java.md

title	Java
description	Turn a Java class into a remotely callable backend service with Graftcode Gateway - no Spring, no REST routes, no OpenAPI specs. Any public method becomes instantly available to call from any language.

Goal

Turn a Java class into a remotely callable backend service using Graftcode Gateway - no Spring, no REST routes, no OpenAPI specs needed.

What You'll See

• Create a small Java class with public methods.
• Host it through Graftcode Gateway using Docker.
• Explore the exposed methods in Graftcode Vision - your service is now accessible from any app as a strongly-typed client.

Prerequisites

• Docker installed and running
• JDK 21 and Maven only if you build outside Docker - the Maven base image below already provides both for docker build

Step 1. Create a project folder

Create a new folder and set up a Maven project:

mkdir java-energy-service
cd java-energy-service

Create a pom.xml:

<?xml version="1.0" encoding="UTF-8"?>
<project xmlns="http://maven.apache.org/POM/4.0.0"
         xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
         xsi:schemaLocation="http://maven.apache.org/POM/4.0.0
                             http://maven.apache.org/xsd/maven-4.0.0.xsd">
    <modelVersion>4.0.0</modelVersion>

    <groupId>com.example</groupId>
    <artifactId>energy-service</artifactId>
    <version>1.0.0</version>

    <properties>
        <maven.compiler.source>21</maven.compiler.source>
        <maven.compiler.target>21</maven.compiler.target>
        <project.build.sourceEncoding>UTF-8</project.build.sourceEncoding>
    </properties>
</project>

Step 2. Write a Java class with public methods

Create the directory structure and a file src/main/java/energy/EnergyPriceCalculator.java:

package energy;

import java.util.Random;

public class EnergyPriceCalculator {
    public static int getPrice() {
        return new Random().nextInt(5) + 100;
    }
}

This is a plain Java class - no annotations, no frameworks, no special interfaces. Any public method you write here will automatically become available for remote consumption once hosted through Graftcode Gateway.

Step 3. Host it with Graftcode Gateway

Create a Dockerfile in the project root:

FROM maven:3.9-eclipse-temurin-21

WORKDIR /usr/app

COPY . /usr/app/

RUN mvn package -q

RUN curl -fsSL grft.dev/get/gg | sh && dpkg -i gg.deb && rm gg.deb

EXPOSE 80
EXPOSE 81

CMD ["gg","--modules", "/usr/app/target/energy-service-1.0.0.jar"]
 

The key line is the last one - gg (Graftcode Gateway) reads the compiled JAR passed via --modules, discovers all public methods in your classes, and exposes them automatically. Port 80 handles service calls, port 81 serves Graftcode Vision.

• FROM maven:3.9-eclipse-temurin-21 - Uses the official Maven image with JDK 21 as the base, which includes everything needed to build and run Java applications.
• COPY . /usr/app/ - Copies your project files (including pom.xml and source files) into the container.
• RUN mvn package -q - Compiles the project and packages it into a JAR in target/.
• RUN curl -fsSL grft.dev/get/gg | sh && dpkg -i gg.deb && rm gg.deb - curl downloads the installer, dpkg -i gg.deb installs Graftcode Gateway, and rm gg.deb removes the package.
• EXPOSE 80 - Declares the port used for service communication (Grafts connect here).
• EXPOSE 81 - Declares the port used by Graftcode Vision, the live portal for exploring and testing exposed methods.
• CMD ["gg", "--modules", "/usr/app/target/energy-service-1.0.0.jar"] - Runs Graftcode Gateway. --modules points Gateway at the compiled JAR to analyze; it discovers public methods and makes them callable. --modules is optional when the same directory as gg contains only one target JAR.

Build and run the container:

docker build --no-cache --pull -t myenergyservice-java:test .
docker run -d -p 80:80 -p 81:81 --name graftcode_demo_java myenergyservice-java:test

Your Java service is now running and exposed through Graftcode Gateway.

Step 4. Explore the service in Graftcode Vision

Open http://localhost:81/GV in your browser.

You will see all public methods from your Java class - their names, parameter types, and return types. Graftcode Vision also provides:

• A "Try it out" button to call methods live, directly from the browser.
• A package manager command (npm, NuGet, PyPI, Maven, etc.) to install this service as a strongly-typed client in any other application.

Step 5. Run with a Project Key (recommended for real-world usage)

Everything above works without any account - perfect for learning and local development. When you're ready for real-world usage, create a free account at portal.graftcode.com, set up a project, and copy its Project Key.

Then pass the key when starting your gateway:

CMD ["gg", "--modules", "/usr/app/target/energy-service-1.0.0.jar", "--projectKey", "YOUR_PROJECT_KEY"]

A Project Key gives you:

• Stable registry URL - consumers always find and update your Graft through a permanent address, so install commands don't change when you redeploy.
• Portal visibility - see all your gateways and exposed services in one place at gateways.graftcode.com.
• Access control - decide who can download your Grafts using package manager authentication and permissions.
• MCP endpoint - Graftcode Gateway automatically exposes an MCP (Model Context Protocol) endpoint alongside your service, making your methods callable by AI agents and LLM-based tools out of the box.

Step 6. Call it from another app

Your service is now accessible from any application. From Graftcode Vision, select your target package type - for example npm - and copy the generated install command. That installs a Graft: a strongly-typed client that lets any app call your service methods directly.

The generated npm package name comes from Maven groupId / artifactId and can be long - for this sample it is typically @graft/maven-groupid_com-example_artifactid_energy-service@1.0.0. Always prefer the exact command from Vision or Gateway logs.

import com.graft.maven.energypricecalculator.EnergyPriceCalculator;

var price = EnergyPriceCalculator.getPrice();
System.out.println(price);

No REST clients, no request/response models, no endpoint URLs in your code. When you add or update a public method, consumers update their Graft with a single package manager command.

Without Graftcode

Exposing backend logic for remote consumption typically requires:

• Designing and implementing REST or gRPC endpoints for each operation
• Defining request and response DTOs / schemas
• Writing or generating an OpenAPI / Protobuf spec
• Building or generating client SDKs for each consumer technology
• Maintaining and versioning the API contract over time
• Updating all clients whenever the backend changes

With Graftcode

• Write your business logic as plain public methods - no Spring, no routes, no specs
• Run it on Graftcode Gateway with one Dockerfile
• Consumers install a typed Graft via their package manager and call methods directly
• When you add or change a method, consumers update with one command - like any other dependency

Your Java class is now a fully accessible backend service - with one Dockerfile and two commands. Any public method you add is instantly discoverable and callable from any language and any platform. No endpoint design, no client SDK maintenance, no integration code.