Introduce @JsonIgnoreUnknownKeys annotation (#2874)

for more fine-grained control over JsonBuilder.ignoreUnknownKeys setting.

Fixes #1420

Also, improve error message and path handling when an 'Unknown key' exception is thrown.

Fixes #2869
Fixes #2637

docs/basic-serialization.md

@@ -411,8 +411,8 @@ Attempts to explicitly specify its value in the serial format, even if the speci
 value is equal to the default one, produces the following exception.
 
 ```text
-Exception in thread "main" kotlinx.serialization.json.internal.JsonDecodingException: Unexpected JSON token at offset 42: Encountered an unknown key 'language' at path: $.name
-Use 'ignoreUnknownKeys = true' in 'Json {}' builder to ignore unknown keys.
+Exception in thread "main" kotlinx.serialization.json.internal.JsonDecodingException: Encountered an unknown key 'language' at offset 42 at path: $
+Use 'ignoreUnknownKeys = true' in 'Json {}' builder or '@JsonIgnoreUnknownKeys' annotation to ignore unknown keys.
 ```
 
 <!--- TEST LINES_START -->

docs/json.md

@@ -13,6 +13,7 @@ In this chapter, we'll walk through features of [JSON](https://www.json.org/json
   * [Pretty printing](#pretty-printing)
   * [Lenient parsing](#lenient-parsing)
   * [Ignoring unknown keys](#ignoring-unknown-keys)
+  * [Ignoring unknown keys per class](#ignoring-unknown-keys-per-class)
   * [Alternative Json names](#alternative-json-names)
   * [Encoding defaults](#encoding-defaults)
   * [Explicit nulls](#explicit-nulls)
@@ -164,6 +165,44 @@ Project(name=kotlinx.serialization)
 
 <!--- TEST -->
 
+### Ignoring unknown keys per class
+
+Sometimes, for cleaner and safer API, it is desirable to ignore unknown properties only for specific classes.
+In that case, you can use [JsonIgnoreUnknownKeys] annotation on such classes while leaving global [ignoreUnknownKeys][JsonBuilder.ignoreUnknownKeys] setting
+turned off:
+
+```kotlin
+@OptIn(ExperimentalSerializationApi::class) // JsonIgnoreUnknownKeys is an experimental annotation for now
+@Serializable
+@JsonIgnoreUnknownKeys
+data class Outer(val a: Int, val inner: Inner)
+
+@Serializable
+data class Inner(val x: String)
+
+fun main() {
+    // 1
+    println(Json.decodeFromString<Outer>("""{"a":1,"inner":{"x":"value"},"unknownKey":42}"""))
+    println()
+    // 2
+    println(Json.decodeFromString<Outer>("""{"a":1,"inner":{"x":"value","unknownKey":"unknownValue"}}"""))
+}
+```
+
+> You can get the full code [here](../guide/example/example-json-04.kt).
+
+Line (1) decodes successfully despite "unknownKey" in `Outer`, because annotation is present on the class. 
+However, line (2) throws `SerializationException` because there is no "unknownKey" property in `Inner`:
+
+```text
+Outer(a=1, inner=Inner(x=value))
+
+Exception in thread "main" kotlinx.serialization.json.internal.JsonDecodingException: Encountered an unknown key 'unknownKey' at offset 29 at path: $.inner
+Use 'ignoreUnknownKeys = true' in 'Json {}' builder or '@JsonIgnoreUnknownKeys' annotation to ignore unknown keys.
+```
+
+<!--- TEST LINES_START-->
+
 ### Alternative Json names
 
 It's not a rare case when JSON fields are renamed due to a schema version change.
@@ -184,7 +223,7 @@ fun main() {
 }
 ```
 
-> You can get the full code [here](../guide/example/example-json-04.kt).
+> You can get the full code [here](../guide/example/example-json-05.kt).
 
 As you can see, both `name` and `title` Json fields correspond to `name` property:
 
@@ -222,7 +261,7 @@ fun main() {
 }
 ```
 
-> You can get the full code [here](../guide/example/example-json-05.kt).
+> You can get the full code [here](../guide/example/example-json-06.kt).
 
 It produces the following output which encodes all the property values including the default ones:
 
@@ -261,7 +300,7 @@ fun main() {
 }
 ```
 
-> You can get the full code [here](../guide/example/example-json-06.kt).
+> You can get the full code [here](../guide/example/example-json-07.kt).
 
 As you can see, `version`, `website` and `description` fields are not present in output JSON on the first line.
 After decoding, the missing nullable property `website` without a default values has received a `null` value,
@@ -319,7 +358,7 @@ fun main() {
 }
 ```
 
-> You can get the full code [here](../guide/example/example-json-07.kt).
+> You can get the full code [here](../guide/example/example-json-08.kt).
 
 The invalid `null` value for the `language` property was coerced into the default value:
 
@@ -348,7 +387,7 @@ fun main() {
 }
 ```
 
-> You can get the full code [here](../guide/example/example-json-08.kt).
+> You can get the full code [here](../guide/example/example-json-09.kt).
 
 Despite that we do not have `Color.pink` and `Color.purple` colors, `decodeFromString` function returns successfully:
 
@@ -384,7 +423,7 @@ fun main() {
 }
 ```
 
-> You can get the full code [here](../guide/example/example-json-09.kt).
+> You can get the full code [here](../guide/example/example-json-10.kt).
 
 The map with structured keys gets represented as JSON array with the following items: `[key1, value1, key2, value2,...]`.
 
@@ -415,7 +454,7 @@ fun main() {
 }
 ```
 
-> You can get the full code [here](../guide/example/example-json-10.kt).
+> You can get the full code [here](../guide/example/example-json-11.kt).
 
 This example produces the following non-stardard JSON output, yet it is a widely used encoding for
 special values in JVM world:
@@ -449,7 +488,7 @@ fun main() {
 }
 ```
 
-> You can get the full code [here](../guide/example/example-json-11.kt).
+> You can get the full code [here](../guide/example/example-json-12.kt).
 
 In combination with an explicitly specified [SerialName] of the class it provides full
 control over the resulting JSON object:
@@ -506,7 +545,7 @@ fun main() {
 }
 ```
 
-> You can get the full code [here](../guide/example/example-json-12.kt).
+> You can get the full code [here](../guide/example/example-json-13.kt).
 
 As you can see, discriminator from the `Base` class is used:
 
@@ -543,7 +582,7 @@ fun main() {
 }
 ```
 
-> You can get the full code [here](../guide/example/example-json-13.kt).
+> You can get the full code [here](../guide/example/example-json-14.kt).
 
 Note that it would be impossible to deserialize this output back with kotlinx.serialization.
 
@@ -579,7 +618,7 @@ fun main() {
 }
 ```
 
-> You can get the full code [here](../guide/example/example-json-14.kt).
+> You can get the full code [here](../guide/example/example-json-15.kt).
 
 It affects serial names as well as alternative names specified with [JsonNames] annotation, so both values are successfully decoded:
 
@@ -612,7 +651,7 @@ fun main() {
 }
 ```
 
-> You can get the full code [here](../guide/example/example-json-15.kt).
+> You can get the full code [here](../guide/example/example-json-16.kt).
 
 As you can see, both serialization and deserialization work as if all serial names are transformed from camel case to snake case:
 
@@ -710,7 +749,7 @@ fun main() {
 }
 ```
 
-> You can get the full code [here](../guide/example/example-json-16.kt)
+> You can get the full code [here](../guide/example/example-json-17.kt)
 
 ```text
 {"base64Input":"Zm9vIHN0cmluZw=="}
@@ -752,7 +791,7 @@ fun main() {
 }
 ```
 
-> You can get the full code [here](../guide/example/example-json-17.kt).
+> You can get the full code [here](../guide/example/example-json-18.kt).
 
 A `JsonElement` prints itself as a valid JSON:
 
@@ -795,7 +834,7 @@ fun main() {
 }
 ```
 
-> You can get the full code [here](../guide/example/example-json-18.kt).
+> You can get the full code [here](../guide/example/example-json-19.kt).
 
 The above example sums `votes` in all objects in the `forks` array, ignoring the objects that have no `votes`:
 
@@ -835,7 +874,7 @@ fun main() {
 }
 ```
 
-> You can get the full code [here](../guide/example/example-json-19.kt).
+> You can get the full code [here](../guide/example/example-json-20.kt).
 
 As a result, you get a proper JSON string:
 
@@ -864,7 +903,7 @@ fun main() {
 }
 ```
 
-> You can get the full code [here](../guide/example/example-json-20.kt).
+> You can get the full code [here](../guide/example/example-json-21.kt).
 
 The result is exactly what you would expect:
 
@@ -910,7 +949,7 @@ fun main() {
 }
 ```
 
-> You can get the full code [here](../guide/example/example-json-21.kt).
+> You can get the full code [here](../guide/example/example-json-22.kt).
 
 Even though `pi` was defined as a number with 30 decimal places, the resulting JSON does not reflect this. 
 The [Double] value is truncated to 15 decimal places, and the String is wrapped in quotes - which is not a JSON number.
@@ -951,7 +990,7 @@ fun main() {
 }
 ```
 
-> You can get the full code [here](../guide/example/example-json-22.kt).
+> You can get the full code [here](../guide/example/example-json-23.kt).
 
 `pi_literal` now accurately matches the value defined.
 
@@ -991,7 +1030,7 @@ fun main() {
 }
 ```
 
-> You can get the full code [here](../guide/example/example-json-23.kt).
+> You can get the full code [here](../guide/example/example-json-24.kt).
 
 The exact value of `pi` is decoded, with all 30 decimal places of precision that were in the source JSON.
 
@@ -1014,7 +1053,7 @@ fun main() {
 }
 ```
 
-> You can get the full code [here](../guide/example/example-json-24.kt).
+> You can get the full code [here](../guide/example/example-json-25.kt).
 
 ```text
 Exception in thread "main" kotlinx.serialization.json.internal.JsonEncodingException: Creating a literal unquoted value of 'null' is forbidden. If you want to create JSON null literal, use JsonNull object, otherwise, use JsonPrimitive
@@ -1090,7 +1129,7 @@ fun main() {
 }
 ```
 
-> You can get the full code [here](../guide/example/example-json-25.kt).
+> You can get the full code [here](../guide/example/example-json-26.kt).
 
 The output shows that both cases are correctly deserialized into a Kotlin [List].
 
@@ -1142,7 +1181,7 @@ fun main() {
 }
 ```
 
-> You can get the full code [here](../guide/example/example-json-26.kt).
+> You can get the full code [here](../guide/example/example-json-27.kt).
 
 You end up with a single JSON object, not an array with one element:
 
@@ -1187,7 +1226,7 @@ fun main() {
 }
 ```
 
-> You can get the full code [here](../guide/example/example-json-27.kt).
+> You can get the full code [here](../guide/example/example-json-28.kt).
 
 See the effect of the custom serializer:
 
@@ -1260,7 +1299,7 @@ fun main() {
 }
 ```
 
-> You can get the full code [here](../guide/example/example-json-28.kt).
+> You can get the full code [here](../guide/example/example-json-29.kt).
 
 No class discriminator is added in the JSON output:
 
@@ -1312,7 +1351,7 @@ fun main() {
 }
 ```
 
-> You can get the full code [here](../guide/example/example-json-29.kt).
+> You can get the full code [here](../guide/example/example-json-30.kt).
 
 `BasicProject` will be printed to the output:
 
@@ -1406,7 +1445,7 @@ fun main() {
 }
 ```
 
-> You can get the full code [here](../guide/example/example-json-30.kt).
+> You can get the full code [here](../guide/example/example-json-31.kt).
 
 This gives you fine-grained control on the representation of the `Response` class in the JSON output:
 
@@ -1471,7 +1510,7 @@ fun main() {
 }
 ```
 
-> You can get the full code [here](../guide/example/example-json-31.kt).
+> You can get the full code [here](../guide/example/example-json-32.kt).
 
 ```text
 UnknownProject(name=example, details={"type":"unknown","maintainer":"Unknown","license":"Apache 2.0"})
@@ -1517,6 +1556,7 @@ The next chapter covers [Alternative and custom formats (experimental)](formats.
 [JsonBuilder.prettyPrint]: https://kotlinlang.org/api/kotlinx.serialization/kotlinx-serialization-json/kotlinx.serialization.json/-json-builder/pretty-print.html
 [JsonBuilder.isLenient]: https://kotlinlang.org/api/kotlinx.serialization/kotlinx-serialization-json/kotlinx.serialization.json/-json-builder/is-lenient.html
 [JsonBuilder.ignoreUnknownKeys]: https://kotlinlang.org/api/kotlinx.serialization/kotlinx-serialization-json/kotlinx.serialization.json/-json-builder/ignore-unknown-keys.html
+[JsonIgnoreUnknownKeys]: https://kotlinlang.org/api/kotlinx.serialization/kotlinx-serialization-json/kotlinx.serialization.json/-json-ignore-unknown-keys/index.html
 [JsonNames]: https://kotlinlang.org/api/kotlinx.serialization/kotlinx-serialization-json/kotlinx.serialization.json/-json-names/index.html
 [JsonBuilder.useAlternativeNames]: https://kotlinlang.org/api/kotlinx.serialization/kotlinx-serialization-json/kotlinx.serialization.json/-json-builder/use-alternative-names.html
 [JsonBuilder.encodeDefaults]: https://kotlinlang.org/api/kotlinx.serialization/kotlinx-serialization-json/kotlinx.serialization.json/-json-builder/encode-defaults.html

docs/serialization-guide.md

@@ -114,6 +114,7 @@ Once the project is set up, we can start serializing some classes.
   * <a name='pretty-printing'></a>[Pretty printing](json.md#pretty-printing)
   * <a name='lenient-parsing'></a>[Lenient parsing](json.md#lenient-parsing)
   * <a name='ignoring-unknown-keys'></a>[Ignoring unknown keys](json.md#ignoring-unknown-keys)
+  * <a name='ignoring-unknown-keys-per-class'></a>[Ignoring unknown keys per class](json.md#ignoring-unknown-keys-per-class)
   * <a name='alternative-json-names'></a>[Alternative Json names](json.md#alternative-json-names)
   * <a name='encoding-defaults'></a>[Encoding defaults](json.md#encoding-defaults)
   * <a name='explicit-nulls'></a>[Explicit nulls](json.md#explicit-nulls)

formats/json-tests/commonTest/src/kotlinx/serialization/json/JsonIgnoreKeysTest.kt

@@ -0,0 +1,53 @@
+/*
+ * Copyright 2017-2024 JetBrains s.r.o. Use of this source code is governed by the Apache 2.0 license.
+ */
+
+package kotlinx.serialization.json
+
+import kotlinx.serialization.Serializable
+import kotlinx.serialization.test.checkSerializationException
+import kotlin.test.Test
+import kotlin.test.assertContains
+import kotlin.test.assertContentEquals
+import kotlin.test.assertEquals
+
+class JsonIgnoreKeysTest : JsonTestBase() {
+    val ignoresKeys = Json(default) { ignoreUnknownKeys = true }
+
+    @Serializable
+    class Outer(val a: Int, val inner: Inner)
+
+    @Serializable
+    @JsonIgnoreUnknownKeys
+    class Inner(val x: String)
+
+    @Test
+    fun testIgnoresKeyWhenGlobalSettingNotSet() = parametrizedTest { mode ->
+        val jsonString = """{"a":1,"inner":{"x":"value","unknownKey":"unknownValue"}}"""
+        val result = default.decodeFromString<Outer>(jsonString, mode)
+        assertEquals(1, result.a)
+        assertEquals("value", result.inner.x)
+    }
+
+    @Test
+    fun testThrowsWithoutAnnotationWhenGlobalSettingNotSet() = parametrizedTest { mode ->
+        val jsonString = """{"a":1,"inner":{"x":"value","unknownKey":"unknownValue"}, "b":2}"""
+        checkSerializationException({
+            default.decodeFromString<Outer>(jsonString, mode)
+        }) { msg ->
+            assertContains(
+                msg,
+                if (mode == JsonTestingMode.TREE) "Encountered an unknown key 'b' at element: \$\n"
+                else "Encountered an unknown key 'b' at offset 59 at path: \$\n"
+            )
+        }
+    }
+
+    @Test
+    fun testIgnoresBothKeysWithGlobalSetting() = parametrizedTest { mode ->
+        val jsonString = """{"a":1,"inner":{"x":"value","unknownKey":"unknownValue"}, "b":2}"""
+        val result = ignoresKeys.decodeFromString<Outer>(jsonString, mode)
+        assertEquals(1, result.a)
+        assertEquals("value", result.inner.x)
+    }
+}

formats/json/api/kotlinx-serialization-json.api

@@ -262,6 +262,13 @@ public final class kotlinx/serialization/json/JsonEncoder$DefaultImpls {
 	public static fun shouldEncodeElementDefault (Lkotlinx/serialization/json/JsonEncoder;Lkotlinx/serialization/descriptors/SerialDescriptor;I)Z
 }
 
+public abstract interface annotation class kotlinx/serialization/json/JsonIgnoreUnknownKeys : java/lang/annotation/Annotation {
+}
+
+public synthetic class kotlinx/serialization/json/JsonIgnoreUnknownKeys$Impl : kotlinx/serialization/json/JsonIgnoreUnknownKeys {
+	public fun <init> ()V
+}
+
 public final class kotlinx/serialization/json/JsonKt {
 	public static final fun Json (Lkotlinx/serialization/json/Json;Lkotlin/jvm/functions/Function1;)Lkotlinx/serialization/json/Json;
 	public static synthetic fun Json$default (Lkotlinx/serialization/json/Json;Lkotlin/jvm/functions/Function1;ILjava/lang/Object;)Lkotlinx/serialization/json/Json;