如何在java中发送由字符串创建的邮件附件？

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

public class EmailAttachmentExample {
    public static void main(String[] args) {
        // 邮件发送者和接收者的邮箱地址
        String senderEmail = "sender@example.com";
        String receiverEmail = "receiver@example.com";
        
        // 邮件发送者的用户名和密码
        String senderUsername = "sender_username";
        String senderPassword = "sender_password";
        
        // 邮件服务器的主机名和端口号
        String host = "smtp.example.com";
        int port = 587;
        
        // 创建邮件会话
        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", port);

        Session session = Session.getInstance(props, new Authenticator() {
            protected PasswordAuthentication getPasswordAuthentication() {
                return new PasswordAuthentication(senderUsername, senderPassword);
            }
        });

        try {
            // 创建邮件消息
            Message message = new MimeMessage(session);
            message.setFrom(new InternetAddress(senderEmail));
            message.setRecipients(Message.RecipientType.TO, InternetAddress.parse(receiverEmail));
            message.setSubject("邮件主题");

            // 创建邮件附件
            MimeBodyPart attachmentPart = new MimeBodyPart();
            attachmentPart.setText("邮件附件内容");
            attachmentPart.setFileName("attachment.txt");

            // 创建邮件正文
            MimeBodyPart textPart = new MimeBodyPart();
            textPart.setText("邮件正文内容");

            // 将附件和正文组合到一个多部分消息中
            Multipart multipart = new MimeMultipart();
            multipart.addBodyPart(textPart);
            multipart.addBodyPart(attachmentPart);

            // 设置邮件内容为多部分消息
            message.setContent(multipart);

            // 发送邮件
            Transport.send(message);

            System.out.println("邮件发送成功！");
        } catch (MessagingException e) {
            e.printStackTrace();
        }
    }
}