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feat: Add a multi-transport sample for A2A Java SDK (#353)
# Description This PR adds a multi-transport sample using the A2A Java SDK. The server agent supports both gRPC and JSON-RPC, with gRPC being the preferred transport. The client supports both as well so the gRPC will be the selected transport. - [x] Follow the [`CONTRIBUTING` Guide](https://github.com/a2aproject/a2a-samples/blob/main/CONTRIBUTING.md).
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‎samples/java/agents/README.md‎

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* [**Weather Agent**](weather_mcp/README.md)
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Sample agent to provide weather information. To make use of this agent in a multi-language, multi-agent system, check out the [weather_and_airbnb_planner](../../python/hosts/weather_and_airbnb_planner/README.md) sample.
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* [**Dice Agent (Multi-Transport)**](dice_agent_multi_transport/README.md)
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Sample agent that can roll dice of different sizes and check if numbers are prime. This agent demonstrates
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multi-transport capabilities.
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## Disclaimer
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Important: The sample code provided is for demonstration purposes and illustrates the

‎samples/java/agents/dice_agent_multi_transport/README.md‎

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# Dice Agent (Multi-Transport)
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This sample agent can roll dice of different sizes and check if numbers are prime. This agent demonstrates
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multi-transport capabilities, supporting both gRPC and JSON-RPC transport protocols. The agent is written
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using Quarkus LangChain4j and makes use of the [A2A Java](https://github.com/a2aproject/a2a-java) SDK.
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## Prerequisites
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- Java 17 or higher
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- Access to an LLM and API Key
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## Running the Sample
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This sample consists of an A2A server agent, which is in the `server` directory, and an A2A client,
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which is in the `client` directory.
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### Running the A2A Server Agent
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1. Navigate to the `dice_agent_multi_transport` sample directory:
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```bash
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cd samples/java/agents/dice_agent_multi_transport/server
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```
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2. Set your Google AI Studio API Key as an environment variable:
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```bash
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export QUARKUS_LANGCHAIN4J_AI_GEMINI_API_KEY=your_api_key_here
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```
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Alternatively, you can create a `.env` file in the `dice_agent_multi_transport` directory:
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```bash
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QUARKUS_LANGCHAIN4J_AI_GEMINI_API_KEY=your_api_key_here
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```
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3. Start the A2A server agent
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**NOTE:**
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By default, the agent will start on port 11000. To override this, add the `-Dquarkus.http.port=YOUR_PORT`
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option at the end of the command below.
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```bash
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mvn quarkus:dev
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```
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### Running the A2A Java Client
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The Java `TestClient` communicates with the Dice Agent using the A2A Java SDK.
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Since the A2A server agent's [preferred transport](server/src/main/java/com/samples/a2a/DiceAgentCardProducer.java) is gRPC and since our client
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also [supports](client/src/main/java/com/samples/a2a/TestClient.java) gRPC, the gRPC transport will be used.
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1. Make sure you have [JBang installed](https://www.jbang.dev/documentation/guide/latest/installation.html)
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2. Run the client using the JBang script:
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```bash
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cd samples/java/agents/dice_agent_multi_transport/client/src/main/java/com/samples/a2a/client
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jbang TestClientRunner.java
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```
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Or specify a custom server URL:
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```bash
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jbang TestClientRunner.java --server-url http://localhost:11001
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```
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Or specify a custom message:
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```bash
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jbang TestClientRunner.java --message "Can you roll a 12-sided die and check if the result is prime?"
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```
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### Running the A2A Python Client
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You can also use a Python client to communicate with the Dice Agent using the A2A
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Python SDK.
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Since the A2A server agent's [preferred transport](server/src/main/java/com/samples/a2a/DiceAgentCardProducer.java) is gRPC and since our [client](client/src/main/java/com/samples/a2a/TestClient.java) also supports gRPC, the gRPC
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transport will be used.
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1. In a separate terminal, run the A2A client and use it to send a message to the
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agent:
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```bash
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cd samples/python/agents/dice_agent_grpc
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uv run test_client.py
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```
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## Expected Client Output
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Both the Java and Python A2A clients will:
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1. Connect to the dice agent
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2. Fetch the agent card
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3. Automatically select gRPC as the transport to be used
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4. Send the message "Can you roll a 5 sided die?"
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5. Display the dice roll result from the agent
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## Multi-Transport Support
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This sample demonstrates multi-transport capabilities by supporting both gRPC and
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JSON-RPC protocols. The A2A server agent is configured to use a unified port
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(11000 by default) for both transport protocols, as specified in the
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`application.properties` file with `quarkus.grpc.server.use-separate-server=false`.
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You can tweak the transports supported by the server or the client to experiment
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with different transport protocols.
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## Disclaimer
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Important: The sample code provided is for demonstration purposes and illustrates the
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mechanics of the Agent-to-Agent (A2A) protocol. When building production applications,
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it is critical to treat any agent operating outside of your direct control as a
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potentially untrusted entity.
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All data received from an external agent—including but not limited to its AgentCard,
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messages, artifacts, and task statuses—should be handled as untrusted input. For
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example, a malicious agent could provide an AgentCard containing crafted data in its
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fields (e.g., description, name, skills.description). If this data is used without
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sanitization to construct prompts for a Large Language Model (LLM), it could expose
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your application to prompt injection attacks. Failure to properly validate and
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sanitize this data before use can introduce security vulnerabilities into your
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application.
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Developers are responsible for implementing appropriate security measures, such as
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input validation and secure handling of credentials to protect their systems and users.

‎samples/java/agents/dice_agent_multi_transport/client/pom.xml‎

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<?xml version="1.0" encoding="UTF-8"?>
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<project xmlns="http://maven.apache.org/POM/4.0.0"
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xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
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xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd">
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<modelVersion>4.0.0</modelVersion>
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<parent>
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<groupId>com.samples.a2a</groupId>
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<artifactId>dice-agent-multi-transport</artifactId>
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<version>0.1.0</version>
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</parent>
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<artifactId>dice-agent-client</artifactId>
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<name>Dice Agent Client</name>
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<description>A2A Dice Agent Test Client</description>
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<dependencies>
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<dependency>
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<groupId>io.github.a2asdk</groupId>
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<artifactId>a2a-java-sdk-client</artifactId>
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<version>${io.a2a.sdk.version}</version>
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</dependency>
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<dependency>
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<groupId>io.github.a2asdk</groupId>
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<artifactId>a2a-java-sdk-client-transport-grpc</artifactId>
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<version>${io.a2a.sdk.version}</version>
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</dependency>
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<dependency>
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<groupId>com.fasterxml.jackson.core</groupId>
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<artifactId>jackson-databind</artifactId>
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</dependency>
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<dependency>
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<groupId>io.grpc</groupId>
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<artifactId>grpc-netty-shaded</artifactId>
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<scope>runtime</scope>
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</dependency>
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</dependencies>
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<build>
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<plugins>
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<plugin>
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<groupId>org.apache.maven.plugins</groupId>
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<artifactId>maven-compiler-plugin</artifactId>
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</plugin>
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<plugin>
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<groupId>org.codehaus.mojo</groupId>
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<artifactId>exec-maven-plugin</artifactId>
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<configuration>
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<mainClass>com.samples.a2a.TestClient</mainClass>
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</configuration>
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</plugin>
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</plugins>
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</build>
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</project>

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