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Generate TypeScript definitions from Kotlin/Java/JVM classes

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TypeScript definition generator for the JVM

This library generates TypeScript definitions that cover a set of Kotlin and Java classes using Kotlin reflection.

TypeScript definitions are useful when data classes are serialized to JSON and handled in a JavaScript or TypeScript web frontend as they enable context-aware type checking and autocompletion in a number of IDEs and editors.

ts-generator supports:

  • Primitive types, with or without explicit int.
  • Kotlin and Java classes.
  • Data classes.
  • Enums.
  • Any type.
  • Generic classes, without type erasure.
  • Generic constraints.
  • Class inheritance.
  • Abstract classes.
  • Lists as JS arrays.
  • Maps as JS objects.
  • Null safety, even inside composite types.
  • Java beans.
  • Mapping types.
  • Nullability annotations, when allowed by the retention policy.
  • Customizing class definitions via transformers.
  • Parenthesis optimization: They are placed only when they are needed to disambiguate.
  • Emitting either null or undefined for JVM nullable types.

Installation

ts-generator requires Kotlin 1.1. Kotlin 1.0 is not compatible as its reflection library is not powerful enough to do this transformation.

Then you need to include this library in your project. The easiest way is to download it from JitPack. For instance, in Gradle you would add this to build.gradle:

repositories {
    maven { url 'https://jitpack.io' }
}

dependencies {
    compile 'com.github.ntrrgc:ts-generator:1.1.1'
}

Basic usage

The first you need is your Kotlin or Java classes or interfaces, for instance:

enum class Rarity(val abbreviation: String) {
    Normal("N"),
    Rare("R"),
    SuperRare("SR"),
}

data class Card(
    val ref: String,
    val rarity: Rarity,
    val name: String,
    val description: String,
    val command: String?,
    val playCard: (() -> Unit)?
) {
    val generatedTitleLine = "*$name* [$rarity]"
}

data class Inventory(
    val cards: List<Card> = listOf()
)

data class Player(
    val name: String,
    val inventory: Inventory = Inventory(),
    val achievementsProgress: List<AchievementCompletionState> = listOf(),
    val notices: List<Notice> = listOf()
)

data class Notice(
    val dateTime: LocalDateTime,
    val text: String
)

data class Achievement(
    val ref: String,
    val title: String,
    val description: String,
    val measuredProperty: (player: Player) -> Int,
    val neededValue: Int
)

data class AchievementCompletionState(
    val achievementRef: String,
    val reachedValue: Int
)

Then use TypeScriptGenerator to generate the TypeScript definitions, like this:

fun main(args: Array<String>) {
    println(TypeScriptGenerator(
        rootClasses = setOf(
            Player::class
        ),
        mappings = mapOf(
            LocalDateTime::class to "Date",
            LocalDate::class to "Date"
        )
    ).definitionsText)
}

You will get an output like this:

interface AchievementCompletionState {
    achievementRef: string;
    reachedValue: number;
}

type Rarity = "Normal" | "Rare" | "SuperRare";

interface Card {
    command: string | null;
    description: string;
    generatedTitleLine: string;
    name: string;
    rarity: Rarity;
    ref: string;
}

interface Inventory {
    cards: Card[];
}

interface Notice {
    dateTime: Date;
    text: string;
}

interface Player {
    achievementsProgress: AchievementCompletionState[];
    inventory: Inventory;
    name: string;
    notices: Notice[];
}

Then you can paste it into a .d.ts file and declare it in your environment, e.g:

Advanced features

This generator can handle more complex data types. Some examples are shown below:

Mapping types

Sometimes you want to map certain Kotlin or Java classes to native JS types, like Date.

This can be done with the mappings argument of TypeScriptGenerator, as show in the first example.

Note the types mapped with this feature are emitted as they were written without any further processing. This is intended to support native JS types not defined in the Kotlin or Java backend.

Int type

Currently TypeScript only supports one number type: number.

This may change if a proposal for int types succeeds. Also, some people may want to be extra explicit and do:

type int = number;

In order to be able to document if a type may or may not be integer. In any case, you can instruct TypeScriptGenerator to use explicit int with the intTypeName parameter. For instance:

fun main(args: Array<String>) {
    println(TypeScriptGenerator(
        rootClasses = setOf(
            AchievementCompletionState::class
        ),
        intTypeName = "int"
    ).definitionsText)
}

The output will be:

interface AchievementCompletionState {
    achievementRef: string;
    reachedValue: int;
}

Inheritance support

open class BaseClass(val a: Int)

class DerivedClass(val b: List<String>): BaseClass(4)

fun main(args: Array<String>) {
    println(TypeScriptGenerator(
        rootClasses = setOf(
            DerivedClass::class
        )
    ).definitionsText)
}

The output is:

interface BaseClass {
    a: number;
}

interface DerivedClass extends BaseClass {
    b: string[];
}

By default Serializable and Comparable interfaces are not emitted. You can filter out more interfaces or classes by using the ignoreSuperclasses parameter of the TypeScriptGenerator constructor.

Generics

class ContrivedExample<A, out B, out C: List<Any>>(
    private val a: A, 
    val b: B, 
    val c: C,
    val listOfPairs: List<Pair<Int, B>>)
    
fun main(args: Array<String>) {
    println(TypeScriptGenerator(
        rootClasses = setOf(
            ContrivedExample::class
        )
    ).definitionsText)
}

The output is:

interface Pair<A, B> {
    first: A;
    second: B;
}

interface ContrivedExample<A, B, C extends any[]> {
    b: B;
    c: C;
    listOfPairs: Pair<number, B>[];
}

Maps as JS objects

data class CardRepository(
    val cardsByRef: Map<String, Card>)

The output is:

type Rarity = "Normal" | "Rare" | "SuperRare";

interface Card {
    command: string | null;
    description: string;
    generatedTitleLine: string;
    name: string;
    rarity: Rarity;
    ref: string;
}

interface CardRepository {
    cardsByRef: { [key: string]: Card };
}

Java beans

Sometimes you want to work with long boring Java classes like this one:

public class JavaClass {
    private String name;
    private int[] results;
    private boolean finished;
    private char[][] multidimensional;

    public String getName() { return name; }
    public void setName(String name) { this.name = name; }

    public int[] getResults() { return results; }
    public void setResults(int[] results) { this.results = results; }

    // setters are not required for this to work!
    public boolean isFinished() { return finished; }

    public char[][] getMultidimensional() { return multidimensional; }
    public void setMultidimensional(char[][] multidimensional) { 
        this.multidimensional = multidimensional; 
    }
}

Even though its fields are private, they are accessible through getter methods. The generator knows this, so they are included in the definition:

interface JavaClass {
    name: string;
    results: number[];
    multidimensional: string[][];
    finished: boolean;
}

Java nullability annotations

Kotlin was designed with null-safety in mind, but the Java land is not so green.

In Java all types are nullable by default, so the programmer needs some way to annotate which may and which will never be null. There are many ways to do this, each with its own set of drawbacks.

The TypeScript generator makes no effort by itself to infer the nullability of Java types. Nevertheless kotlin-reflect is capable of decoding it if the classes are annotated with JSR305 annotations (javax.annotation.*). If no annotations are found, the types are assumed to be not null.

Note that org.jetbrains.annotations.* and android.support.annotation.* cannot work for this purpose, as they don't have runtime retention and therefore are stripped by the compiler without leaving a way to read them through reflection.

The following an example of a class with supported annotations:

import javax.annotation.Nullable;
import javax.annotation.ParametersAreNonnullByDefault;

// Add this to Gradle/Maven to get the annotations:
// compile 'com.google.code.findbugs:jsr305:3.0.1'

@ParametersAreNonnullByDefault
public class JavaClassWithNonnullAsDefault {
    private int[] results;

    @Nullable
    private int[] nextResults;

    JavaClassWithNonnullAsDefault(
        int[] results, 
        @Nullable int[] nextResults)
    {
        this.results = results;
        this.nextResults = nextResults;
    }

    public int[] getResults() { return results; }
    public void setResults(int[] results) { this.results = results; }

    @Nullable
    public int[] getNextResults() { return nextResults; }
    public void setNextResults(@Nullable int[] nextResults) {
        this.nextResults = nextResults;
    }
}

The output is the following:

interface JavaClassWithNonnullAsDefault {
    name: string;
    results: number[];
    nextResults: number[] | null;
}

Transformers

Sometimes they objects you use in TypeScript or JavaScript are not exactly the same you use in your backend, but have some differences, for instance:

  • You may transform one type into another.
  • Your classes may use camelCase in the backend but being turned into snake_case in the frontend by the JSON serializer.
  • Some properties of some classes may be not be sent to the frontend.

To support cases like these, TypeScriptGenerator supports class transformers. They are objects implementing the ClassTransformer interface, arranged in a pipeline. They can be used to customize the list of properties of a class and their name and type.

Below are some examples:

Filtering unwanted properties

In the following example, assume we don't want to emit ref:

data class Achievement(
    val ref: String,
    val title: String,
    val description: String,
    val measuredProperty: (player: Player) -> Int,
    val neededValue: Int
)

We can use the transformPropertyList() to remove it.

fun main(args: Array<String>) {
    println(TypeScriptGenerator(
        rootClasses = setOf(
            Achievement::class
        ),
        classTransformers = listOf(
            object : ClassTransformer {
                override fun transformPropertyList(
                    properties: List<KProperty<*>>,
                    klass: KClass<*>
                ): List<KProperty<*>> {
                    return properties.filter { property ->
                        property.name != "ref"
                    }
                }
            }
        )
    ).definitionsText)
}

The output is:

interface Achievement {
    description: string;
    neededValue: number;
    title: string;
}

Renaming to snake_case

You can use transformPropertyName() to rename any property.

The functions camelCaseToSnakeCase() and snakeCaseToCamelCase() are included in this library.

data class AchievementCompletionState(
    val achievementRef: String,
    val reachedValue: Int)

fun main(args: Array<String>) {
    println(TypeScriptGenerator(
        rootClasses = setOf(
            AchievementCompletionState::class
        ),
        classTransformers = listOf(
            object : ClassTransformer {
                override fun transformPropertyName(
                    propertyName: String,
                    property: KProperty<*>,
                    klass: KClass<*>
                ): String {
                    return camelCaseToSnakeCase(propertyName)
                }
            }
        )
    ).definitionsText)
}

The output is:

interface AchievementCompletionState {
    achievement_ref: string;
    reached_value: number;
}

Replacing types for some properties

Imagine in our previous example we don't want to emit achievement_ref with type string, but rather achievement, with type Achievement.

We can use a combination of transformPropertyName() and transformPropertyType() for this purpose:

fun main(args: Array<String>) {
    println(TypeScriptGenerator(
        rootClasses = setOf(
            AchievementCompletionState::class
        ),
        classTransformers = listOf(
            object : ClassTransformer {
                override fun transformPropertyName(
                    propertyName: String, 
                    property: KProperty<*>, 
                    klass: KClass<*>
                ): String {
                    if (propertyName == "achievementRef") {
                        return "achievement"
                    } else {
                        return propertyName
                    }
                }

                override fun transformPropertyType(
                    type: KType, 
                    property: KProperty<*>, 
                    klass: KClass<*>
                ): KType {
                    // Note: property is the actual property from the class
                    // (unless replaced in transformPropertyList()), so
                    // it maintains the original property name declared
                    // in the code.
                    if (property.name == "achievementRef") {
                        return Achievement::class.createType(nullable = false)
                    } else {
                        return type
                    }
                }
            }
        )
    ).definitionsText)
}

The output is:

interface Achievement {
    description: string;
    neededValue: number;
    ref: string;
    title: string;
}

interface AchievementCompletionState {
    achievement: Achievement;
    reachedValue: number;
}

Note how Achievement class is emitted recursively after the transformation has taken place, even though it was not declared in the original AchievementCompletionState class nor specified in rootClasses.

Applying transformers only to some classes

Transformers are applied to all classes by default. If you want your transformers to apply only to classes matching a certain predicate, you can wrap them in an instance of FilteredClassTransformer. This is its definition:

class FilteredClassTransformer(
   val wrappedTransformer: ClassTransformer,
   val filter: (klass: KClass<*>) -> Boolean
): ClassTransformer

For the common case of applying a transformer only on a class and its subclasses if any, an extension method is provided, .onlyOnSubclassesOf():

fun main(args: Array<String>) {
    println(TypeScriptGenerator(
        rootClasses = setOf(
            Achievement::class
        ),
        classTransformers = listOf(
            object : ClassTransformer {
                override fun transformPropertyList(
                    properties: List<KProperty<*>>,
                    klass: KClass<*>
                ): List<KProperty<*>> {
                    return properties.filter { property ->
                        property.name != "ref"
                    }
                }
            }.onlyOnSubclassesOf(Achievement::class)
        )
    ).definitionsText)
}

Optional<T> unwrapping

This is an example of a more complex transformer that can be used to unwrap Optional<T> into T | null.

Let's suppose a Java class like this:

public class JavaClassWithOptional {
    private String name;
    private String surname;

    public Optional<String> getSurname() {
        return Optional.ofNullable(surname);
    }

    public String getName() {
        return name;
    }
}

We could use this transformer:

object : ClassTransformer {
    override fun transformPropertyType(
        type: KType,
        property: KProperty<*>,
        klass: KClass<*>
    ): KType {
        val bean = Introspector.getBeanInfo(klass.java)
            .propertyDescriptors
            .find { it.name == property.name }

        val getterReturnType = bean?.readMethod?.kotlinFunction?.returnType
        if (getterReturnType?.classifier == Optional::class) {
            val wrappedType = getterReturnType.arguments.first().type!!
            return wrappedType.withNullability(true)
        } else {
            return type
        }
    }
}

The result would be this:

interface JavaClassWithOptional {
    name: string;
    surname: string | null;
}

License

Copyright 2017 Alicia Boya García

Licensed under the Apache License, Version 2.0 (the "License");
you may not use this file except in compliance with the License.
You may obtain a copy of the License at

   http://www.apache.org/licenses/LICENSE-2.0

Unless required by applicable law or agreed to in writing, software
distributed under the License is distributed on an "AS IS" BASIS,
WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
See the License for the specific language governing permissions and
limitations under the License.

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