Change default value of encodeDefaults config flag (#1084)

Motivated by the fact that this behavior is what users usually want from framework;
GSON and Moshi already do that.

* Switch `encodeDefaults` flag in default JsonConfiguration to false.
* Flip encodeDefaults flag for Protobuf & Cbor
* Bring back accidentally lost tests after runtime->core moving

Author: sandwwraith

docs/basic-serialization.md

@@ -20,7 +20,7 @@ This chapter shows the basic use of Kotlin Serialization and explains its core c
   * [Optional property initializer call](#optional-property-initializer-call)
   * [Required properties](#required-properties)
   * [Transient properties](#transient-properties)
-  * [Defaults are encoded](#defaults-are-encoded)
+  * [Defaults are not encoded](#defaults-are-not-encoded)
   * [Nullable properties](#nullable-properties)
   * [Type safety is enforced](#type-safety-is-enforced)
   * [Referenced objects](#referenced-objects)
@@ -421,9 +421,10 @@ Use 'ignoreUnknownKeys = true' in 'Json {}' builder to ignore unknown keys.
 
 > 'ignoreUnknownKeys' feature is explained in the [Ignoring Unknown Keys section](json.md#ignoring-unknown-keys) section.
 
-### Defaults are encoded
+### Defaults are not encoded
 
-Default values are still encoded by default.
+Default values are not encoded by default in JSON. This behavior is motivated by the fact that in most real-life scenarios,
+such configuration reduces visual clutter and saves amount of data being serialized. 
 
 ```kotlin
 @Serializable 
@@ -437,10 +438,10 @@ fun main() {
 
 > You can get the full code [here](../guide/example/example-classes-09.kt).
 
-It produces the following output, which has `language` property, even though its value is equal to the default one.
+It produces the following output, which does not have `language` property, because its value is equal to the default one.
 
 ```text
-{"name":"kotlinx.serialization","language":"Kotlin"}
+{"name":"kotlinx.serialization"}
 ```                 
 
 > See [Encoding defaults](json.md#encoding-defaults) section on how this behavior can be configured for JSON. 
@@ -463,10 +464,10 @@ fun main() {
 
 > You can get the full code [here](../guide/example/example-classes-10.kt).
 
-This example explicitly encodes `null` in JSON because [Defaults are encoded](#defaults-are-encoded).
+This example does not encode `null` in JSON because [Defaults are not encoded](#defaults-are-not-encoded).
 
 ```text
-{"name":"kotlinx.serialization","renamedTo":null}
+{"name":"kotlinx.serialization"}
 ```     
 
 <!--- TEST -->

docs/polymorphism.md

@@ -209,8 +209,9 @@ sealed class Project {
 class OwnedProject(override val name: String, val owner: String) : Project()
 
 fun main() {
+    val json = Json { encodeDefaults = true } // "status" will be skipped otherwise
     val data: Project = OwnedProject("kotlinx.coroutines", "kotlin")
-    println(Json.encodeToString(data))
+    println(json.encodeToString(data))
 }  
 ```
 

docs/serialization-guide.md

@@ -24,7 +24,7 @@ Once the project is set up, we can start serializing some classes.
   * <a name='optional-property-initializer-call'></a>[Optional property initializer call](basic-serialization.md#optional-property-initializer-call)
   * <a name='required-properties'></a>[Required properties](basic-serialization.md#required-properties)
   * <a name='transient-properties'></a>[Transient properties](basic-serialization.md#transient-properties)
-  * <a name='defaults-are-encoded'></a>[Defaults are encoded](basic-serialization.md#defaults-are-encoded)
+  * <a name='defaults-are-not-encoded'></a>[Defaults are not encoded](basic-serialization.md#defaults-are-not-encoded)
   * <a name='nullable-properties'></a>[Nullable properties](basic-serialization.md#nullable-properties)
   * <a name='type-safety-is-enforced'></a>[Type safety is enforced](basic-serialization.md#type-safety-is-enforced)
   * <a name='referenced-objects'></a>[Referenced objects](basic-serialization.md#referenced-objects)

formats/cbor/commonMain/src/kotlinx/serialization/cbor/Cbor.kt

@@ -25,6 +25,7 @@ import kotlinx.serialization.modules.*
  * from corresponding Kotlin objects. However, other 3rd-party parsers (e.g. `jackson-dataformat-cbor`) may not accept such maps.
  *
  * @param encodeDefaults specifies whether default values of Kotlin properties are encoded.
+ *                       False by default; meaning that properties with values equal to defaults will be elided.
  */
 @ExperimentalSerializationApi
 public sealed class Cbor(
@@ -36,7 +37,7 @@ public sealed class Cbor(
     /**
      * The default instance of [Cbor]
      */
-    public companion object Default : Cbor(true, EmptySerializersModule, null)
+    public companion object Default : Cbor(false, EmptySerializersModule, null)
 
     override fun <T> encodeToByteArray(serializer: SerializationStrategy<T>, value: T): ByteArray {
         val output = ByteArrayOutput()

formats/cbor/jvmTest/src/kotlinx/serialization/cbor/TestUtilities.kt

@@ -1,5 +1,5 @@
 /*
- * Copyright 2017-2019 JetBrains s.r.o. Use of this source code is governed by the Apache 2.0 license.
+ * Copyright 2017-2020 JetBrains s.r.o. Use of this source code is governed by the Apache 2.0 license.
  */
 
 package kotlinx.serialization.cbor
@@ -10,11 +10,12 @@ import com.fasterxml.jackson.module.kotlin.*
 import kotlinx.serialization.*
 
 internal val cborJackson = ObjectMapper(CBORFactory()).apply { registerKotlinModule() }
+internal val defaultCbor = Cbor { encodeDefaults = true }
 
 internal inline fun <reified T : Any> dumpCborCompare(it: T, alwaysPrint: Boolean = false): Boolean {
     var parsed: T?
     val c = try {
-        val bytes = Cbor.encodeToByteArray(it)
+        val bytes = defaultCbor.encodeToByteArray(it)
         parsed = cborJackson.readValue<T>(bytes)
         it == parsed
     } catch (e: Exception) {
@@ -30,7 +31,7 @@ internal inline fun <reified T: Any> readCborCompare(it: T, alwaysPrint: Boolean
     var obj: T?
     val c = try {
         val hex = cborJackson.writeValueAsBytes(it)
-        obj = Cbor.decodeFromByteArray(hex)
+        obj = defaultCbor.decodeFromByteArray(hex)
         obj == it
     } catch (e: Exception) {
         obj = null

formats/json/build.gradle

@@ -19,6 +19,7 @@ kotlin {
         jvmTest {
             dependencies {
                 implementation group: 'com.google.code.gson', name: 'gson', version: '2.8.5'
+                implementation "org.jetbrains.kotlinx:kotlinx-coroutines-core:$coroutines_version"
             }
         }
     }

formats/json/commonMain/src/kotlinx/serialization/json/JsonConfiguration.kt

@@ -15,7 +15,7 @@ import kotlin.jvm.*
             "'Json(MyJsonConfiguration.copy(prettyPrint = true))' can be replaced with 'Json(from = MyApplicationJson) { prettyPrint = true }'"
 )
 public open class JsonConfiguration(
-    internal val encodeDefaults: Boolean = true,
+    internal val encodeDefaults: Boolean = false,
     internal val ignoreUnknownKeys: Boolean = false,
     internal val isLenient: Boolean = false,
     internal val serializeSpecialFloatingPointValues: Boolean = false,

formats/json/commonMain/src/kotlinx/serialization/json/internal/JsonConf.kt

@@ -11,7 +11,7 @@ import kotlin.jvm.*
 // Mirror of the deprecated JsonConfiguration. Not for external use.
 @OptIn(ExperimentalSerializationApi::class)
 internal data class JsonConf(
-    @JvmField public val encodeDefaults: Boolean = true,
+    @JvmField public val encodeDefaults: Boolean = false,
     @JvmField public val ignoreUnknownKeys: Boolean = false,
     @JvmField public val isLenient: Boolean = false,
     @JvmField public val allowStructuredMapKeys: Boolean = false,

formats/json/commonTest/src/kotlinx/serialization/SerializerForNullableTypeTest.kt

@@ -85,7 +85,7 @@ public class SerializerForNullableTypeTest : JsonTestBase() {
 
     @Serializable
     data class NullablePrimitive(
-        @Serializable(with = NullableLongSerializer::class) val value: Long? = null
+        @Serializable(with = NullableLongSerializer::class) val value: Long?
     )
 
     @Test

formats/json/commonTest/src/kotlinx/serialization/features/ContextAndPolymorphicTest.kt

@@ -7,12 +7,10 @@ package kotlinx.serialization.features
 import kotlinx.serialization.*
 import kotlinx.serialization.builtins.*
 import kotlinx.serialization.descriptors.*
-import kotlinx.serialization.descriptors.PrimitiveSerialDescriptor
 import kotlinx.serialization.encoding.*
-import kotlinx.serialization.internal.*
 import kotlinx.serialization.json.*
 import kotlinx.serialization.modules.*
-import kotlinx.serialization.test.InternalHexConverter
+import kotlinx.serialization.test.*
 import kotlin.test.*
 
 class ContextAndPolymorphicTest {
@@ -58,6 +56,7 @@ class ContextAndPolymorphicTest {
         val bPolymorphicModule = SerializersModule { polymorphic(Any::class) { subclass(PayloadSerializer) } }
         json = Json {
             useArrayPolymorphism = true
+            encodeDefaults = true
             serializersModule = scope + bPolymorphicModule
         }
     }
@@ -131,7 +130,7 @@ class ContextAndPolymorphicTest {
 
     @Test
     fun testContextualSerializerUsesDefaultIfModuleIsEmpty() {
-        val s = Json { useArrayPolymorphism = true }.encodeToString(EnhancedData.serializer(), value)
+        val s = Json { useArrayPolymorphism = true; encodeDefaults = true }.encodeToString(EnhancedData.serializer(), value)
         assertEquals("""{"data":{"a":100500,"b":42},"stringPayload":{"s":"string"},"binaryPayload":"62696E617279"}""", s)
     }
 }