writing-extension-devui

name	writing-extension-devui
description	How to add a Dev UI page to a Quarkus extension: deployment processors, runtime-dev JSON-RPC services, and Lit web components.

Writing a Dev UI for a Quarkus Extension

Dev UI is the interactive dashboard at /q/dev-ui during quarkus:dev. Extensions add pages via build items, runtime JSON-RPC services, and Lit web components. See the Dev UI guide for full documentation.

Directory Layout

my-extension/
  deployment/src/main/java/.../deployment/devui/
    MyFeatureDevUIProcessor.java          # Build steps
  deployment/src/main/resources/dev-ui/
    qwc-myfeature-dashboard.js            # Lit web components
  runtime-dev/src/main/java/.../runtime/dev/ui/
    MyFeatureJsonRpcService.java          # JSON-RPC service

• JS naming: qwc-<extensionname>-<pagename>.js
• JSON-RPC services go in runtime-dev/, not runtime/. Register as a conditional dev dependency — see the classloading-and-runtime-dev skill.

Deployment Processor

Gate all Dev UI build steps with @BuildStep(onlyIf = IsDevelopment.class) or use @BuildSteps(onlyIf = IsLocalDevelopment.class) at the class level.

import io.quarkus.devui.spi.page.CardPageBuildItem;
import io.quarkus.devui.spi.page.Page;

@BuildStep(onlyIf = IsDevelopment.class)
CardPageBuildItem devUI() {
    CardPageBuildItem card = new CardPageBuildItem();

    card.addPage(Page.webComponentPageBuilder()
            .title("Dashboard")
            .componentLink("qwc-myfeature-dashboard.js")
            .icon("font-awesome-solid:robot"));

    // Build-time data — available in JS via: import { items } from 'build-time-data';
    card.addBuildTimeData("items", someList);

    return card;
}

Register the JSON-RPC provider in a separate build step. This does not need the IsDevelopment guard because the build item is only consumed in dev mode:

import io.quarkus.devui.spi.JsonRPCProvidersBuildItem;

@BuildStep
JsonRPCProvidersBuildItem jsonRpcProvider() {
    return new JsonRPCProvidersBuildItem(MyFeatureJsonRpcService.class);
}

Maven dependency for the deployment module:

<dependency>
    <groupId>io.quarkus</groupId>
    <artifactId>quarkus-devui-spi</artifactId>
</dependency>

Runtime JSON-RPC Service

A plain class in runtime-dev. Every public method becomes a JSON-RPC endpoint automatically — registration happens via JsonRPCProvidersBuildItem.

public class MyFeatureJsonRpcService {
    @Inject
    SomeBean bean;

    public List<Item> getItems() { return bean.listAll(); }
    public boolean doAction(String id) { return bean.execute(id); }
}

• Use @Inject for CDI; @PostConstruct for initialization.
• Return JSON-serializable data, not HTML.
• For streaming, return Multi<JsonObject> (Smallrye Mutiny).

Frontend Web Components

Components extend QwcHotReloadElement (not LitElement directly) and use Vaadin Web Components for consistent styling.

import { QwcHotReloadElement, html, css } from 'qwc-hot-reload-element';
import { JsonRpc } from 'jsonrpc';
import { items } from 'build-time-data';

export class QwcMyfeatureDashboard extends QwcHotReloadElement {
    jsonRpc = new JsonRpc(this);
    static properties = { _items: { state: true } };

    constructor() {
        super();
        this._items = items;
    }

    connectedCallback() {
        super.connectedCallback();
        this.hotReload();
    }

    hotReload() {
        this.jsonRpc.getItems().then(r => { this._items = r.result; });
    }

    render() {
        if (!this._items)
             html`;
         html`;
    }
}
customElements.(, );

• build-time-data keys must match what was passed to card.addBuildTimeData(key, value).
• JsonRpc method names must match the Java service method names exactly.
• Access results via response.result.
• For state updates, use spread: this._items = [...this._items, newItem].
• Unsubscribe streaming observers in disconnectedCallback().

Testing

Extend DevUIJsonRPCTest (io.quarkus.devui.tests). Pass the extension namespace to the super constructor, then call executeJsonRPCMethod():

public class MyFeatureDevUITest extends DevUIJsonRPCTest {
    @RegisterExtension
    static final QuarkusDevModeTest config = new QuarkusDevModeTest()
            .withApplicationRoot((jar) -> jar.addClass(MyBean.class));

    public MyFeatureDevUITest() { super("quarkus-myfeature"); }

    @Test
    public void testGetItems() throws Exception {
        JsonNode result = super.executeJsonRPCMethod("getItems");
        assertNotNull(result);
    }
}

Key Rules

• Correct imports: CardPageBuildItem is in io.quarkus.devui.spi.page, JsonRPCProvidersBuildItem is in io.quarkus.devui.spi.
• JSON-RPC services belong in runtime-dev/, never in runtime/.
• JS files go in deployment/src/main/resources/dev-ui/.
• Extend QwcHotReloadElement, not LitElement — it provides the hotReload() hook that re-runs on dev-mode restarts.
• Return JSON from services, not HTML. Use Page.externalPageBuilder() for external content like Swagger UI.