[#491] Encapsulate the payload of Push notifications

The payload of push notifications is encapsulated by the
`kind` of payload as specified in [§ 4.3.3. Push Notification Payload](https://a2a-protocol.org/latest/specification/#433-push-notification-payload)

This commit only works for `task` payload and will required more work to support additional payloads (tracked by #490).

This fixes #491
Signed-off-by: Jeff Mesnil <[email protected]>

Author: jmesnil

server-common/src/main/java/io/a2a/server/tasks/BasePushNotificationSender.java

@@ -80,7 +80,7 @@ private boolean dispatchNotification(Task task, PushNotificationConfig pushInfo)
 
         String body;
         try {
-            body = Utils.OBJECT_MAPPER.writeValueAsString(task);
+            body = Utils.marshalFrom(task);
         } catch (JsonProcessingException e) {
             LOGGER.debug("Error writing value as string: {}", e.getMessage(), e);
             return false;

server-common/src/test/java/io/a2a/server/requesthandlers/AbstractA2ARequestHandlerTest.java

@@ -13,6 +13,8 @@
 import java.util.concurrent.Executors;
 import java.util.function.Consumer;
 
+import com.fasterxml.jackson.core.JsonFactory;
+import com.fasterxml.jackson.core.JsonParser;
 import jakarta.enterprise.context.Dependent;
 
 import io.a2a.client.http.A2AHttpClient;
@@ -199,7 +201,17 @@ public PostBuilder body(String body) {
 
             @Override
             public A2AHttpResponse post() throws IOException, InterruptedException {
-                tasks.add(Utils.OBJECT_MAPPER.readValue(body, Task.TYPE_REFERENCE));
+                Task task;
+                JsonFactory factory = new JsonFactory();
+                try (JsonParser parser = factory.createParser(body)) {
+                    parser.nextToken(); // Should be START_OBJECT
+                    parser.nextToken(); // Move to first FIELD_NAME
+                    // FIXME when we expand the kind that can be pushed in #490
+                    String kind = parser.currentName();
+                    parser.nextToken(); // Move to VALUE token
+                    task = Utils.OBJECT_MAPPER.readValue(parser, Task.TYPE_REFERENCE);
+                }
+                tasks.add(task);
                 try {
                     return new A2AHttpResponse() {
                         @Override

server-common/src/test/java/io/a2a/server/tasks/PushNotificationSenderTest.java

@@ -16,6 +16,10 @@
 import java.util.concurrent.TimeUnit;
 import java.util.function.Consumer;
 
+import com.fasterxml.jackson.core.JsonFactory;
+import com.fasterxml.jackson.core.JsonParser;
+import com.fasterxml.jackson.databind.JsonNode;
+import com.fasterxml.jackson.databind.ObjectMapper;
 import org.junit.jupiter.api.BeforeEach;
 import org.junit.jupiter.api.Test;
 
@@ -77,7 +81,16 @@ public A2AHttpResponse post() throws IOException, InterruptedException {
                 }
 
                 try {
-                    Task task = Utils.OBJECT_MAPPER.readValue(body, Task.TYPE_REFERENCE);
+                    Task task;
+                    JsonFactory factory = new JsonFactory();
+                    try (JsonParser parser = factory.createParser(body)) {
+                        parser.nextToken(); // Should be START_OBJECT
+                        parser.nextToken(); // Move to first FIELD_NAME
+                        // FIXME when we expand the kind that can be pushed in #490
+                        String kind = parser.currentName();
+                        parser.nextToken(); // Move to VALUE token
+                        task = Utils.OBJECT_MAPPER.readValue(parser, Task.TYPE_REFERENCE);
+                    }
                     tasks.add(task);
                     urls.add(url);
                     headers.add(new java.util.HashMap<>(requestHeaders));

spec/src/main/java/io/a2a/util/Utils.java

@@ -2,15 +2,18 @@
 
 import java.util.ArrayList;
 import java.util.List;
+import java.util.Map;
 import java.util.logging.Logger;
 
 import com.fasterxml.jackson.core.JsonProcessingException;
 import com.fasterxml.jackson.core.type.TypeReference;
 import com.fasterxml.jackson.databind.ObjectMapper;
+import com.fasterxml.jackson.databind.node.ObjectNode;
 import com.fasterxml.jackson.datatype.jsr310.JavaTimeModule;
 
 import io.a2a.spec.Artifact;
 import io.a2a.spec.Part;
+import io.a2a.spec.StreamingEventKind;
 import io.a2a.spec.Task;
 import io.a2a.spec.TaskArtifactUpdateEvent;
 
@@ -69,6 +72,21 @@ public static <T> T unmarshalFrom(String data, TypeReference<T> typeRef) throws
         return OBJECT_MAPPER.readValue(data, typeRef);
     }
 
+    /**
+     * Serializes a StreamingEventKind in a JSON string
+     * <p>
+     * The StreamingEventKind object is wrapped in a JSON field named from its kind (e.g. "task") before
+     * it is serialized
+     *
+     * @param kind the StreamingEventKind to deserialize
+     * @return a JSON String
+     * @throws JsonProcessingException if JSON parsing fails
+     */
+    public static String marshalFrom(StreamingEventKind kind) throws JsonProcessingException {
+        Map<String, StreamingEventKind> wrapper = Map.of(kind.getKind(), kind);
+        return OBJECT_MAPPER.writeValueAsString(wrapper);
+    }
+
     /**
      * Returns the provided value if non-null, otherwise returns the default value.
      * <p>