Add content-type header and empty body for empty structure payload in restJSON request

aws/client/aws-client-restjson/src/it/java/software/amazon/smithy/java/client/aws/restjson/RestJson1ProtocolTests.java

@@ -35,9 +35,6 @@ public class RestJson1ProtocolTests {
     @HttpClientRequestTests
     @ProtocolTestFilter(
             skipTests = {
-                    // TODO: These tests require a payload even when the httpPayload member is null. Should it?
-                    "RestJsonHttpWithHeadersButNoPayload",
-                    "RestJsonHttpWithEmptyStructurePayload",
                     "RestJsonHttpEmptyPrefixHeadersRequestClient" //FIXME https://github.com/smithy-lang/smithy-java/issues/647
             })
     public void requestTest(DataStream expected, DataStream actual) {

aws/client/aws-client-restjson/src/main/java/software/amazon/smithy/java/aws/client/restjson/RestJsonClientProtocol.java

@@ -5,6 +5,7 @@
 
 package software.amazon.smithy.java.aws.client.restjson;
 
+import java.net.URI;
 import software.amazon.smithy.aws.traits.protocols.RestJson1Trait;
 import software.amazon.smithy.java.aws.events.AwsEventDecoderFactory;
 import software.amazon.smithy.java.aws.events.AwsEventEncoderFactory;
@@ -17,14 +18,20 @@
 import software.amazon.smithy.java.client.http.binding.HttpBindingClientProtocol;
 import software.amazon.smithy.java.client.http.binding.HttpBindingErrorFactory;
 import software.amazon.smithy.java.context.Context;
+import software.amazon.smithy.java.core.schema.ApiOperation;
 import software.amazon.smithy.java.core.schema.InputEventStreamingApiOperation;
 import software.amazon.smithy.java.core.schema.OutputEventStreamingApiOperation;
+import software.amazon.smithy.java.core.schema.SerializableStruct;
+import software.amazon.smithy.java.core.schema.TraitKey;
 import software.amazon.smithy.java.core.serde.Codec;
 import software.amazon.smithy.java.core.serde.event.EventDecoderFactory;
 import software.amazon.smithy.java.core.serde.event.EventEncoderFactory;
 import software.amazon.smithy.java.core.serde.event.EventStreamingException;
+import software.amazon.smithy.java.http.api.HttpRequest;
+import software.amazon.smithy.java.io.datastream.DataStream;
 import software.amazon.smithy.java.json.JsonCodec;
 import software.amazon.smithy.model.shapes.ShapeId;
+import software.amazon.smithy.model.shapes.ShapeType;
 
 /**
  * Implements aws.protocols#restJson1.
@@ -55,6 +62,32 @@ public RestJsonClientProtocol(ShapeId service) {
                 .build();
     }
 
+    @Override
+    public <I extends SerializableStruct, O extends SerializableStruct> HttpRequest createRequest(
+            ApiOperation<I, O> operation,
+            I input,
+            Context context,
+            URI endpoint
+    ) {
+        HttpRequest request = super.createRequest(operation, input, context, endpoint);
+
+        if (request.body().contentLength() == 0) {
+            var payloadMember = input.schema()
+                    .members()
+                    .stream()
+                    .filter(m -> m.hasTrait(TraitKey.HTTP_PAYLOAD_TRAIT))
+                    .findFirst();
+            if (payloadMember.isPresent() && payloadMember.get().type().equals(ShapeType.STRUCTURE)) {
+                return request.toBuilder()
+                        .body(DataStream.ofString("{}"))
+                        .withAddedHeader("Content-Type", "application/json")
+                        .build();
+            }
+        }
+
+        return request;
+    }
+
     @Override
     public Codec payloadCodec() {
         return codec;

http/http-binding/src/main/java/software/amazon/smithy/java/http/binding/RequestSerializer.java

@@ -129,7 +129,6 @@ public HttpRequest serializeRequest() {
         Objects.requireNonNull(operation, "operation is not set");
         Objects.requireNonNull(payloadCodec, "payloadCodec is not set");
         Objects.requireNonNull(endpoint, "endpoint is not set");
-        Objects.requireNonNull(shapeValue, "value is not set");
         Objects.requireNonNull(payloadMediaType, "payloadMediaType is not set");
 
         var matcher = bindingCache.computeIfAbsent(operation.inputSchema(), BindingMatcher::requestMatcher);