Programming language: Kotlin
License: MIT License
Tags: Web    

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The invisible REST (and WEB) framework. It is "invisible" since it does not invade your code, and only uses annotations for exception cases (or view rendering).

Kovert is a simple framework that binds your Kotlin classes into your Vert.x 3 (and soon Undertow) routers. It does not try to replace or rebuild these frameworks and only handles the task of providing the "last mile" binding to your controllers. From a fairly normal looking Kotlin class, Kovert can infer the route path and parameters.

This framework is opinionated and optimized for easy coding and default behavior typical of most apps. If you want more control, please use Vertx-Nubes for Vert-x and Kikaha for Undertow, or take a peek at SparkJava.

For starting an application with Kovert, you have two options:

  • Configure, Startup Vert-x, deploy a Vert-x verticle, add your routes with Vert-x Web, and then ask Kovert to bind a controller to an existing route. For that sentence to make sense, you should be familiar with Vertx-Web and the basics of Vertx
  • Alternatively, if you just want to get started without knowing too much, Kovert provides KovertVertx and KovertVerticle classes that can bootstrap a base application, but this acts more as an example starting point from which you should build your own.

In addition, Kovert uses Klutter/Vertx3 module which contains helper classes for working with Vert-x that use Kovenant promises -- including ensuring that the dispatcher for Kovenant is unified with the thread dispatching in Vert.x so that Vert.x context is maintained on dispatch threads, and callbacks come as expected by Vert.x as well. There are additional helpers for Vert.x JSON objects, the logging facade, web, and integration with Injekt, and more.

Maven Dependnecy (Vert.x Version, requires JDK 8 or newer)

Include the dependency in your Gradle / Maven projects that are setup with Kotlin.


compile "uy.kohesive.kovert:kovert-vertx:1.5.+"



Learn by Example

For a full sample, view the [sample REST application](vertx-example/src/main/kotlin/uy/kohesive/kovert/vertx/sample/) to see an example that uses the helper classes KovertVertx and KovertVerticle to startup and run a vertx server using a configuration file.

It helps if you are familiar with Injekt to completely understand the sample. Injekt is not required for use of Kovert and can be bypassed, but it helps! The sample also uses small libraries from Klutter such as to control configuration loading with Typesafe Config.

Binding a Controller

To bind a controller is simple. When you have a route object, call the extension method bindController:

route.bindController(MyControllerClass(), "/api/mystuff")

A controller class is any class that contains methods that are extension functions on a class that you wish to be your dispatch context. So you can either decide that dispatch context is the raw RoutingContext of Vert.x, or you can isolate your code from the raw Vert.x and use simple classes that wrap elements of the RoutingContext to make them available in a type-safe way. Any class that has a 1 parameter constructor of type RoutingContext can be a context class, or RoutingContext itself. An example of a custom context:

class RestContext(private val routingContext: RoutingContext) {
    public val user: User by Delegates.lazy { routingContext.user() }

Or one that uses an API key to validate the request using an Auth service provided by Injekt (really you should build a Vert.x auth service, this is just for an example of a context that can reject a request instead of using an intercept):

class ApiKeySecured(private val routingContext: RoutingContext) {
    public val user: User =  Injekt.get<AuthService>()
                 .apiKeyToUser(routingContext.request().getHeader(HttpHeaders.AUTHORIZATION.toString()) ?: "") 
                 ?: throw HttpErrorUnauthorized()

With the dispatch object selected, write your controller. Here is the CompanyRestController from the sample app:

class CompanyRestController(val companyService: CompanyService = Injekt.get()) {
    public fun ApiKeySecured.getCompanyByName(name: String): Company = companyService.findCompanyByName(name) ?: throw HttpErrorNotFound()

    public fun ApiKeySecured.putCompanyByName(name: String, company: Company): Company {
        if (!name.equals(company.name, ignoreCase = true)) {
            throw HttpErrorBadRequest()
        return company

    public fun ApiKeySecured.listCompanyByNameEmployees(name: String): List<Person> {
        return companyService.listEmployeesOfCompany(name) ?: throw HttpErrorNotFound()

    public fun ApiKeySecured.findCompaniesNamedByName(name: String): Company = companyService.findCompanyByName(name) ?: throw HttpErrorNotFound()

    public fun ApiKeySecured.findCompaniesLocatedInCountry(country: String): List<Company> {
        val found = companyService.findCompaniesByCountry(country)
        if (found.isEmpty()) throw HttpErrorNotFound()
        return found

    public fun ApiKeySecured.getCompaniesSearch(name: String?, country: String?): Promise<Set<Company>, Exception> {
        return task {
            val byName: List<Company> = name.whenNotNull { companyService.findCompanyByName(name!!) }.whenNotNull { listOf(it) } ?: emptyList()
            val byCountry: List<Company> = country.whenNotNull { companyService.findCompaniesByCountry(country!!) } ?: emptyList()
            (byName + byCountry).toSet()

Great, that class looks like it contains normal Kotlin methods, just that they are extension methods on RestContext class. That means each method has access to values of the context and only those values. You can actually have a different context class for each method if you want, and the correct context will be created for dispatching to that method. A context for public methods, one for logged in, another for temporary state during a process flow, etc.

When binding the controller class, HTTP verb and path names are derived from method names unless you use special annotations to override the path, path parameters, HTTP verb and success status code. Kovert is designed to avoid using those annotations altogether, and they should appear ONLY in special cases.

Infering HTTP Verb and Path

So without any annotations, how does Kovert decide the path names and parameters?!?

First, if the name contains underscores it is split on each underscore maintaining case of each fragment in between. Otherwise it camelCase parses the name lower casing each fragment.

After parsing the first fragment indicates the HTTP verb, and the rest are segments of the path. When it encounters special words, it creates path parameters.

The parsing looks like:

// thisIsATestOfSplitting = this is a test of splitting
// AndWhatAboutThis = and what about this
// aURIIsPresent = a uri is present
// SomethingBySomething = something :something
// something20BySomething30 = something20 :something30
// 20ThisAndThat = 20 this and that
// 20thisAndThat = 20this and that
// What_about_underscores = What about underscores
// 20_ThisAndThat_And_What = 20 ThisAndThat And What
// 20________thisAndThat__What = 20 thisAndThat What

Using the prefix part of the method, a HTTP verb is inferred. Obviously prefixes of "get", "put", "post", "delete", "patch" will generate a route that is for the HTTP verb of the same name. You can see in KovertConfig that other aliases are defined such as "list" and "view" for HTTP GET, and "remove" also works same as HTTP DELETE. You can change the alias list in KovertConfig using the addVerbAlias or removeVerbAlias methods. You can also specify aliases in the bindController method as an optional parameter, or as annotations @VerbAliases and @VerbAlias on your controller class. The sample application modifies KovertConfig to add "find" as an alias to HTPT GET:

KovertConfig.addVerbAlias("find", HttpVerb.GET)

Other annotations can be used on classes to tune their behavior and override the path, path parameters and HTTP verbs.

All routing path and parameter decisions are logged to the current logger, so you can easily see the results of the bindController method. The example above, would generate these paths when bound at "/api" route:

Method Verb Path (w/parameters)
getCompanyByName(name: String) GET api/company/:name
putCompanyByName(name: String) PUT api/company/:name
listCompanyByNameEmployees(name: String) GET api/company/:name/employees
findCompaniesNamedByName(name: String) GET api/companies/named/:name
findCompaniesLocatedInCountry(country: String) GET api/companies/located/:country
getCompaniesSearch(name: String, country: String) GET api/companies/search?name=xyz&country=abc

Which I can confirm by viewing my log output (notice it logs from my controller class):

11:41:20.880 [vert.x-eventloop-thread-2] INFO  u.k.k.vertx.sample.CompanyRestController - Binding getCompanyByName to HTTP GET:200 /api/company/:name w/context ApiKeySecured
11:41:20.882 [vert.x-eventloop-thread-2] INFO  u.k.k.vertx.sample.CompanyRestController - Binding listCompanyByNameEmployees to HTTP GET:200 /api/company/:name/employees w/context ApiKeySecured
11:41:20.883 [vert.x-eventloop-thread-2] INFO  u.k.k.vertx.sample.CompanyRestController - Binding findCompaniesNamedByName to HTTP GET:200 /api/companies/named/:name w/context ApiKeySecured
11:41:20.884 [vert.x-eventloop-thread-2] INFO  u.k.k.vertx.sample.CompanyRestController - Binding putCompanyByName to HTTP PUT:200 /api/company/:name w/context ApiKeySecured
11:41:20.885 [vert.x-eventloop-thread-2] INFO  u.k.k.vertx.sample.CompanyRestController - Binding findCompaniesLocatedInCountry to HTTP GET:200 /api/companies/located/:country w/context ApiKeySecured
11:41:20.886 [vert.x-eventloop-thread-2] INFO  u.k.k.vertx.sample.CompanyRestController - Binding getCompaniesSearch to HTTP GET:200 /api/companies/search w/context ApiKeySecured

Path Parameters

Previously, we mentioned that you can use special words to create path parameters, here they are:

word description example result
By next word is path parameter getCompanyByName(name:String) HTTP GET company/:name
In same as By getCompaniesInCountry(country:String) HTTP GET companies/:country
With next word is path segment and then repeated as path parameter getPersonWithName(name:String) HTTP GET person/name/:name

The parameter name will then be bound into your method parameters if one of them has a mathing name. Optional parameters should be nullable.

Soon, we will allow configuring additional replacement rules for word patterns so you can customize the behavior.

Query and Form Parameters

Just add the parameter to your method signature, and it is looked for in the path parameters, query and form parameters. Nothing is needed. You can do simple parameters such as:

public fun MyContext.getPeopleByName(name: String): List<People>

Or you can use complex parameter such as an object:

public fun MyContext.getPeopleByQuery(query: Query): List<People>

In this case, it must receive parameters prefixed by query. such as query.text, query.name, query.country to fill in the values of the Query object in this example.

Body as JSON

If a parameter is not satisfied from path, query, or form parameters, and it has Content-Type of application/json then if it is a complex parameter type it will bound from the body of the request using Jackson data binding.

You can freely mix all parameter types, and the body will only be used if the others do not provide values for a parameter and it will only be used once. An error will result if a complex parameter exists that cannot be satsified from the request.

JSON Response

Any non-String return type becomes JSON automatically using Jackson to serialize the result. This includes classes, lists, maps, and anything else Jackson can detect and serialize as JSON.

Async and Longer Running Handlers

Returning a Kovenant Promise will unwrap the promise when completed and use the resulting value as the response object. This allows async methods. EVERYTHING that isn't immediately resonsive should use Promises otherwise you block Vert.x IO thread, whereas a Promise dispathces on the Vert.x worker thread. In the sample application, you can see in the company controller that the query method uses a promise return type and returns an task {} block of code. You can also create a Deferred instead with more control over your Promise.

public fun RestContext.getCompaniesSearch(name: String?, country: String?): Promise<Set<Company>, Exception> {
    return task {
        val byName: List<Company> = name.whenNotNull { companyService.findCompanyByName(name!!) }.whenNotNull { listOf(it) } ?: emptyList()
        val byCountry: List<Company> = country.whenNotNull { companyService.findCompaniesByCountry(country!!) } ?: emptyList()
        (byName + byCountry).toSet()

When using Kovenant promises, please see the section below about Vert.x + Kovenant.


Controllers, context classes and controller methods can be protected by a list of Authorities that must be present on the current user or the controller method(s) cannot be called. These are set using the `@Authority annotation and provides one or more authorities to match with mode ANY or ALL. If authorities are set on the controller then they apply to all methods fo the controller and must match along with any added directly to the context or methods. Same for adding authorities to a context, it must match and any method specific authorities.

Authorization handlers must be set when creating the routes to which the controllers are bound. The authority annotations basically cause the user.isAuthorised(authoriy) method to be called for each authority specified, then combined based on the mode of ANY or ALL. See the Vert.x documentation for Authentication and Authorisation for more on configuration of Vert.x.

An empty @Authority() annotation indicates soley that a valid user must be present, they must be authenticated.

An example:

class AdminApiController {
    @Authority("resource:users:read") // requires both role:admin and resource:users:read
    fun SecuredContext.listUsers(): Promise<PageableList<User>, Exception> { ... }

    @Authority("resource:users:write") // requires both role:admin and resource:users:write
    fun SecuredContext.postUser(user: User): Promise<Unit, Exception> { ... }

    // requires only role:admin
    fun SecuredContext.getAdminStats(): Promise<AdminStats, Exception> { ... }

    @Authority("resource:secrets")  // requires role:admin from controller, and role:root from the context, and resource:secrets
    fun RootContext.getSecretThing(): Promise<SuperSecret, Exception> { ... }

class RootContext(val routingContext: RoutingContext) { ... }

HTML Views (or rendering any text based content type)

For rendering HTML or other rendered content you have two options:

  • Return anything of type String and it will set Content-Type of text/html
  • Setup a renderer for any controller method

To use a renderer, add the @Rendered annotation to any controller method, and provide the template name to render. The return value of the method will be used as the model and will be provided to the template. This looks like:

public fun UserContext.getCompaniesSearch(name: String?, country: String?): Promise<CompanySearchResults, Exception> {

In this example, the template engine will be selected by the file extension ".ftl", and the rendering will be passed a model containing and instance of CompanySearchResults.

If the code dynamically changes the template to be used, it can provide a empty @Rendered annotation and return a ModelAndRenderTemplate instance which provides the information about how to render the results.

public fun UserContext.getCompaniesSearch(name: String?, country: String?): Promise<ModelAndRenderTemplate, Exception> {
    return task {
        ModelAndRenderTemplate(searchResults, "search-${searchType}-results.ftl")

Before using rendering, add the dependency for your rendering engine, and register an instance of TemplateEngine via the KovertConfig.registerTemplateEngine() method.

val freemarker = KovertFreemarkerTemplateEngine(configuredFreemarker)
KovertConfig.registerTemplateEngine(freemarker, ".html.ftl", "text/html") // content-type is optional for text/html
KovertConfig.registerTemplateEngine(freemarker, ".xml.ftl", "application/xml")
KovertConfig.registerTemplateEngine(freemarker, ".json.ftl", "application/json")

Template engines are matched from the longest extension to the shortest, so the most specific wins. Note that each extension is registered separately with its appropriate Content-Type type. You can override the content type on any controller method in the @Rendered annotation:

@Rendered("search-results.ftl", "application/xml")
public fun UserContext.getCompaniesSearch(name: String?, country: String?): Promise<CompanySearchResults, Exception> {

WARNING: content type expressed in registration of template engine, or in the @Rendered annotation does not currently affect route matching, but might in the future when it is known at binding time.

Creating your own template engine is simple, just implement a simple interface TemplateEngine, and register an instance of your new engine. The template engine is always run async so it does not block. Most implementations are a few lines of code.

public interface TemplateEngine {
    fun render(template: String, model: Any): String
    // throw unknown exception causes error 500,
    // intentionally throw HttpErrorNotFound if you want a 404

NOTE: Some engines may need to wrap the model and the use of the model will vary by engine (some allow it to be at the "root", others always require it to be named).

Available template engines:

  • Apache FreeMarker - KovertFreemarkerTemplateEngine ([source](template-engine-freemarker/src/main/kotlin/uy/kohesive/kovert/template/freemarker/KovertFreemarkerTemplateEngine.kt), [tests](template-engine-freemarker/src/test/kotlin/uy/kohesive/kovert/template/freemarker/TestKovertFreemarkerTemplateEngine.kt))
compile group: 'uy.kohesive.kovert', name: 'kovert-template-engine-freemarker', version: "${versionKovert}"
  • Handlebars.java - KovertHandlebarsTemplateEngine ([source](template-engine-handlebars/src/main/kotlin/uy/kohesive/kovert/template/handlebars/KovertHandlebarsTemplateEngine.kt), [tests](template-engine-handlebars/src/test/kotlin/uy/kohesive/kovert/template/handlebars/TestKovertHandlebarsTemplateEngine.kt))
compile group: 'uy.kohesive.kovert', name: 'kovert-template-engine-handlebars', version: "${versionKovert}"

It is up to you to configure the raw template system that the engine uses, see the samples above for ideas.


For a redirect, just throw an HttpRedirect(toUrl) exception from anywhere in your controller to cause a redirect. Our rational is that redirects are rare, or exception cases so we didn't want to force a standardized return type (such as ActionResult) just for these special cases, which typically never occur in REST api's and rarely in web frontend controllers.

HTTP Errors

The following prebuilt exceptions are available to return HTTP errors. Any other exception will always result in an HTTP status code 500.

open class HttpErrorUnauthorized() : HttpErrorCode("unauthorized", 401)
open class HttpErrorForbidden() : HttpErrorCode("forbidden", 403)
open class HttpErrorBadRequest() : HttpErrorCode("bad request", 400)
open class HttpErrorNotFound() : HttpErrorCode("not found", 404)

open class HttpErrorCode(message: String, val code: Int = 500, causedBy: Throwable? = null)
open class HttpErrorCodeWithBody(message: String, code: Int = 500, val body: Any, causedBy: Throwable? = null) : HttpErrorCode(message, code, causedBy)

The HttpErrorCodeWithBody allows returning a body (String as HTML, complex object as JSON) with the error code.


You can intercept by putting another Vert.x handler before you bind the controller and that handler will be called before the controller, and it can decide whether the next handler is called or not. Or, a controller can implement traits to intercept requests, failures, dispatching and also to create a custom context object factory. See [VertxTraits.kt](vertx-jdk8/src/main/kotlin/uy/kohesive/kovert/vertx/VertxTraits.kt) for more information.


Name Where Purpose
@VerbAlias Controller Set one method prefix alias to be used only by this controller
@VerbAliases Controller Set a list of method perfix aliases to be used only by this controller
@Location Method Set a specific path for a method, ignoring the method name other than for the prefix to infer the HTTP Verb. Path parameters should be prefixed by a : such as my/path/with/:param
@Verb Method Set the HTTP Verb and default status success code for a method, optionally skipping part of the method name when infering the path
@Authority Controller, Method, Context Set the authorities (role, permission, etc) required for the controller, context or method. And the mode to match ANY or ALL

If is typical to use @Location and @Verb together on a method, although they can be used individually.

If you use the @Verb annotation on a method, by default the prefix of the method name is parsed and thrown away so it really can be anything. Or if you want to use the prefix as the first path segment you may use the skipPrefix parameter with value false such as @Verb(HttpVerb.GET, skipPrefix = false) public fun SomeContext.someHappyMethod(): MyResult would bind to some/happy/method whereas skipPrefix = true would bind to happy/method.

Kovert Helpers

Vert.x + Kovenant Promises

For using Vert.x with Kovenant promises, you should launch Vert.x using one of the Klutter/Vertx3 helper functions. If you are NOT using these methods, then call VertxInit.ensure() before using your first Kovenant promise, and before using anything that involves data binding with Kovert. Otherwise, using a helper startup function will do this for you automatically. Note that you can also use the prettier task { } instead of Vert.x executeBlocking() when using Kovenant integration.

See Klutter/Vertx3 for all Vert.x helper functions include JSON, Vertx-Web, Logging and Injekt modules.

Vert.x and KovertVerticle startup

Really, you should configure and launch Vert.x yourself (use helpers above, Klutter for config loading, etc.). But to act as both a sample, and a quick start helper, There are two classes you can use to startup Vert.x enabled for everything described in this documentation. Or use these as a samples to write your own:

  • [KovertVertx.kt](vertx-jdk8/src/main/kotlin/uy/kohesive/kovert/vertx/boot/KovertVertx.kt)
  • [KovertVerticle.kt](vertx-jdk8/src/main/kotlin/uy/kohesive/kovert/vertx/boot/KovertVerticle.kt)

The sample application [App.kt](vertx-example/src/main/kotlin/uy/kohesive/kovert/vertx/sample/App.kt) shows one use of these classes.


Both for setting up JSON integrated with Kotlin, JDK 8 and Vert.x; and for integrated logging you may import Injekt modules.

Importing module VertxInjektables will provide an ObjectMapper singleton available for Jackson data binding that is shared with Vert.x, and Kovenant will be initialized correctly so that promises and async calls work in conjunction with Vert.x thread dispathcing, and you will have a logger factory configured routing any of your injected logging calls through the Vertx logging Facade. Alterantively you can import the VertxWithSlf4jInjektables module for the same benefits, although your logging factory will be setup to be direct to SLF4j for application code.

See the sample application [App.kt](vertx-example/src/main/kotlin/uy/kohesive/kovert/vertx/sample/App.kt) which uses Injekt for configuration, KovertVertx and KovertVerticle classes to launch Vert.x and Kovert, data binding, logging and providing services.

More Examples

View the [sample application](vertx-example/src/main/kotlin/uy/kohesive/kovert/vertx/sample/), and the [unit tests](vertx-jdk8/src/test/kotlin/uy/kohesive/kovert/vertx/) for more combinations of the previously described topics.

Road Map (random order)

  • Undertow support as alternative to Vert.x
  • SparkJava support as alternative to Vert.x and Undertow
  • Configurable clauses in method names for substitution patterns (i.e. "By", "In", "With" are substitution patterns)
  • With View support, people will want to ask for the HREF from a given controller method, should be able to provide that in Kotlin M13, or can provide using the val getSomeThing = fun MyContext.(param: String): MyObject { ... } form of declaring a controller method already since MyClass::getSomething can reference that value, whereas in the other form, it is not referenceable in M12.
  • Ignore annotation for extension methods in controller that are not desired

Special Thanks

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YourKit supports open source projects with its full-featured Java Profiler. YourKit, LLC is the creator of YourKit Java Profiler and YourKit .NET Profiler, innovative and intelligent tools for profiling Java and .NET applications.

*Note that all licence references and agreements mentioned in the kovert README section above are relevant to that project's source code only.