# 生成MD5值
#### 媒资上传和更新
调用[创建媒资：上传方式](https://support.huaweicloud.com/api-vod/vod_04_0196.html)和[视频更新](https://support.huaweicloud.com/api-vod/vod_04_0207.html)接口时，可以通过"video_md5"设置媒资文件的MD5值。设置后，OBS会对媒资的MD5值进行检验，具体可参考[设置对象属性](https://support.huaweicloud.com/sdk-ios-devg-obs/obs_27_0405.html)。
该MD5值是经过标准的MD5哈希算法计算后，再进行base64编码的。示例代码如下所示：
```
import org.apache.commons.codec.binary.Base64;
import java.io.File;
import java.io.FileInputStream;
import java.io.IOException;
import java.security.DigestInputStream;
import java.security.MessageDigest;
import java.security.NoSuchAlgorithmException;
public class VodDemoObsCheckMd5 {
    public String md5Generate4ObsCheck(String fileUrl)
        throws IOException, NoSuchAlgorithmException {
        String md5Content = null;
        if ((fileUrl != null) && (fileUrl.length() != 0)) {
            File file = new File(fileUrl);
            if (!file.exists()) {
                System.out.println("文件不存在");
            } else if (file.isDirectory()) {
                System.out.println(file.getCanonicalPath() + "是一个目录,无法计算");
            } else {
                FileInputStream fis = new FileInputStream(file);
                DigestInputStream dis = new DigestInputStream(fis,
                        MessageDigest.getInstance("MD5"));
                byte[] buffer = new byte[8192];
                while (dis.read(buffer) > 0) {
                }
                md5Content = new String(Base64.encodeBase64(
                            dis.getMessageDigest().digest()));
                fis.getChannel().position(0L);
                System.out.println("该文件MD5为： " + md5Content);
                fis.close();
            }
        } else {
            System.out.println("缺少文件名");
        }
        return md5Content;
    }
}
```
#### 上传校验
调用[上传检验](https://support.huaweicloud.com/api-vod/vod_04_0050.html)接口时，点播服务会根据媒资的MD5值来检查是否已有重复的媒资文件。MD5值的生成方式是取媒资文件的1024字节，并进行MD5计算，示例代码如下所示：
```
import java.io.File;
import java.io.IOException;
import java.nio.ByteBuffer;
import java.nio.channels.SeekableByteChannel;
import java.nio.file.Files;
import java.nio.file.Path;
import java.nio.file.Paths;
import java.nio.file.StandardOpenOption;
import org.apache.commons.codec.digest.DigestUtils;
public class VodDemoDuplicateCheckMd5 {
    public static String computeMd5ByFile(String fileUrl) {
        String md5Content = null;
        Path targetFile = Paths.get(fileUrl);
        try (SeekableByteChannel channel = Files.newByteChannel(targetFile, StandardOpenOption.READ)) {
            ByteBuffer byteBuffer = ByteBuffer.allocate(1025);
            channel.read(byteBuffer);
            byteBuffer.flip();
            byte[] data = new byte[byteBuffer.limit()];
            byteBuffer.get(data);
            md5Content = DigestUtils.md5Hex(data);
        } catch (IOException e) {
            throw new RuntimeException(String.format("Read file %s failed.", fileUrl));
        }
        return md5Content;
    }
}
```