对于在内存中生成的文件，您可以使用ByteArrayResource。只需使用Apache Commons IO库中的IOUtils转换InputStream对象。

这很简单：

helper.addAttachment("attachement",
new ByteArrayResource(IOUtils.toByteArray(inputStream)));

使用Java Mail MimeMessageHelper查看Spring参考第24.3章

这个例子就是从那里开始的，我认为它确实想让您这样做：

JavaMailSenderImpl sender = new JavaMailSenderImpl();
sender.setHost("mail.host.com");

MimeMessage message = sender.createMimeMessage();

// use the true flag to indicate you need a multipart message
MimeMessageHelper helper = new MimeMessageHelper(message, true);
helper.setTo("test@host.com");

helper.setText("Check out this image!");

// let's attach the infamous windows Sample file (this time copied to c:/)
FileSystemResource resource = new FileSystemResource(new File("c:/Sample.jpg"));

helper.addAttachment("CoolImage.jpg", resource );

sender.send(message);

如果要使用Stream，则可以使用

ByteArrayResource resource = new ByteArrayResource(IOUtils.toByteArray(inputStream)));

而不是文件系统资源

您可以简单地实现InputStreamSource，并根据请求在其中传递新的InputStream：

InputStreamSource iss = new InputStreamSource() {
    @Override
    public InputStream getInputStream() throws IOException {
        // provide fresh InputStream
        return new FileInputStream("c:\a.txt");
    }
}
MimeMessageHelper message = new MimeMessageHelper(mimeMessage, true, "UTF-8");
message.addAttachment("attachment", iss);

//inlineFileObjectCreated--可以为示例创建StringBuilder对象

ByteArrayDataSource source = new ByteArrayDataSource("file name", "contentType", inlineFileObjectCreated.getBytes() );

                JavaMailSender mailSender = (JavaMailSender) ServicesHome.getService("javaMailSender");
                MimeMessage mimeMessage = mailSender.createMimeMessage();
                MimeMessageHelper mimeMessageHelper = new MimeMessageHelper(mimeMessage, true);
                mimeMessageHelper.setTo(toArray);           
                mimeMessageHelper.setSubject("");
                mimeMessageHelper.setText("");
                mimeMessageHelper.addAttachment("filename", source);
                mailSender.send(mimeMessageHelper.getMimeMessage());

/

import javax.activation.DataSource;

    public class ByteArrayDataSource implements DataSource {
        byte[] bytes;
        String contentType;
        String name;

        public ByteArrayDataSource( String name, String contentType, byte[] bytes ) {
          this.name = name;
          this.bytes = bytes;
          this.contentType = contentType;
        }

        public String getContentType() {
          return contentType;
        }

        public InputStream getInputStream() {
          return new ByteArrayInputStream(bytes);
        }

        public String getName() {
          return name;
        }

        public OutputStream getOutputStream() throws IOException {
          throw new FileNotFoundException();
        }
      }

具体的工作示例包括：

1)附件为InputStreamSource接口

public void send() throws IOException, MessagingException {
    final ByteArrayOutputStream stream = createInMemoryDocument("body");
    final InputStreamSource attachment = new ByteArrayResource(stream.toByteArray());
    final MimeMessage message = javaMailSender.createMimeMessage();
    final MimeMessageHelper helper = new MimeMessageHelper(message, true);
    helper.setSubject("subject");
    helper.setFrom("from@from.com");
    helper.setTo("to@to.com");
    helper.setReplyTo("replyTo@replyTo.com");
    helper.setText("stub", false);
    helper.addAttachment("document.txt", attachment);
    javaMailSender.send(message);
}

2)附件为DataSource接口

public void send() throws IOException, MessagingException {
        final ByteArrayOutputStream document = createInMemoryDocument("body");
        final InputStream inputStream = new ByteArrayInputStream(document.toByteArray());
        final DataSource attachment = new ByteArrayDataSource(inputStream, "application/octet-stream");
        final MimeMessage message = javaMailSender.createMimeMessage();
        final MimeMessageHelper helper = new MimeMessageHelper(message, true);
        helper.setSubject("subject");
        helper.setFrom("from@from.com");
        helper.setTo("to@to.com");
        helper.setReplyTo("replyTo@replyTo.com");
        helper.setText("stub", false);
        helper.addAttachment("document.txt", attachment);
        javaMailSender.send(message);
    }

解释：

传入的资源包含开放流：无效参数。JavaMail需要一个InputStreamSource来为每个调用创建一个新的流。

如果开发人员使用在isOpen()方法中返回true的InputStreamSource实现，则可能会出现此消息。

方法MimeMessageHelper#addAttacment()中有一个特殊检查：

public void addAttachment(String attachmentFilename, InputStreamSource inputStreamSource, String contentType) {
    //...
    if (inputStreamSource instanceof Resource && ((Resource) inputStreamSource).isOpen()) {
        throw new IllegalArgumentException(
        "Passed-in Resource contains an open stream: invalid argument. " +
        "JavaMail requires an InputStreamSource that creates a fresh stream for every call.");
    }
    //...
}

InputStreamResource#isOpen()始终返回true，这使得无法将此实现用作附件：

public class InputStreamResource extends AbstractResource {
   //...
   @Override
   public boolean isOpen() {
      return true;
   }
   //...
}