feat: Add pluggable A2AHttpClient with Vert.x implementation

Introduce ServiceLoader-based A2AHttpClient client architecture with two implementations:
- JDK HttpClient (core, always available)
- Vert.x WebClient (optional, high-performance)

Changes:
- Create http-client-vertx module for Vert.x implementation
- Add ServiceLoader infrastructure (A2AHttpClientFactory, providers)
- Update dependent modules to use Vert.x client by default
- Add comprehensive tests and usage examples

Users can choose implementation via Maven dependencies.
Priority-based selection: Vert.x (100) > JDK (0).

Signed-off-by: Emmanuel Hugonnet <ehugonne@redhat.com>

Author: ehsavoie

boms/sdk/pom.xml

@@ -49,6 +49,11 @@
                 <artifactId>a2a-java-sdk-http-client</artifactId>
                 <version>${project.version}</version>
             </dependency>
+            <dependency>
+                <groupId>${project.groupId}</groupId>
+                <artifactId>a2a-java-sdk-http-client-vertx</artifactId>
+                <version>${project.version}</version>
+            </dependency>
 
             <!-- Server modules -->
             <dependency>

client/base/pom.xml

@@ -22,6 +22,11 @@
             <groupId>${project.groupId}</groupId>
             <artifactId>a2a-java-sdk-http-client</artifactId>
         </dependency>
+        <dependency>
+            <groupId>${project.groupId}</groupId>
+            <artifactId>a2a-java-sdk-http-client-vertx</artifactId>
+            <scope>provided</scope>
+        </dependency>
         <dependency>
             <groupId>${project.groupId}</groupId>
             <artifactId>a2a-java-sdk-client-transport-spi</artifactId>

client/base/src/main/java/io/a2a/A2A.java

@@ -6,7 +6,7 @@
 
 import io.a2a.client.http.A2ACardResolver;
 import io.a2a.client.http.A2AHttpClient;
-import io.a2a.client.http.JdkA2AHttpClient;
+import io.a2a.client.http.A2AHttpClientFactory;
 import io.a2a.spec.A2AClientError;
 import io.a2a.spec.A2AClientJSONError;
 import io.a2a.spec.AgentCard;
@@ -286,7 +286,7 @@ private static Message toMessage(List<Part<?>> parts, Message.Role role, String
      * @see AgentCard
      */
     public static AgentCard getAgentCard(String agentUrl) throws A2AClientError, A2AClientJSONError {
-        return getAgentCard(new JdkA2AHttpClient(), agentUrl);
+        return getAgentCard(A2AHttpClientFactory.create(), agentUrl);
     }
 
     /**
@@ -357,7 +357,7 @@ public static AgentCard getAgentCard(A2AHttpClient httpClient, String agentUrl)
      * @throws io.a2a.spec.A2AClientJSONError if the response body cannot be decoded as JSON or validated against the AgentCard schema
      */
     public static AgentCard getAgentCard(String agentUrl, String relativeCardPath, Map<String, String> authHeaders) throws A2AClientError, A2AClientJSONError {
-        return getAgentCard(new JdkA2AHttpClient(), agentUrl, relativeCardPath, authHeaders);
+        return getAgentCard(A2AHttpClientFactory.create(), agentUrl, relativeCardPath, authHeaders);
     }
 
     /**

client/base/src/test/java/io/a2a/client/ClientBuilderTest.java

@@ -6,7 +6,7 @@
 import java.util.List;
 
 import io.a2a.client.config.ClientConfig;
-import io.a2a.client.http.JdkA2AHttpClient;
+import io.a2a.client.http.A2AHttpClientFactory;
 import io.a2a.client.transport.grpc.GrpcTransport;
 import io.a2a.client.transport.grpc.GrpcTransportConfigBuilder;
 import io.a2a.client.transport.jsonrpc.JSONRPCTransport;
@@ -89,7 +89,7 @@ public void shouldCreateClient_differentConfigurations() throws A2AClientExcepti
         Client client = Client
                 .builder(card)
                 .withTransport(JSONRPCTransport.class, new JSONRPCTransportConfigBuilder())
-                .withTransport(JSONRPCTransport.class, new JSONRPCTransportConfig(new JdkA2AHttpClient()))
+                .withTransport(JSONRPCTransport.class, new JSONRPCTransportConfig(A2AHttpClientFactory.create()))
                 .build();
 
         Assertions.assertNotNull(client);

client/transport/jsonrpc/pom.xml

@@ -21,6 +21,10 @@
             <groupId>${project.groupId}</groupId>
             <artifactId>a2a-java-sdk-http-client</artifactId>
         </dependency>
+        <dependency>
+            <groupId>${project.groupId}</groupId>
+            <artifactId>a2a-java-sdk-http-client-vertx</artifactId>
+        </dependency>
         <dependency>
             <groupId>${project.groupId}</groupId>
             <artifactId>a2a-java-sdk-client-transport-spi</artifactId>

client/transport/jsonrpc/src/main/java/io/a2a/client/transport/jsonrpc/JSONRPCTransport.java

@@ -23,8 +23,8 @@
 import com.google.protobuf.MessageOrBuilder;
 import io.a2a.client.http.A2ACardResolver;
 import io.a2a.client.http.A2AHttpClient;
+import io.a2a.client.http.A2AHttpClientFactory;
 import io.a2a.client.http.A2AHttpResponse;
-import io.a2a.client.http.JdkA2AHttpClient;
 import io.a2a.client.transport.jsonrpc.sse.SSEEventListener;
 import io.a2a.client.transport.spi.ClientTransport;
 import io.a2a.client.transport.spi.interceptors.ClientCallContext;
@@ -84,7 +84,7 @@ public JSONRPCTransport(AgentCard agentCard) {
 
     public JSONRPCTransport(@Nullable A2AHttpClient httpClient, @Nullable AgentCard agentCard,
                             AgentInterface agentInterface, @Nullable List<ClientCallInterceptor> interceptors) {
-        this.httpClient = httpClient == null ? new JdkA2AHttpClient() : httpClient;
+        this.httpClient = httpClient == null ? A2AHttpClientFactory.create() : httpClient;
         this.agentCard = agentCard;
         this.agentInterface = agentInterface;
         this.interceptors = interceptors;

client/transport/jsonrpc/src/main/java/io/a2a/client/transport/jsonrpc/JSONRPCTransportConfigBuilder.java

@@ -1,7 +1,7 @@
 package io.a2a.client.transport.jsonrpc;
 
 import io.a2a.client.http.A2AHttpClient;
-import io.a2a.client.http.JdkA2AHttpClient;
+import io.a2a.client.http.A2AHttpClientFactory;
 import io.a2a.client.transport.spi.ClientTransportConfigBuilder;
 import org.jspecify.annotations.Nullable;
 
@@ -11,7 +11,7 @@
  * This builder provides a fluent API for configuring the JSON-RPC transport protocol.
  * All configuration options are optional - if not specified, sensible defaults are used:
  * <ul>
- *   <li><b>HTTP client:</b> {@link JdkA2AHttpClient} (JDK's built-in HTTP client)</li>
+ *   <li><b>HTTP client:</b> Auto-selected via {@link A2AHttpClientFactory} (prefers Vert.x, falls back to JDK)</li>
  *   <li><b>Interceptors:</b> None</li>
  * </ul>
  * <p>
@@ -78,7 +78,7 @@ public class JSONRPCTransportConfigBuilder extends ClientTransportConfigBuilder<
      *   <li>Custom header handling</li>
      * </ul>
      * <p>
-     * If not specified, the default {@link JdkA2AHttpClient} is used.
+     * If not specified, a client is auto-selected via {@link A2AHttpClientFactory}.
      * <p>
      * Example:
      * <pre>{@code
@@ -101,16 +101,16 @@ public JSONRPCTransportConfigBuilder httpClient(A2AHttpClient httpClient) {
     /**
      * Build the JSON-RPC transport configuration.
      * <p>
-     * If no HTTP client was configured, the default {@link JdkA2AHttpClient} is used.
+     * If no HTTP client was configured, one is auto-selected via {@link A2AHttpClientFactory}.
      * Any configured interceptors are transferred to the configuration.
      *
      * @return the configured JSON-RPC transport configuration
      */
     @Override
     public JSONRPCTransportConfig build() {
-        // No HTTP client provided, fallback to the default one (JDK-based implementation)
+        // No HTTP client provided, use factory to get best available implementation
         if (httpClient == null) {
-            httpClient = new JdkA2AHttpClient();
+            httpClient = A2AHttpClientFactory.create();
         }
 
         JSONRPCTransportConfig config = new JSONRPCTransportConfig(httpClient);

client/transport/jsonrpc/src/main/java/io/a2a/client/transport/jsonrpc/JSONRPCTransportProvider.java

@@ -1,6 +1,6 @@
 package io.a2a.client.transport.jsonrpc;
 
-import io.a2a.client.http.JdkA2AHttpClient;
+import io.a2a.client.http.A2AHttpClientFactory;
 import io.a2a.client.transport.spi.ClientTransportProvider;
 import io.a2a.spec.A2AClientException;
 import io.a2a.spec.AgentCard;
@@ -14,7 +14,7 @@ public class JSONRPCTransportProvider implements ClientTransportProvider<JSONRPC
     public JSONRPCTransport create(@Nullable JSONRPCTransportConfig clientTransportConfig, AgentCard agentCard, AgentInterface agentInterface) throws A2AClientException {
         JSONRPCTransportConfig currentClientTransportConfig = clientTransportConfig;
         if (currentClientTransportConfig == null) {
-            currentClientTransportConfig = new JSONRPCTransportConfig(new JdkA2AHttpClient());
+            currentClientTransportConfig = new JSONRPCTransportConfig(A2AHttpClientFactory.create());
         }
         return new JSONRPCTransport(currentClientTransportConfig.getHttpClient(), agentCard, agentInterface, currentClientTransportConfig.getInterceptors());
     }

client/transport/rest/pom.xml

@@ -42,6 +42,10 @@
             <groupId>io.github.a2asdk</groupId>
             <artifactId>a2a-java-sdk-http-client</artifactId>
         </dependency>
+        <dependency>
+            <groupId>io.github.a2asdk</groupId>
+            <artifactId>a2a-java-sdk-http-client-vertx</artifactId>
+        </dependency>
         <dependency>
             <groupId>com.google.protobuf</groupId>
             <artifactId>protobuf-java-util</artifactId>

client/transport/rest/src/main/java/io/a2a/client/transport/rest/RestTransport.java

@@ -29,8 +29,8 @@
 import com.google.protobuf.util.JsonFormat;
 import io.a2a.client.http.A2ACardResolver;
 import io.a2a.client.http.A2AHttpClient;
+import io.a2a.client.http.A2AHttpClientFactory;
 import io.a2a.client.http.A2AHttpResponse;
-import io.a2a.client.http.JdkA2AHttpClient;
 import io.a2a.client.transport.rest.sse.RestSSEEventListener;
 import io.a2a.client.transport.spi.ClientTransport;
 import io.a2a.client.transport.spi.interceptors.ClientCallContext;
@@ -74,7 +74,7 @@ public RestTransport(AgentCard agentCard) {
 
     public RestTransport(@Nullable A2AHttpClient httpClient, AgentCard agentCard,
             AgentInterface agentInterface, @Nullable List<ClientCallInterceptor> interceptors) {
-        this.httpClient = httpClient == null ? new JdkA2AHttpClient() : httpClient;
+        this.httpClient = httpClient == null ? A2AHttpClientFactory.create() : httpClient;
         this.agentCard = agentCard;
         this.agentInterface = agentInterface;
         this.interceptors = interceptors;