When it comes to integrating with your API, Software Development Kits (SDKs) can be a game-changer. Whether you're distributing them to customers or using them internally, SDKs simplify the integration process, boosting product adoption and developer productivity. One of the most important languages to support with SDKs is Java (or Kotlin), as it's a widely adopted backend language that also underpins the entire Android ecosystem. By providing SDKs for Java and Kotlin, you'll be able to tap into the vast community of developers and companies who use this technology, making it easier for them to build apps and services that seamlessly interact with your API.

The power of an SDK

An SDK simplifies API integration by providing a clean abstraction layer over raw HTTP calls. Without an SDK, developers must manage numerous implementation details and frequently consult API documentation to understand:

• Endpoint paths and routing
• Required and optional parameters
• Parameter validation rules
• Request payload structure
• Response format parsing
• HTTP header requirements

An SDK encapsulates these technical details into intuitive method calls, dramatically simplifying development. Consider these two approaches for creating a pet record:

Without an SDK: Manual HTTP request handling

HttpClient client = HttpClient.newHttpClient();

String petJson = """
    {
        "name": "Link",
        "species": "cat",
        "age": 1
    }
""";

HttpRequest request = HttpRequest.newBuilder()
    .uri(URI.create("http://example.com/pets"))
    .header("Content-Type", "application/json")
    .POST(HttpRequest.BodyPublishers.ofString(petJson))
    .build();

HttpResponse<String> response = client.send(request, HttpResponse.BodyHandlers.ofString());
System.out.println("Response body: " + response.body());

With an SDK: Clean, type-safe interface

PetstoreSdk client = new PetstoreSdk();
Pet pet = new Pet.builder()
    .name("Link")
    .species("cat")
    .age(1)
    .build();
Pet response = client.pets.createPet(pet);

Key advantages of the SDK approach:

1. Simplified Integration: Handles HTTP requests, routing, and headers automatically
2. Type Safety: Uses strongly-typed models (Pet) instead of raw JSON strings
3. IDE Support: Enables code completion and inline documentation
4. Validation: Enforces correct parameter types and required fields at compile-time
5. Maintainability: Reduces boilerplate code and potential for errors

These benefits make SDKs essential for any API that prioritizes developer experience and productivity. Let's tap into how you can build a Java SDK for your API.

1. Model your data

An SDK's primary value comes from providing type-safe representations of your API's data structures.

Before implementing functionality, we should define model classes that match both the JSON objects sent to and returned from your API endpoints. These models serve as a structured interface between your API's raw JSON requests/responses and the SDK's typed environment. Say your endpoint returns a JSON body in this format:

{
	"name": "Nika",
	"species": "cat",
	"age": 1
}

You should create a model class to represent that data in your Java SDK:

class Pet {
	private String name;
	private String species;
	private int age;
	// Constructors, Getters and setters
}

tip

Consider creating a builder class for your model (Eg. PetBuilder for the Pet model) for models with long lists of optional parameters. Read more about the Builder Pattern here.

2. From operations to methods

In REST APIs, each endpoint combines a path with an HTTP method (like GET or POST) to form an "operation". For example:

• GET /users → List users
• POST /users → Create a user
• GET /pets → List pets
• POST /pets → Create a pet

Your SDK should provide intuitive methods that map to these operations. The method names should clearly convey their purpose:

• getUsers() → GET /users
• createUser() → POST /users
• getPets() → GET /pets
• createPet() → POST /pets

To maintain a clean, organized structure - especially in larger APIs with many operations - it's best to group related methods into "services" based on the resource they handle. This is similar to how controllers are organized in MVC architecture. Here's how it could look in a Java SDK for that example API:

// UsersService.java
class UsersService {
	public List<User> getUsers() { ... }
	public User createUser(User user) { ... }
}

// PetsService.java
class PetsService {
	public List<Pet> getPets() { ... }
	public Pet createPet(Pet pet) { ... }
}

3. The entry-point

If we want to achieve a developer experience as shown in our SDK example, we need to consolidate our SDK in a single class. We call that the SDK client. That client will aggregate all of the services, as well as store any shared state, like authentication tokens, for example.

// PetstoreSdk.java
class PetstoreSdk {
	public UsersService users;
	public PetsService pets;
}

4. Request execution

Now that you have your data models and service structure in place, you can implement each operation's actual behavior.

For starters, if your operation has any dynamic data like path segments, query parameters, headers or request body, it should be defined as input to its matching service method. The end user doesn't need to know what each input represents in the final request (Eg. if pet in createPet(Pet pet) goes in the request body or in a query parameter).

Then, using your favorite HTTP client library, build and execute the request following the API documentation. Using a library like jackson, we can convert from model to JSON and vice-versa.

Here's an example for the createPet method:

class PetsService {
	public Pet createPet(Pet pet) {
		HttpClient client = HttpClient.newHttpClient();
		Pet pet = new Pet("Link", "cat", 1);
		String petJson = new ObjectMapper().writeValueAsString(pet);
		HttpRequest request = HttpRequest.newBuilder()
		    .uri(URI.create("<http://example.com/pets>"))
		    .header("Content-Type", "application/json")
		    .POST(HttpRequest.BodyPublishers.ofString(petJson))
		    .build();

		HttpResponse<String> response = client.send(request, HttpResponse.BodyHandlers.ofString());
		return ObjectMapper().readValue(response.body(), Pet.class);
	}
}

Do the same for every API operation, and you've got yourself a working Java SDK! But, as you may have guessed, there's much more that an SDK could do. Here are a few things:

• Store authentication state between requests.
• Validate models with specific rules before sending a request.
• Automatic retries on failed requests.
• And much more.

You can leverage this base architecture and keep building functionality into your SDK to make it better and better.

Going beyond

You might have noticed that creating SDKs for large APIs is a really laborious task. Not to mention that some advanced SDK features can be really challenging to implement, and your SDK gets deprecated with each API update you make. As an alternative, you might want to consider using liblab. liblab's SDK generator delivers high quality SDKs in seconds, in all of the most popular languages, and can be integrated in your CI/CD. We focus on developer experience, and support advanced features like authentication, pagination, streaming, retries, customization with hooks, and much more. liblab also generates SDK documentation, allowing you to level-up your API documentation with SDK snippets. Let's dive into how to generate SDKs using liblab.

Using liblab

liblab generates SDKs from OpenAPI specification files. OpenAPI is the industry standard specification for describing HTTP APIs. There are dozens of community driven tools for working with OpenAPI, from documentation UIs, to framework integrations to generate the specification file for you. If you don't currently have an OpenAPI specification in hands for your API, consider looking for a library that integrates with your API's framework of choice.

With your OpenAPI spec in hands, we can jump straight to action. You can install the liblab cli using NPM:

npm install -g liblab

Or using homebrew:

brew tap liblaber/homebrew-liblab
brew install liblab

Login or register with liblab using:

liblab login

Check-out our detailed Get started with liblab guide if you have any issues

Initialize a liblab.config.json file by running:

liblab init

Open the generated liblab.config.json file, and update some settings. The main ones your looking for are:

• sdkName: the name of your generated SDK. Will be reflected in the SDK Client name.
• specFilePath: the path to your OpenAPI spec file. Supports local and remote paths.
• languages: what programming languages to generate SDKs for.
• auth: which authentication type your API expects.

Here's an example:

{
	"sdkName": "PetstoreSDK",
	"specFilePath": "./my-openapi-spec.json",
	"languages": [ "java", "python" ],
	"auth": [ "apikey" ],
	...
}

You can read a more in-depth explanation in Generate an SDK from your OpenAPI spec

You can set java specific configurations like groupId and artifactId under languageOptions → java:

{
	...
	"languageOptions": {
		"java": {
			"groupId": "com.example",
			"artifactId": "petstore-sdk"
		}
	}
}

When you're satisfied with your configuration, you can generate the SDKs by running:

liblab build

That's it! Your generated SDKs will be delivered to ./output/. liblab can be integrated with your CI/CD, so that new updated SDKs are generated whenever your API specification is updated. You can read more about it in our CI/CD integration overview.

Build Better SDKs, Faster

Offering an SDK gives you complete control over implementation details and lets offer the functionalities your developers need. You've learned how to structure models, create intuitive service methods, handle requests, and deliver a clean developer experience.

For production environments, liblab offers an efficient way to generate and maintain high-quality SDKs. It handles the complex implementation details while providing:

• Automatic SDK updates when your API changes
• Built-in support for authentication, pagination, and retries
• Generated documentation with code snippets
• Multi-language support from a single OpenAPI spec
• CI/CD integration for continuous delivery

Whether you choose to build your SDK manually or generate it with liblab, following these best practices will help you create SDKs that developers love to use. Start implementing these patterns today to improve your API's adoption and developer experience.