Use PutBucketEncryption with an AWS SDK

Set bucket encryption to a directory bucket.

import org.slf4j.Logger;
import org.slf4j.LoggerFactory;
import software.amazon.awssdk.regions.Region;
import software.amazon.awssdk.services.kms.KmsClient;
import software.amazon.awssdk.services.s3.S3Client;
import software.amazon.awssdk.services.s3.model.PutBucketEncryptionRequest;
import software.amazon.awssdk.services.s3.model.S3Exception;
import software.amazon.awssdk.services.s3.model.ServerSideEncryption;
import software.amazon.awssdk.services.s3.model.ServerSideEncryptionByDefault;
import software.amazon.awssdk.services.s3.model.ServerSideEncryptionConfiguration;
import software.amazon.awssdk.services.s3.model.ServerSideEncryptionRule;

import static com.example.s3.util.S3DirectoryBucketUtils.createDirectoryBucket;
import static com.example.s3.util.S3DirectoryBucketUtils.createKmsClient;
import static com.example.s3.util.S3DirectoryBucketUtils.createKmsKey;
import static com.example.s3.util.S3DirectoryBucketUtils.deleteDirectoryBucket;
import static com.example.s3.util.S3DirectoryBucketUtils.scheduleKeyDeletion;

    /**
     * Sets the default encryption configuration for an S3 bucket as SSE-KMS.
     *
     * @param s3Client   The S3 client used to interact with S3
     * @param bucketName The name of the directory bucket
     * @param kmsKeyId   The ID of the customer-managed KMS key
     */
    public static void putDirectoryBucketEncryption(S3Client s3Client, String bucketName, String kmsKeyId) {
        // Define the default encryption configuration to use SSE-KMS. For directory
        // buckets, AWS managed KMS keys aren't supported. Only customer-managed keys
        // are supported.
        ServerSideEncryptionByDefault encryptionByDefault = ServerSideEncryptionByDefault.builder()
                .sseAlgorithm(ServerSideEncryption.AWS_KMS)
                .kmsMasterKeyID(kmsKeyId)
                .build();

        // Create a server-side encryption rule to apply the default encryption
        // configuration. For directory buckets, the bucketKeyEnabled field is enforced
        // to be true.
        ServerSideEncryptionRule rule = ServerSideEncryptionRule.builder()
                .bucketKeyEnabled(true)
                .applyServerSideEncryptionByDefault(encryptionByDefault)
                .build();

        // Create the server-side encryption configuration for the bucket
        ServerSideEncryptionConfiguration encryptionConfiguration = ServerSideEncryptionConfiguration.builder()
                .rules(rule)
                .build();

        // Create the PutBucketEncryption request
        PutBucketEncryptionRequest putRequest = PutBucketEncryptionRequest.builder()
                .bucket(bucketName)
                .serverSideEncryptionConfiguration(encryptionConfiguration)
                .build();

        // Set the bucket encryption
        try {
            s3Client.putBucketEncryption(putRequest);
            logger.info("SSE-KMS Bucket encryption configuration set for the directory bucket: {}", bucketName);
        } catch (S3Exception e) {
            logger.error("Failed to set bucket encryption: {} - Error code: {}", e.awsErrorDetails().errorMessage(),
                    e.awsErrorDetails().errorCode());
            throw e;
        }
    }