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Getting Started with the OpenFeature Java SDK and Dropwizard

Introduction​

This walk-through teaches you the basics of using OpenFeature in Java in the context of a Dropwizard application.

You'll learn how to:

  • Integrate the OpenFeature Java SDK
  • Install and configure the OpenFeature provider
  • Perform basic feature flagging

Requirements​

This walk-through assumes that:

  • You have a basic knowledge of Java and Dropwizard
  • You have Java 8 or later
  • You have Docker installed and running on the host system

This guide uses Java 21 syntax, adjustments might be needed for earlier Java versions.

Walk-through​

Step 1: Create a Dropwizard application​

Create a new Dropwizard application following the guide on dropwizard.io.

For this example we are using the maven archetype

$ mvn archetype:generate -DarchetypeGroupId=io.dropwizard.archetypes -DarchetypeArtifactId=java-simple -DarchetypeVersion=4.0.13
...
Define value for property 'name': OpenFeatureExample
...
Define value for property 'groupId': dev.openfeature
Define value for property 'artifactId': openfeature-example
Define value for property 'version' 1.0-SNAPSHOT:
Define value for property 'package' dev.openfeature:
Confirm properties configuration:
name: OpenFeatureExample
description: null
shaded: true
groupId: dev.openfeature
artifactId: openfeature-example
version: 1.0-SNAPSHOT
package: dev.openfeature
 Y: Y
...

We now have an application we can open

cd openfeature-example

Step 2: Add dependencies​

For dropwizard we can either use the SDK directly, or use a community module dropwizard-openfeature, and while dropwizard-openfeature provides some benefits like built-in healthchecks, and managing the startup and shutdown of resources associated with the OpenFeature Java SDK, it is not however not officially supported, so use at your own risk.

Depending what you choose add the following to your pom.xml

<dependency>
    <groupId>dev.openfeature</groupId>
    <artifactId>sdk</artifactId>
    <version>1.15.1</version>
</dependency>
<dependency>
    <groupId>dev.openfeature.contrib.providers</groupId>
    <artifactId>flagd</artifactId>
    <version>0.11.8</version>
</dependency>

Step 3: Create a resource​

We create a resource to serve the /welcome endpoint. Depending on the value of the feature flag named "welcome-message", it serves different messages.

@Path("/welcome")
@Produces(TEXT_PLAIN)
public class WelcomeResource {

    private final Client client;

    public WelcomeResource(Client client) {
        this.client = client;
    }

    @GET
    public String getWelcome() {
        if(client.getBooleanValue("welcome-message", false)) {
            return "Welcome to OpenFeature in Dropwizard!\n";
        }

        return "Welcome!\n";
    }
}

to make this resource available we need to register it, and inject the OpenFeature Client.

public class OpenFeatureExampleApplication extends Application<OpenFeatureExampleConfiguration> {

    public static void main(final String[] args) throws Exception {
        new OpenFeatureExampleApplication().run(args);
    }

    @Override
    public String getName() {
        return "OpenFeatureExample";
    }

    @Override
    public void initialize(final Bootstrap<OpenFeatureExampleConfiguration> bootstrap) {
        // TODO: application initialization
    }

    @Override
    public void run(final OpenFeatureExampleConfiguration configuration, final Environment environment) {
        // TODO: implement application
        var client = OpenFeatureAPI.getInstance().getClient("dev.openfeature.OpenFeatureExample");
        var welcomeResource = new WelcomeResource(client);
        environment.jersey().register(welcomeResource);
    }
}

Step 4: Run the application​

Now we can build and run the initial version of the application.

$ mvn clean package
$ java -jar target/openfeature-example-1.0-SNAPSHOT.jar server config.yml
INFO  [2025-06-13 10:00:00,000] io.dropwizard.core.server.DefaultServerFactory: Registering jersey handler with root path prefix: /
INFO  [2025-06-13 10:00:00,000] io.dropwizard.core.server.DefaultServerFactory: Registering admin handler with root path prefix: /
INFO  [2025-06-13 10:00:00,000] io.dropwizard.core.server.ServerFactory: Starting OpenFeatureExample
================================================================================

                              OpenFeatureExample

================================================================================

INFO  [2025-06-13 10:00:00,000] org.eclipse.jetty.setuid.SetUIDListener: Opened application@748e9b20{HTTP/1.1, (http/1.1)}{0.0.0.0:8080}
INFO  [2025-06-13 10:00:00,000] org.eclipse.jetty.setuid.SetUIDListener: Opened admin@2063c53e{HTTP/1.1, (http/1.1)}{0.0.0.0:8081}
INFO  [2025-06-13 10:00:00,000] org.eclipse.jetty.server.Server: jetty-11.0.25; built: 2025-03-13T00:15:57.301Z; git: a2e9fae3ad8320f2a713d4fa29bba356a99d1295; jvm 21+35

INFO  [2025-06-13 10:00:00,000] io.dropwizard.jersey.DropwizardResourceConfig: The following paths were found for the configured resources:

    GET     /welcome (com.sideshowcoder.resources.WelcomeResource)
...

In the output we can see the resource being available under /welcome as well as the application listening to http://0.0.0.0:8080, where 0.0.0.0 refers to all interfaces. Using curl we can see the application working.

$ curl -i http://localhost:8080/welcome
HTTP/1.1 200 OK
Date: Fri, 13 Jun 2025 10:00:00 GMT
Content-Type: text/plain
Vary: Accept-Encoding
Content-Length: 9

Welcome!

"Why I'm I seeing that value?", you may ask. Well, it's because a provider hasn't been configured yet. Without a provider to actually evaluate flags, OpenFeature will return the default value. In the next step, you'll learn how to add a provider.

Step 5: Configure flagd as a provider​

Providers are an important concept in OpenFeature because they are responsible for the flag evaluation itself. As we saw in the previous step, OpenFeature without a provider always returns the default value. If we want to actually perform feature flagging, we'll need to register a provider.

Create a new file named flags.flagd.json and add the following JSON. Notice that there's a flag called welcome-message which matches the flag key referenced earlier. The welcome-message flag has on and off variants that return true and false respectively. The state property controls whether the feature flag is active or not. Finally, the defaultVariant property controls the variant that should be returned. In this case, the defaultVariant is off, therefore the value false would be returned.

{
  "flags": {
    "welcome-message": {
      "variants": {
        "on": true,
        "off": false
      },
      "state": "ENABLED",
      "defaultVariant": "off"
    }
  }
}

NOTE: This configuration is specific for flagd and varies across providers.

With the flagd configuration in place, start flagd service with the following docker command.

NOTE: On Windows WSL is required both for running docker and to store the file. This is a limitation of Docker (https://github.com/docker/for-win/issues/8479)

docker run -p 8013:8013 -v $(pwd)/:/etc/flagd/ -it ghcr.io/open-feature/flagd:latest start --uri file:/etc/flagd/flags.flagd.json

Now we configure flagd as the provider in our application

public class OpenFeatureExampleApplication extends Application<OpenFeatureExampleConfiguration> {

    public static void main(final String[] args) throws Exception {
        new OpenFeatureExampleApplication().run(args);
    }

    @Override
    public String getName() {
        return "OpenFeatureExample";
    }

    @Override
    public void initialize(final Bootstrap<OpenFeatureExampleConfiguration> bootstrap) {
        // nothing to do here
    }

    @Override
    public void run(final OpenFeatureExampleConfiguration configuration, final Environment environment) {
        // Use flagd as the OpenFeature provider and use default configurations
        try {
            OpenFeatureAPI.getInstance().setProviderAndWait(new FlagdProvider());
        } catch (OpenFeatureError e) {
            throw new RuntimeException("Failed to set OpenFeature provider", e);
        }

        var client = OpenFeatureAPI.getInstance().getClient("dev.openfeature.OpenFeatureExample");
        var welcomeResource = new WelcomeResource(client);
        environment.jersey().register(welcomeResource);
    }
}

Step 6: Rerun the application​

We can now rerun the application

$ mvn clean package
$ java -jar target/openfeature-example-1.0-SNAPSHOT.jar server config.yml
INFO  [2025-06-13 10:00:00,000] io.dropwizard.core.server.DefaultServerFactory: Registering jersey handler with root path prefix: /
INFO  [2025-06-13 10:00:00,000] io.dropwizard.core.server.DefaultServerFactory: Registering admin handler with root path prefix: /
INFO  [2025-06-13 10:00:00,000] io.dropwizard.core.server.ServerFactory: Starting OpenFeatureExample
================================================================================

OpenFeatureExample

================================================================================

INFO  [2025-06-13 10:00:00,000] org.eclipse.jetty.setuid.SetUIDListener: Opened application@748e9b20{HTTP/1.1, (http/1.1)}{0.0.0.0:8080}
INFO  [2025-06-13 10:00:00,000] org.eclipse.jetty.setuid.SetUIDListener: Opened admin@2063c53e{HTTP/1.1, (http/1.1)}{0.0.0.0:8081}
INFO  [2025-06-13 10:00:00,000] org.eclipse.jetty.server.Server: jetty-11.0.25; built: 2025-03-13T00:15:57.301Z; git: a2e9fae3ad8320f2a713d4fa29bba356a99d1295; jvm 21+35

INFO  [2025-06-13 10:00:00,000] io.dropwizard.jersey.DropwizardResourceConfig: The following paths were found for the configured resources:

GET     /welcome (com.sideshowcoder.resources.WelcomeResource)
...

Using curl to again fetch the /welcome resource will show again the default message

$ curl -i http://localhost:8080/welcome
HTTP/1.1 200 OK
Date: Fri, 13 Jun 2025 10:00:00 GMT
Content-Type: text/plain
Vary: Accept-Encoding
Content-Length: 9

Welcome!

Let's change the feature flag in our flags.flagd.json, making defaultVariant to on

{
  "flags": {
    "welcome-message": {
      "variants": {
        "on": true,
        "off": false
      },
      "state": "ENABLED",
      "defaultVariant": "off"
      "defaultVariant": "on"
    }
  }
}

fetching /welcome now will show the message for "welcome-message" being true.

$ curl -i http://localhost:8080/welcome
HTTP/1.1 200 OK
Date: Fri, 13 Jun 2025 10:00:00 GMT
Content-Type: text/plain
Vary: Accept-Encoding
Content-Length: 38

Welcome to OpenFeature in Dropwizard!

Conclusion​

In this walkthrough we learned how to integrate OpenFeature into a Dropwizard application, using flagd to provide the feature flags at runtime. We saw how changing the flags definition can change the runtime behaviour of our application, without the need to redeploy or restart the application.