Map可以将java类ByteArrayoutputstream作为值吗？ Can List可以保存javax.mail.MimeMessage对象吗？

Question

I am using JavaMail API to send emails in my java web application. My use case is to send multiple emails to different recipients with the user-specific content. the content includes the pdf file attachment. I want to do code as follows,

Map<Long, ByteArrayOutputStream> pdffiles = new HashMap<Long, ByteArrayOutputStream>();
Map<Long, String> contentMap = new HashMap<Long,String>();
start of loop
{
   String userId = //uniqId;
   ByteArrayOutputStream outFile= new ByteArrayOutputStream();
   outFile  = // statement to invoke a method to create the customer 
    specific pdf file 
   String fileName = "Invoice_<company_name>"+".pdf";   
   MimeBodyPart pdfBodyPart = new MimeBodyPart();
   pdffiles.put(userId, outFile);
   String content   = //Some user specific content loaded here.
   contentMap.put(userId, content);

}

Session session = Session.getInstance(props, new Authenticator() {
@Override
protected PasswordAuthentication getPasswordAuthentication() {
return new PasswordAuthentication(username, password);
}
});

List<MimeMessage> msgList= new ArrayList<MimeMessage>();
for(Long userid : contentMap.keySet()){
   String content = contentMap.get();
   String contentType  ="text/html;charset=UTF-8";
   MimeMessage msg = new MimeMessage(session);
   ByteArrayOutputStream outFile = // get the pdf file from map using the userid as key 
   byte[] bytes = outFile.toByteArray();
   DataSource dataSource = new ByteArrayDataSource(bytes, 
                                             "application/pdf");
   pdfBodyPart.setDataHandler(new DataHandler(dataSource));

   Multipart multipart = new MimeMultipart();
   try {
         //adding the passed multipart content to the mail that to send 
      as an inline attachment.
       messageBodyPart.setContent(content, contentType);
       multipart.addBodyPart(messageBodyPart);
       multipart.addBodyPart(pdfBodyPart);
       msg.setContent(multipart);
       // have to add this 'msg' Object in List.

       InternetAddress[] addressTo = null;
       try {
         addressTo = InternetAddress.parse(eo.getTo());
         msg.setRecipients(Message.RecipientType.TO, addressTo);
        } catch (AddressException e) {
         // excpetion handled here
        } catch (MessagingException e) {
         // excpetion handled here
        }
   }catch (MessagingException e) {
   } catch (Exception e) {//expetion handled here
  }
}

Transport transport = null;
try {
    transport = session.getTransport("smtp");
} catch (NoSuchProviderException e) {
  //exception handled here
}

try {
  transport.connect();
  for(MimeMessage msg : msgList){
     transport.sendMessage(msg, msg.getAllRecipients());
  }
  transport.close();
}catch (Exception ex) {
   //exception handled here
}

My question is, Does HashMap accept the ByteArrayOutputStream class instance as values? if so how to get it from Map by using the key?

Does ArrayList accept to hold the MimeMessage Object? if so, What if the MimeMessage has large files in its Bodyparts? what will happen when holding large files in memory as List?

Answer 1

Yes, HashMap and ArrayList can store any subtype of Object , including ByteArrayOutputStream or MimeMessage as values.

Perhaps answering part of your confusion: These data structures do not store the actual objects "inside" themselves; rather HashMap and ArrayList store references to objects that must already exist in memory.

So as long as you have enough memory to create the ByteArrayOutputStream , the extra memory to store a reference to it is unlikely to be a problem.