diff --git a/.changes/next-release/bugfix-NettyNIOHTTPClient-b9465c8.json b/.changes/next-release/bugfix-NettyNIOHTTPClient-b9465c8.json
new file mode 100644
index 000000000000..b124c6a4b41a
--- /dev/null
+++ b/.changes/next-release/bugfix-NettyNIOHTTPClient-b9465c8.json
@@ -0,0 +1,6 @@
+{
+ "type": "bugfix",
+ "category": "Netty NIO HTTP Client",
+ "contributor": "",
+ "description": "Fixed a `NullPointerException` in `HandlerSubscriber` that could intermittently fail async requests (such as S3 `PutObject`/`UploadPart`) when a channel writability change occurred during the `Expect: 100-continue` window before the request body subscription was established. See [#7271](https://github.com/aws/aws-sdk-java-v2/issues/7271)."
+}
diff --git a/http-clients/netty-nio-client/src/main/java/software/amazon/awssdk/http/nio/netty/internal/nrs/HandlerSubscriber.java b/http-clients/netty-nio-client/src/main/java/software/amazon/awssdk/http/nio/netty/internal/nrs/HandlerSubscriber.java
index 81e2a648a145..28c0f2933046 100644
--- a/http-clients/netty-nio-client/src/main/java/software/amazon/awssdk/http/nio/netty/internal/nrs/HandlerSubscriber.java
+++ b/http-clients/netty-nio-client/src/main/java/software/amazon/awssdk/http/nio/netty/internal/nrs/HandlerSubscriber.java
@@ -154,7 +154,12 @@ private void verifyRegisteredWithRightExecutor(ChannelHandlerContext ctx) {
@Override
public void channelWritabilityChanged(ChannelHandlerContext ctx) throws Exception {
- maybeRequestMore();
+ // Only request demand once RUNNING. A writability change can arrive while the subscription is still pending
+ // (the Expect: 100-continue deferral window, where the subscribe is held back until the server answers), when
+ // maybeRequestMore() would dereference a null subscription. See #7271.
+ if (state == HandlerSubscriber.State.RUNNING) {
+ maybeRequestMore();
+ }
ctx.fireChannelWritabilityChanged();
}
diff --git a/http-clients/netty-nio-client/src/test/java/software/amazon/awssdk/http/nio/netty/internal/nrs/HandlerSubscriberExpectContinueTest.java b/http-clients/netty-nio-client/src/test/java/software/amazon/awssdk/http/nio/netty/internal/nrs/HandlerSubscriberExpectContinueTest.java
new file mode 100644
index 000000000000..df2f74db062b
--- /dev/null
+++ b/http-clients/netty-nio-client/src/test/java/software/amazon/awssdk/http/nio/netty/internal/nrs/HandlerSubscriberExpectContinueTest.java
@@ -0,0 +1,163 @@
+/*
+ * Copyright Amazon.com, Inc. or its affiliates. All Rights Reserved.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License").
+ * You may not use this file except in compliance with the License.
+ * A copy of the License is located at
+ *
+ * http://aws.amazon.com/apache2.0
+ *
+ * or in the "license" file accompanying this file. This file is distributed
+ * on an "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either
+ * express or implied. See the License for the specific language governing
+ * permissions and limitations under the License.
+ */
+
+package software.amazon.awssdk.http.nio.netty.internal.nrs;
+
+import static java.nio.charset.StandardCharsets.UTF_8;
+import static org.assertj.core.api.Assertions.assertThat;
+
+import io.netty.buffer.ByteBuf;
+import io.netty.buffer.Unpooled;
+import io.netty.channel.ChannelHandlerContext;
+import io.netty.channel.ChannelInboundHandlerAdapter;
+import io.netty.channel.embedded.EmbeddedChannel;
+import io.netty.handler.codec.http.DefaultFullHttpResponse;
+import io.netty.handler.codec.http.DefaultHttpContent;
+import io.netty.handler.codec.http.HttpContent;
+import io.netty.handler.codec.http.HttpMethod;
+import io.netty.handler.codec.http.HttpResponse;
+import io.netty.handler.codec.http.HttpResponseStatus;
+import io.netty.handler.codec.http.HttpUtil;
+import io.netty.handler.codec.http.HttpVersion;
+import java.util.concurrent.atomic.AtomicBoolean;
+import java.util.concurrent.atomic.AtomicReference;
+import org.junit.jupiter.api.AfterEach;
+import org.junit.jupiter.api.BeforeEach;
+import org.junit.jupiter.api.Test;
+import org.reactivestreams.Publisher;
+import org.reactivestreams.Subscriber;
+import org.reactivestreams.Subscription;
+
+/**
+ * Regression coverage for the NPE reported in
+ * #7271: when a request carries
+ * {@code Expect: 100-continue}, {@link HttpStreamsClientHandler} defers subscribing the {@link HandlerSubscriber} to the
+ * body until the server answers {@code 100 Continue}. A {@code channelWritabilityChanged} event fired during that window
+ * used to dereference the still-null subscription and throw.
+ */
+public class HandlerSubscriberExpectContinueTest {
+
+ private EmbeddedChannel channel;
+ private ExceptionCapturingHandler exceptionCapture;
+
+ @BeforeEach
+ public void setup() {
+ channel = new EmbeddedChannel(new HttpStreamsClientHandler());
+ exceptionCapture = new ExceptionCapturingHandler();
+ channel.pipeline().addLast(exceptionCapture);
+ }
+
+ @AfterEach
+ public void teardown() {
+ channel.finishAndReleaseAll();
+ }
+
+ @Test
+ public void channelWritabilityChanged_whenSubscriptionPending_doesNotRouteExceptionToPipeline() {
+ writeExpectContinueRequest(new SingleChunkPublisher("body"));
+
+ channel.pipeline().fireChannelWritabilityChanged();
+
+ assertThat(exceptionCapture.captured()).isNull();
+ }
+
+ @Test
+ public void channelWritabilityChanged_whenSubscriptionPending_bodyStreamsAfter100Continue() {
+ SingleChunkPublisher body = new SingleChunkPublisher("body");
+ writeExpectContinueRequest(body);
+
+ channel.pipeline().fireChannelWritabilityChanged();
+ deliver100Continue();
+
+ assertThat(body.subscribed()).isTrue();
+ assertThat(streamedBody()).isEqualTo("body");
+ assertThat(exceptionCapture.captured()).isNull();
+ }
+
+ private void writeExpectContinueRequest(Publisher body) {
+ DefaultStreamedHttpRequest request =
+ new DefaultStreamedHttpRequest(HttpVersion.HTTP_1_1, HttpMethod.PUT, "/", body);
+ HttpUtil.set100ContinueExpected(request, true);
+ channel.writeOutbound(request);
+ }
+
+ private void deliver100Continue() {
+ HttpResponse continueResponse =
+ new DefaultFullHttpResponse(HttpVersion.HTTP_1_1, HttpResponseStatus.CONTINUE);
+ channel.writeInbound(continueResponse);
+ channel.runPendingTasks();
+ }
+
+ private String streamedBody() {
+ StringBuilder body = new StringBuilder();
+ Object msg;
+ while ((msg = channel.readOutbound()) != null) {
+ if (msg instanceof HttpContent) {
+ body.append(((HttpContent) msg).content().toString(UTF_8));
+ }
+ }
+ return body.toString();
+ }
+
+ private static final class ExceptionCapturingHandler extends ChannelInboundHandlerAdapter {
+ private final AtomicReference captured = new AtomicReference<>();
+
+ @Override
+ public void exceptionCaught(ChannelHandlerContext ctx, Throwable cause) {
+ captured.compareAndSet(null, cause);
+ }
+
+ Throwable captured() {
+ return captured.get();
+ }
+ }
+
+ private static final class SingleChunkPublisher implements Publisher {
+ private final byte[] payload;
+ private final AtomicBoolean subscribed = new AtomicBoolean(false);
+
+ private SingleChunkPublisher(String payload) {
+ this.payload = payload.getBytes(UTF_8);
+ }
+
+ boolean subscribed() {
+ return subscribed.get();
+ }
+
+ @Override
+ public void subscribe(Subscriber subscriber) {
+ subscribed.set(true);
+ subscriber.onSubscribe(new Subscription() {
+ private boolean delivered;
+
+ @Override
+ public void request(long n) {
+ if (n <= 0 || delivered) {
+ return;
+ }
+ delivered = true;
+ ByteBuf buf = Unpooled.wrappedBuffer(payload);
+ subscriber.onNext(new DefaultHttpContent(buf));
+ subscriber.onComplete();
+ }
+
+ @Override
+ public void cancel() {
+ delivered = true;
+ }
+ });
+ }
+ }
+}