feat: add example on how to expose agent via A2A protocol

a2a/pom.xml

@@ -104,6 +104,11 @@
       <version>${truth.version}</version>
       <scope>test</scope>
     </dependency>
+    <dependency>
+      <groupId>org.mockito</groupId>
+      <artifactId>mockito-core</artifactId>
+      <scope>test</scope>
+    </dependency>
   </dependencies>
   <build>
     <plugins>

contrib/samples/a2a_server/README.md

@@ -0,0 +1,70 @@
+# Google ADK A2A Agent Server Sample
+
+This sample demonstrates how to expose a Google ADK (Agent Development Kit)
+agent via the A2A (Agent-to-Agent) protocol using A2A SDK and Quarkus service.
+
+## Overview
+
+The application implements a simple conversational agent that checks whether
+given numbers are prime numbers. It uses the `LlmAgent` from the Google ADK and
+exposes it via an A2A server.
+
+### Key Components
+
+*   **`Agent.java`**: Defines the `LlmAgent` instance (`check_prime_agent`) and
+    the `checkPrime` tool function it uses to verify numbers.
+*   **`AgentCardProducer.java`**: Loads and provides the `AgentCard` metadata
+    (from `agent.json`) which defines the agent's identity and capabilities in
+    the A2A network.
+*   **`AgentExecutorProducer.java`**: Configures and provides the A2A
+    `AgentExecutor`, implemented by the ADK library to wire ADK-owned agents
+    automatically.
+*   **`StartupConfig.java`**: Contains initialization logic, such as registering
+    JSON modules for the Vert.x/Quarkus runtime.
+*   **`application.properties`**: Contains a configuration for the Quarkus
+    service and A2A, such as port where application will be exposed, application
+    name and event processing timeouts.
+
+## Building the Project
+
+You can build the project using Maven:
+
+```shell
+mvn clean install
+```
+
+The Java server can be started using `mvn` as follow (don't forget to set your
+GOOGLE_API_KEY before running the service):
+
+```bash
+export GOOGLE_API_KEY=<YOUR_API_KEY>
+
+cd contrib/samples/a2a_server
+mvn quarkus:dev
+```
+
+## Sample request
+
+```bash
+curl -X POST http://localhost:9090 \
+  -H 'Content-Type: application/json' \
+  -d '{
+        "jsonrpc": "2.0",
+        "id": "cli-check-2",
+        "method": "message/stream",
+        "params": {
+          "message": {
+            "kind": "message",
+            "contextId": "cli-demo-context",
+            "messageId": "cli-check-2",
+            "role": "user",
+            "parts": [
+              {
+                "kind": "text",
+                "text": "Is 2 prime?"
+              }
+            ]
+          }
+        }
+      }'
+```

contrib/samples/a2a_server/pom.xml

@@ -0,0 +1,142 @@
+<?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>
+
+  <parent>
+    <groupId>com.google.adk</groupId>
+    <artifactId>google-adk-samples</artifactId>
+    <version>0.7.1-SNAPSHOT</version><!-- {x-version-update:google-adk:current} -->
+    <relativePath>..</relativePath>
+  </parent>
+
+  <artifactId>google-adk-sample-a2a-agent</artifactId>
+  <packaging>jar</packaging>
+
+  <name>Google ADK - Sample - A2A Agent Server</name>
+  <description>Demonstrates exposing ADK agent via A2A.</description>
+
+  <properties>
+    <project.build.sourceEncoding>UTF-8</project.build.sourceEncoding>
+    <java.version>17</java.version>
+    <google-adk.version>${project.version}</google-adk.version>
+    <google-adk-a2a.version>${project.version}</google-adk-a2a.version>
+    <a2a.sdk.version>0.3.0.Beta1</a2a.sdk.version>
+    <quarkus.platform.version>3.30.6</quarkus.platform.version>
+    <flogger.version>0.8</flogger.version>
+  </properties>
+
+    <dependencyManagement>
+        <dependencies>
+            <dependency>
+                <groupId>io.quarkus</groupId>
+                <artifactId>quarkus-bom</artifactId>
+                <version>${quarkus.platform.version}</version>
+                <type>pom</type>
+                <scope>import</scope>
+            </dependency>
+        </dependencies>
+    </dependencyManagement>
+
+  <dependencies>
+    <dependency>
+      <groupId>io.github.a2asdk</groupId>
+      <artifactId>a2a-java-sdk-reference-jsonrpc</artifactId>
+      <version>${a2a.sdk.version}</version>
+    </dependency>
+    <dependency>
+      <groupId>io.quarkus</groupId>
+      <artifactId>quarkus-resteasy-jackson</artifactId>
+    </dependency>
+    <dependency>
+        <groupId>com.fasterxml.jackson.datatype</groupId>
+        <artifactId>jackson-datatype-jsr310</artifactId>
+    </dependency>
+    <dependency>
+      <groupId>com.google.adk</groupId>
+      <artifactId>google-adk</artifactId>
+      <version>${google-adk.version}</version>
+    </dependency>
+    <dependency>
+      <groupId>com.google.adk</groupId>
+      <artifactId>google-adk-a2a</artifactId>
+      <version>${google-adk-a2a.version}</version>
+    </dependency>
+
+    <dependency>
+      <groupId>io.github.a2asdk</groupId>
+      <artifactId>a2a-java-sdk-spec</artifactId>
+      <version>${a2a.sdk.version}</version>
+    </dependency>
+
+    <dependency>
+        <groupId>io.quarkus</groupId>
+        <artifactId>quarkus-arc</artifactId>
+    </dependency>
+    <dependency>
+        <groupId>io.quarkus</groupId>
+        <artifactId>quarkus-reactive-routes</artifactId>
+    </dependency>
+    <dependency>
+        <groupId>io.quarkus</groupId>
+        <artifactId>quarkus-jackson</artifactId>
+    </dependency>
+
+    <dependency>
+      <groupId>io.github.a2asdk</groupId>
+      <artifactId>a2a-java-sdk-client</artifactId>
+      <version>${a2a.sdk.version}</version>
+    </dependency>
+
+    <dependency>
+      <groupId>com.google.flogger</groupId>
+      <artifactId>flogger</artifactId>
+      <version>${flogger.version}</version>
+    </dependency>
+
+    <dependency>
+      <groupId>com.google.flogger</groupId>
+      <artifactId>google-extensions</artifactId>
+      <version>${flogger.version}</version>
+    </dependency>
+
+    <dependency>
+      <groupId>com.google.flogger</groupId>
+      <artifactId>flogger-system-backend</artifactId>
+      <version>${flogger.version}</version>
+    </dependency>
+
+  </dependencies>
+
+  <build>
+    <resources>
+        <resource>
+            <directory>src/main/java</directory>
+            <includes>
+                <include>**/*.json</include>
+            </includes>
+        </resource>
+        <resource>
+            <directory>src/main/resources</directory>
+        </resource>
+    </resources>
+        <plugins>
+            <plugin>
+              <groupId>io.quarkus</groupId>
+                  <artifactId>quarkus-maven-plugin</artifactId>
+                  <version>${quarkus.platform.version}</version> 
+                  <extensions>true</extensions>
+                  <executions>
+                      <execution>
+                          <goals>
+                              <goal>build</goal>
+                              <goal>generate-code</goal>
+                              <goal>generate-code-tests</goal>
+                          </goals>
+                      </execution>
+                  </executions>
+              </plugin>
+        </plugins>
+    </build>
+</project>

contrib/samples/a2a_server/src/main/java/com/google/adk/samples/a2aagent/AgentCardProducer.java

@@ -0,0 +1,33 @@
+package com.google.adk.samples.a2aagent;
+
+import io.a2a.server.PublicAgentCard;
+import io.a2a.spec.AgentCard;
+import io.a2a.util.Utils;
+import jakarta.enterprise.context.ApplicationScoped;
+import jakarta.enterprise.inject.Produces;
+import java.io.InputStream;
+import java.nio.charset.StandardCharsets;
+
+/** Produces the {@link AgentCard} from the bundled JSON resources. */
+@ApplicationScoped
+public class AgentCardProducer {
+
+  @Produces
+  @PublicAgentCard
+  public AgentCard agentCard() {
+    try (InputStream is = getClass().getResourceAsStream("/agent/agent.json")) {
+      if (is == null) {
+        throw new RuntimeException("agent.json not found in resources");
+      }
+
+      // Read the JSON file content
+      String json = new String(is.readAllBytes(), StandardCharsets.UTF_8);
+
+      // Use the SDK's built-in mapper to convert JSON string to AgentCard record
+      return Utils.OBJECT_MAPPER.readValue(json, AgentCard.class);
+
+    } catch (Exception e) {
+      throw new RuntimeException("Failed to load AgentCard from JSON", e);
+    }
+  }
+}

contrib/samples/a2a_server/src/main/java/com/google/adk/samples/a2aagent/AgentExecutorProducer.java

@@ -0,0 +1,28 @@
+package com.google.adk.samples.a2aagent;
+
+import com.google.adk.a2a.executor.AgentExecutorConfig;
+import com.google.adk.samples.a2aagent.agent.Agent;
+import com.google.adk.sessions.InMemorySessionService;
+import io.a2a.server.agentexecution.AgentExecutor;
+import jakarta.enterprise.context.ApplicationScoped;
+import jakarta.enterprise.inject.Produces;
+import org.eclipse.microprofile.config.inject.ConfigProperty;
+
+/** Produces the {@link AgentExecutor} instance that handles agent interactions. */
+@ApplicationScoped
+public class AgentExecutorProducer {
+
+  @ConfigProperty(name = "my.adk.app.name", defaultValue = "default-app")
+  String appName;
+
+  @Produces
+  public AgentExecutor agentExecutor() {
+    InMemorySessionService sessionService = new InMemorySessionService();
+    return new com.google.adk.a2a.executor.AgentExecutor.Builder()
+        .agent(Agent.ROOT_AGENT)
+        .appName(appName)
+        .sessionService(sessionService)
+        .agentExecutorConfig(AgentExecutorConfig.builder().build())
+        .build();
+  }
+}

contrib/samples/a2a_server/src/main/java/com/google/adk/samples/a2aagent/StartupConfig.java

@@ -0,0 +1,17 @@
+package com.google.adk.samples.a2aagent;
+
+import com.fasterxml.jackson.datatype.jsr310.JavaTimeModule;
+import io.quarkus.runtime.StartupEvent;
+import io.vertx.core.json.jackson.DatabindCodec;
+import jakarta.enterprise.context.ApplicationScoped;
+import jakarta.enterprise.event.Observes;
+
+/** Configuration applied on startup, such as Jackson module registrations. */
+@ApplicationScoped
+public class StartupConfig {
+
+  void onStart(@Observes StartupEvent ev) {
+    // Register globally for Vert.x's internal JSON handling
+    DatabindCodec.mapper().registerModule(new JavaTimeModule());
+  }
+}