gmail javax.mail.MessagingException: 530 5.7.0 Must issue a STARTTLS command fi

下面是用gmail smtp发送邮件的

import java.util.Properties;
import javax.mail.Message;
import javax.mail.Session;
import javax.mail.Transport;
import javax.mail.internet.InternetAddress;
import javax.mail.internet.MimeMessage;

public class SendMail implements Runnable {
	private final String smtp_host = "smtp.gmail.com";  					
	private final String from_userName = "[email protected]";		
	private final String from_passWord = "xx";						
	private final String show_name = "xxx";
	
	public String email_address;
	public String email_subject;
	public String email_content;	
	
	public SendMail(String email_address, String email_subject, String email_content) {
		this.email_address = email_address;
		this.email_subject = email_subject;
		this.email_content = email_content;
	}

	public void run() {
		executeMailSend(email_address, email_subject, email_content);
	}
	
	/**
	 * 
	 * @param recipients: the mail send to
	 * @param sendSubject: the mail subject
	 * @param sendText: the mail content
	 */
	public void executeMailSend(String recipients, String sendSubject, String sendText) {
		
		try {			
			Properties props = System.getProperties();
			props.put("mail.smtp.starttls.enable", "true");
			props.put("mail.smtp.host", smtp_host);
			props.put("mail.smtp.user", from_userName);
			props.put("mail.smtp.password", from_passWord);
			props.put("mail.smtp.port", "587");  //gmail smtp port 587
			props.put("mail.smtp.auth", "true");
			

			
			Session session = Session.getDefaultInstance(props, null);
			session.setDebug(false);
			
			MimeMessage message = new MimeMessage(session);
			message.setFrom(new InternetAddress(from_userName));
			message.addRecipient(Message.RecipientType.TO, new InternetAddress(recipients));
			message.setFrom(new InternetAddress(show_name + "<" + from_userName + ">"));	
			message.setSubject(sendSubject);
			message.setContent(sendText, "text/html;charset=utf-8");			
			
			Transport transport = session.getTransport("smtp");
			transport.connect(smtp_host, from_userName, from_passWord);
			transport.sendMessage(message, message.getAllRecipients());
			transport.close();
			
			System.out.println("send an email to " + recipients + " success");
		} catch (Exception e) {
			e.printStackTrace();
			System.out.println("failure! ");

		}
	}
	public static void main (String args[]){
		new SendMail("[email protected]","test","hehe").run();
	}
}

结果老是报如下的错误

javax.mail.MessagingException: 530 5.7.0 Must issue a STARTTLS command first. 36sm443504yxh.67
failure! 

	at com.sun.mail.smtp.SMTPTransport.issueCommand(SMTPTransport.java:1020)
	at com.sun.mail.smtp.SMTPTransport.mailFrom(SMTPTransport.java:716)
	at com.sun.mail.smtp.SMTPTransport.sendMessage(SMTPTransport.java:388)
	at SendMail.executeMailSend(SendMail.java:59)
	at SendMail.run(SendMail.java:25)
	at SendMail.main(SendMail.java:70)
 

折腾了个把小时，苦苦寻求后发现一个解决办法
把上述代码中的

props.put("mail.smtp.port", "587");

替换成

props.setProperty("mail.smtp.socketFactory.class", "javax.net.ssl.SSLSocketFactory"); 
			props.setProperty("mail.smtp.socketFactory.fallback", "false"); 
			props.setProperty("mail.smtp.port", "465"); 
			props.setProperty("mail.smtp.socketFactory.port", "465"); 

究其原因也不太清楚，网上有资料说port 587也可以，不知结果不行