Deserialization of kotlin singleton as singletons

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

@@ -0,0 +1,26 @@
+package com.fasterxml.jackson.module.kotlin
+
+import com.fasterxml.jackson.databind.BeanDescription
+import com.fasterxml.jackson.databind.DeserializationConfig
+import com.fasterxml.jackson.databind.JsonDeserializer
+import com.fasterxml.jackson.databind.deser.BeanDeserializerModifier
+
+object KotlinBeanDeserializerModifier: BeanDeserializerModifier() {
+    override fun modifyDeserializer(
+            config: DeserializationConfig,
+            beanDesc: BeanDescription,
+            deserializer: JsonDeserializer<*>
+    ) = super.modifyDeserializer(config, beanDesc, deserializer)
+            .maybeSingletonDeserializer(objectSingletonInstance(beanDesc.beanClass))
+}
+
+fun JsonDeserializer<*>.maybeSingletonDeserializer(singleton: Any?) = when (singleton) {
+    null -> this
+    else -> this.asSingletonDeserializer(singleton)
+}
+
+private fun objectSingletonInstance(beanClass: Class<*>): Any? = if (!beanClass.isKotlinClass()) {
+    null
+} else {
+    beanClass.kotlin.objectInstance
+}

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

@@ -43,6 +43,8 @@ class KotlinModule(val reflectionCacheSize: Int = 512, val nullToEmptyCollection
 
         context.addValueInstantiators(KotlinInstantiators(cache, nullToEmptyCollection, nullToEmptyMap))
 
+        context.addBeanDeserializerModifier(KotlinBeanDeserializerModifier)
+
         fun addMixIn(clazz: Class<*>, mixin: Class<*>) {
             context.setMixInAnnotations(clazz, mixin)
         }

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

@@ -0,0 +1,42 @@
+package com.fasterxml.jackson.module.kotlin
+
+import com.fasterxml.jackson.core.JsonParser
+import com.fasterxml.jackson.databind.BeanProperty
+import com.fasterxml.jackson.databind.DeserializationContext
+import com.fasterxml.jackson.databind.JsonDeserializer
+import com.fasterxml.jackson.databind.deser.ContextualDeserializer
+import com.fasterxml.jackson.databind.deser.ResolvableDeserializer
+
+internal fun JsonDeserializer<*>.asSingletonDeserializer(singleton: Any) =
+        KotlinObjectSingletonDeserializer(singleton, this)
+
+/** deserialize as normal, but return the canonical singleton instance. */
+internal class KotlinObjectSingletonDeserializer(
+        private val singletonInstance: Any,
+        private val defaultDeserializer: JsonDeserializer<*>
+) : JsonDeserializer<Any>(),
+        // Additional interfaces of a specific 'JsonDeserializer' must be supported
+        // Kotlin objectInstances are currently handled by a BeanSerializer which
+        // implements 'ContextualDeserializer' and 'ResolvableDeserializer'.
+        ContextualDeserializer,
+        ResolvableDeserializer {
+
+    override fun resolve(ctxt: DeserializationContext?) {
+        if (defaultDeserializer is ResolvableDeserializer) {
+            defaultDeserializer.resolve(ctxt)
+        }
+    }
+
+    override fun createContextual(ctxt: DeserializationContext?, property: BeanProperty?): JsonDeserializer<*> =
+            if (defaultDeserializer is ContextualDeserializer) {
+                defaultDeserializer.createContextual(ctxt, property)
+                        .asSingletonDeserializer(singletonInstance)
+            } else {
+                this
+            }
+
+    override fun deserialize(p: JsonParser, ctxt: DeserializationContext): Any {
+        defaultDeserializer.deserialize(p, ctxt)
+        return singletonInstance
+    }
+}

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

@@ -0,0 +1,57 @@
+package com.fasterxml.jackson.module.kotlin.test
+
+import com.fasterxml.jackson.databind.ObjectMapper
+import com.fasterxml.jackson.databind.SerializationFeature
+import com.fasterxml.jackson.module.kotlin.jacksonObjectMapper
+import com.fasterxml.jackson.module.kotlin.readValue
+import org.hamcrest.CoreMatchers.equalTo
+import org.hamcrest.CoreMatchers.not
+import org.hamcrest.MatcherAssert.assertThat
+import org.junit.Test
+
+class ObjectSingletonTest {
+
+    val mapper: ObjectMapper = jacksonObjectMapper()
+            .configure(SerializationFeature.INDENT_OUTPUT, false)
+
+    object Singleton {
+        var content = 1 // mutable state
+    }
+
+    @Test
+    fun deserializationPreservesSingletonProperty() {
+        val js = mapper.writeValueAsString(Singleton)
+        val newSingleton = mapper.readValue<Singleton>(js)
+
+        assertThat(newSingleton, equalTo(Singleton))
+    }
+
+    @Test
+    fun deserializationResetsSingletonObjectState() {
+        // persist current singleton state
+        val js = mapper.writeValueAsString(Singleton)
+        val initial = Singleton.content
+
+        // mutate the in-memory singleton state
+        val after = initial + 1
+        Singleton.content = after
+        assertThat(Singleton.content, equalTo(after))
+
+        // read back persisted state resets singleton state
+        val newSingleton = mapper.readValue<Singleton>(js)
+        assertThat(newSingleton.content, equalTo(initial))
+        assertThat(Singleton.content, equalTo(initial))
+    }
+
+    @Test
+    fun deserializedObjectsBehaveLikeSingletons() {
+        val js = mapper.writeValueAsString(Singleton)
+        val newSingleton = mapper.readValue<Singleton>(js)
+        assertThat(newSingleton.content, equalTo(Singleton.content))
+
+        newSingleton.content += 1;
+
+        assertThat(Singleton.content, equalTo(newSingleton.content))
+    }
+
+}