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Note: while this uploads the the OSS Nexus, it does not automatically promote to release. According to the sonatype documentation, it's possible to script promoting, but I'll save that for future improvements.

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Unleash Client SDK for Kotlin

This is the Unleash Client SDK for Kotlin. This was converted from unleash-client-java and had Java 8 functionality that Android 21+ doesn't support removed.

Create a new Unleash instance

It is easy to get a new instance of Unleash. In your app you typically just want one instance of Unleash, and inject that where you need it. You will typically use a dependency injection frameworks such as Dagger to manage this.

To create a new instance of Unleash you need to pass in a config object:

val config: UnleashConfig = unleashConfig { 
  instanceId("instance x")

val unleash = new DefaultUnleash(config)

Awesome feature toggle API

It is really simple to use unleash.

if(unleash.isEnabled("AwesomeFeature")) {
  //do some magic
} else {
  //do old boring stuff

Calling unleash.isEnabled("AwesomeFeature") is the equivalent of calling unleash.isEnabled("AwesomeFeature", false). Which means that it will return false if it cannot find the named toggle.

If you want it to default to true instead, you can pass true as the second argument:

unleash.isEnabled("AwesomeFeature", true)

Activation strategies

The Kotlin client comes with implementations for the built-in activation strategies provided by unleash.

  • DefaultStrategy
  • UserWithIdStrategy
  • GradualRolloutRandomStrategy
  • GradualRolloutUserWithIdStrategy
  • GradualRolloutSessionIdStrategy
  • RemoteAddressStrategy
  • ApplicationHostnameStrategy

Read more about the strategies in

Custom strategies

You may also specify and implement your own strategy. The specification must be registered in the Unleash UI and you must register the strategy implementation when you wire up unleash.

val s1 = MyAwesomeStrategy()
val s2 = MySuperAwesomeStrategy()
val unleash = DefaultUnleash(config, s1, s2)

Unleash context

In order to use some of the common activation strategies you must provide a unleash-context. This client SDK provides two ways of provide the unleash-context:

1. As part of isEnabled call

This is the simplest and most explicit way of providing the unleash context. You just add it as an argument to the isEnabled call.

val context = unleashContext {

unleash.isEnabled("someToggle", context)

2. Via a UnleashContextProvider

This is a bit more advanced approach, where you configure a unleash-context provider. By doing this you do not have rebuild or pass the unleash-context object to every place you are calling unleash.isEnabled.

The provider typically binds the context to the same thread as the request. If you are using Spring the UnleashContextProvider will typically be a 'request scoped' bean.

val contextProvider = MyAwesomeContextProvider()

val config = unleashConfig {
  instanceId("instance x")

val unleash = DefaultUnleash(config)

// Anywhere in the code unleash will get the unleash context from your registered provider. 

Custom HTTP headers

If you want the client to send custom HTTP Headers with all requests to the Unleash API you can define that by setting them via the UnleashConfig.

val config = unleashConfig {
  customHttpHeader("Authorization", "12312Random")

Dynamic custom HTTP headers

If you need custom http headers that change during the lifetime of the client, a provider can be defined via the UnleashConfig.

class CustomHttpHeadersProviderImpl : CustomHttpHeadersProvider {
  override fun getCustomHeaders(): Map<String, String> {
    val token = "Acquire or refresh token"
    return mutableMapOf(Pair("Authorization", "Bearer $token"))
val provider = CustomHttpHeadersProviderImpl()

val unleashConfig = unleashConfig { 

Subscriber API

Sometimes you want to know when Unleash updates internally. This can be achieved by registering a subscriber. An example on how to configure a custom subscriber is shown below. Have a look at UnleashSubscriber.kt to get a complete overview of all methods you can override.

val unleashConfig = unleashConfig {
  subscriber(object : UnleashSubscriber { 
    override fun onReady(ready: UnleashReady) {
      System.out.println("Unleash is ready")
    override fun togglesFetched(response: FeatureToggleResponse) {
      System.out.println("Fetch toggles with status: " + toggleResponse.getStatus())

    override fun togglesBackedUp(toggleCollection: ToggleCollection) {
      System.out.println("Backup stored.")


  • appName - Required. Should be a unique name identifying the client application using Unleash.
  • synchronousFetchOnInitialisation - Allows the user to specify that the unleash-client should do one synchronous fetch to the unleash-api at initialisation. This will slow down the initialisation (the client must wait for a http response). If the unleash-api is unavailable the client will silently move on and assume the api will be available later.

HTTP Proxy with Authentication

The Unleash Kotlin client uses HttpURLConnection as HTTP Client which already recognizes the common JVM proxy settings such as http.proxyHost and http.proxyPort. So if you are using a Proxy without authentication, everything works out of the box. However, if you have to use Basic Auth authentication with your proxy, the related settings such as http.proxyUser and http.proxyPassword do not get recognized by default. In order to enable support for basic auth against a http proxy, you can simply enable the following option on the configuration builder:

config = unleashConfig {

Local backup

By default unleash-client fetches the feature toggles from unleash-server every 10s, and stores the result in unleash-repo.json which is located in the directory. This means that if the unleash-server becomes unavailable, the unleash-client will still be able to toggle the features based on the values stored in unleash-repo.json. As a result of this, the second argument of isEnabled will be returned in two cases:

  • When unleash-repo.json does not exists
  • When the named feature toggle does not exist in unleash-repo.json

Unit testing

You might want to control the state of the toggles during unit-testing. Unleash do come with a FakeUnleash implementation for doing this.

Some examples on how to use it below:

// example 1: everything on
var fakeUnleash = FakeUnleash()


// example 2
var fakeUnleash = FakeUnleash()
fakeUnleash.enable("t1", "t2")


// example 3: variants
var fakeUnleash = FakeUnleash()
fakeUnleash.enable("t1", "t2")
fakeUnleash.setVariant("t1", Variant("a", null, true))

assertEquals(fakeUnleash.getVariant("t1").getName(), "a")

See more in



gradlew clean assemble 


Unofficial Unleash Kotlin Client








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