Add documentation for JsonNamingStrategy to the Json guide (#2305)

to increase adoption.

Fixes #2206

Co-authored-by: Vsevolod Tolstopyatov <qwwdfsad@gmail.com>

docs/json.md

@@ -20,6 +20,7 @@ In this chapter, we'll walk through features of [JSON](https://www.json.org/json
   * [Allowing structured map keys](#allowing-structured-map-keys)
   * [Allowing special floating-point values](#allowing-special-floating-point-values)
   * [Class discriminator for polymorphism](#class-discriminator-for-polymorphism)
+  * [Global naming strategy](#global-naming-strategy)
 * [Json elements](#json-elements)
   * [Parsing to Json element](#parsing-to-json-element)
   * [Types of Json elements](#types-of-json-elements)
@@ -468,6 +469,53 @@ As you can see, discriminator from the `Base` class is used:
 
 <!--- TEST -->
 
+### Global naming strategy
+
+If properties' names in Json input are different from Kotlin ones, it is recommended to specify the name 
+for each property explicitly using [`@SerialName` annotation](basic-serialization.md#serial-field-names).
+However, there are certain situations where transformation should be applied to every serial name — such as migration
+from other frameworks or legacy codebase. For that cases, it is possible to specify a [namingStrategy][JsonBuilder.namingStrategy]
+for a [Json] instance. `kotlinx.serialization` provides one strategy implementation out of the box, the [JsonNamingStrategy.SnakeCase](https://kotlinlang.org/api/kotlinx.serialization/kotlinx-serialization-json/kotlinx.serialization.json/-json-naming-strategy/-builtins/-snake-case.html):
+
+```kotlin
+@Serializable
+data class Project(val projectName: String, val projectOwner: String)
+
+val format = Json { namingStrategy = JsonNamingStrategy.SnakeCase }
+
+fun main() {
+    val project = format.decodeFromString<Project>("""{"project_name":"kotlinx.coroutines", "project_owner":"Kotlin"}""")
+    println(format.encodeToString(project.copy(projectName = "kotlinx.serialization")))
+}
+```
+
+> You can get the full code [here](../guide/example/example-json-12.kt).
+
+As you can see, both serialization and deserialization work as if all serial names are transformed from camel case to snake case:
+
+```text
+{"project_name":"kotlinx.serialization","project_owner":"Kotlin"}
+```
+
+There are some caveats one should remember while dealing with a [JsonNamingStrategy]:
+
+* Due to the nature of the `kotlinx.serialization` framework, naming strategy transformation is applied to all properties regardless 
+of whether their serial name was taken from the property name or provided by [SerialName] annotation.
+Effectively, it means one cannot avoid transformation by explicitly specifying the serial name. To be able to deserialize
+non-transformed names, [JsonNames] annotation can be used instead.
+
+* Collision of the transformed name with any other (transformed) properties serial names or any alternative names
+specified with [JsonNames] will lead to a deserialization exception.
+
+* Global naming strategies are very implicit: by looking only at the definition of the class,
+it is impossible to determine which names it will have in the serialized form.
+As a consequence, naming strategies are not friendly to actions like Find Usages/Rename in IDE, full-text search by grep, etc.
+For them, the original name and the transformed are two different things;
+changing one without the other may introduce bugs in many unexpected ways and lead to greater maintenance efforts for code with global naming strategies.
+
+Therefore, one should carefully weigh the pros and cons before considering adding global naming strategies to an application.
+
+<!--- TEST -->
 
 ## Json elements
 
@@ -493,7 +541,7 @@ 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).
 
 A `JsonElement` prints itself as a valid JSON:
 
@@ -536,7 +584,7 @@ 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).
 
 The above example sums `votes` in all objects in the `forks` array, ignoring the objects that have no `votes`:
 
@@ -576,7 +624,7 @@ 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).
 
 As a result, you get a proper JSON string:
 
@@ -605,7 +653,7 @@ 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).
 
 The result is exactly what you would expect:
 
@@ -651,7 +699,7 @@ 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).
 
 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.
@@ -691,7 +739,7 @@ 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).
 
 `pi_literal` now accurately matches the value defined.
 
@@ -731,7 +779,7 @@ 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 exact value of `pi` is decoded, with all 30 decimal places of precision that were in the source JSON.
 
@@ -753,7 +801,7 @@ 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).
 
 ```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
@@ -829,7 +877,7 @@ 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 output shows that both cases are correctly deserialized into a Kotlin [List].
 
@@ -881,7 +929,7 @@ 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).
 
 You end up with a single JSON object, not an array with one element:
 
@@ -926,7 +974,7 @@ 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).
 
 See the effect of the custom serializer:
 
@@ -999,7 +1047,7 @@ 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).
 
 No class discriminator is added in the JSON output:
 
@@ -1095,7 +1143,7 @@ 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).
 
 This gives you fine-grained control on the representation of the `Response` class in the JSON output:
 
@@ -1160,7 +1208,7 @@ 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).
 
 ```text
 UnknownProject(name=example, details={"type":"unknown","maintainer":"Unknown","license":"Apache 2.0"})
@@ -1214,6 +1262,8 @@ The next chapter covers [Alternative and custom formats (experimental)](formats.
 [JsonBuilder.allowSpecialFloatingPointValues]: https://kotlinlang.org/api/kotlinx.serialization/kotlinx-serialization-json/kotlinx.serialization.json/-json-builder/allow-special-floating-point-values.html
 [JsonBuilder.classDiscriminator]: https://kotlinlang.org/api/kotlinx.serialization/kotlinx-serialization-json/kotlinx.serialization.json/-json-builder/class-discriminator.html
 [JsonClassDiscriminator]: https://kotlinlang.org/api/kotlinx.serialization/kotlinx-serialization-json/kotlinx.serialization.json/-json-class-discriminator/index.html
+[JsonBuilder.namingStrategy]: https://kotlinlang.org/api/kotlinx.serialization/kotlinx-serialization-json/kotlinx.serialization.json/-json-builder/naming-strategy.html
+[JsonNamingStrategy]: https://kotlinlang.org/api/kotlinx.serialization/kotlinx-serialization-json/kotlinx.serialization.json/-json-naming-strategy/index.html
 [JsonElement]: https://kotlinlang.org/api/kotlinx.serialization/kotlinx-serialization-json/kotlinx.serialization.json/-json-element/index.html
 [Json.parseToJsonElement]: https://kotlinlang.org/api/kotlinx.serialization/kotlinx-serialization-json/kotlinx.serialization.json/-json/parse-to-json-element.html
 [JsonPrimitive]: https://kotlinlang.org/api/kotlinx.serialization/kotlinx-serialization-json/kotlinx.serialization.json/-json-primitive/index.html

docs/serialization-guide.md

@@ -119,6 +119,7 @@ Once the project is set up, we can start serializing some classes.
   * <a name='allowing-structured-map-keys'></a>[Allowing structured map keys](json.md#allowing-structured-map-keys)
   * <a name='allowing-special-floating-point-values'></a>[Allowing special floating-point values](json.md#allowing-special-floating-point-values)
   * <a name='class-discriminator-for-polymorphism'></a>[Class discriminator for polymorphism](json.md#class-discriminator-for-polymorphism)
+  * <a name='global-naming-strategy'></a>[Global naming strategy](json.md#global-naming-strategy)
 * <a name='json-elements'></a>[Json elements](json.md#json-elements)
   * <a name='parsing-to-json-element'></a>[Parsing to Json element](json.md#parsing-to-json-element)
   * <a name='types-of-json-elements'></a>[Types of Json elements](json.md#types-of-json-elements)

guide/example/example-json-12.kt

@@ -4,9 +4,12 @@ package example.exampleJson12
 import kotlinx.serialization.*
 import kotlinx.serialization.json.*
 
+@Serializable
+data class Project(val projectName: String, val projectOwner: String)
+
+val format = Json { namingStrategy = JsonNamingStrategy.SnakeCase }
+
 fun main() {
-    val element = Json.parseToJsonElement("""
-        {"name":"kotlinx.serialization","language":"Kotlin"}
-    """)
-    println(element)
+    val project = format.decodeFromString<Project>("""{"project_name":"kotlinx.coroutines", "project_owner":"Kotlin"}""")
+    println(format.encodeToString(project.copy(projectName = "kotlinx.serialization")))
 }

guide/example/example-json-13.kt

@@ -6,13 +6,7 @@ import kotlinx.serialization.json.*
 
 fun main() {
     val element = Json.parseToJsonElement("""
-        {
-            "name": "kotlinx.serialization",
-            "forks": [{"votes": 42}, {"votes": 9000}, {}]
-        }
+        {"name":"kotlinx.serialization","language":"Kotlin"}
     """)
-    val sum = element
-        .jsonObject["forks"]!!
-        .jsonArray.sumOf { it.jsonObject["votes"]?.jsonPrimitive?.int ?: 0 }
-    println(sum)
+    println(element)
 }

guide/example/example-json-14.kt

@@ -5,19 +5,14 @@ import kotlinx.serialization.*
 import kotlinx.serialization.json.*
 
 fun main() {
-    val element = buildJsonObject {
-        put("name", "kotlinx.serialization")
-        putJsonObject("owner") {
-            put("name", "kotlin")
+    val element = Json.parseToJsonElement("""
+        {
+            "name": "kotlinx.serialization",
+            "forks": [{"votes": 42}, {"votes": 9000}, {}]
         }
-        putJsonArray("forks") {
-            addJsonObject {
-                put("votes", 42)
-            }
-            addJsonObject {
-                put("votes", 9000)
-            }
-        }
-    }
-    println(element)
+    """)
+    val sum = element
+        .jsonObject["forks"]!!
+        .jsonArray.sumOf { it.jsonObject["votes"]?.jsonPrimitive?.int ?: 0 }
+    println(sum)
 }

guide/example/example-json-15.kt

@@ -4,14 +4,20 @@ package example.exampleJson15
 import kotlinx.serialization.*
 import kotlinx.serialization.json.*
 
-@Serializable
-data class Project(val name: String, val language: String)
-
 fun main() {
     val element = buildJsonObject {
         put("name", "kotlinx.serialization")
-        put("language", "Kotlin")
+        putJsonObject("owner") {
+            put("name", "kotlin")
+        }
+        putJsonArray("forks") {
+            addJsonObject {
+                put("votes", 42)
+            }
+            addJsonObject {
+                put("votes", 9000)
+            }
+        }
     }
-    val data = Json.decodeFromJsonElement<Project>(element)
-    println(data)
+    println(element)
 }

guide/example/example-json-16.kt

@@ -4,20 +4,14 @@ package example.exampleJson16
 import kotlinx.serialization.*
 import kotlinx.serialization.json.*
 
-import java.math.BigDecimal
-
-val format = Json { prettyPrint = true }
+@Serializable
+data class Project(val name: String, val language: String)
 
 fun main() {
-    val pi = BigDecimal("3.141592653589793238462643383279")
-
-    val piJsonDouble = JsonPrimitive(pi.toDouble())
-    val piJsonString = JsonPrimitive(pi.toString())
-
-    val piObject = buildJsonObject {
-        put("pi_double", piJsonDouble)
-        put("pi_string", piJsonString)
+    val element = buildJsonObject {
+        put("name", "kotlinx.serialization")
+        put("language", "Kotlin")
     }
-
-    println(format.encodeToString(piObject))
+    val data = Json.decodeFromJsonElement<Project>(element)
+    println(data)
 }

guide/example/example-json-17.kt

@@ -10,15 +10,11 @@ val format = Json { prettyPrint = true }
 
 fun main() {
     val pi = BigDecimal("3.141592653589793238462643383279")
-
-    // use JsonUnquotedLiteral to encode raw JSON content
-    val piJsonLiteral = JsonUnquotedLiteral(pi.toString())
-
+
     val piJsonDouble = JsonPrimitive(pi.toDouble())
     val piJsonString = JsonPrimitive(pi.toString())
 
     val piObject = buildJsonObject {
-        put("pi_literal", piJsonLiteral)
         put("pi_double", piJsonDouble)
         put("pi_string", piJsonString)
     }

guide/example/example-json-18.kt

@@ -6,18 +6,22 @@ import kotlinx.serialization.json.*
 
 import java.math.BigDecimal
 
+val format = Json { prettyPrint = true }
+
 fun main() {
-    val piObjectJson = """
-          {
-              "pi_literal": 3.141592653589793238462643383279
-          }
-      """.trimIndent()
-
-    val piObject: JsonObject = Json.decodeFromString(piObjectJson)
-
-    val piJsonLiteral = piObject["pi_literal"]!!.jsonPrimitive.content
-
-    val pi = BigDecimal(piJsonLiteral)
-
-    println(pi)
+    val pi = BigDecimal("3.141592653589793238462643383279")
+
+    // use JsonUnquotedLiteral to encode raw JSON content
+    val piJsonLiteral = JsonUnquotedLiteral(pi.toString())
+
+    val piJsonDouble = JsonPrimitive(pi.toDouble())
+    val piJsonString = JsonPrimitive(pi.toString())
+
+    val piObject = buildJsonObject {
+        put("pi_literal", piJsonLiteral)
+        put("pi_double", piJsonDouble)
+        put("pi_string", piJsonString)
+    }
+
+    println(format.encodeToString(piObject))
 }

guide/example/example-json-19.kt

@@ -4,7 +4,20 @@ package example.exampleJson19
 import kotlinx.serialization.*
 import kotlinx.serialization.json.*
 
+import java.math.BigDecimal
+
 fun main() {
-    // caution: creating null with JsonUnquotedLiteral will cause an exception! 
-    JsonUnquotedLiteral("null")
+    val piObjectJson = """
+          {
+              "pi_literal": 3.141592653589793238462643383279
+          }
+      """.trimIndent()
+
+    val piObject: JsonObject = Json.decodeFromString(piObjectJson)
+
+    val piJsonLiteral = piObject["pi_literal"]!!.jsonPrimitive.content
+
+    val pi = BigDecimal(piJsonLiteral)
+
+    println(pi)
 }