怎么导入 java.util.Base64 到 Android

java 的提取证书的方法，其中用到了java 中的 base64包

  import java.io.File;
  import java.io.FileInputStream;
  import java.io.IOException;
  import java.io.InputStream;
  import java.security.MessageDigest;
  import java.security.cert.CertificateFactory;
  import java.security.cert.X509Certificate;
  import java.util.Arrays;
  import java.util.Base64;

  public class PeerCertificateExtractor {
/**
 * Get peer certificate(Public key to sha256 to base64)
 * @param certificate Crt or der or pem file with a valid certificate
 * @return
 */
public static String extract(File certificate){

    FileInputStream inputStream = null;

    try{
        inputStream = new FileInputStream(certificate);
        X509Certificate x509Certificate = (X509Certificate) CertificateFactory.getInstance("X509")
                .generateCertificate(inputStream);

        byte[] publicKeyEncoded = x509Certificate.getPublicKey().getEncoded();
        MessageDigest messageDigest = MessageDigest.getInstance("SHA-256");
        byte[] publicKeySha256 = messageDigest.digest(publicKeyEncoded);
        byte[] publicKeyShaBase64 = Base64.getEncoder().encode(publicKeySha256);

       return "sha256/" + new String(publicKeyShaBase64);
    }catch (Exception e){
        e.printStackTrace();
    } finally {
        try {
            if (inputStream != null) {
                inputStream.close();
            }
        } catch (IOException e) {
            e.printStackTrace();
        }
    }
    return "";
}
}

而在 android 当中，你应该删除下列代码

 byte[] publicKeyShaBase64 = Base64.getEncoder().encode(publicKeySha256);
 return "sha256/" + new String(publicKeyShaBase64);

然后替换成

return "sha256/" + Base64.encodeToString(publicKeySha256, Base64.DEFAULT);