java mail发送邮件出现乱码

MimeBodyPart  body = new MimeBodyPart();
body.setContent(content, "text/html;charset=GB2312");

MimeMultipart mmp=new MimeMultipart();
mmp.setSubject(subject, "GB2312");

body.setHeader("Content-Transfer-Encoding", "base64"); // 指定使用base64编码

import java.util.Properties;
import javax.mail.*;
import javax.mail.internet.*;
public class MailSender {
    public static void main(String[] args) {
        try {
     String host = "staff.tixa.com"; // smtp主机
     String username = "sample@staff.tixa.com"; // 认证用户名
     String password = "sample"; // 认证密码
     String from = "例子<sample@staff.tixa.com>"; // 发送者
     String to = "toOne@staff.tixa.com, toAnother@staff.tixa.com"; // 接受者，用“,”分隔
     String subject = "测试例子";
     String content = "仅仅是个供测试的例子。";
     // 建立session
     Properties prop = new Properties();
     prop.put("mail.smtp.host", host);
     prop.put("mail.smtp.auth", "true"); //是否需要认证
     Session session = Session.getDefaultInstance(prop, null);
     // 创建MIME邮件对象
     MimeMessage mimeMsg = new MimeMessage(session);
     MimeMultipart mp = new MimeMultipart();
     // 设置信息
     mimeMsg.setFrom(new InternetAddress(from));
     mimeMsg.setSubject(subject, "GB2312"); // ！！！注意设置编码
     mimeMsg.setRecipients(
         Message.RecipientType.TO,
  InternetAddress.parse(to));
     // 设置正文
     BodyPart body = new MimeBodyPart();
     body.setContent(content, "text/plain;charset=GB2312"); // ！！！注意设置编码
     mp.addBodyPart(body);
     mimeMsg.setContent(mp);
     // 发送邮件
     Transport transport = session.getTransport("smtp");
     transport.connect(host, username, password);
     transport.sendMessage(mimeMsg,mimeMsg.getRecipients(Message.RecipientType.TO));
     transport.close();
 }
 catch(Exception exp) {
     exp.printStackTrace();
 }
    }