Added support for explicit null values during deserialization

codecs/json-codec/src/main/java/software/amazon/smithy/java/json/jackson/JacksonJsonDeserializer.java

@@ -261,8 +261,9 @@ public <T> void readStruct(Schema schema, T state, StructMemberConsumer<T> struc
         try {
             var fieldToMember = settings.fieldMapper().fieldToMember(schema);
             for (var memberName = parser.nextName(); memberName != null; memberName = parser.nextName()) {
-                if (parser.nextToken() != VALUE_NULL) {
-                    var member = fieldToMember.member(memberName);
+                var member = fieldToMember.member(memberName);
+                if (parser.nextToken() != VALUE_NULL
+                        || (member != null && structMemberConsumer.supportsNullValues(member))) {
                     if (member != null) {
                         structMemberConsumer.accept(state, member, this);
                     } else if (schema.type() == ShapeType.STRUCTURE) {

codecs/json-codec/src/main/java/software/amazon/smithy/java/json/smithy/SmithyJsonDeserializer.java

@@ -616,7 +616,8 @@ public <T> void readStruct(Schema schema, T state, StructMemberConsumer<T> struc
                         && p + 4 <= localEnd
                         && localBuf[p + 1] == 'u'
                         && localBuf[p + 2] == 'l'
-                        && localBuf[p + 3] == 'l') {
+                        && localBuf[p + 3] == 'l'
+                        && !structMemberConsumer.supportsNullValues(member)) {
                     p += 4;
                 } else {
                     // Write pos back before callback, reload after

codecs/json-codec/src/test/java/software/amazon/smithy/java/json/JsonDeserializerTest.java

@@ -1340,6 +1340,64 @@ public void nullMemberValueSkippedInStruct(JsonSerdeProvider provider) {
         }
     }
 
+    @PerProvider
+    public void nullMemberValuePassedToConsumerWhenSupported(JsonSerdeProvider provider) {
+        try (var codec = codecBuilder(provider).useJsonName(true).build()) {
+            var de = codec.createDeserializer(
+                    "{\"name\":null,\"Color\":\"red\"}".getBytes(StandardCharsets.UTF_8));
+            Set<String> members = new LinkedHashSet<>();
+            List<Boolean> nullFlags = new ArrayList<>();
+            de.readStruct(JsonTestData.BIRD, members, new ShapeDeserializer.StructMemberConsumer<>() {
+                @Override
+                public void accept(Set<String> state, Schema member, ShapeDeserializer deser) {
+                    state.add(member.memberName());
+                    nullFlags.add(deser.isNull());
+                    if (deser.isNull()) {
+                        deser.readNull();
+                    } else {
+                        deser.readString(member);
+                    }
+                }
+
+                @Override
+                public boolean supportsNullValues(Schema memberSchema) {
+                    return true;
+                }
+            });
+            // "name" was null but consumer was still called because supportsNullValues is true
+            assertThat(members, contains("name", "color"));
+            assertThat(nullFlags, contains(true, false));
+        }
+    }
+
+    @PerProvider
+    public void nullMemberValueSkippedWhenNotSupportedBySpecificMember(JsonSerdeProvider provider) {
+        try (var codec = codecBuilder(provider).useJsonName(true).build()) {
+            var de = codec.createDeserializer(
+                    "{\"name\":null,\"Color\":null,\"nested\":\"hi\"}".getBytes(StandardCharsets.UTF_8));
+            Set<String> members = new LinkedHashSet<>();
+            de.readStruct(JsonTestData.BIRD, members, new ShapeDeserializer.StructMemberConsumer<>() {
+                @Override
+                public void accept(Set<String> state, Schema member, ShapeDeserializer deser) {
+                    state.add(member.memberName());
+                    if (deser.isNull()) {
+                        deser.readNull();
+                    } else {
+                        deser.readString(member);
+                    }
+                }
+
+                @Override
+                public boolean supportsNullValues(Schema memberSchema) {
+                    // Only support null for "name", not for "color"
+                    return memberSchema.memberName().equals("name");
+                }
+            });
+            // "name" null is passed through, "color" null is skipped
+            assertThat(members, contains("name", "nested"));
+        }
+    }
+
     @ParameterizedTest
     @MethodSource("smithyOnly")
     public void rejectsNonObjectForMap(JsonSerdeProvider provider) {

codegen/codegen-core/src/main/java/software/amazon/smithy/java/codegen/SymbolProperties.java

@@ -99,5 +99,14 @@ public final class SymbolProperties {
      */
     public static final Property<Boolean> IS_NULLABLE = Property.named("is-nullable");
 
+    /**
+     * Indicates if a member supports explicit null values during deserialization.
+     *
+     * <p>When set to {@code true}, the generated deserializer will pass null values through to the
+     * struct member consumer rather than skipping them, allowing the member to distinguish between
+     * absent and explicitly null.
+     */
+    public static final Property<Boolean> SUPPORTS_NULL_VALUES = Property.named("supports-null-values");
+
     private SymbolProperties() {}
 }

codegen/codegen-core/src/main/java/software/amazon/smithy/java/codegen/generators/StructureDeserializerGenerator.java

@@ -8,10 +8,12 @@
 import java.util.Map;
 import software.amazon.smithy.codegen.core.SymbolProvider;
 import software.amazon.smithy.java.codegen.CodegenUtils;
+import software.amazon.smithy.java.codegen.SymbolProperties;
 import software.amazon.smithy.java.codegen.writer.JavaWriter;
 import software.amazon.smithy.java.core.schema.Schema;
 import software.amazon.smithy.java.core.serde.ShapeDeserializer;
 import software.amazon.smithy.model.Model;
+import software.amazon.smithy.model.shapes.MemberShape;
 import software.amazon.smithy.model.shapes.Shape;
 import software.amazon.smithy.model.shapes.ShapeId;
 import software.amazon.smithy.model.traits.ErrorTrait;
@@ -56,7 +58,15 @@ public void accept(Builder builder, ${sdkSchema:T} member, ${shapeDeserializer:T
                             @Override
                             public void unknownMember(Builder builder, ${string:T} memberName) {
                                 builder.$$unknownMember(memberName);
-                            }${/union}
+                            }${/union}${?supportsNullValues}
+
+                            @Override
+                            public boolean supportsNullValues(${sdkSchema:T} member) {
+                                return switch (member.memberIndex()) {
+                                    ${nullValueCases:C|}
+                                    default -> false;
+                                };
+                            }${/supportsNullValues}
                         }""";
         writer.putContext("shapeDeserializer", ShapeDeserializer.class);
         writer.putContext("sdkSchema", Schema.class);
@@ -66,6 +76,8 @@ public void unknownMember(Builder builder, ${string:T} memberName) {
         writer.putContext("union", shape.isUnionShape());
         writer.putContext("illegalArg", IllegalArgumentException.class);
         writer.putContext("isError", shape.hasTrait(ErrorTrait.class));
+        writer.putContext("supportsNullValues", hasMembersThatSupportsNullValues(shape));
+        writer.putContext("nullValueCases", writer.consumer(this::generateMemberNullValuesSwitchCases));
         writer.write(template);
         writer.popState();
     }
@@ -83,4 +95,31 @@ private void generateMemberSwitchCases(JavaWriter writer) {
             writer.popState();
         }
     }
+
+    private void generateMemberNullValuesSwitchCases(JavaWriter writer) {
+        int idx = 0;
+        for (var iter = CodegenUtils.getSortedMembers(shape).iterator(); iter.hasNext(); idx++) {
+            var member = iter.next();
+
+            if (memberSupportsNullValues(member)) {
+                writer.write("case $L -> true;", idx);
+            }
+        }
+    }
+
+    private boolean hasMembersThatSupportsNullValues(Shape shape) {
+        return shape.getAllMembers()
+                .values()
+                .stream()
+                .anyMatch(this::memberSupportsNullValues);
+    }
+
+    private boolean memberSupportsNullValues(MemberShape member) {
+        var memberSymbol = symbolProvider.toSymbol(member);
+        if (memberSymbol == null) {
+            return false;
+        }
+
+        return memberSymbol.getProperty(SymbolProperties.SUPPORTS_NULL_VALUES).orElse(false);
+    }
 }

core/src/main/java/software/amazon/smithy/java/core/serde/ShapeDeserializer.java

@@ -254,6 +254,20 @@ interface StructMemberConsumer<T> {
         void accept(T state, Schema memberSchema, ShapeDeserializer memberDeserializer);
 
         default void unknownMember(T state, String memberName) {}
+
+        /**
+         * Returns whether a given member supports explicit null values during deserialization.
+         *
+         * <p>When this returns {@code true}, the deserializer will invoke {@link #accept} for the member
+         * even when the serialized value is null, allowing the consumer to distinguish between an absent
+         * field and an explicitly null value.
+         *
+         * @param memberSchema Schema of the member to check.
+         * @return {@code true} if the member should receive null values, {@code false} to skip them.
+         */
+        default boolean supportsNullValues(Schema memberSchema) {
+            return false;
+        }
     }
 
     /**