Sending email SMTP via GMAIL (OAUTH2 and JavaMail) - oauth-2.0

Question

I'm using a web application through which I'm sending an email.
The SMTP host is GMAIL.
I'm using Java 1.8 and JavaMail 1.6.2.
Is there any alternative to the code written below? (credits: https://hellokoding.com/sending-email-through-gmail-smtp-server-with-java-mail-api-and-oauth-2-authorization/)
void sendMail(String smtpServerHost, String smtpServerPort, String smtpUserName, String smtpUserAccessToken, String fromUserEmail, String fromUserFullName, String toEmail, String subject, String body) {
try {
Properties props = System.getProperties();
props.put("mail.transport.protocol", "smtp");
props.put("mail.smtp.port", smtpServerPort);
props.put("mail.smtp.starttls.enable", "true");
Session session = Session.getDefaultInstance(props);
session.setDebug(true);
MimeMessage msg = new MimeMessage(session);
msg.setFrom(new InternetAddress(fromUserEmail, fromUserFullName));
msg.setRecipient(Message.RecipientType.TO, new InternetAddress(toEmail));
msg.setSubject(subject);
msg.setContent(body, "text/html");
SMTPTransport transport = new SMTPTransport(session, null);
transport.connect(smtpServerHost, smtpUserName, null);
transport.issueCommand("AUTH XOAUTH2 " + new String(BASE64EncoderStream.encode(String.format("user=%s\1auth=Bearer %s\1\1", smtpUserName, smtpUserAccessToken).getBytes())), 235);
transport.sendMessage(msg, msg.getAllRecipients());
} catch (Exception ex) {
Logger.getLogger(this.getClass().getName()).log(Level.SEVERE, ex.getMessage(), ex);
}
}
Specifically I'm very confused about these two issues:
is the following line truly the only way to set the access token in the Transport?
transport.issueCommand("AUTH XOAUTH2 " + new String(BASE64EncoderStream.encode(String.format("user=%s\1auth=Bearer %s\1\1", smtpUserName, smtpUserAccessToken).getBytes())), 235);
I've been searching throughout the web but I don't seem to find an answer, basically because every other way I've attempted has resulted in NO success.
Is it true that Google has not (yet) implemented a CLIENT credentials grant type?
How else could I send emails through my web application without the user interaction (which I don't have)?
Thank you

Answer 1

Try this:
//TLS and OAuth2
String address = "my.smtpserver.com";
Integer port = 587;
String user = "my_username";
String accesstoken = "my_accesstoken";
String sender = "me#mycompany.com";
String recipients = "you#yourcompany.com;someone#theircompany.com";
String subject = "Test";
String body = "This is a test.";
Properties properties = new Properties();
properties.put("mail.smtp.host", address);
properties.put("mail.smtp.port", port.toString());
properties.put("mail.smtp.starttls.enable", "true");
properties.put("mail.smtp.auth.mechanisms", "XOAUTH2");
properties.put("mail.debug.auth", "true");
Session session = Session.getInstance(properties);
session.setDebug(true);
MimeMessage mimeMessage = new MimeMessage(session);
mimeMessage.setFrom(new InternetAddress(sender));
String s = recipients.replace(';', ',');
mimeMessage.addRecipients(MimeMessage.RecipientType.TO,
InternetAddress.parse(s));
mimeMessage.setSubject(subject);
MimeMultipart mimeMultipart = new MimeMultipart();
MimeBodyPart mimeBodyPart = new MimeBodyPart();
mimeBodyPart.setText(body);
mimeMultipart.addBodyPart(mimeBodyPart);
mimeMessage.setContent(mimeMultipart);
Transport transport = session.getTransport();
transport.connect(user, accesstoken);
transport.sendMessage(mimeMessage, mimeMessage.getAllRecipients());
transport.close();
session = null;