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Kotson: Gson for Kotlin

Kotson enables you to parse and write JSON with Google’s Gson using a conciser and easier syntax.

Kotson is a set of extension functions, meaning that it adds utility functions and syntactic sugars to Gson in Kotlin. It does not add new features to Gson nor does it creates new types. It is therefore usable on any Gson object, whether created from java or kotlin in source or library.

Install

Maven:

<dependency>
	<groupId>com.github.salomonbrys.kotson</groupId>
	<artifactId>kotson</artifactId>
	<version>2.5.0</version>
</dependency>

Gradle:

compile 'com.github.salomonbrys.kotson:kotson:2.5.0'

Table Of Contents

Usage

Creating Json

JsonObject:

import com.github.salomonbrys.kotson.*

val obj: JsonObject = jsonObject(
    "name" to "kotson",
    "creation" to Date().getTime(),
    "files" to 4
)

JsonArray:

import com.github.salomonbrys.kotson.*

val arr: JsonArray = jsonArray("one", "two", 42, 21.5)

Of course, a combination of both:

import com.github.salomonbrys.kotson.*

val obj: JsonObject = jsonObject(
    "property" to "value",
    "array" to jsonArray(
        21,
        "fourty-two",
        jsonObject("go" to "hell")
    )
)

JsonPrimitives:

import com.github.salomonbrys.kotson.*

val pi = 42.toJson()         // java: new JsonPrimitive(42);
val pf = 42.21f.toJson()     // java: new JsonPrimitive(42.21f);
val pd = 42.21.toJson()      // java: new JsonPrimitive(42.21d);
val pc = 'c'.toJson()        // java: new JsonPrimitive('c');
val pz = true.toJson()       // java: new JsonPrimitive(true);
val os = "coucou".toJson()   // java: new JsonPrimitive("coucou");

Setting custom (de)serializers

If you need to register a serializer / deserializer / InstanceCreator, you can use these “builder” APIs:

import com.github.salomonbrys.kotson.*

val gson = GsonBuilder()
    .registerTypeAdapter<MyType> {
    
        serialize {
            /*
                it.type: Type to serialize from
                it.context: GSon context
                it.src : Object to serialize
            */
        }
        
        deserialize {
            /*
                it.type: Type to deserialize to
                it.context: GSon context
                it.json : JsonElement to deserialize from
            */
        }
        
        createInstances {
            /*
                it: Type of instance to create
            */
        }
    
    }
    .registerTypeHierarchyAdapter<MyOtherType> {
        serialize { /* Same a above */ }
        deserialize { /* Same a above */ }
        createInstances { /* Same a above */ }
    }
    .create()

Of course, you can declare only a serializer, a deserializer or an instance creator. You don’t have to declare all three.

You don’t have to declare all your JsonSerializers, JsonDeserializers and InstanceCreators where you initialize Gson. Kotson allows you to create those type adapters in different files using the functions jsonSerializer, jsonDeserializer and instanceCreator. You will then be able to register those when creating the Gson object:

//TypeAdapters.kt
import com.github.salomonbrys.kotson.*

val personSerializer = jsonSerializer { /* Same arguments as before */ }
//Main.kt
import com.github.salomonbrys.kotson.*

val gson = GsonBuilder().registerTypeAdapter<Person>(personSerializer).create()

Setting custom Readers and Writers

Gson has another API (named Stream API) that allows to register writers (to JsonWriter) and readers (from JsonReader).
Here is an example for a simple Person class:

import com.github.salomonbrys.kotson.*

val gson = GsonBuilder()
    .registerTypeAdapter<Person> {
    
        write {
            beginArray()
            value(it.name)
            value(it.age)
            endArray()
        }
        
        read {
            beginArray()
            val name = nextString()
            val age = nextInt()
            endArray()
    
            Person(name, age)
        }
    
    }
    .create()

While a bit more complex and difficult to handle, this API is also better optimized. So if you’re after performance, I recommend you use this API.

Using this API has a few drawbacks:

  • You must define both write and read
  • You cannot use a mix (serialize + read or write + deserialize).
  • In read, you cannot access the exact object type that you are deserializing to (the it.type of deserialize).

If you wish to register a nullable reader or writer, you can use registerNullableTypeAdapter instead.

You don’t have to declare all your TypeAdapters where you initialize Gson. Kotson allows you to create those type adapters in different files using the functions typeAdapter and nullableTypeAdapter. You will then be able to register those when creating the Gson object:

//TypeAdapters.kt
import com.github.salomonbrys.kotson.*

val personTypeAdapter = typeAdapter<Person> {
    write { /*...*/ }
    read { /*...*/ }
}
//Main.kt
import com.github.salomonbrys.kotson.*

val gson = GsonBuilder().registerTypeAdapter<Person>(personSerializer).create()

Kotson provides no utility function for the TypeAdapterFactory interface. Because this interface defines a generic function, there is currently no other way to use it other than implementing it on a regular object or class.

Parsing JSON

Kotson provides a simple API that does not suffer from Java’s type erasure. That means that whatever the output type, it will be parsed correctly and eliminates the need for TypeToken.

import com.github.salomonbrys.kotson.*

val gson = Gson()

// java: List<User> list = gson.fromJson(src, new TypeToken<List<User>>(){}.getType());
val list1 = gson.fromJson<List<User>>(jsonString)
val list2 = gson.fromJson<List<User>>(jsonElement)
val list3 = gson.fromJson<List<User>>(jsonReader)
val list4 = gson.fromJson<List<User>>(reader)

Attention: gson.fromJson<MyType> will return a non-nullable type whereas gson.fromJson<MyType?> will return a nullable type. Therefore the code gson.fromJson<MyType>("null") is correct and will throw a null-pointer exception!

A lot of Gson’s APIs are relying on java.lang.reflect.Type to specify a type, but Kotlin’s javaClass returns a java.lang.Class which is a Type but suffers from type erasure. To mediate this issue, Gson uses TypeToken to create java.lang.reflect.Type objects without type erasure. If you need such a Type object, you can simply use the typeToken function the same way you use the javaClass function. For example: typeToken<Map<String, List<User>>>()

Careful: the typeToken function behaves slightly differently then Gson’s TypeToken class. When providing a non-specialized generic type, typeToken<List<*>> will return Class<List> (while Gson’s mechanism will return a ParameterizedType). If you really need a ParameterizedType for a non-specialized generic type, you can use the gsonTypeToken function.

Browsing Json Elements

Kotson allows you to simply convert a jsonElement to a primitive, a JsonObject or a JsonArray:

import com.github.salomonbrys.kotson.*

val s = json.string // java: String s = json.getAsString();
val i = json.int    // java: int i = json.getAsInt();
val a = json.array  // java: JsonArray = json.getAsJsonArray();
val o = json.obj    // java: JsonObject = json.getAsJsonObject();

val ns = json.nullString // java: String s = json.isJsonNull() ? null : json.getAsString();
val ni = json.nullInt    // java: Integer i = json.isJsonNull() ? null : json.getAsInt();
val na = json.nullArray  // java: JsonArray = json.isJsonNull() ? null : json.getAsJsonArray();
val no = json.nullObj    // java: JsonObject = json.isJsonNull() ? null : json.getAsJsonObject();

The same APIs exist for .string, .bool, .byte, .char, .short, .int, .long, .float, .double, .number, .bigInteger, .bigDecimal, .array, .obj

Kotson provides a simple API that allows you to easily browse JsonElement, JsonObject and JsonArray:

import com.github.salomonbrys.kotson.*

// java: JsonElement components = colors.getAsJsonObject().get("orange");
val components = colors["orange"]

// java: JsonElement greenComp = components.getAsJsonArray().get(1);
val greenComp = components[1]


// java: int greenComp = json .getAsJsonObject()
//                            .getAsJsonObject("colors")
//                            .getAsJsonArray("orange")
//                            .get(1)
//                            .getAsInt();

val greenComp = json["colors"]["orange"][1].int

Mutating Json Elements

Kotson allows you to mutate a JsonObject or a JsonArray:

import com.github.salomonbrys.kotson.*

fun test() {
    val array = jsonArray("zero", "x", "two")
    array[1] = "one"
    array += "three"
    array -= "zero"

    val obj = jsonObject()
    obj["this"] = "that"
    obj += "answer" to 42
    obj -= "this"
}

Copying Json Elements

Kotson allows you to make a shallow copy (single-level copy) or a deep copy (recursive copy) of a JsonObject or a JsonArray:

import com.github.salomonbrys.kotson.*

val shallow = json.shallowCopy()
val deep = json.deepCopy()

Accessing object fields via property delegates

Kotson allows you to delegate properties to JsonObject fields:

import com.github.salomonbrys.kotson.*

class Person(public val obj: JsonObject) {
    val id: String by obj.byString               // Maps to obj["id"]
    val name: String by obj.byString("fullName") // Maps to obj["fullName"]
    val birthDate: Int by obj["dates"].byInt(0)  // Maps to obj["dates"][0]
}

Let’s talk!

You’ve read so far ?! You’re awsome! Why don’t you drop by the Kotson Slack channel on Kotlin’s Slack group?

Donate

Kotson is free to use for both non-profit and commercial use and always will be.

If you wish to show some support or appreciation to my work, you are free to donate!

This would be (of course) greatly appreciated but is by no means necessary to receive help or support, which I’ll be happy to provide for free :)

compile "com.github.salomonbrys.kotson:kotson:2.5.0"

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