使用JavaMail发送邮件-no object DCH for MIME type multipart/mixed报错解决

最近需要实现一个使用Spring schedule按一定时间间隔自动触发条件发送邮件的功能，在开发的过程中，是按照先测试能发出text/html文本邮件，然后测试添加附件发送邮件，我碰到的问题是，文本邮件能正常发送出来，但是添加附件的邮件却发不出来，这个问题困扰了我很久，所以有必要记录下。

问题点

报错内容："javax.activation.UnsupportedDataTypeException: no object DCH for MIME type multipart/mixed"

初步解释：是不支持数据类型，没有复合邮件对象的专用信息渠道（MIME type multipart/mixed）”，字面意思是没有发送MIME类型邮件的对象专用信息通道。

参考博客：https://blog.csdn.net/q1035331653/article/details/80839853；

报错内容

以下是百度对MIME邮件的解释：

MIME(Multipurpose Internet Mail Extensions)多用途互联网邮件扩展类型。是设定某种扩展名的文件用一种应用程序来打开的方式类型，当该扩展名文件被访问的时候，浏览器会自动使用指定应用程序来打开。多用于指定一些客户端自定义的文件名，以及一些媒体文件打开方式。

它是一个互联网标准，扩展了电子邮件标准，使其能够支持：非ASCII字符文本；非文本格式附件（二进制、声音、图像等）；由多部分（multiple parts）组成的消息体；包含非ASCII字符的头信息（Header information）

问题解决过程

a.发送邮件前可能缺少mime配置代码

CSDN,overstackflow等上有很多帖子，说在使用实例化Transport对象send()方法发送邮件前，需要添加下面这一段代码

MailcapCommandMap mc = (MailcapCommandMap) CommandMap.getDefaultCommandMap(); mc.addMailcap("text/html;; x-java-content-handler=com.sun.mail.handlers.text_html"); mc.addMailcap("text/xml;; x-java-content-handler=com.sun.mail.handlers.text_xml"); mc.addMailcap("text/plain;; x-java-content-handler=com.sun.mail.handlers.text_plain"); mc.addMailcap("multipart/*;; x-java-content-handler=com.sun.mail.handlers.multipart_mixed; x-java-fallback-entry=true"); mc.addMailcap("message/rfc822;; x-java-content-handler=com.sun.mail.handlers.message_rfc822"); CommandMap.setDefaultCommandMap(mc); Thread.currentThread().setContextClassLoader( getClass().getClassLoader() ); 

overstackflow上面对这段代码的解释是：javaMail depends on some configuration files to map MIME types to Java classes (e.g., multipart/mixed to javax.mail.internet.MimeMultipart). These configuration files are loaded using the ClassLoader for the application. If the ClassLoader doesn't function properly, these configuration files won't be found中文意思大概是：javaMail发送MIME类型的邮件需要依赖某些配置文件，比如如果使用MimeMultipart需要找到multipart/mixed的对应配置。这些配置文件将会被类装载器加载，如果不能被正常加载，这些配置文件将无法生效。javamail包下有MANFEST文件，里面有一个mailcap或者mailcap.default文件，读取的就是这个文件，发现里面有对邮件格式的要求，如普通html邮件text/html，纯文本邮件text/plain，包含附件的邮件multipart/*等，如下图所示：参考博客：https://stackoverflow.com/questions/21856211/javax-activation-unsupporteddatatypeexception-no-object-dch-for-mime-type-multi?r=SearchResults；结果：添加完成后，依然报错，无效。

b.怀疑mail.jar包和activation.jar包版本问题

由于项目中使用的mail.jar包版本1.4.1，网上建议升级到1.4.7版本或更高的版本尝试解决，由于项目服务器是使用OSGI来建立的，对OSGI几乎不了解，使用常规的方式导入包后发现没有任何效果，项目下只是新多了一个dependency libraries，里面包含自己添加的包，但是这个包里面的资源对于项目好像没有效果，测试依然报错。后面根据帖子又重新build path，将新jar包import然后又export后，还是没有效果。结果：包更换未成功，对OSGI不了解参考博客：http://osgi.com.cn/article/7289374； 

c.怀疑自己代码本身存在问题

在尝试了以上方法还是没有解决后，参考博客 https://www.cnblogs.com/xdp-gacl/p/4216311.html，使用博客里能发出邮件附件的代码来进行测试。测试时在OSGI服务端项目里新增了一个package，里面写了一个包含main方法的类，直接发送邮件测试，以下是代码

package TestMailSend;
import java.io.FileNotFoundException;
import java.io.FileOutputStream;
import java.io.IOException;
import java.util.Properties;
import javax.activation.DataHandler;
import javax.activation.FileDataSource;
import javax.mail.Message;
import javax.mail.MessagingException;
import javax.mail.Session;
import javax.mail.Transport;
import javax.mail.internet.AddressException;
import javax.mail.internet.InternetAddress;
import javax.mail.internet.MimeBodyPart;
import javax.mail.internet.MimeMessage;
import javax.mail.internet.MimeMultipart;

public class SendMail {

    public static void main(String[] args) throws Exception{
        //创建Properties文件
        Properties prop=new Properties();
        prop.setProperty("mail.host", "mail.boe.com.cn");//主机
        prop.setProperty("mail.transport.protocol", "smtp");
        prop.setProperty("mail.smtp.auth", "true");
        //使用JavaMail发送邮件的5个step
        //step1 创建session
        //Session session=Session.getInstance(prop);
        Session session1=Session.getDefaultInstance(prop);
        //开启session的debug模式，可以看到程序发送email的运行状态
        session1.setDebug(true);
        //step2 通过session得到transport对象
        //Transport trans=session.getTransport();
        Transport trans=session1.getTransport();
        //step3 使用邮件的用户名和密码连上邮箱服务器，发送邮件时，需要验证发件人的邮箱名和密码正确后才可以发送
        trans.connect("mail.boe.com.cn", "发件人邮箱", "发件人邮箱密码");
        //step4 创建邮件
        //Message message=createSimpleMail(session);//发送纯文本
        Message messageAttach=createAttachMail(session1);
        //step5 发送邮件

        //trans.sendMessage(message, message.getAllRecipients());
        trans.sendMessage(messageAttach, messageAttach.getAllRecipients());
        trans.close();
    }
    /**
     * 创建一个封只包含文本的邮件
     * @param session
     * @return
     * @throws MessagingException
     * @throws AddressException
     */
    public static MimeMessage createSimpleMail(Session session) throws AddressException, MessagingException{
        //创建邮箱对象
        MimeMessage message=new MimeMessage(session);
        //指明邮件的发件人
        message.setFrom(new InternetAddress("发件人邮箱"));
        //指明邮件的收件人
        message.setRecipient(Message.RecipientType.TO, new InternetAddress("收件人邮箱"));
        //邮件的标题
        message.setSubject("只包含文本的简单邮件");
        //邮件的文本内容
        message.setContent("这是一封测试邮件！","text/html;charset=utf-8");
        //返回创建好的邮件对象
        return message;
    }
    /**
     * 创建一封包含附件的邮件
     * @param session
     * @return
     * @throws MessagingException
     * @throws AddressException
     * @throws IOException
     * @throws FileNotFoundException
     */
    public static MimeMessage createAttachMail(Session session) throws AddressException, MessagingException, FileNotFoundException, IOException{
        //创建邮件
        MimeMessage message=new MimeMessage(session);
        //指明邮件发件人
        message.setFrom(new InternetAddress("发件人邮箱"));
        //指明邮件收件人
        message.setRecipient(Message.RecipientType.TO, new InternetAddress("收件人邮箱"));
        //邮件标题
        message.setSubject("这是一封包含附件的邮件");    

        //创建邮件正文，为了避免邮件正文中文乱码，使用utf-8
        MimeBodyPart text=new MimeBodyPart();
        text.setContent("使用JavaMail创建的包含附件的邮件","application/x-xls;charset=utf-8");

        //创建附件邮件
        MimeBodyPart attachment=new MimeBodyPart();
        DataHandler dh=new DataHandler(new FileDataSource("C:\\测试Excel.xls"));
        attachment.setDataHandler(dh);
        attachment.setFileName(dh.getName());

        //创建容器描述数据关系
        MimeMultipart mmp=new MimeMultipart();
        mmp.addBodyPart(text);
        mmp.addBodyPart(attachment);
        mmp.setSubType("mixed");

        message.setContent(mmp);
        message.saveChanges();
        //存盘
        message.writeTo(new FileOutputStream("C:\\attachmentEmail.eml"));
        //返回生成的邮件
        return message;
    }

}

结果：将这部分代码执行，发现能正常发出包含附件的邮件，然后将同样的代码放到OSGI服务器下运行，发现还是发不出来。通过以上发现，邮件发不出附件排除了jar包版本的问题，也可排除了代码本身的问题（前面测试过了没有问题），然后怀疑到了OSGI服务器设置的问题，查询overstackflow上也有一个人跟我情况类似，参考博客：https://stackoverflow.com/questions/8923324/java-throws-an-unsupporteddatatypeexception-when-running-osgi-with-javax-mail?r=SearchResults，但是依然没有找到解决办法。

d.尝试将mail包通过MANIFEST添加到class path里

最后通过MANIFEST，在runtime下添加了class path，将mail.jar包添加到类路径，重新测试竟然OK了！runtime下添加mail.jar类路径。

Classpath下面有一段介绍：

Specify the libraries and folders that constitute the plug-in classpath.  If unspecified, the classes and resources are assumed to be at the root of the plug-in.

英语蹩脚，大概意思就是，指定组成plug-in 类路径下的文件夹，如本项目文件夹为lib。如果不指定，资源类可能还在plug-in的根目录，这样配置后，MANIFEST.MF里在Bundle-Class下面新增lib/mail.jar 。

然后在build.properties下发现自动新增lib/mail.jar。

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" alt="" />

网上有一篇帖子有大概的说明MANIFEST.MF配置，里面有一段话： .”指程序运行目录，即导出的JAR 包所在目录。程序运行时依据Class-Path 项的设置路径来查找支持库。每一个支持库之间用空格隔开。

结论

将mail.jar包在MANIFEST文件下，通过runtime导入包的类路径后，重新发送添加附件的邮件能成功。

参考博客：https://blog.csdn.net/maihilton/article/details/80234866

总结

Javamail发送有附件的邮件功能不是很熟悉，走了很多弯路，小小的记录一下，有些不需要的步骤没有写。另外如果想对发送邮件过程有详细的了解，可以学习segmentfault上面的一篇博文：https://segmentfault.com/a/1190000008030346