Generating Taxi from source

Instantiate Taxi from its original source

Java & Kotlin

You can generate Taxi models and services from Java and Kotlin.

Examples are shown in Kotlin throughout this guide, as it makes for shorter code snippets. However, the same approaches will work for Java too.

@DataType & @Namespace

data class Person(...)  // generates demo.Person

data class Person(...)  // generates demo.Person

@DataType is the main annotation to indicate that a class should have a Taxi schema generated.

Namespaces may be defined either through the @Namespace("foo.bar") annotation, or within a @DataType directly.

If the namespace isn't provided through either approach, the package name is used.

Fields on types are exported. By default, the corresponding primitive type in Taxi is used. However, the output type can be customized by specifying the type in a @DataType annotation on the field, which will result in a Type alias within Taxi being generates.

data class Client(@field:DataType("demo.ClientId") val clientId: String,
        val clientName: String,
        @field:DataType("isic.uk.SIC2008") val sicCode: String


// Generates:
namespace demo {
    type Client {
        clientId : ClientId as String
        clientName : String
        sicCode : isic.uk.SIC2008

namespace isic.uk {
    type alias SIC2008 as String

If you're working in Kotlin, using typealias with the @DataType annotation is recommended over annotating fields directly, as it leads to more readable & maintainable code.

Support for Kotlin Type Aliases

Type aliases can be directly annotated with the @DataType annotation.

package com.foo.bar

data class Bus(val passengers: Passengers)

typealias Passengers = List<Person>

data class Person(val name: PersonName)

typealias PersonName = String

val taxiDef = TaxiGenerator().forClasses(Bus::class.java).generateAsStrings()

// Generates:
model Bus {
    passengers : Passengers

model Person {
    name : PersonName

type PersonName inherits String
type alias Passengers as Person[]

If using the @DataType against a Kotlin type alias, you need to declare the package(s) at startup.

This is as simple as:

import lang.taxi.generators.kotlin.TypeAliasRegister


Any annotated type aliases in the provided package will be discovered and have their names generated correctly.


Constraints are supported via the @Constraint annotation.

data class Money(val amount: BigDecimal, val currency: String)

data class SomeRequest(
        @Constraint("currency == 'USD'")
        val amount: Money)

TaxiGenerator().forClasses(Money::class.java, SomeRequest::class.java).generateAsStrings()

// Generates:
namespace vyne {
     parameter type SomeRequest {
        amount : Money(currency == "USD")
     type Money {
        amount : Decimal
        currency : String

Parameter Type

Parameter types are indicated through the @ParameterType annotation.

Services & Operations

Services and Operations are exposed via the @Service and @Operation annotations respectively.

class MyService {
    fun findPerson(@DataType("taxi.example.PersonId") personId: String): Person {

TaxiGenerator().forClasses(MyService::class.java, Person::class.java).generateAsStrings()

// Generates:
namespace taxi.example {
    type Person {
        personId : PersonId as String
    service PersonService {
        operation findPerson(PersonId) : Person

Operations may customize the generated name of operation parameters through the @Parameter annotation:

fun findPerson(@Parameter(name = "personId") id: String): Person ...

Service constraints & contracts

Constraints on parameters are defined by adding constraints to the @Parameter annotation.

Contracts for operations are defined on the method, using the @ResponseContract annotation.

class MyService {
    @ResponseContract(basedOn = "source",
            constraints = ResponseConstraint("currency = targetCurrency")
    fun convertRates(@Parameter(constraints = Constraint("currency = 'GBP'")) source: Money, @Parameter(name = "targetCurrency") targetCurrency: String): Money {
        TODO("Not a real service")


// Generates
namespace taxi.example

type Money {
    currency : Currency as String
    value : MoneyAmount as Decimal
service MoneyService {
    operation convertRates( Money( currency = "GBP" ),
        targetCurrency : String ) : Money( from source, currency = targetCurrency )

Taxi Generator customisation & extensions

Taxi's generator can be customized, adding additional annotations or modifying the tree structure before source is output.

The most common scenario here is to provide extensions to modify how services are generated. However, you can inject an entirely separate TypeMapper and ServiceMapper for complete control over the generation process. Take a look at TaxiGenerator to get started.

ServiceMapperExtension and OperationMapperExtension provide the ability to modify the way services are generated, injecting custom metadata (eg., to describe HTTP operations, or how to subscribe to a message bus).

The SpringMVC extensions (discussed below) give a good example of how to leverage this.

Spring MVC Extensions

java2taxi ships with extensions for Spring MVC, to add metadata about how services are discovered and invoked. They add the following features:

  • Support for mapping @GetMapping/@PostMapping etc to an HttpOperation() annotation on the output.
    • All HTTP methods are supported
    • Path variable substitution is supported
  • Support for mapping springs @RequestBody annotation to a corresponding @RequestBody annotation on the generated Taxi source.
  • Support for integration with service discovery from Spring Cloud's DiscoveryClient abstraction
Currently, the SpringMVC extensions are part of java2taxi. However, we expect to move these out to their own library in a future release.

Enable the extensions by configuring the TaxiGenerator as follows:

val taxiGenerator = TaxiGenerator(serviceMapper = DefaultServiceMapper(
    operationExtensions = listOf(SpringMvcHttpOperationExtension()),
    serviceExtensions = listOf(SpringMvcHttpServiceExtension(ServiceDiscoveryAddressProvider("mockService")))

Example usage:

class CreditCostService {
    fun getInterestRate(@PathVariable("clientId") @DataType("vyne.demo.ClientId") clientId: String): BigDecimal = BigDecimal.ONE

    fun calculateCreditCosts(@PathVariable("clientId") @DataType("vyne.demo.ClientId") clientId: String, @RequestBody request: CreditCostRequest): CreditCostResponse = CreditCostResponse("TODO")


// Generates:
namespace vyne.demo {

    type alias ClientId as String

     type CreditCostRequest {
        deets : String

     type CreditCostResponse {
        stuff : String

    @ServiceDiscoveryClient(serviceName = "mockService")
    service CreditCostService {
        @HttpOperation(method = "GET" , url = "/costs/interestRates/{vyne.demo.ClientId}")
        operation getInterestRate(  ClientId ) : Decimal
        @HttpOperation(method = "POST" , url = "/costs/{vyne.demo.ClientId}/doCalculate")
        operation calculateCreditCosts(  ClientId, @RequestBody CreditCostRequest ) : CreditCostResponse

The tests give the best example of usage.

Maven / Gradle

The binaries are hosted by the Vyne team in their maven repository. Be sure to add the repository:


Then grab the artifacts:

<!-- To generate Taxi, you need java2taxi -->

<!-- To annotate your models, you need the annotations package -->

Swagger / OpenApi

It's possible to convert a swagger document to Taxi, using the TaxiGenerator from the openApi package.

This is especially useful if you want to take advantage of Taxi's type extensions, to mix in annotations. (eg., persistence or validation).

Grab the taxi generator from maven:


Then run the converter as follows:

This example loads source from this reference example from the OpenApi project.

// The actual swagger source is omitted for brevity
val source = IOUtils.toString(URI.create("https://gitlab.com/taxi-lang/taxi-lang/raw/master/swagger2taxi/src/test/resources/openApiSpec/v2.0/yaml/petstore-simple.yaml"))
TaxiGenerator().generateAsStrings(source, "vyne.openApi")

// generated:
namespace vyne.openApi  {

   model NewPet {
      name : String
      tag : String?

   model Pet inherits NewPet {
      id : Int

   model ErrorModel {
      code : Int
      message : String

   service PetsService {
      [[ Returns all pets from the system that the user has access to ]]
      @HttpOperation(method = "GET" , url = "http://petstore.swagger.io/api/pets")
      operation findPets(  tags : String,  limit : Int ) : Pet[]
      [[ Creates a new pet in the store.  Duplicates are allowed ]]
      @HttpOperation(method = "POST" , url = "http://petstore.swagger.io/api/pets")
      operation addPet( @RequestBody pet : NewPet ) : Pet
   service PetsIdService {
      [[ Returns a user based on a single ID, if the user does not have access to the pet ]]
      @HttpOperation(method = "GET" , url = "http://petstore.swagger.io/api/pets/{id}")
      operation findPetById( @PathVariable(value = "id") id : Int ) : Pet
      [[ deletes a single pet based on the ID supplied ]]
      @HttpOperation(method = "DELETE" , url = "http://petstore.swagger.io/api/pets/{id}")
      operation deletePet( @PathVariable(value = "id") id : Int )


OpenApi has different ways of expressing nullability on attributes

  • required attributes present on a model
  • the nullable attribute of a field definition (where nullable is false by default)

The Taxi processor applies the following order of precedence to determine nullability:

  • Does the model specify required attributes?
    • If so, is this field declared as required?
      • Yes -> field is not nullable
      • No -> field is nullable
  • Is the attribute defined as nullable?
    • Yes -> field is nullable
    • No -> field is not nullable

OpenAPI x-taxi-type Extension

Like any generator, TaxiGenerator can only do its best with what is provided. Both Swagger and OpenAPI specifications rely heavily on unnamed schemas. TaxiGenerator has to turn these into types, and so has to invent names for them.

In addition TaxiGenerator cannot know about your existing Taxi taxonomy, nor could it reliably match schemas from the OpenAPI specification with types in that taxonomy without further help.

To solve this, TaxiGenerator supports an OpenAPI Extension x-taxi-type, allowing you to specify the name, fully qualified or otherwise, of the type, and whether it should be generated, or assumed to exist.

The x-taxi-type extension can be applied to the schema of parameters, request bodies, and response contents of OpenAPI operations.

The x-taxi-type extension can be applied in both json and yaml OpenAPI specification documents. For example: OpenAPI JSON specification with x-taxi-type OpenAPI YAML specification with x-taxi-type Resulting generated taxi


  • Any OpenAPI schema object may contain an x-taxi-type object at the top level
  • An x-taxi-type object must contain a name property of type string
  • If the value of the name is unqualified, the generator considers it to be in the default namespace provided as an argument to generateAsStrings
  • The generator will use that name as the name of the type in preference to the name the generator would otherwise have generated.
  • An x-taxi-type object may contain a create property of type boolean
  • The create property defaults to true if the schema would generate a model (i.e., it either has properties or is composed via an allOf)
  • The create property defaults to false if the schema would generate a type (i.e., it neither has properties nor is composed via an allOf)


openapi: "3.0.0"
  version: 1.0.0
  title: Swagger Petstore
      type: object
            name: org.other.Name
          type: string
            name: DateOfBirth
            create: true
          type: string
          format: date
            name: org.other.Address
            create: false
              type: string
            name: Career
              type: string
paths: {}


namespace vyne.openApi {

   type DateOfBirth inherits Date

   model Career {
      title : String?

   model Person {
      name : org.other.Name?
      dob : DateOfBirth?
      address : org.other.Address?
      job : Career?
Edit on GitLab