Introduction

To read a file from Amazon S3 in Java, use the AWS SDK for Java and request the object by bucket name and key. In modern Java projects, the AWS SDK for Java v2 is the usual choice because it has a cleaner API and clearer client configuration than older examples built around SDK v1.

Add the AWS SDK for Java v2 Dependency

With Maven, add the S3 module.

1<dependency>
2  <groupId>software.amazon.awssdk</groupId>
3  <artifactId>s3</artifactId>
4  <version>2.25.0</version>
5</dependency>

In a real project, keep the SDK version aligned with the rest of your AWS dependencies rather than copying a version string blindly from an example.

Read the Object as Bytes

A straightforward way to read a small file is getObjectAsBytes.

1import software.amazon.awssdk.core.ResponseBytes;
2import software.amazon.awssdk.regions.Region;
3import software.amazon.awssdk.services.s3.S3Client;
4import software.amazon.awssdk.services.s3.model.GetObjectRequest;
5import software.amazon.awssdk.services.s3.model.GetObjectResponse;
6
7public class ReadS3File {
8    public static void main(String[] args) {
9        try (S3Client s3 = S3Client.builder().region(Region.US_EAST_1).build()) {
10            GetObjectRequest request = GetObjectRequest.builder()
11                    .bucket("my-bucket")
12                    .key("reports/data.txt")
13                    .build();
14
15            ResponseBytes<GetObjectResponse> objectBytes = s3.getObjectAsBytes(request);
16            String content = objectBytes.asUtf8String();
17            System.out.println(content);
18        }
19    }
20}

This is easy to understand and works well for smaller text files.

Stream Larger Objects Instead of Loading Them All at Once

If the object may be large, stream it rather than pulling the whole response into memory.

1import java.io.BufferedReader;
2import java.io.InputStreamReader;
3import java.nio.charset.StandardCharsets;
4import software.amazon.awssdk.services.s3.model.GetObjectRequest;
5
6try (S3Client s3 = S3Client.builder().region(Region.US_EAST_1).build();
7     var input = s3.getObject(GetObjectRequest.builder().bucket("my-bucket").key("big/file.txt").build());
8     var reader = new BufferedReader(new InputStreamReader(input, StandardCharsets.UTF_8))) {
9
10    reader.lines().forEach(System.out::println);
11}

Streaming is the better default when file size is uncertain.

The Bucket Key Is Not a Local File Path

An S3 object is addressed by bucket plus key. The key can look path-like, but it is not the same as a normal file-system path.

That matters when debugging because “folder” assumptions often cause wrong-key errors. If the object is stored under reports/data.txt, then the full key must match exactly.

Read Text Only After You Know the Encoding

Calling asUtf8String is convenient for UTF-8 text files, but not every S3 object is text. If the object is binary or uses another encoding, treat the response as bytes or stream content with the correct decoder.

That distinction prevents subtle corruption bugs when the object is an image, archive, or non-UTF-8 document.

Handle Missing Objects Explicitly

A wrong key or bucket can raise an S3 service exception rather than returning an empty stream. Good client code should treat missing objects as a normal failure mode and log enough information to diagnose the wrong bucket, key, or environment.

Credentials and Region Must Be Correct

The code works only if the client can authenticate and the region matches the bucket’s setup. In many environments, the SDK resolves credentials automatically from standard providers such as environment variables, AWS profiles, IAM roles, or container credentials.

That means many “cannot read S3” problems are really about:

• missing credentials
• wrong region
• missing s3:GetObject permission
• wrong bucket or key name

Common Pitfalls

• Using old AWS SDK for Java examples without noticing they target a different SDK generation.
• Loading a large object fully into memory when streaming would be safer.
• Assuming credentials are valid without checking the active AWS identity and permissions.
• Confusing the S3 object key with a local file-system path.
• Forgetting that bucket region and client region configuration must be compatible.

Summary

• Use the AWS SDK for Java v2 to read S3 objects in modern Java code.
• 'getObjectAsBytes is convenient for small files.'
• Stream larger objects to avoid unnecessary memory pressure.
• Remember that S3 object keys are exact identifiers, not normal file paths.
• Ensure region, credentials, and s3:GetObject permissions are correct.