Java SDK reference

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Last edited: Jun 18, 2024

Recent major versions

Version 7 of the Java SDK supports migration feature flags. These are temporary flags used to migrate data or systems while keeping your application available and disruption free. To learn more about upgrading, read Java SDK 6.x to 7.0 migration guide.

Version 6 of the Java SDK replaces users with contexts. A context is a generalized way of referring to the people, services, machines, or other resources that encounter feature flags in your product. Contexts replace another data object in LaunchDarkly: "users." To learn more about upgrading, read Java SDK 5.x to 6.0 migration guide and Best practices for upgrading users to contexts.

Code samples on this page are from the three most recent SDK versions where they differ.

Overview

This topic documents how to get started with the Java SDK, and links to reference information on all of the supported features.

SDK quick links

LaunchDarkly's SDKs are open source. In addition to this reference guide, we provide source, API reference documentation, and sample applications:

Resource	Location
SDK API documentation	SDK API docs
GitHub repository	java-server-sdk
Sample applications	Java Scala Clojure You can also use this SDK with Kotlin
Published module	Maven

For use in server-side applications only

This SDK is intended for use in multi-user Java server applications. If you have an Android application and want to set up LaunchDarkly in a mobile, desktop, or embedded application, read the Android SDK reference.

To learn more about LaunchDarkly's different SDK types, read Client-side, server-side, and edge SDKs.

SDK version compatibility

The LaunchDarkly Java SDK, version 5.0 and higher, is compatible with Java 8 and higher.

Prior to version 5.0, the LaunchDarkly Java SDK also supported Java 7.

Get started

After you complete the Getting Started process, follow these instructions to start using the LaunchDarkly SDK in your Java application.

Install the SDK

First, install the LaunchDarkly SDK as a dependency in your application using your application's dependency manager. Refer to the SDK releases page to identify the latest version.

In this example, it uses version 7.0.0:

<dependency>
  <groupId>com.launchdarkly</groupId>
  <artifactId>launchdarkly-java-server-sdk</artifactId>
  <version>7.0.0</version>
</dependency>

Next, import the LaunchDarkly client in your application code.

import com.launchdarkly.sdk.*;
import com.launchdarkly.sdk.server.*;

The Java SDK uses an SDK key

The Java SDK uses an SDK key. Keys are specific to each project and environment. They are available from the Environments list for each project. To learn more about key types, read Keys.

Initialize the client

After you install and import the SDK, create a single, shared instance of LDClient. Specify your SDK key here to authorize your application to connect to a particular environment within LaunchDarkly.

Here's how:

LDClient client = new LDClient("sdk-key-123abc");

The client attempts to connect to LaunchDarkly as soon as you call the constructor. The constructor will return when it successfully connects, or when the default timeout of five seconds expires, whichever comes first. If the client does not succeed, your feature flags will serve default values, and the client will continue trying to connect in the background.

To learn more about the specific configuration properties that are available in this SDK, read LDConfig.Builder.

Working with reactive libraries

If you are working with reactive libraries, then you can create an instance of LDReactorClient, instead of LDClient. The LDReactorClient is a thin wrapper of the LDClient that adapts it to reactive stream programming. LDReactorClient uses an elastic scheduler internally, and may allocate a number of threads that is proportional to the number of concurrent API calls being made on the LDClient.

To learn more, read LDReactorClient.

If you receive an "unable to find valid certification path" error when you initialize the SDK, you must update the Java truststore. To learn how, read the troubleshooting article in the LaunchDarkly Customer Knowledge Base.

LDClient must be a singleton

It's important to make LDClient a singleton for each LaunchDarkly project. The client instance maintains internal state that allows LaunchDarkly to serve feature flags without making any remote requests. Do not instantiate a new client with every request.

If you have multiple LaunchDarkly projects, you can create one LDClient for each. In this situation, the clients operate independently. For example, they do not share a single connection to LaunchDarkly.

Evaluate a context

You can use client to check which variation a particular context will receive for a given feature flag. To learn more, read Evaluating flags and Flag evaluation reasons. For more information about how contexts are specified, read Context configuration.

In the v6 example, the context key is the string "context-key-123abc". In the v5 example, the user key is the string "user-key-123abc":

LDContext context = LDContext.builder("context-key-123abc")
  .name("Sandy")
  .build();
boolean flagValue = client.boolVariation("flag-key-123abc", context, false);
if (flagValue) {
  // Application code to show the feature
}
else {
  // The code to run if the feature is off
}

Using the Java SDK in OSGi

You can install versions 4.6.0 and higher of the SDK as OSGi bundles.

The SDK's default jar, which it gets from Maven or Gradle if you do not specify a "classifier," does not contain Gson or SLF4j because applications are often built with their own specific versions of those libraries. Using the default jar in OSGi requires Gson and SLF4j to be provided by some other bundle.

However, there is also a distribution that includes Gson and SLF4j as part of the SDK bundle. You can use this if you do not need to control the versions of those libraries separately.

To do so, add the classifier "all":

<!-- in Maven -->
<dependency>
   <groupId>com.launchdarkly</groupId>
   <artifactId>launchdarkly-java-server-sdk</artifactId>
   <version>7.0.0</version>
   <classifier>all</classifier>
</dependency>
<!-- or in Gradle -->
"com.launchdarkly:launchdarkly-java-server-sdk:7.0.0:all"

Potential network connectivity issues caused by DNS caching

There is a potential problem for any Java application that communicates with a web service, such as LaunchDarkly, that also uses a load-balancing framework. If a service starts to use a different set of IP addresses, a Java application could continue trying to use an old IP address, causing connection attempts to fail. In most environments, this is unlikely to be a problem because IP addresses are not cached for very long.

However, Java has special behavior if the runtime environment has a security manager. In that case, it caches IP addresses indefinitely and doesn't update them until the application is restarted. If you are running in an environment that has a security manager, or if you're not sure whether that is the case, we recommend that you set the cache duration (TTL) explicitly. To learn how, read Setting the JVM TTL for DNS Name Lookups.

Shut down the client

Shut down the client when your application terminates. To learn more, read Shutting down.

Supported features

This SDK supports the following features: