Optimize HTTP binding and XML overhead

benchmarks/serde-benchmarks/src/jmh/java/software/amazon/smithy/java/benchmarks/serde/AwsQueryDeserializeBenchmark.java

@@ -31,6 +31,14 @@ public class AwsQueryDeserializeBenchmark {
     private static final String VERSION = "1999-12-31";
     private static final String CONTENT_TYPE = "text/xml";
 
+    /**
+     * Empty body fixture for response tests that have no body. AWS Query responses are XML, so an empty
+     * {@code byte[]} cleanly hits the {@code XmlCodec} empty-buffer fast path. None of the current
+     * {@code GetMetricDataResponse_*} test cases have empty bodies, but keeping the fixture consistent with
+     * {@link RestXmlDeserializeBenchmark} makes the auto-derive workaround in {@link DeserializeState} unnecessary.
+     */
+    private static final byte[] EMPTY_XML_BODY = new byte[0];
+
     @Param({
             "awsQuery_GetMetricDataResponse_S",
             "awsQuery_GetMetricDataResponse_M",
@@ -44,7 +52,7 @@ public class AwsQueryDeserializeBenchmark {
     @Setup
     public void setup() {
         protocol = new AwsQueryClientProtocol(SERVICE_ID, VERSION);
-        state = DeserializeState.forTestCase(testCaseId, GENERATED_PACKAGE, null, CONTENT_TYPE, false);
+        state = DeserializeState.forTestCase(testCaseId, GENERATED_PACKAGE, EMPTY_XML_BODY, CONTENT_TYPE, false);
     }
 
     @Benchmark

benchmarks/serde-benchmarks/src/jmh/java/software/amazon/smithy/java/benchmarks/serde/RestXmlDeserializeBenchmark.java

@@ -28,8 +28,11 @@ public class RestXmlDeserializeBenchmark {
             "software.amazon.smithy.java.benchmarks.serde.generated.restxml.model";
     private static final ShapeId SERVICE_ID =
             ShapeId.from("com.amazonaws.sdk.benchmark#AwsRestXmlDataPlane");
-    /** Pass null to DeserializeState to use the auto-derived <ShapeName/> default. */
-    private static final byte[] EMPTY_XML_BODY = null;
+    /**
+     * Empty body fixture for response tests that have no body. Feeding zero bytes here accurately models the wire
+     * reality of an empty 200 response and exercises that fast path.
+     */
+    private static final byte[] EMPTY_XML_BODY = new byte[0];
     private static final String CONTENT_TYPE = "application/xml";
 
     @Param({

codecs/xml-codec/src/main/java/software/amazon/smithy/java/xml/EmptyXmlDeserializer.java

@@ -0,0 +1,109 @@
+/*
+ * Copyright Amazon.com, Inc. or its affiliates. All Rights Reserved.
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+package software.amazon.smithy.java.xml;
+
+import java.math.BigDecimal;
+import java.math.BigInteger;
+import java.nio.ByteBuffer;
+import java.time.Instant;
+import software.amazon.smithy.java.core.schema.Schema;
+import software.amazon.smithy.java.core.serde.SerializationException;
+import software.amazon.smithy.java.core.serde.ShapeDeserializer;
+import software.amazon.smithy.java.core.serde.document.Document;
+
+/**
+ * A no-op {@link ShapeDeserializer} for empty XML payloads.
+ */
+final class EmptyXmlDeserializer implements ShapeDeserializer {
+
+    static final EmptyXmlDeserializer INSTANCE = new EmptyXmlDeserializer();
+
+    private EmptyXmlDeserializer() {}
+
+    @Override
+    public boolean isNull() {
+        return true;
+    }
+
+    @Override
+    public <T> void readStruct(Schema schema, T state, StructMemberConsumer<T> consumer) {}
+
+    @Override
+    public <T> void readList(Schema schema, T state, ListMemberConsumer<T> consumer) {}
+
+    @Override
+    public <T> void readStringMap(Schema schema, T state, MapMemberConsumer<String, T> consumer) {}
+
+    // Scalar reads are not valid at the top level of an empty XML body.
+    @Override
+    public boolean readBoolean(Schema schema) {
+        throw empty(schema);
+    }
+
+    @Override
+    public ByteBuffer readBlob(Schema schema) {
+        throw empty(schema);
+    }
+
+    @Override
+    public byte readByte(Schema schema) {
+        throw empty(schema);
+    }
+
+    @Override
+    public short readShort(Schema schema) {
+        throw empty(schema);
+    }
+
+    @Override
+    public int readInteger(Schema schema) {
+        throw empty(schema);
+    }
+
+    @Override
+    public long readLong(Schema schema) {
+        throw empty(schema);
+    }
+
+    @Override
+    public float readFloat(Schema schema) {
+        throw empty(schema);
+    }
+
+    @Override
+    public double readDouble(Schema schema) {
+        throw empty(schema);
+    }
+
+    @Override
+    public BigInteger readBigInteger(Schema schema) {
+        throw empty(schema);
+    }
+
+    @Override
+    public BigDecimal readBigDecimal(Schema schema) {
+        throw empty(schema);
+    }
+
+    @Override
+    public String readString(Schema schema) {
+        throw empty(schema);
+    }
+
+    @Override
+    public Document readDocument() {
+        throw new SerializationException("Cannot read a document value from an empty XML payload");
+    }
+
+    @Override
+    public Instant readTimestamp(Schema schema) {
+        throw empty(schema);
+    }
+
+    private static SerializationException empty(Schema schema) {
+        return new SerializationException("Cannot read " + schema.id() + " value from an empty XML payload");
+    }
+}

codecs/xml-codec/src/main/java/software/amazon/smithy/java/xml/XmlCodec.java

@@ -63,6 +63,10 @@ public ShapeSerializer createSerializer(OutputStream sink) {
 
     @Override
     public ShapeDeserializer createDeserializer(ByteBuffer source) {
+        if (source == null || !source.hasRemaining()) {
+            return EmptyXmlDeserializer.INSTANCE;
+        }
+
         try {
             var reader = xmlInputFactory.createXMLStreamReader(ByteBufferUtils.byteBufferInputStream(source));
             return XmlDeserializer.topLevel(

codecs/xml-codec/src/test/java/software/amazon/smithy/java/xml/XmlCodecTest.java

@@ -48,6 +48,17 @@ public void deserializesXml() {
         }
     }
 
+    @Test
+    public void deserializesEmptyBody() {
+        try (var codec = XmlCodec.builder().build()) {
+            // Empty ByteBuffer: top-level readStruct should be a no-op
+            var pojo = codec.deserializeShape(ByteBuffer.allocate(0), new TestPojo.Builder());
+            assertThat(pojo.name, equalTo(null));
+            assertThat(pojo.date, equalTo(null));
+            assertThat(pojo.numbers, equalTo(List.of()));
+        }
+    }
+
     @Test
     public void serializesXml() {
         try (var codec = XmlCodec.builder().build()) {

core/src/main/java/software/amazon/smithy/java/core/schema/DeferredRootSchema.java

@@ -95,6 +95,12 @@ public Schema member(String memberName) {
         return resolvedMembers.members.get(memberName);
     }
 
+    @Override
+    public Schema member(int memberIndex) {
+        resolveInternal();
+        return resolvedMembers.memberList.get(memberIndex);
+    }
+
     @Override
     public Set<Integer> intEnumValues() {
         return intEnumValues;

core/src/main/java/software/amazon/smithy/java/core/schema/ResolvedRootSchema.java

@@ -46,6 +46,11 @@ public Schema member(String memberName) {
         return members.get(memberName);
     }
 
+    @Override
+    public Schema member(int memberIndex) {
+        return memberList.get(memberIndex);
+    }
+
     @Override
     public Set<Integer> intEnumValues() {
         return intEnumValues;

core/src/main/java/software/amazon/smithy/java/core/schema/RootSchema.java

@@ -108,6 +108,11 @@ public Schema member(String memberName) {
         return members.get(memberName);
     }
 
+    @Override
+    public Schema member(int memberIndex) {
+        return memberList.get(memberIndex);
+    }
+
     @Override
     public Set<Integer> intEnumValues() {
         return intEnumValues;

core/src/main/java/software/amazon/smithy/java/core/schema/Schema.java

@@ -547,6 +547,22 @@ public Schema member(String memberName) {
         return null;
     }
 
+    /**
+     * Get a member by its index.
+     *
+     * <p>Index is the position of the member in declaration order, matching
+     * what {@link Schema#memberIndex()} returns for that member. Direct array
+     * indexing — significantly faster than {@link #member(String)} when the
+     * caller already has a member's index in hand (for example, when walking
+     * a precomputed plan).
+     *
+     * @param memberIndex Index of the member, 0-based in declaration order.
+     * @return the found member, or null if this schema has no such index.
+     */
+    public Schema member(int memberIndex) {
+        return null;
+    }
+
     /**
      * Get the member of a list.
      *