【Java】實作透過Gmail SMTP Server 發送信件(利用javamail API)

連線加密方式有三種：

1. None(不加密)
2. TLS , port number:587
3. SSL , port number:465 (參考網址)

這裡實作以SSL加密寄送信件。
需要包裝javamail API到lib，下例用JavaMail 1.4.2進行包裝。

注意： 若發送信件內容，不只是純文字，而有用到html、CSS。
經過測試後，確定gmail 不支援部分block css內容，若可行，請寫inline css。
其他html email設計的要素，參考：

1. Guide to CSS support in email 
2. How HTML email works, basic concepts, best practices, tips and tricks

package option;

import java.text.DateFormat;
import java.util.Date;
import java.util.Properties;

import javax.mail.Authenticator;
import javax.mail.Message;
import javax.mail.PasswordAuthentication;
import javax.mail.Session;
import javax.mail.Transport;
import javax.mail.internet.InternetAddress;
import javax.mail.internet.MimeBodyPart;
import javax.mail.internet.MimeMessage;
import javax.mail.internet.MimeMultipart;

public class Email {

 String subject;
 String content;
 String receiver;

 Email() {
  subject = "";
  content = "";
  receiver = "";
 }

 public void setSubject(String subject) {
  this.subject = subject;
 }

 public void setContents(String content) {
  this.content = content;
 }

 public void setReceiverAddress(String address) {
  this.receiver = address;
 }

 static Properties generateRequireProp() {

  final String SSL_FACTORY = "javax.net.ssl.SSLSocketFactory";
  
  // set up gmail information
  Properties props = new Properties();
  props.setProperty("mail.smtp.host", "smtp.gmail.com");
  props.setProperty("mail.smtp.socketFactory.class", SSL_FACTORY);
  props.setProperty("mail.smtp.socketFactory.fallback", "false");
  props.setProperty("mail.smtp.socketFactory.port", "465");
  props.setProperty("mail.smtp.socketFactory.class", "javax.net.ssl.SSLSocketFactory");
  props.setProperty("mail.smtp.auth", "true");
  props.setProperty("mail.smtp.port", "465");

  return props;
 }

 boolean send() {
  if (receiver.equals("")) {
   System.out.println("Email Receiver is empty.");
   return false;
  }

  Properties props = generateRequireProp();
  final String username = "sender@gmail.com";
  final String password = "sender password";

  // build the new session service
  Session session = Session.getInstance(props, new Authenticator() {
   protected PasswordAuthentication getPasswordAuthentication() {
    return new PasswordAuthentication(username, password);
   }
  });

  try {   
   Message message = new MimeMessage(session);
   
   //build the mail of sender , receiver and subject info 
   message.setFrom(new InternetAddress(username));
   message.setRecipients(Message.RecipientType.TO, InternetAddress.parse(receiver, false));
   message.setSubject(subject);
   
   // set mime content
   MimeMultipart multipart = new MimeMultipart();
   MimeBodyPart messageBodyPart = new MimeBodyPart();
   messageBodyPart.setContent(content, "text/html;charset=utf-8");
   multipart.addBodyPart(messageBodyPart);
   message.setContent(multipart);

   Transport.send(message);
   System.out.println("Done");

  } catch (Exception e) {
   throw new RuntimeException(e);
  }
  return true;

 }

 public static void main(String args[]) {

  Email email = new Email();
  email.setSubject("Test mail "+DateFormat.getDateTimeInstance().format(new Date(System.currentTimeMillis())));
  email.setContents("This is a test e-mail by javamail library");
  email.setReceiverAddress("receiver@mail");

  if (email.send())
   System.out.println("Sending is successing");

  else
   System.out.println("Error");

 }
}