feat: Update the ClientTransport interface, introducing ClientCallContext, ClientConfig, and ClientCallInterceptor similar to the Python SDK. Introduce a ClientTransportProvider and update the JSONRPC and gRPC transport implementations. Introduce a new Client and ClientFactory implementations.

Author: fjuma

client-config/src/main/java/io/a2a/client/config/ClientCallContext.java

@@ -0,0 +1,27 @@
+package io.a2a.client.config;
+
+import java.util.Map;
+
+/**
+ * A context passed with each client call, allowing for call-specific.
+ * configuration and data passing. Such as authentication details or
+ * request deadlines.
+ */
+public class ClientCallContext {
+
+    private final Map<String, Object> state;
+    private final Map<String, String> headers;
+
+    public ClientCallContext(Map<String, Object> state, Map<String, String> headers) {
+        this.state = state;
+        this.headers = headers;
+    }
+
+    public Map<String, Object> getState() {
+        return state;
+    }
+
+    public Map<String, String> getHeaders() {
+        return headers;
+    }
+}

client-config/src/main/java/io/a2a/client/config/ClientCallInterceptor.java

@@ -0,0 +1,26 @@
+package io.a2a.client.config;
+
+import java.util.Map;
+
+import io.a2a.spec.AgentCard;
+
+/**
+ * An abstract base class for client-side call interceptors.
+ * Interceptors can inspect and modify requests before they are sent,
+ * which is ideal for concerns like authentication, logging, or tracing.
+ */
+public abstract class ClientCallInterceptor {
+
+    /**
+     * Intercept a client call before the request is sent.
+     *
+     * @param methodName the name of the protocol method (e.g., 'message/send')
+     * @param payload the request payload
+     * @param headers the headers to use
+     * @param agentCard the agent card (may be {@code null})
+     * @param clientCallContext the {@code ClientCallContext} for this call (may be {@code null})
+     * @return the potentially modified payload and headers
+     */
+    public abstract PayloadAndHeaders intercept(String methodName, Object payload, Map<String, String> headers,
+                                                AgentCard agentCard, ClientCallContext clientCallContext);
+}

client-config/src/main/java/io/a2a/client/config/ClientConfig.java

@@ -0,0 +1,150 @@
+package io.a2a.client.config;
+
+import java.util.List;
+import java.util.Map;
+
+import io.a2a.client.http.A2AHttpClient;
+import io.a2a.spec.PushNotificationConfig;
+import io.grpc.Channel;
+
+/**
+ * Configuration for the A2A client factory.
+ */
+public class ClientConfig {
+
+    private final Boolean streaming;
+    private final Boolean polling;
+    private final A2AHttpClient httpClient;
+    private final Channel channel;
+    private final List<String> supportedTransports;
+    private final Boolean useClientPreference;
+    private final List<String> acceptedOutputModes;
+    private final PushNotificationConfig pushNotificationConfig;
+    private final Integer historyLength;
+    private final Map<String, Object> metadata;
+
+    public ClientConfig(Boolean streaming, Boolean polling, A2AHttpClient httpClient, Channel channel,
+                        List<String> supportedTransports, Boolean useClientPreference,
+                        List<String> acceptedOutputModes, PushNotificationConfig pushNotificationConfig,
+                        Integer historyLength, Map<String, Object> metadata) {
+        this.streaming = streaming == null ? true : streaming;
+        this.polling = polling == null ? false : polling;
+        this.httpClient = httpClient;
+        this.channel = channel;
+        this.supportedTransports = supportedTransports;
+        this.useClientPreference = useClientPreference == null ? false : useClientPreference;
+        this.acceptedOutputModes = acceptedOutputModes;
+        this.pushNotificationConfig = pushNotificationConfig;
+        this.historyLength = historyLength;
+        this.metadata = metadata;
+    }
+
+    public boolean isStreaming() {
+        return streaming;
+    }
+
+    public boolean isPolling() {
+        return polling;
+    }
+
+    public A2AHttpClient getHttpClient() {
+        return httpClient;
+    }
+
+    public Channel getChannel() {
+        return channel;
+    }
+
+    public List<String> getSupportedTransports() {
+        return supportedTransports;
+    }
+
+    public boolean isUseClientPreference() {
+        return useClientPreference;
+    }
+
+    public List<String> getAcceptedOutputModes() {
+        return acceptedOutputModes;
+    }
+
+    public PushNotificationConfig getPushNotificationConfig() {
+        return pushNotificationConfig;
+    }
+
+    public Integer getHistoryLength() {
+        return historyLength;
+    }
+
+    public Map<String, Object> getMetadata() {
+        return metadata;
+    }
+
+    public static class Builder {
+        private Boolean streaming;
+        private Boolean polling;
+        private A2AHttpClient httpClient;
+        private Channel channel;
+        private List<String> supportedTransports;
+        private Boolean useClientPreference;
+        private List<String> acceptedOutputModes;
+        private PushNotificationConfig pushNotificationConfig;
+        private Integer historyLength;
+        private Map<String, Object> metadata;
+
+        public Builder setStreaming(Boolean streaming) {
+            this.streaming = streaming;
+            return this;
+        }
+
+        public Builder setPolling(Boolean polling) {
+            this.polling = polling;
+            return this;
+        }
+
+        public Builder setHttpClient(A2AHttpClient httpClient) {
+            this.httpClient = httpClient;
+            return this;
+        }
+
+        public Builder setChannel(Channel channel) {
+            this.channel = channel;
+            return this;
+        }
+
+        public Builder setSupportedTransports(List<String> supportedTransports) {
+            this.supportedTransports = supportedTransports;
+            return this;
+        }
+
+        public Builder setUseClientPreference(Boolean useClientPreference) {
+            this.useClientPreference = useClientPreference;
+            return this;
+        }
+
+        public Builder setAcceptedOutputModes(List<String> acceptedOutputModes) {
+            this.acceptedOutputModes = acceptedOutputModes;
+            return this;
+        }
+
+        public Builder setPushNotificationConfig(PushNotificationConfig pushNotificationConfig) {
+            this.pushNotificationConfig = pushNotificationConfig;
+            return this;
+        }
+
+        public Builder setHistoryLength(Integer historyLength) {
+            this.historyLength = historyLength;
+            return this;
+        }
+
+        public Builder setMetadata(Map<String, Object> metadata) {
+            this.metadata = metadata;
+            return this;
+        }
+
+        public ClientConfig build() {
+            return new ClientConfig(streaming, polling, httpClient, channel,
+                    supportedTransports, useClientPreference, acceptedOutputModes,
+                    pushNotificationConfig, historyLength, metadata);
+        }
+    }
+}

client-config/src/main/java/io/a2a/client/config/PayloadAndHeaders.java

@@ -0,0 +1,22 @@
+package io.a2a.client.config;
+
+import java.util.Map;
+
+public class PayloadAndHeaders {
+
+    private final Object payload;
+    private final Map<String, String> headers;
+
+    public PayloadAndHeaders(Object payload, Map<String, String> headers) {
+        this.payload = payload;
+        this.headers = headers;
+    }
+
+    public Object getPayload() {
+        return payload;
+    }
+
+    public Map<String, String> getHeaders() {
+        return headers;
+    }
+}

client-transport/grpc/pom.xml

@@ -17,6 +17,11 @@
     <description>Java SDK for the Agent2Agent Protocol (A2A) - gRPC Client Transport</description>
 
     <dependencies>
+        <dependency>
+            <groupId>${project.groupId}</groupId>
+            <artifactId>a2a-java-sdk-client-config</artifactId>
+            <version>${project.version}</version>
+        </dependency>
         <dependency>
             <groupId>${project.groupId}</groupId>
             <artifactId>a2a-java-sdk-common</artifactId>

client-transport/grpc/src/main/java/io/a2a/client/transport/grpc/EventStreamObserver.java

@@ -47,7 +47,9 @@ public void onNext(StreamResponse response) {
 
     @Override
     public void onError(Throwable t) {
-        errorHandler.accept(t);
+        if (errorHandler != null) {
+            errorHandler.accept(t);
+        }
     }
 
     @Override