.jackson.databind.ObjectMapper
 import com.fasterxml.jackson.databind.SerializationFeature
+import com.fasterxml.jackson.databind.json.JsonMapper
 import com.fasterxml.jackson.databind.node.JsonNodeFactory
+import com.fasterxml.jackson.module.kotlin.addMixIn
 import com.fasterxml.jackson.module.kotlin.contains
 import com.fasterxml.jackson.module.kotlin.convertValue
 import com.fasterxml.jackson.module.kotlin.jacksonObjectMapper
+import com.fasterxml.jackson.module.kotlin.jsonMapper
 import com.fasterxml.jackson.module.kotlin.minusAssign
 import com.fasterxml.jackson.module.kotlin.plusAssign
 import com.fasterxml.jackson.module.kotlin.readValue
@@ -82,4 +86,16 @@ private class TestExtensionMethods {
         val convertValueResult: List<Person> = mapper.convertValue(tree)
         assertEquals(listOf(Person("Neo")), convertValueResult)
     }
+
+    @Test fun mixInExtensionTest() {
+
+        data class Person(val name: String)
+        abstract class PersonMixIn { @JsonIgnore
+        var name: String = "" }
+
+        val mapper: JsonMapper = jsonMapper { addMixIn<Person, PersonMixIn>() }
+        val serializedPerson: String = mapper.writeValueAsString(Person("test"))
+
+        assertEquals("{}", serializedPerson)
+    }
 }