How to Fix AWS TransferManager uploadFileList Truncating File Names in S3 Batch Uploads

Table of Contents#

1. Understanding the Problem: Filename Truncation in S3 Batch Uploads
2. Root Causes of Truncation
3. Solutions to Fix Truncated Filenames
4. Step-by-Step Implementation Guide
5. Testing the Fix
6. Best Practices to Prevent Future Truncation
7. References

1. Understanding the Problem: Filename Truncation in S3 Batch Uploads#

When using TransferManager.uploadFileList, you might observe that local files with long or special-character names are uploaded to S3 with truncated keys. For example:

• Local filename: quarterly-report-2024-finance-department-final-v1.2.pdf
• S3 key (truncated): quarterly-report-2024-finance~1.pdf

This truncation mimics the 8.3 filename convention (e.g., longname.txt → longn~1.txt) used in legacy filesystems, but it’s unexpected in S3, which supports object keys up to 1,024 characters. Truncation breaks workflows like data pipelines, where downstream systems (e.g., Lambda, ETL tools) depend on exact filenames.

2. Root Causes of Truncation#

To resolve the issue, we first identify why uploadFileList truncates filenames:

2.1 Default Key Inference from File Objects#

uploadFileList infers the S3 key from the File object’s name by default. If the File object’s getName() method returns a truncated name (e.g., due to OS-level filesystem limitations like Windows’ 8.3 naming for legacy compatibility), the S3 key will also be truncated.

2.2 Outdated AWS SDK Versions#

Older versions of the AWS SDK for Java (e.g., 1.x or early 2.x releases) had bugs in TransferManager where filenames exceeding certain lengths were truncated during key generation, even if the File object’s name was correct.

2.3 Reliance on Implicit Key Generation#

Many users rely on uploadFileList’s default behavior, which does not explicitly set the S3 key. This leaves key generation vulnerable to hidden logic (e.g., legacy filename sanitization) in the SDK.

3. Solutions to Fix Truncated Filenames#

The core fix is to explicitly control the S3 key generation instead of relying on TransferManager’s defaults. Here are the most effective solutions:

3.1 Upgrade to the Latest AWS SDK#

Update to the latest AWS SDK for Java 2.x, as newer releases (v2.20.0+) include fixes for filename truncation bugs.

3.2 Explicitly Set S3 Keys with a KeyProvider#

Use UploadFileListRequest (in SDK 2.x) and define a custom keyProvider to explicitly set the S3 key for each file. This bypasses default filename inference.

3.3 Avoid File Object Limitations#

If the File object’s getName() is truncated (e.g., due to OS constraints), use Path objects instead, as they preserve full filenames more reliably.

4. Step-by-Step Implementation Guide#

Follow these steps to fix truncation in your batch uploads:

Step 1: Verify Your AWS SDK Version#

Check your project’s AWS SDK version. If using SDK 1.x (legacy) or an older 2.x release (e.g., <2.20.0), upgrade to the latest 2.x version.

Example pom.xml (Maven) for SDK 2.x:

<dependency>  
  <groupId>software.amazon.awssdk</groupId>  
  <artifactId>s3transfermanager</artifactId>  
  <version>2.25.0</version> <!-- Use the latest version -->  
</dependency>  

Step 2: Replace Implicit Key Generation with Explicit keyProvider#

In SDK 2.x, S3TransferManager’s uploadFileList uses UploadFileListRequest, which accepts a keyProvider to define S3 keys. Use this to explicitly set keys.

Before (Problematic Code):#

import software.amazon.awssdk.transfer.s3.S3TransferManager;  
import java.nio.file.Paths;  
import java.util.List;  
 
public class BatchUploader {  
  public void uploadFiles() {  
    S3TransferManager transferManager = S3TransferManager.create();  
 
    // Upload files from "local-directory" to "my-bucket"  
    var request = UploadFileListRequest.builder()  
        .source(Paths.get("local-directory")) // Local directory with files  
        .bucket("my-bucket") // S3 bucket name  
        .build(); // Relies on default keyProvider (may truncate)  
 
    transferManager.uploadFileList(request).completionFuture().join();  
    transferManager.close();  
  }  
}  

After (Fixed Code):#

Define a keyProvider to explicitly set the S3 key to the full filename of each file:

import software.amazon.awssdk.transfer.s3.S3TransferManager;  
import software.amazon.awssdk.transfer.s3.model.UploadFileListRequest;  
import java.nio.file.Path;  
import java.nio.file.Paths;  
 
public class BatchUploader {  
  public void uploadFiles() {  
    S3TransferManager transferManager = S3TransferManager.create();  
 
    var request = UploadFileListRequest.builder()  
        .source(Paths.get("local-directory"))  
        .bucket("my-bucket")  
        // Explicit keyProvider: Use the full filename as the S3 key  
        .keyProvider(filePath -> filePath.getFileName().toString())  
        .build();  
 
    transferManager.uploadFileList(request).completionFuture().join();  
    transferManager.close();  
  }  
}  

Key Change: keyProvider(filePath -> filePath.getFileName().toString()) ensures the S3 key is set to the full filename of the local file (e.g., quarterly-report-2024-finance-department-final-v1.2.pdf), avoiding truncation.

Step 3: Handle Edge Cases (Optional)#

For complex scenarios (e.g., nested directories, special characters), customize the keyProvider to sanitize or format keys explicitly:

.keyProvider(filePath -> {  
  // Example: Prefix keys with "uploads/" and replace spaces with underscores  
  String fileName = filePath.getFileName().toString().replace(" ", "_");  
  return "uploads/" + fileName;  
})  

5. Testing the Fix#

Verify filenames are no longer truncated with these steps:

Test 1: Upload a Batch with Long Filenames#

Create test files with long names (e.g., this-is-a-very-long-filename-1234567890abcdefghijklmnopqrstuvwxyz.pdf) and run your upload code.

Test 2: Validate S3 Keys#

Check the S3 bucket via:

• AWS Console: Navigate to your bucket and confirm filenames match local names exactly.
• AWS CLI: Run aws s3 ls s3://my-bucket/ to list objects and verify keys.
• Code: Use S3Client.listObjectsV2 to programmatically check keys:

import software.amazon.awssdk.services.s3.S3Client;  
import software.amazon.awssdk.services.s3.model.ListObjectsV2Request;  
 
public class S3Validator {  
  public void checkKeys() {  
    S3Client s3Client = S3Client.create();  
    var request = ListObjectsV2Request.builder().bucket("my-bucket").build();  
    s3Client.listObjectsV2Paginator(request).stream()  
        .flatMap(r -> r.contents().stream())  
        .forEach(obj -> System.out.println("S3 Key: " + obj.key())); // Print keys  
  }  
}  

6. Best Practices to Prevent Future Truncation#

6.1 Always Explicitly Define S3 Keys#

Never rely on TransferManager’s default key inference. Use keyProvider (SDK 2.x) or PutObjectRequest (for single files) to set keys explicitly.

6.2 Use Path Over File#

java.nio.file.Path preserves full filenames better than java.io.File, especially on systems with legacy filesystem limitations.

6.3 Test with Edge Cases#

Upload files with:

• Long names (up to 1,024 characters, S3’s key limit).
• Special characters (e.g., !@#$%^&*()_+).
• OS-specific edge cases (e.g., Windows vs. Linux paths).

6.4 Monitor Uploads with Logging#

Log S3 keys during upload to debug truncation issues:

.keyProvider(filePath -> {  
  String key = filePath.getFileName().toString();  
  System.out.println("Uploading to S3 key: " + key); // Log the key  
  return key;  
})  

7. References#

• AWS SDK for Java 2.x Documentation
• UploadFileListRequest Javadoc
• AWS SDK 2.x Release Notes (check for truncation fixes)
• S3 Object Key Naming Guidelines

By explicitly controlling S3 key generation and keeping your SDK updated, you can ensure batch uploads preserve filenames accurately. Let us know in the comments if you encountered other truncation edge cases!