Unleash Client SDK for Java

This is the Unleash Client SDK for Java. It is compatible with the Unleash-hosted.com SaaS offering and Unleash Open-Source.

Getting started

This section shows you how to get started quickly and explains some common configuration scenarios. For a full overview of Unleash configuration options, check out the Configuration options section.

Step 1: Install the Unleash Java SDK

You need to add the Unleash SDK as a dependency for your project. Here's how you would add it to your pom.xml and build.gradle file:

pom.xml

<dependency>
    <groupId>io.getunleash</groupId>
    <artifactId>unleash-client-java</artifactId>
    <version>Latest version here</version>
</dependency>

build.gradle

 implementation("io.getunleash:unleash-client-java:$unleashedVersion")

Step 2: Create a new Unleash instance

⚠️ Important: In almost every case, you only want a single, shared instance of the Unleash class (a singleton) in your application . You would typically use a dependency injection framework (such as Spring or Guice) to inject it where you need it. Having multiple instances of the client in your application could lead to inconsistencies and performance degradation.

To help you detect cases where you configure multiple instances by mistake, the SDK will print an error message if you create multiple instances with the same configuration values. You can also tell Unleash to fail when this happens by setting the constructor parameter failOnMultipleInstantiations to true.

When instantiating an Unleash client, you can choose to do it either synchronously or asynchronously: The SDK will synchronize with the Unleash API on initialization, so it can take a few hundred milliseconds for the client to reach the correct state. This is usually not an issue and Unleash will do this in the background as soon as you initialize it. However, if it's important that you not continue execution until the SDK has synchronized, then you should use the synchronousFetchOnInitialisation option to block the client until it has successfully synchronized with the server.

Example configurations

💡 Tip: Refer to the section on configuration options for a more complete explanation of all the options.

Here's two examples of how you might initialize the Unleash SDK in your applications. The examples use dummy values and are almost identical. The only difference is that the first example is asynchronous, while the second example is synchronous.

Asynchronous initialization example:

UnleashConfig config = UnleashConfig.builder()
        .appName("my.java-app")
        .instanceId("your-instance-1")
        .unleashAPI("<unleash-api-url>")
        .apiKey("<client-api-token>")
        .build();

Unleash unleash = new DefaultUnleash(config);

Synchronous initialization example:

UnleashConfig config = UnleashConfig.builder()
        .appName("my.java-app")
        .instanceId("your-instance-1")
        .unleashAPI("<unleash-api-url>")
        .apiKey("<client-api-token>")
        .synchronousFetchOnInitialisation(true)
        .build();

Unleash unleash = new DefaultUnleash(config);

Step 3: Use the feature toggle

With the SDK initialized, you can use the isEnabled method to check the state of your feature toggles. The method returns a boolean indicating whether a feature is enabled for the current request or not.

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

The isEnabled method also accepts a second, boolean argument. The SDK uses this as a fallback value if it can't find the feature you're trying to check. In other words, unleash.isEnabled("non-existing-toggle") would usually return false (assuming that "non-existing-toggle") doesn't exist). If you instead do unleash.isEnabled("non-existing-toggle", true), then Unleash would return true if it didn't find the toggle.

You can also provide an Unleash context to the isEnabled method. Refer to the Unleash context section for more information about using the Unleash context in the Java SDK.

Activation strategies

The Java 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 the activation strategies reference documentation.

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.

Strategy s1 = new MyAwesomeStrategy();
Strategy s2 = new MySuperAwesomeStrategy();
Unleash unleash return new DefaultUnleash(config, s1, s2);

Unleash context

In order to use some of the common activation strategies you must provide an 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.

UnleashContext context = UnleashContext.builder()
  .userId("user@mail.com").build();

unleash.isEnabled("someToggle", context);

2. Via an `UnleashContextProvider`

This is a more advanced approach, where you configure an Unleash context provider. With a context provider, you don't need to rebuild or pass the Unleash context to every unleash.isEnabled call.

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

UnleashContextProvider contextProvider = new MyAwesomeContextProvider();

UnleashConfig config = new UnleashConfig.Builder()
            .appName("java-test")
            .instanceId("instance x")
            .unleashAPI("http://unleash.herokuapp.com/api/")
            .apiKey("<client-api-token>")
            .unleashContextProvider(contextProvider)
            .build();

Unleash unleash = new DefaultUnleash(config);

// Anywhere in the code unleash will get the unleash context from your registered provider.
unleash.isEnabled("someToggle");

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.

UnleashConfig unleashConfig = UnleashConfig.builder()
                .appName("my-app")
                .instanceId("my-instance-1")
                .unleashAPI(unleashAPI)
                .apiKey("12312Random")
                .customHttpHeader("<name>", "<value>")
                .build();

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.

public class CustomHttpHeadersProviderImpl implements CustomHttpHeadersProvider {
    @Override
    public Map<String, String> getCustomHeaders() {
        String token = "Acquire or refresh token";
        return new HashMap() {{ put("Authorization", "Bearer "+token); }};
    }
}

CustomHttpHeadersProvider provider = new CustomHttpHeadersProviderImpl();

UnleashConfig unleashConfig = UnleashConfig.builder()
                .appName("my-app")
                .instanceId("my-instance-1")
                .unleashAPI(unleashAPI)
                .apiKey("API token")
                .customHttpHeadersProvider(provider)
                .build();

Subscriber API

(Introduced in 3.2.2)

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.java to get a complete overview of all methods you can override.

UnleashConfig unleashConfig = UnleashConfig.builder()
    .appName("my-app")
    .instanceId("my-instance-1")
    .unleashAPI(unleashAPI)
    .apiKey("API token")
    .subscriber(new UnleashSubscriber() {
        @Override
        public void onReady(UnleashReady ready) {
            System.out.println("Unleash is ready");
        }
        @Override
        public void togglesFetched(FeatureToggleResponse toggleResponse) {
            System.out.println("Fetch toggles with status: " + toggleResponse.getStatus());
        }

        @Override
        public void togglesBackedUp(ToggleCollection toggleCollection) {
            System.out.println("Backup stored.");
        }

    })
    .build();

Options

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.
disablePolling - Stops the client from polling. If used without synchronousFetchOnInitialisation will cause the client to never fetch toggles from the unleash-api.
fetchTogglesInterval - Sets the interval (in seconds) between each poll to the unleash-api. Set this to 0 to do a single fetch and then stop refreshing while the process lives.

HTTP Proxy with Authentication

The Unleash Java 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:

UnleashConfig config = UnleashConfig.builder()
    .appName("my-app")
    .unleashAPI("http://unleash.org")
    .apiKey("API token")
    .enableProxyAuthenticationByJvmProperties()
    .build();

Toggle fetcher

The Unleash Java client now supports using your own toggle fetcher. The Config builder has been expanded to accept a io.getunleash.util.UnleashFeatureFetcherFactory which should be a Function<UnleashConfig, FeatureFetcher>. If you want to use OkHttp instead of HttpURLConnection you'll need a dependency on okhttp

<dependency>
    <groupId>com.squareup.okhttp3</groupId>
    <artifactId>okhttp</artifactId>
    <version>4.10+</version>
</dependency>

Then you can change your config to

UnleashConfig config = UnleashConfig.builder()
    .appName("my-app")
    .unleashAPI("http://unleash.org")
    .apiKey("API token")
    .unleashFeatureFetcherFactory(OkHttpFeatureFetcher::new)
    .build();

This will then start using OkHttp instead of HttpURLConnection.

Metrics sender

The Unleash Java client now supports using your own metrics sender. The Config builder has been expanded to accept a io.getunleash.util.MetricsSenderFactory which should be a Function<UnleashConfig, MetricsSender>.

If you want to use OkHttp instead of HttpURLConnection you'll need a dependency on okhttp

<dependency>
    <groupId>com.squareup.okhttp3</groupId>
    <artifactId>okhttp</artifactId>
    <version>4.10+</version>
</dependency>

Then you can change your config to

UnleashConfig config = UnleashConfig.builder()
    .appName("my-app")
    .unleashAPI("http://unleash.org")
    .customHttpHeader("Authorization", "API token")
    .unleashMetricsSenderFactory(OkHttpMetricsSender::new)
    .build();

This will then start using OkHttp instead of HttpURLConnection to send metrics

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 java.io.tmpdir 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

Bootstrapping

Unleash supports bootstrapping from a JSON string.
Configure your own custom provider implementing the ToggleBootstrapProvider interface's single method String read(). This should return a JSON string in the same format returned from /api/client/features
Example bootstrap files can be found in the json files located in src/test/resources
Our assumption is this can be use for applications deployed to ephemeral containers or more locked down file systems where Unleash's need to write the backup file is not desirable or possible.

Provided Bootstrappers

ToggleBootstrapFileProvider

Unleash comes configured with a ToggleBootstrapFileProvider which implements the ToggleBootstrapProvider interface.
It is the default implementation used if not overridden via the setToggleBootstrapProvider on UnleashConfig.

Configure ToggleBootstrapFileProvider

The ToggleBootstrapFileProvider reads the file located at the path defined by the UNLEASH_BOOTSTRAP_FILE environment variable.
It supports both classpath: paths and absolute file paths.

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
FakeUnleash fakeUnleash = new FakeUnleash();
fakeUnleash.enableAll();

assertThat(fakeUnleash.isEnabled("unknown"), is(true));
assertThat(fakeUnleash.isEnabled("unknown2"), is(true));

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

assertThat(fakeUnleash.isEnabled("t1"), is(true));
assertThat(fakeUnleash.isEnabled("t2"), is(true));
assertThat(fakeUnleash.isEnabled("unknown"), is(false));

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

assertThat(fakeUnleash.getVariant("t1").getName(), is("a"));

Se more in FakeUnleashTest.java

Development

Build:

mvn clean install

Jacoco coverage reports:

mvn jacoco:report

The generated report will be available at target/site/jacoco/index.html

Formatting

We're using AOSP Java code style, enforced by spotless. For formatting, use mvn spotless:apply and spotless will do the job for you
It should apply spotless automatically upon compilation for you, but only on code that is changed since origin/main
For Intelij you can use https://plugins.jetbrains.com/plugin/8527-google-java-format
For VSCode you can use https://marketplace.visualstudio.com/items?itemName=wx-chevalier.google-java-format
For VIM there's https://github.com/google/vim-codefmt

Releasing

Deployment

You'll need an account with Sonatype's JIRA - https://issues.sonatype.org
In addition your account needs access to publish under io.getunleash

GPG signing

You'll need gpg installed and a configured gpg key for signing the artifacts

Example settings.xml

In ~/.m2/settings.xml put

<settings>
    ...
    <profiles>
        ...
        <profile>
            <id>ossrh</id>
            <activation>
                <activeByDefault>true</activeByDefault>
            </activation>
            <properties>
                <gpg.executable>gpg</gpg.executable> <!-- Where to find gpg -->
                <gpg.passphrase>[PASSPHRASE FOR YOUR GPG KEY]</gpg.passphrase>
            </properties>
        </profile>
    </profiles>
    ...
    <servers>
        ...
        <server>
            <id>sonatype-nexus-snapshots</id>
            <username>[YOUR_SONATYPE_JIRA_USERNAME]</username>
            <password>[YOUR_SONATYPE_JIRA_PASSWORD]</password>
        </server>
        <server>
            <id>ossrh</id>
            <username>[YOUR_SONATYPE_JIRA_USERNAME]</username>
            <password>[YOUR_SONATYPE_JIRA_PASSWORD]</password>
        </server>
        <server>
            <id>sonatype-nexus-staging</id>
            <username>[YOUR_SONATYPE_JIRA_USERNAME]</username>
            <password>[YOUR_SONATYPE_JIRA_PASSWORD]</password>
        </server>
    </servers>
</settings>

More information

https://central.sonatype.org/pages/ossrh-guide.html

Configuration options

The UnleashConfig$Builder class (created via UnleashConfig.builder()) exposes a set of builder methods to configure your Unleash client. The available options are listed below with a description of what they do. For the full signatures, take a look at the UnleashConfig class definition.

Method name	Description	Required	Default value
`apiKey`	The api key to use for authenticating against the Unleash API.	Yes	`null`
`appName`	The name of the application as shown in the Unleash UI. Registered applications are listed on the Applications page.	Yes	`null`
`backupFile`	The path to the file where local backups get stored.	No	Synthesized from your system's `java.io.tmpdir` and your `appName`: `"<java.io.tmpdir>/unleash-<appName>-repo.json"`
`customHttpHeader`	Add a custom HTTP header to the list of HTTP headers that will the client sends to the Unleash API. Each method call will add a new header. Note: in most cases, you'll need to use this method to provide an API token.	No	N/A
`customHttpHeadersProvider`	Add a custom HTTP header provider. Useful for dynamic custom HTTP headers.	No	`null`
`disablePolling`	A boolean indicating whether the client should poll the unleash api for updates to toggles.
`disableMetrics`	A boolean indicating whether the client should disable sending usage metrics to the Unleash server.	No	`false`
`enableProxyAuthenticationByJvmProperties`	Enable support for using JVM properties for HTTP proxy authentication.	No	`false`
`environment`	The value to set for the Unleash context's `environment` property. Not the same as Unleash's environments.	No	`null`
`fallbackStrategy`	A strategy implementation that the client can use if it doesn't recognize the strategy type returned from the server.	No	`null`
`fetchTogglesInterval`	How often (in seconds) the client should check for toggle updates. Set to `0` if you want to only check once.	No	`10`
`instanceId`	A unique(-ish) identifier for your instance. Typically a hostname, pod id or something similar. Unleash uses this to separate metrics from the client SDKs with the same `appName`.	Yes	`null`
`namePrefix`	If provided, the client will only fetch toggles whose name starts with the provided value.	No	`null`
`projectName`	If provided, the client will only fetch toggles from the specified project. (This can also be achieved with an API token).	No	`null`
`proxy`	A `Proxy` object. Use this to configure a third-party proxy that sits between your client and the Unleash server.	No	`null`
`scheduledExecutor`	A custom executor to control timing and running of tasks (such as fetching toggles, sending metrics).	No	`UnleashScheduledExecutorImpl`
`sendMetricsInterval`	How often (in seconds) the client should send metrics to the Unleash server. Ignored if you disable metrics with the `disableMetrics` method.	No	`60`
`subscriber`	Register a subscriber to Unleash client events.	No	`null`
`synchronousFetchOnInitialisation`	Whether the client should fetch toggle configuration synchronously (in a blocking manner) on initialisation.	No	`false`
`toggleBootstrapProvider`	Add a bootstrap provider (must implement the `ToggleBootstrapProvider` interface)	No
`unleashAPI`	The URL of the Unleash API.	Yes	`null`
`unleashContextProvider`	An Unleash context provider used to configure Unleash.	No	`null`
`unleashFeatureFetcherFactory`	A factory providing a FeatureFetcher implementation.	No	`HttpFeatureFetcher::new`
`unleashMetricsSenderFactory`	A factory providing a MetricSender implementation.	No	`DefaultHttpMetricsSender::new`
`startupExceptionHandler`	Handler for the behavior in the event of an error when starting the client.	No	`null`

When you have set all the desired options, initialize the configuration with the build method. You can then pass the configuration to the Unleash client constructor. As an example:

UnleashConfig config = UnleashConfig.builder()
            .appName("your app name")
            .instanceId("instance id")
            .unleashAPI("http://unleash.herokuapp.com/api/")
            .apiKey("API token")
            // ... more configuration options
            .build();

Unleash unleash = new DefaultUnleash(config);

Other information

Check out our guide for more information on how to build and scale feature flag systems

Unleash/unleash-client-java