Java Mail API setContent()未在邮件正文中编写为HTML

import javax.mail.*;
import javax.mail.internet.*;
import java.util.*;

public class SendMail {
    public static void main(String[] args) {
        // 邮件发送者和接收者的信息
        String from = "sender@example.com";
        String to = "recipient@example.com";
        String host = "smtp.example.com";
        
        // 邮件的用户名和密码
        final String username = "username";
        final String password = "password";
        
        // 邮件的属性设置
        Properties props = new Properties();
        props.put("mail.smtp.auth", "true");
        props.put("mail.smtp.starttls.enable", "true");
        props.put("mail.smtp.host", host);
        props.put("mail.smtp.port", "587");
        
        // 创建Session对象
        Session session = Session.getInstance(props, new Authenticator() {
            protected PasswordAuthentication getPasswordAuthentication() {
                return new PasswordAuthentication(username, password);
            }
        });
        
        try {
            // 创建MimeMessage对象
            MimeMessage message = new MimeMessage(session);
            
            // 设置发件人和收件人
            message.setFrom(new InternetAddress(from));
            message.setRecipient(Message.RecipientType.TO, new InternetAddress(to));
            
            // 设置邮件主题
            message.setSubject("Java Mail API Test");
            
            // 创建MimeMultipart对象
            MimeMultipart multipart = new MimeMultipart();
            
            // 创建纯文本部分
            MimeBodyPart textPart = new MimeBodyPart();
            textPart.setText("This is the plain text part of the email.");
            multipart.addBodyPart(textPart);
            
            // 创建HTML部分
            MimeBodyPart htmlPart = new MimeBodyPart();
            htmlPart.setContent("<h1>This is the HTML part of the email.</h1>", "text/html");
            multipart.addBodyPart(htmlPart);
            
            // 将MimeMultipart对象设置为邮件的内容
            message.setContent(multipart);
            
            // 发送邮件
            Transport.send(message);
            
            System.out.println("Email sent successfully.");
        } catch (MessagingException e) {
            e.printStackTrace();
        }
    }
}