Java Feature Flags

Java Feature Flag Resources/Solutions

  • LaunchDarkly Java Feature Flag SDK – LaunchDarkly
    • An installable feature flag software development kit for Java apps.  This SDK harnesses the LaunchDarkly app and allows you to target users and manage rollouts.  Feature flags are evaluated in microseconds.
    • Main Website
    • GitHub Repo
  • Togglz – Christian Kaltepoth
    • “Togglz is an implementation of the Feature Toggles pattern for Java. Feature Toggles are a very common agile development practices in the context of continuous deployment and delivery. The basic idea is to associate a toggle with each new feature you are working on. This allows you to enable or disable these features at application runtime, even for individual users.”
    • GitHub Repo
  • FF4J – Cédrick Lunven
    • “Feature Toggle is the capability for an application, a system or a product to enable and/or disable features through configuration (files, databases,…) and possibly at runtime. FF4J, stands as Feature Flipping for Java, is an implementation of this pattern for the Java platform.”
    • GitHub Repo
  • Flip – Petric Coroli
    • “Flip makes it simple to use Feature Toggle, (also known as Feature Flags, Feature Bits, Feature Switches, etc) in your Java application. Feature toggles can be used for many purposes, including: Reducing the need to branch by feature in your source control system by having all work done on the main code line. Features that are not ready for production can simply be disabled. New features can be rolled out to a subset of users (e.g. by group membership, canary testing, etc) Should a feature become problematic, we can turn it off without requiring a build and deploy.”
    • GitHub Repo
  • Unleash – Unleash
    • “Unleash is a feature toggle system, that gives you a great overview over all feature toggles across all your applications and services.

      The main motivation for doing feature toggling is to decouple the process for deploying code to production and releasing new features. This helps reducing risk, and allow us to easily manage which features to enable”

    • GitHub Repo


LaunchDarkly Java SDK Reference

This reference guide documents all of the methods available in LaunchDarkly’s Java SDK, and explains in detail how these methods work. If you want to dig even deeper, our SDKs are open source– head to our Java SDK GitHub repository or our Javadocs to look under the hood.

Android support

Our Java SDK is intended for use in trusted server environments only, and should not be distributed to untrusted mobile users.

If you want to use LaunchDarkly in an Android application, see our Android SDK Reference Guide.

Getting started

If you haven’t taken a look at our Quickstart guide yet, we recommend starting there to see how install our SDK into your Java application.

Once the SDK is installed, you’ll want to create a single, shared instance of LDClient:

LDClient ldClient = new LDClient("YOUR_SDK_KEY");

LDClient must be a singleton

It’s important to make this a singleton– internally, the client instance maintains internal state that allows us to serve feature flags without making any remote requests. Be sure that you’re not instantiating a new client with every request.

Customizing your client

You can also pass custom parameters to the client by creating a custom configuration object:

LDConfig config = new LDConfig.Builder()
LDClient ldClient = new LDClient("YOUR_SDK_KEY", config);

Here, we’ve customized the client connect and socket timeout parameters. See the Javadoc for LDConfig.Builder for the complete list of configuration options for the client.


Feature flag targeting and rollouts are all determined by the user you pass to your variation calls. In our Java SDK, we use a builder pattern to make it easy to construct users. Here’s an example:

 LDUser user = new LDUser.Builder("aa0ceb")
      .custom("groups", Arrays.asList("Google", "Microsoft"))

Let’s walk through this snippet. The first argument to the builder is the user’s key– in this case we’ve used the hash"aa0ceb". The user key is the only mandatory user attribute. The key should also uniquely identify each user. You can use a primary key, an e-mail address, or a hash, as long as the same user always has the same key. We recommend using a hash if possible.

All of the other attributes (like firstName, email, and the custom attributes) are optional. The attributes you specify will automatically appear on our dashboard, meaning that you can start segmenting and targeting users with these attributes.

Our Javadoc for LDUser.Builder shows you all the attributes that LaunchDarkly supports by default. In addition to these, you can pass us any of your own user data by passing custom attributes, like the groups attribute in the example above.

A note on types

Most of our built-in attributes (like names and e-mail addresses) expect string values. Custom attributes values can be strings, booleans (like true or false), numbers, or lists of strings, booleans or numbers.

If you enter a custom value on our dashboard that looks like a number or a boolean, it’ll be interpreted that way. The Java SDK is strongly typed, so be aware of this distinction.

Custom attributes are one of the most powerful features of LaunchDarkly. They let you target users according to any data that you want to send to us– organizations, groups, account plans– anything you pass to us becomes available instantly on our dashboard.

Anonymous users

You can also distinguish logged-in users from anonymous users in the SDK, as follows:

 LDUser user = new LDUser.Builder("aa0ceb")

You will still need to generate a unique key for anonymous users– session IDs or UUIDs work best for this.

Anonymous users work just like regular users, except that they won’t appear on your Users page in LaunchDarkly. You also can’t search for anonymous users on your Features page, and you can’t search or autocomplete by anonymous user keys. This is actually a good thing– it keeps anonymous users from polluting your Users page!


The variation method determines which variation of a feature flag a user receives. In Java, there is a variationmethod for each type (e.g. boolVariation, stringVariation):

ldClient.boolVariation("your.feature.key", user, false);

variation calls take the feature flag key, an LDUser, and a default value.

The default value will only be returned if an error is encountered– for example, if the feature flag key doesn’t exist or the user doesn’t have a key specified.

The variation call will automatically create a user in LaunchDarkly if a user with that user key doesn’t exist already. There’s no need to create users ahead of time (but if you do need to, take a look at Identify).

All flags

The allFlags method produces a map of feature flag keys to their values for a specific user.

This method can be useful for passing feature flags to your front-end. In particular, it can be used to provide bootstrap flag settings for our JavaScript SDK.



The track method allows you to record actions your users take on your site. This lets you record events that take place on your server. In LaunchDarkly, you can tie these events to goals in A/B tests. Here’s a simple example:

ldClient.track("Signed up", user);

You can also attach custom JSON data to your event by passing an extra parameter to track:

JsonObject data = new JsonObject();
data.add("price", 320);
ldClient.track("Completed purchase", data);

You can attach any JSON object (using Google’s GSON API) to your events.


The identify creates or updates users on LaunchDarkly, making them available for targeting and autocomplete on the dashboard. In most cases, you won’t need to call identify— the variation call will automatically create users on the dashboard for you. identify can be useful if you want to pre-populate your dashboard before launching any features.


Secure mode hash

The secureModeHash method computes an HMAC signature of a user signed with the client’s SDK key. If you’re using our JavaScript SDK for client-side flags, this method generates the signature you need for secure mode.



Internally, the LaunchDarkly SDK keeps an event buffer for track and identify calls. These are flushed periodically in a background thread. In some situations (for example, if you’re testing out the SDK in a REPL), you may want to manually call flush to process events immediately.


Note that the flush interval is configurable– if you need to change the interval, you can do so viaLDConfig.Builder.

Offline mode

In some situations, you might want to stop making remote calls to LaunchDarkly and fall back to default values for your feature flags. For example, if your software is both cloud-hosted and distributed to customers to run on premise, it might make sense to fall back to defaults when running on premise. You can do this by setting offlinemode in the client’s Configuration.

LDConfig config = new LDConfig.Builder()
LDClient ldClient = new LDClient("YOUR_SDK_KEY", config);
ldClient.boolVariation("any.feature.flag", user, false) // will always return the default value (false)


Close safely shuts down the client instance and releases all resources associated with the client. In most long-running applications, you should not have to call close.