feat(http-client): add Android support and implement SPI for A2AHttpClient

- Introduce `AndroidA2AHttpClient` using `HttpURLConnection` for Android compatibility.
- Implement `A2AHttpClientFactory` and `A2AHttpClientProvider` using `ServiceLoader` to decouple implementations.
- Update `A2A` and transport providers to use the factory instead of hardcoding `JdkA2AHttpClient`.
- Add `@JsonProperty` annotations to spec records to prevent parsing failures on Android.

Author: sherryfox

README.md

@@ -394,7 +394,7 @@ Different transport protocols can be configured with specific settings using spe
 
 ##### JSON-RPC Transport Configuration
 
-For the JSON-RPC transport, to use the default `JdkA2AHttpClient`, provide a `JSONRPCTransportConfig` created with its default constructor.
+For the JSON-RPC transport, to use the default HTTP client (resolved automatically by `A2AHttpClientFactory`), provide a `JSONRPCTransportConfig` created with its default constructor.
 
 To use a custom HTTP client implementation, simply create a `JSONRPCTransportConfig` as follows:
 
@@ -441,7 +441,7 @@ Client client = Client
 
 ##### HTTP+JSON/REST Transport Configuration
 
-For the HTTP+JSON/REST transport, if you'd like to use the default `JdkA2AHttpClient`, provide a `RestTransportConfig` created with its default constructor.
+For the HTTP+JSON/REST transport, to use the default HTTP client (resolved automatically by `A2AHttpClientFactory`), provide a `RestTransportConfig` created with its default constructor.
 
 To use a custom HTTP client implementation, simply create a `RestTransportConfig` as follows:
 

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

@@ -3,11 +3,10 @@
 import java.util.Collections;
 import java.util.List;
 import java.util.Map;
-import java.util.UUID;
 
 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;
@@ -139,7 +138,7 @@ private static Message toMessage(List<Part<?>> parts, Message.Role role, String
      * @throws A2AClientJSONError If the response body cannot be decoded as JSON or validated against the AgentCard schema
      */
     public static AgentCard getAgentCard(String agentUrl) throws A2AClientError, A2AClientJSONError {
-        return getAgentCard(new JdkA2AHttpClient(), agentUrl);
+        return getAgentCard(A2AHttpClientFactory.create(), agentUrl);
     }
 
     /**
@@ -167,7 +166,7 @@ public static AgentCard getAgentCard(A2AHttpClient httpClient, String agentUrl)
      * @throws 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/transport/jsonrpc/src/main/java/io/a2a/client/transport/jsonrpc/JSONRPCTransport.java

@@ -15,8 +15,8 @@
 import io.a2a.client.transport.spi.interceptors.ClientCallInterceptor;
 import io.a2a.client.transport.spi.interceptors.PayloadAndHeaders;
 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.spi.ClientTransport;
 import io.a2a.spec.A2AClientError;
 import io.a2a.spec.A2AClientException;
@@ -87,7 +87,7 @@ public JSONRPCTransport(AgentCard agentCard) {
 
     public JSONRPCTransport(A2AHttpClient httpClient, AgentCard agentCard,
                             String agentUrl, List<ClientCallInterceptor> interceptors) {
-        this.httpClient = httpClient == null ? new JdkA2AHttpClient() : httpClient;
+        this.httpClient = httpClient == null ? A2AHttpClientFactory.create() : httpClient;
         this.agentCard = agentCard;
         this.agentUrl = agentUrl;
         this.interceptors = interceptors;

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

@@ -1,6 +1,7 @@
 package io.a2a.client.transport.jsonrpc;
 
 import io.a2a.client.http.A2AHttpClient;
+import io.a2a.client.http.A2AHttpClientFactory;
 import io.a2a.client.http.JdkA2AHttpClient;
 import io.a2a.client.transport.spi.ClientTransportConfigBuilder;
 
@@ -18,7 +19,7 @@ public JSONRPCTransportConfigBuilder httpClient(A2AHttpClient httpClient) {
     public JSONRPCTransportConfig build() {
         // No HTTP client provided, fallback to the default one (JDK-based 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;
@@ -11,7 +11,7 @@ public class JSONRPCTransportProvider implements ClientTransportProvider<JSONRPC
     @Override
     public JSONRPCTransport create(JSONRPCTransportConfig clientTransportConfig, AgentCard agentCard, String agentUrl) throws A2AClientException {
         if (clientTransportConfig == null) {
-            clientTransportConfig = new JSONRPCTransportConfig(new JdkA2AHttpClient());
+            clientTransportConfig = new JSONRPCTransportConfig(A2AHttpClientFactory.create());
         }
 
         return new JSONRPCTransport(clientTransportConfig.getHttpClient(), agentCard, agentUrl, clientTransportConfig.getInterceptors());

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

@@ -8,8 +8,8 @@
 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;
@@ -62,7 +62,7 @@ public RestTransport(AgentCard agentCard) {
 
     public RestTransport(@Nullable A2AHttpClient httpClient, AgentCard agentCard,
             String agentUrl, @Nullable List<ClientCallInterceptor> interceptors) {
-        this.httpClient = httpClient == null ? new JdkA2AHttpClient() : httpClient;
+        this.httpClient = httpClient == null ? A2AHttpClientFactory.create() : httpClient;
         this.agentCard = agentCard;
         this.agentUrl = agentUrl.endsWith("/") ? agentUrl.substring(0, agentUrl.length() - 1) : agentUrl;
         this.interceptors = interceptors;

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

@@ -1,6 +1,7 @@
 package io.a2a.client.transport.rest;
 
 import io.a2a.client.http.A2AHttpClient;
+import io.a2a.client.http.A2AHttpClientFactory;
 import io.a2a.client.http.JdkA2AHttpClient;
 import io.a2a.client.transport.spi.ClientTransportConfigBuilder;
 import org.jspecify.annotations.Nullable;
@@ -16,9 +17,8 @@ public RestTransportConfigBuilder httpClient(A2AHttpClient httpClient) {
 
     @Override
     public RestTransportConfig build() {
-        // No HTTP client provided, fallback to the default one (JDK-based implementation)
         if (httpClient == null) {
-            httpClient = new JdkA2AHttpClient();
+            httpClient = A2AHttpClientFactory.create();
         }
 
         RestTransportConfig config = new RestTransportConfig(httpClient);

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

@@ -1,6 +1,6 @@
 package io.a2a.client.transport.rest;
 
-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;
@@ -17,7 +17,7 @@ public String getTransportProtocol() {
     public RestTransport create(RestTransportConfig clientTransportConfig, AgentCard agentCard, String agentUrl) throws A2AClientException {
         RestTransportConfig transportConfig = clientTransportConfig;
          if (transportConfig == null) {
-            transportConfig = new RestTransportConfig(new JdkA2AHttpClient());
+            transportConfig = new RestTransportConfig(A2AHttpClientFactory.create());
         }
         return new RestTransport(clientTransportConfig.getHttpClient(), agentCard, agentUrl, transportConfig.getInterceptors());
     }

http-client/src/main/java/io/a2a/client/http/A2ACardResolver.java

@@ -23,16 +23,16 @@ public class A2ACardResolver {
 
     private static final TypeReference<AgentCard> AGENT_CARD_TYPE_REFERENCE = new TypeReference<>() {};
 
-    /**
-     * Get the agent card for an A2A agent.
-     * The {@code JdkA2AHttpClient} will be used to fetch the agent card.
-     *
-     * @param baseUrl the base URL for the agent whose agent card we want to retrieve
-     * @throws A2AClientError if the URL for the agent is invalid
-     */
-    public A2ACardResolver(String baseUrl) throws A2AClientError {
-        this(new JdkA2AHttpClient(), baseUrl, null, null);
-    }
+  /**
+   * Get the agent card for an A2A agent.
+   * The {@link A2AHttpClientFactory#create()} will be used to fetch the agent card if available.
+   *
+   * @param baseUrl the base URL for the agent whose agent card we want to retrieve
+   * @throws A2AClientError if the URL for the agent is invalid
+   */
+  public A2ACardResolver(String baseUrl) throws A2AClientError {
+    this(A2AHttpClientFactory.create(), baseUrl, null, null);
+  }
 
     /**
      * Constructs an A2ACardResolver with a specific HTTP client and base URL.

http-client/src/main/java/io/a2a/client/http/A2AHttpClientFactory.java

@@ -0,0 +1,46 @@
+package io.a2a.client.http;
+
+import java.util.Comparator;
+import java.util.ServiceLoader;
+import java.util.stream.StreamSupport;
+
+/**
+ * Factory for creating {@link A2AHttpClient} instances using the ServiceLoader mechanism.
+ */
+public final class A2AHttpClientFactory {
+
+    private A2AHttpClientFactory() {
+        // Utility class
+    }
+
+    /**
+     * Creates a new A2AHttpClient instance using the highest priority provider available.
+     * If no providers are found, it throws an {@link IllegalStateException}.
+     */
+    public static A2AHttpClient create() {
+        ServiceLoader<A2AHttpClientProvider> loader = ServiceLoader.load(A2AHttpClientProvider.class);
+
+        return StreamSupport.stream(loader.spliterator(), false)
+                .max(Comparator.comparingInt(A2AHttpClientProvider::priority))
+                .map(A2AHttpClientProvider::create)
+                .orElseThrow(() -> new IllegalStateException("No A2AHttpClientProvider found"));
+    }
+
+    /**
+     * Creates a new A2AHttpClient instance using a specific provider by name.
+     */
+    public static A2AHttpClient create(String providerName) {
+        if (providerName == null || providerName.isEmpty()) {
+            throw new IllegalArgumentException("Provider name must not be null or empty");
+        }
+
+        ServiceLoader<A2AHttpClientProvider> loader = ServiceLoader.load(A2AHttpClientProvider.class);
+
+        return StreamSupport.stream(loader.spliterator(), false)
+                .filter(provider -> providerName.equals(provider.name()))
+                .findFirst()
+                .map(A2AHttpClientProvider::create)
+                .orElseThrow(() -> new IllegalArgumentException(
+                        "No A2AHttpClientProvider found with name: " + providerName));
+    }
+}