aws-sdk-java

You must generate an Access Key before getting started. All examples will utilize access_key_id and access_key_secret variables which represent the Access Key ID and Secret Access Key values you generated.

This example uses version 2 of the aws-sdk-java ↗ package. You must pass in the R2 configuration credentials when instantiating your S3 service client:

import software.amazon.awssdk.auth.credentials.AwsBasicCredentials;
import software.amazon.awssdk.auth.credentials.StaticCredentialsProvider;
import software.amazon.awssdk.regions.Region;
import software.amazon.awssdk.services.s3.S3Client;
import software.amazon.awssdk.services.s3.model.*;
import software.amazon.awssdk.services.s3.S3Configuration;
import java.net.URI;
import java.util.List;

/**
 * Client for interacting with Cloudflare R2 Storage using AWS SDK S3 compatibility
 */
public class CloudflareR2Client {
    private final S3Client s3Client;

    /**
     * Creates a new CloudflareR2Client with the provided configuration
     */
    public CloudflareR2Client(S3Config config) {
        this.s3Client = buildS3Client(config);
    }

    /**
     * Configuration class for R2 credentials and endpoint
     */
    public static class S3Config {
        private final String accountId;
        private final String accessKey;
        private final String secretKey;
        private final String endpoint;

        public S3Config(String accountId, String accessKey, String secretKey) {
            this.accountId = accountId;
            this.accessKey = accessKey;
            this.secretKey = secretKey;
            this.endpoint = String.format("https://%s.r2.cloudflarestorage.com", accountId);
        }

        public String getAccessKey() { return accessKey; }
        public String getSecretKey() { return secretKey; }
        public String getEndpoint() { return endpoint; }
    }

    /**
     * Builds and configures the S3 client with R2-specific settings
     */
    private static S3Client buildS3Client(S3Config config) {
        AwsBasicCredentials credentials = AwsBasicCredentials.create(
            config.getAccessKey(),
            config.getSecretKey()
        );

        S3Configuration serviceConfiguration = S3Configuration.builder()
            .pathStyleAccessEnabled(true)
            .build();

        return S3Client.builder()
            .endpointOverride(URI.create(config.getEndpoint()))
            .credentialsProvider(StaticCredentialsProvider.create(credentials))
            .region(Region.of("auto"))
            .serviceConfiguration(serviceConfiguration)
            .build();
    }

    /**
     * Lists all buckets in the R2 storage
     */
    public List<Bucket> listBuckets() {
        try {
            return s3Client.listBuckets().buckets();
        } catch (S3Exception e) {
            throw new RuntimeException("Failed to list buckets: " + e.getMessage(), e);
        }
    }

    /**
     * Lists all objects in the specified bucket
     */
    public List<S3Object> listObjects(String bucketName) {
        try {
            ListObjectsV2Request request = ListObjectsV2Request.builder()
                .bucket(bucketName)
                .build();

            return s3Client.listObjectsV2(request).contents();
        } catch (S3Exception e) {
            throw new RuntimeException("Failed to list objects in bucket " + bucketName + ": " + e.getMessage(), e);
        }
    }

    public static void main(String[] args) {
        S3Config config = new S3Config(
            "your_account_id",
            "your_access_key",
            "your_secret_key"
        );

        CloudflareR2Client r2Client = new CloudflareR2Client(config);

        // List buckets
        System.out.println("Available buckets:");
        r2Client.listBuckets().forEach(bucket ->
            System.out.println("* " + bucket.name())
        );

        // List objects in a specific bucket
        String bucketName = "demos";
        System.out.println("\nObjects in bucket '" + bucketName + "':");
        r2Client.listObjects(bucketName).forEach(object ->
            System.out.printf("* %s (size: %d bytes, modified: %s)%n",
                object.key(),
                object.size(),
                object.lastModified())
        );
    }
}

Generate presigned URLs

You can also generate presigned links that can be used to temporarily share public write access to a bucket.

// import required packages for presigning
// Rest of the packages are same as above
import software.amazon.awssdk.services.s3.presigner.S3Presigner;
import software.amazon.awssdk.services.s3.presigner.model.PutObjectPresignRequest;
import software.amazon.awssdk.services.s3.presigner.model.PresignedPutObjectRequest;
import java.time.Duration;

public class CloudflareR2Client {
  private final S3Client s3Client;
  private final S3Presigner presigner;

    /**
     * Creates a new CloudflareR2Client with the provided configuration
     */
    public CloudflareR2Client(S3Config config) {
        this.s3Client = buildS3Client(config);
        this.presigner = buildS3Presigner(config);
    }

    /**
     * Builds and configures the S3 presigner with R2-specific settings
     */
    private static S3Presigner buildS3Presigner(S3Config config) {
        AwsBasicCredentials credentials = AwsBasicCredentials.create(
            config.getAccessKey(),
            config.getSecretKey()
        );

        return S3Presigner.builder()
            .endpointOverride(URI.create(config.getEndpoint()))
            .credentialsProvider(StaticCredentialsProvider.create(credentials))
            .region(Region.of("auto"))
            .serviceConfiguration(S3Configuration.builder()
                .pathStyleAccessEnabled(true)
                .build())
            .build();
    }

    public String generatePresignedUploadUrl(String bucketName, String objectKey, Duration expiration) {
        PutObjectPresignRequest presignRequest = PutObjectPresignRequest.builder()
            .signatureDuration(expiration)
            .putObjectRequest(builder -> builder
                .bucket(bucketName)
                .key(objectKey)
                .build())
            .build();

        PresignedPutObjectRequest presignedRequest = presigner.presignPutObject(presignRequest);
        return presignedRequest.url().toString();
    }

    // Rest of the methods remains the same

    public static void main(String[] args) {
      // config the client as before

      // Generate a pre-signed upload URL valid for 15 minutes
        String uploadUrl = r2Client.generatePresignedUploadUrl(
            "demos",
            "README.md",
            Duration.ofMinutes(15)
        );
        System.out.println("Pre-signed Upload URL (valid for 15 minutes):");
        System.out.println(uploadUrl);
    }

}

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