review comments

core/sdk-core/src/main/java/software/amazon/awssdk/core/SplittingTransformerConfiguration.java

@@ -19,6 +19,7 @@
 import software.amazon.awssdk.annotations.SdkPublicApi;
 import software.amazon.awssdk.core.async.AsyncResponseTransformer;
 import software.amazon.awssdk.core.internal.async.SplittingTransformer;
+import software.amazon.awssdk.utils.ToString;
 import software.amazon.awssdk.utils.Validate;
 import software.amazon.awssdk.utils.builder.CopyableBuilder;
 import software.amazon.awssdk.utils.builder.ToCopyableBuilder;
@@ -72,6 +73,13 @@ public int hashCode() {
         return bufferSizeInBytes != null ? bufferSizeInBytes.hashCode() : 0;
     }
 
+    @Override
+    public String toString() {
+        return ToString.builder("SplittingTransformerConfiguration")
+            .add("bufferSizeInBytes", bufferSizeInBytes)
+            .build();
+    }
+
     @Override
     public Builder toBuilder() {
         return new DefaultBuilder(this);

core/sdk-core/src/main/java/software/amazon/awssdk/core/async/AsyncResponseTransformer.java

@@ -142,6 +142,13 @@ default SplitResult<ResponseT, ResultT> split(SplittingTransformerConfiguration
                                             .build();
     }
 
+    default SplitResult<ResponseT, ResultT> split(Consumer<SplittingTransformerConfiguration.Builder> splitConfig) {
+        SplittingTransformerConfiguration conf = SplittingTransformerConfiguration.builder()
+                                                                                  .applyMutation(splitConfig)
+                                                                                  .build();
+        return split(conf);
+    }
+
     /**
      * Creates an {@link AsyncResponseTransformer} that writes all the content to the given file. In the event of an error, the
      * SDK will attempt to delete the file (whatever has been written to it so far). If the file already exists, an exception will

services-custom/s3-transfer-manager/src/main/java/software/amazon/awssdk/transfer/s3/S3TransferManager.java

@@ -75,9 +75,23 @@
  *
  *         S3TransferManager transferManager =
  *             S3TransferManager.builder()
- *                              .s3AsyncClient(s3AsyncClient)
+ *                              .s3Client(s3AsyncClient)
  *                              .build();
  * }
+ *
+ * <b>Create an S3 Transfer Manager with S3 Multipart Async Client</b>
+ * {@snippet :
+ *      S3AsyncClient s3AsyncClient = s3AsyncClient.builder()
+ *                                                 .multipartEnabled(true)
+ *                                                 .multipartConfiguration(conf -> conf.apiCallBufferSizeInBytes(32 * MB))
+ *                                                 .build();
+ *
+ *      S3TransferManager transferManager =
+ *          S3TransferManager.builder()
+ *                           .s3Client(s3AsyncClient)
+ *                           .build();
+ * }
+ *
  * <h2>Common Usage Patterns</h2>
  * <b>Upload a file to S3</b>
  * {@snippet :
@@ -157,9 +171,14 @@
  *         // Wait for the transfer to complete
  *         CompletedCopy completedCopy = copy.completionFuture().join();
  * }
- * <b> Warns the user if a multi-part operation disabled client is provided for TransferManager to use
- * If no client is provided, a multi-part enabled async client is instantiated and used.
- * In the event of a multi-part disabled defaultS3AsyncClient being used, a warning is logged </b>
+ * <b> The automatic parallel transfer feature (multipart upload/download) is available
+ * through the AWS-CRT based S3 client {@link S3AsyncClient#crtBuilder().build)}
+ * and Java-based S3 multipart client {@link S3AsyncClient#builder().multipartEnabled(true).build()}.
+ * If no client is configured, AWS-CRT based S3 client will be used if AWS CRT is in the classpath,
+ * otherwise, Java-based S3 multipart client will be used. </b>
+ * {@snippet
+ *
+ * }
  */
 @SdkPublicApi
 @ThreadSafe

services-custom/s3-transfer-manager/src/main/java/software/amazon/awssdk/transfer/s3/internal/TransferManagerFactory.java

@@ -54,7 +54,9 @@ public static S3TransferManager createTransferManager(DefaultBuilder tmBuilder)
 
         if (!s3AsyncClient.getClass().getName().equals("software.amazon.awssdk.services.s3.internal.multipart"
                                                        + ".MultipartS3AsyncClient")) {
-            log.debug(() -> "The provided S3AsyncClient is neither an instance of S3CrtAsyncClient or MultipartS3AsyncClient, "
+            log.debug(() -> "The provided S3AsyncClient is neither "
+                            + "an AWS CRT-based S3 async client (S3AsyncClient.crtBuilder().build()) or "
+                            + "a Java-based S3 async client (S3AsyncClient.builder().multipartEnabled(true).build()), "
                             + "and thus multipart upload/download feature may not be enabled and resumable file upload may not "
                             + "be supported. To benefit from maximum throughput, consider using "
                             + "S3AsyncClient.crtBuilder().build() or "

services/s3/src/main/java/software/amazon/awssdk/services/s3/internal/multipart/DownloadObjectHelper.java

@@ -24,9 +24,12 @@
 import software.amazon.awssdk.services.s3.model.ChecksumMode;
 import software.amazon.awssdk.services.s3.model.GetObjectRequest;
 import software.amazon.awssdk.services.s3.model.GetObjectResponse;
+import software.amazon.awssdk.utils.Logger;
 
 @SdkInternalApi
 public class DownloadObjectHelper {
+    private static final Logger log = Logger.loggerFor(DownloadObjectHelper.class);
+
     private final S3AsyncClient s3AsyncClient;
     private final long bufferSizeInBytes;
 
@@ -38,6 +41,7 @@ public DownloadObjectHelper(S3AsyncClient s3AsyncClient, long bufferSizeInBytes)
     public <T> CompletableFuture<T> downloadObject(
         GetObjectRequest getObjectRequest, AsyncResponseTransformer<GetObjectResponse, T> asyncResponseTransformer) {
         if (getObjectRequest.range() != null || getObjectRequest.partNumber() != null) {
+            logSinglePartWarning(getObjectRequest);
             return s3AsyncClient.getObject(getObjectRequest, asyncResponseTransformer);
         }
         GetObjectRequest requestToPerform = getObjectRequest.toBuilder().checksumMode(ChecksumMode.ENABLED).build();
@@ -57,4 +61,17 @@ private MultipartDownloaderSubscriber subscriber(GetObjectRequest getObjectReque
             .map(ctx -> new MultipartDownloaderSubscriber(s3AsyncClient, getObjectRequest, ctx.highestSequentialCompletedPart()))
             .orElseGet(() -> new MultipartDownloaderSubscriber(s3AsyncClient, getObjectRequest));
     }
+
+    private void logSinglePartWarning(GetObjectRequest getObjectRequest) {
+        String reason = "";
+        if (getObjectRequest.range() != null) {
+            reason = " because getObjectRequest range is included in the request."
+                     + " range = " + getObjectRequest.range();
+        } else if (getObjectRequest.partNumber() != null) {
+            reason = " because getObjectRequest part number is included in the request."
+                     + " part number = " + getObjectRequest.partNumber();
+        }
+        String finalReason = reason;
+        log.debug(() -> "Using single part download" + finalReason);
+    }
 }