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Cover image for It Has Never Been This Easy to Build Gen AI Features in Java
Google Developer Experts profile image Xavier Portilla Edo
Xavier Portilla Edo for Google Developer Experts

Posted on • Originally published at xavidop.me

It Has Never Been This Easy to Build Gen AI Features in Java

#genkit #gemini #ai #genai

Introduction

Building generative AI applications in Java used to be a complex, boilerplate-heavy endeavor. You'd wrestle with raw HTTP clients, hand-craft JSON payloads, parse streaming responses, manage API keys, and stitch together observability, all before writing a single line of actual AI logic. Those days are over.

Genkit Java is an open-source framework that makes building AI-powered applications in Java as straightforward as defining a function. Pair it with Google's Gemini models and Google Cloud Run, and you can go from zero to a production-deployed generative AI service in minutes, not days.

This is a complete, working example. Clone it, set your API key, and run.

Why Genkit Java?

If you're a Java developer, you've probably watched the Gen AI revolution unfold mostly in Python and TypeScript. The tooling, the frameworks, the tutorials, all skewed toward those ecosystems. Java developers were left to either build everything from scratch or use verbose, low-level SDKs.

What We're Building

A Java application with a translation AI flow powered by Gemini via Genkit, showcasing:

  • Typed flow inputs, TranslateRequest class with @JsonProperty annotations as the flow input
  • Structured LLM output, Gemini returns a TranslateResponse Java object directly (no manual JSON parsing)
  • Typed flow outputs, The flow returns a fully typed TranslateResponse to the caller

All of this in a single Java file + two model classes. No Spring Boot. No annotations soup. No XML configuration. Just clean, readable, type-safe code.

Prerequisites

  • Java 21+ (Eclipse Temurin recommended)
  • Maven 3.6+
  • Node.js 18+ (for the Genkit CLI)
  • A Google GenAI API key (free from Google AI Studio)
  • Google Cloud SDK (only for Cloud Run deployment)

Install the Genkit CLI

The Genkit CLI is your command-line companion for developing and testing AI flows. Install it globally:

npm install -g genkit
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Verify the installation:

genkit --version
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The CLI is what powers the Dev UI and provides a seamless development experience, more on that below.

Project Structure 

genkit-java-getting-started/
├── src/
│   └── main/
│       ├── java/
│       │   └── com/example/
│       │       ├── App.java                # ← The main application
│       │       ├── TranslateRequest.java   # ← Typed flow input
│       │       └── TranslateResponse.java  # ← Typed flow + LLM output
│       └── resources/
│           └── logback.xml                 # Logging configuration
├── pom.xml                                 # Maven config with Genkit + Jib
├── run.sh                                  # Quick-start script
└── README.md                               # This article
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Getting Started

1. Clone and Set Your API Key 

git clone https://github.com/xavidop/genkit-java-getting-started.git
cd genkit-java-getting-started

export GOOGLE_API_KEY=your-api-key-here
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2. Run with the Genkit Dev UI (Recommended) 

genkit start -- mvn compile exec:java
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That's it. Two commands. Your AI-powered Java server is running on http://localhost:8080, and the Genkit Dev UI is available at http://localhost:4000.

3. Or Run Directly (Without Dev UI) 

mvn compile exec:java
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The Code, It's Stupidly Simple

Step 1: Define Typed Input/Output Classes

Instead of using raw Map or String, define proper Java classes with Jackson annotations. Genkit uses these annotations to generate JSON schemas that tell Gemini exactly what structure to return.

TranslateRequest.java, the flow input:

import com.fasterxml.jackson.annotation.JsonProperty;
import com.fasterxml.jackson.annotation.JsonPropertyDescription;

/**
 * Input for the translate flow.
 */
public class TranslateRequest {

    @JsonProperty(required = true)
    @JsonPropertyDescription("The text to translate")
    private String text;

    @JsonProperty(required = true)
    @JsonPropertyDescription("The target language (e.g., Spanish, French, Japanese)")
    private String language;

    public TranslateRequest() {}

    public TranslateRequest(String text, String language) {
        this.text = text;
        this.language = language;
    }

    public String getText() {
        return text;
    }

    public void setText(String text) {
        this.text = text;
    }

    public String getLanguage() {
        return language;
    }

    public void setLanguage(String language) {
        this.language = language;
    }

    @Override
    public String toString() {
        return String.format("TranslateRequest{text='%s', language='%s'}", text, language);
    }
}
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TranslateResponse.java, the flow output and the LLM structured output:

import com.fasterxml.jackson.annotation.JsonProperty;
import com.fasterxml.jackson.annotation.JsonPropertyDescription;

/**
 * Structured output for the translate flow.
 */
public class TranslateResponse {

    @JsonProperty(required = true)
    @JsonPropertyDescription("The original text that was translated")
    private String originalText;

    @JsonProperty(required = true)
    @JsonPropertyDescription("The translated text")
    private String translatedText;

    @JsonProperty(required = true)
    @JsonPropertyDescription("The target language")
    private String language;

    public TranslateResponse() {}

    public TranslateResponse(String originalText, String translatedText, String language) {
        this.originalText = originalText;
        this.translatedText = translatedText;
        this.language = language;
    }

    public String getOriginalText() {
        return originalText;
    }

    public void setOriginalText(String originalText) {
        this.originalText = originalText;
    }

    public String getTranslatedText() {
        return translatedText;
    }

    public void setTranslatedText(String translatedText) {
        this.translatedText = translatedText;
    }

    public String getLanguage() {
        return language;
    }

    public void setLanguage(String language) {
        this.language = language;
    }

    @Override
    public String toString() {
        return String.format(
            "TranslateResponse{originalText='%s', translatedText='%s', language='%s'}",
            originalText, translatedText, language);
    }
}
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The @JsonPropertyDescription annotations are key, Genkit passes them to Gemini as part of the JSON schema, so the model knows exactly what each field means.

Step 2: Initialize Genkit 

Genkit genkit = Genkit.builder()
    .options(GenkitOptions.builder()
        .devMode(true)
        .reflectionPort(3100)
        .build())
    .plugin(GoogleGenAIPlugin.create())
    .plugin(jetty)
    .build();
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That's the entire setup. The GoogleGenAIPlugin reads your GOOGLE_API_KEY automatically. The JettyPlugin handles HTTP. Genkit wires everything together.

Step 3: Define a Flow with Typed Classes and Structured Output 

genkit.defineFlow(
    "translate",
    TranslateRequest.class,     // ← typed input
    TranslateResponse.class,    // ← typed output
    (ctx, request) -> {
        String prompt = String.format(
            "Translate the following text to %s.\n\nText: %s",
            request.getLanguage(), request.getText()
        );

        return genkit.generate(
            GenerateOptions.<TranslateResponse>builder()
                .model("googleai/gemini-3-flash-preview")
                .prompt(prompt)
                .outputClass(TranslateResponse.class)  // ← Gemini returns a typed object!
                .config(GenerationConfig.builder()
                    .temperature(0.1)
                    .build())
                .build()
        );
    }
);
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Look at what's happening here:

  1. TranslateRequest.class as the flow input, Genkit automatically deserializes incoming JSON into a TranslateRequest object. No Map.get() casting.
  2. TranslateResponse.class as the flow output, the flow returns a typed object, serialized automatically to JSON for the HTTP response.
  3. outputClass(TranslateResponse.class) on the generate call, this is the magic. Genkit sends the JSON schema derived from TranslateResponse to Gemini, and Gemini returns structured JSON that Genkit deserializes into a TranslateResponse object. No response.getText() + manual parsing.

That single defineFlow call:

  • Registers the flow in Genkit's internal registry
  • Exposes it as a POST /api/flows/translate HTTP endpoint
  • Makes it visible in the Dev UI
  • Adds full OpenTelemetry tracing automatically
  • Tracks token usage, latency, and error rates

Compare that to writing a Spring Boot controller + service + DTO + config + exception handler for the same functionality.

The Genkit Dev UI - Your AI Playground

This is where Genkit truly shines for development. When you run with genkit start, the CLI launches a visual Dev UI at http://localhost:4000.

What Can You Do in the Dev UI?

  • Browse all flows, See every flow you've registered, like translate, with its typed input/output schemas.
  • Run flows interactively, Fill in a TranslateRequest JSON, click "Run", see the TranslateResponse instantly. No cURL needed.
  • Inspect traces, Every flow execution is traced. See exactly which model was called, what the input/output was, how long it took, and how many tokens were used.
  • View registered models & tools, See all available Gemini models and any tools you've defined.
  • Test tool calling, Watch Gemini decide to call your tools in real-time.
  • Manage datasets & evaluations, Create test datasets and evaluate your AI outputs.

Deploying to Google Cloud Run

The project uses Jib to build and push container images directly from Maven, no Dockerfile and no Docker daemon required. Jib is configured in the pom.xml and builds optimized, layered container images.

Step-by-Step Deployment 

# Set your GCP project
export PROJECT_ID=$(gcloud config get-value project)
export REGION=us-central1

# Build the container image and push it to Google Container Registry
# No Docker needed, Jib does it all from Maven!
mvn compile jib:build -Djib.to.image=gcr.io/$PROJECT_ID/genkit-java-app

# Deploy to Cloud Run
gcloud run deploy genkit-java-app \
  --image gcr.io/$PROJECT_ID/genkit-java-app \
  --region $REGION \
  --platform managed \
  --allow-unauthenticated \
  --set-env-vars "GOOGLE_API_KEY=$GOOGLE_API_KEY" \
  --memory 512Mi \
  --cpu 1
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Two commands. No Docker. Your Java GenAI application is now live on a globally-distributed, auto-scaling, serverless platform.

Why Jib?

  • No Dockerfile, Container image is built directly from your Maven project
  • No Docker daemon, Doesn't require Docker installed or running on your machine
  • Fast rebuilds, Separates dependencies, classes, and resources into layers, so only changed layers are rebuilt
  • Reproducible, Builds are deterministic and don't depend on the local Docker environment
  • Direct push, Sends the image straight to GCR/Artifact Registry without a local docker push

You can also build a local Docker image (requires Docker running) with:

mvn compile jib:dockerBuild -Djib.to.image=genkit-java-app
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Available Flows & API Examples

Once the server is running, test the translate flow:

Translate Text

Send a TranslateRequest JSON object and receive a structured TranslateResponse:

curl -X POST http://localhost:8080/api/flows/translate \
  -H 'Content-Type: application/json' \
  -d '{"text": "Building AI applications has never been easier", "language": "Spanish"}'
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Example response (a TranslateResponse object):

{
  "originalText": "Building AI applications has never been easier",
  "translatedText": "Construir aplicaciones de IA nunca ha sido tan fácil",
  "language": "Spanish"
}
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Try other languages:

# French
curl -X POST http://localhost:8080/api/flows/translate \
  -H 'Content-Type: application/json' \
  -d '{"text": "Genkit makes Java AI development simple", "language": "French"}'

# Japanese
curl -X POST http://localhost:8080/api/flows/translate \
  -H 'Content-Type: application/json' \
  -d '{"text": "Hello world", "language": "Japanese"}'
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Notice how the response is always a structured JSON object, not a raw string. That's the power of outputClass(TranslateResponse.class). Gemini returns structured data that Genkit deserializes into your Java class automatically.

What Genkit Gives You for Free

When you use Genkit, you're not just getting a wrapper around API calls. You get a production-grade framework:

Observability (Zero Config)

Every flow execution is automatically traced with OpenTelemetry:

  • Latency tracking per flow, per model call
  • Token usage (input/output/thinking tokens)
  • Error rates and failure tracking
  • Span hierarchy showing the full execution path

Plugin Ecosystem

Need to swap Gemini for another model? Change one line:

// Switch from Gemini to OpenAI
.plugin(OpenAIPlugin.create())

// Or use Anthropic Claude
.plugin(AnthropicPlugin.create())

// Or run locally with Ollama
.plugin(OllamaPlugin.create())
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Genkit supports 10+ model providers, vector databases (Pinecone, Weaviate, PostgreSQL), Firebase integration, and more.

Type Safety

This is where Genkit really shines for Java developers. Flows, generate calls, and even LLM responses are fully typed:

// The flow takes a TranslateRequest and returns a TranslateResponse
genkit.defineFlow("translate", TranslateRequest.class, TranslateResponse.class, ...);

// The LLM returns a TranslateResponse directly, no string parsing
genkit.generate(
    GenerateOptions.<TranslateResponse>builder()
        .outputClass(TranslateResponse.class)
        .build()
);
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Genkit derives JSON schemas from your @JsonProperty and @JsonPropertyDescription annotations and sends them to Gemini, so the model returns structured data that maps directly to your Java classes. No Object casting, no response.getText() + objectMapper.readValue(), no runtime surprises.

What's Next?

This getting-started project covers the fundamentals. Genkit Java can do much more:

  • RAG, Retrieval-Augmented Generation with vector stores (Firestore, Pinecone, pgvector, Weaviate)
  • Multi-agent orchestration, Coordinate multiple AI agents
  • Chat sessions, Multi-turn conversations with session persistence
  • Evaluations, RAGAS-style metrics to measure your AI output quality
  • MCP Integration, Connect to Model Context Protocol servers
  • Spring Boot, Use the Spring plugin instead of Jetty for existing Spring apps
  • Firebase, Deploy as Cloud Functions with Firestore vector search

Explore the full Genkit Java documentation and the samples directory to dive deeper.

Conclusion

As you can see, it is very easy to use Genkit Java and Gemini to build powerful generative AI applications with minimal code. The combination of typed inputs/outputs, structured LLM responses, built-in observability, and seamless deployment makes Genkit Java the best way to build GenAI features in Java.

You can find the full code of this example in the GitHub repository

Happy coding!

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