Set explicit payload members to null if the input is empty

.changes/next-release/bugfix-AWSSDKforJavav2-e2de7f3.json

@@ -0,0 +1,6 @@
+{
+    "category": "AWS SDK for Java v2", 
+    "contributor": "", 
+    "type": "bugfix", 
+    "description": "Fix a regression for the JSON REST protocol for which an structure explicit payload member was set to the empty object instead of null"
+}

core/protocols/aws-json-protocol/src/main/java/software/amazon/awssdk/protocols/json/internal/unmarshall/JsonProtocolUnmarshaller.java

@@ -27,6 +27,7 @@
 import java.util.List;
 import java.util.Map;
 import java.util.Optional;
+import java.util.function.Supplier;
 import software.amazon.awssdk.annotations.SdkInternalApi;
 import software.amazon.awssdk.annotations.ThreadSafe;
 import software.amazon.awssdk.core.SdkBytes;
@@ -266,6 +267,11 @@ private <T extends SdkPojo> T unmarshallFromJson(SdkPojo sdkPojo, InputStream in
         return (T) unmarshallingParser.parse(sdkPojo, inputStream);
     }
 
+    @SuppressWarnings("unchecked")
+    private <T extends SdkPojo> T unmarshallMemberFromJson(Supplier<SdkPojo> constructor, InputStream inputStream) {
+        return (T) unmarshallingParser.parseMember(constructor, inputStream);
+    }
+
     private <TypeT extends SdkPojo> TypeT unmarshallResponse(SdkPojo sdkPojo,
                                                              SdkHttpFullResponse response) throws IOException {
         JsonUnmarshallerContext context = JsonUnmarshallerContext.builder()
@@ -290,7 +296,7 @@ private <TypeT extends SdkPojo> TypeT unmarshallResponse(SdkPojo sdkPojo,
             } else if (isExplicitPayloadMember(field) && field.marshallingType() == MarshallingType.SDK_POJO) {
                 Optional<AbortableInputStream> responseContent = context.response().content();
                 if (responseContent.isPresent()) {
-                    field.set(sdkPojo, unmarshallFromJson(field.constructor().get(), responseContent.get()));
+                    field.set(sdkPojo, unmarshallMemberFromJson(field.constructor(), responseContent.get()));
                 } else {
                     field.set(sdkPojo, null);
                 }

core/protocols/aws-json-protocol/src/main/java/software/amazon/awssdk/protocols/json/internal/unmarshall/JsonUnmarshallingParser.java

@@ -24,6 +24,7 @@
 import java.util.LinkedHashMap;
 import java.util.List;
 import java.util.Map;
+import java.util.function.Supplier;
 import software.amazon.awssdk.annotations.SdkInternalApi;
 import software.amazon.awssdk.annotations.ThreadSafe;
 import software.amazon.awssdk.core.SdkBytes;
@@ -72,6 +73,33 @@ public static Builder builder() {
         return new Builder();
     }
 
+    /**
+     * Parse the provided {@link InputStream} and return the deserialized {@link SdkPojo}. Unlike
+     * {@link #parse(SdkPojo, InputStream)} this method returns null if the input stream is empty. This is used to unmarshall
+     * payload members that can be null unlike top-level response pojos.
+     */
+    public SdkPojo parseMember(Supplier<SdkPojo> constructor, InputStream content) {
+        return invokeSafely(() -> {
+            try (JsonParser parser = jsonFactory.createParser(content)
+                                                .configure(JsonParser.Feature.AUTO_CLOSE_SOURCE, false)) {
+
+                JsonUnmarshallerContext c = JsonUnmarshallerContext.builder().build();
+                JsonToken token = parser.nextToken();
+                if (token == null) {
+                    return null;
+                }
+                if (token == JsonToken.VALUE_NULL) {
+                    return null;
+                }
+                if (token != JsonToken.START_OBJECT) {
+                    throw new JsonParseException("expecting start object, got instead: " + token);
+                }
+                SdkPojo pojo = constructor.get();
+                return parseSdkPojo(c, pojo, parser);
+            }
+        });
+    }
+
     /**
      * Parse the provided {@link InputStream} and return the deserialized {@link SdkPojo}.
      */
@@ -95,7 +123,6 @@ public SdkPojo parse(SdkPojo pojo, InputStream content) {
             }
         });
     }
-
     /**
      * Parses an sdk pojo and fills its fields. The given SdkPojo instance is expected to be a {@link Buildable} instance. This
      * method expects that the START_OBJECT token has been already consumed, so the next token should be either a field name or an

test/protocol-tests-core/src/main/resources/software/amazon/awssdk/protocol/suites/cases/rest-json-output.json

@@ -19,6 +19,24 @@
       }
     }
   },
+  {
+    "description": "Operation with explicit payload structure, with emtpy output is unmarshalled as null value",
+    "given": {
+      "response": {
+        "status_code": 200,
+        "body": ""
+      }
+    },
+    "when": {
+      "action": "unmarshall",
+      "operation": "OperationWithExplicitPayloadStructure"
+    },
+    "then": {
+      "deserializedAs": {
+        "PayloadMember": null
+      }
+    }
+  },
   {
     "description": "Operation with streaming payload in output is unmarshalled correctly",
     "given": {