要计算AWS S3签名的UploadPart请求头的内容MD5，可以使用AWS SDK或者手动计算。

1. 使用AWS SDK: 使用AWS SDK可以更方便地计算请求头的内容MD5。以下是使用Java AWS SDK的示例代码：

   import com.amazonaws.services.s3.AmazonS3;
   import com.amazonaws.services.s3.AmazonS3ClientBuilder;
   import com.amazonaws.services.s3.model.PartETag;
   import com.amazonaws.services.s3.model.UploadPartRequest;
   import com.amazonaws.services.s3.model.UploadPartResult;
   import org.apache.commons.codec.binary.Base64;
   
   import java.io.ByteArrayInputStream;
   import java.io.IOException;
   import java.io.InputStream;
   import java.security.DigestInputStream;
   import java.security.MessageDigest;
   import java.security.NoSuchAlgorithmException;
   
   public class S3UploadPartMD5 {
       public static void main(String[] args) throws IOException, NoSuchAlgorithmException {
           // 创建Amazon S3客户端
           AmazonS3 s3Client = AmazonS3ClientBuilder.defaultClient();
   
           // 上传文件分块
           String bucketName = "your-bucket-name";
           String key = "your-object-key";
           String uploadId = "your-upload-id";
           int partNumber = 1; // 分块号
           byte[] data = "This is the part data".getBytes(); // 分块数据
           InputStream inputStream = new ByteArrayInputStream(data);
   
           // 计算请求头的内容MD5
           String contentMD5 = calculateContentMD5(data);
   
           // 创建UploadPart请求
           UploadPartRequest uploadPartRequest = new UploadPartRequest()
                   .withBucketName(bucketName)
                   .withKey(key)
                   .withUploadId(uploadId)
                   .withPartNumber(partNumber)
                   .withInputStream(inputStream)
                   .withPartSize(data.length)
                   .withMD5Digest(contentMD5);
   
           // 执行UploadPart请求
           UploadPartResult uploadPartResult = s3Client.uploadPart(uploadPartRequest);
   
           // 获取分块标识
           PartETag partETag = uploadPartResult.getPartETag();
           System.out.println("Part ETag: " + partETag.getETag());
       }
   
       private static String calculateContentMD5(byte[] data) throws NoSuchAlgorithmException, IOException {
           // 计算MD5
           MessageDigest md = MessageDigest.getInstance("MD5");
           InputStream is = new ByteArrayInputStream(data);
           DigestInputStream dis = new DigestInputStream(is, md);
           while (dis.read(data) != -1) {
           }
           byte[] digest = md.digest();
   
           // 将MD5转换为Base64编码
           byte[] base64Bytes = Base64.encodeBase64(digest);
           return new String(base64Bytes);
       }
   }
   

   在上面的示例代码中，使用了AWS Java SDK来上传文件分块，并计算请求头的内容MD5。calculateContentMD5方法使用MessageDigest类来计算MD5，并使用Apache Commons Codec库将MD5转换为Base64编码。

2. 手动计算: 如果不使用AWS SDK，也可以手动计算请求头的内容MD5。以下是一个使用Java手动计算的示例代码：

   import org.apache.commons.codec.binary.Base64;
   
   import java.io.ByteArrayInputStream;
   import java.io.IOException;
   import java.io.InputStream;
   import java.security.DigestInputStream;
   import java.security.MessageDigest;
   import java.security.NoSuchAlgorithmException;
   
   public class S3UploadPartMD5 {
       public static void main(String[] args) throws IOException, NoSuchAlgorithmException {
           // 上传文件分块
           byte[] data = "This is the part data".getBytes(); // 分块数据
           InputStream inputStream = new ByteArrayInputStream(data);
   
           // 计算请求头的内容MD5
           String contentMD5 = calculateContentMD5(data);
           System.out.println("Content-MD5: " + contentMD5);
       }
   
       private static String calculateContentMD5(byte[] data) throws NoSuchAlgorithmException, IOException {
           // 计算MD5
           MessageDigest md = MessageDigest.getInstance("MD5");
           InputStream is = new ByteArrayInputStream(data);
           DigestInputStream dis = new DigestInputStream(is, md);
           while (dis.read(data) != -1) {
           }
           byte[] digest = md.digest();
   
           // 将MD5转换为Base64编码
           byte[] base64Bytes = Base64.encodeBase64(digest);
           return new String(base64Bytes);
       }
   }