java.lang.SecurityException

All the classes and jars were copied from the other location and they weren't "signed". (Sorry I am not even sure what that means.) I made some changes to the java files, such as default values of some variables etc. and that's it. I don't think they need to be signed.

I did some testing and found the problem is with a class called Data.  Data has two email sending functions together with a bunch of public static variables, which are referenced each time an object is created. If I commented out these two email functions, the problem would be gone, but of course the problem would have problem sending emails.  Any thoughts?

public static void sendEmail(String title, String content, String[] mailingList) {
            
            try {
                  Address[] addresses_to = new InternetAddress[mailingList.length];
                  
                  for (int i=0; i<mailingList.length; i++) {
                        addresses_to[i]= new InternetAddress(mailingList[i]);
                  }
                                    
                  Properties props = new Properties();
                  props.put("mail.host", "mailserver");
                  
                  Session mailConnection = Session.getInstance(props, null);
                  Message msg = new MimeMessage(mailConnection);      
                  Address sender = new InternetAddress ("notifications@camenergy.com", "system");
                  msg.setContent(content, "text/plain");
                  msg.setFrom(sender);
                  msg.setRecipients(Message.RecipientType.TO, addresses_to);
                  msg.setSubject(title);
                  
                  Transport.send(msg);
                              
                  System.out.println("email sent.");

            } catch (Exception e) {
                  e.printStackTrace();
                  System.out.println("fail to send email.");
            }
      }
      
      public static void sendEmail(String title, String content,
                  File attachment, String[] mailingList) {
            
            try {
                  Address[] addresses_to = new InternetAddress[mailingList.length];
                  
                  for (int i=0; i<mailingList.length; i++) {
                        addresses_to[i]= new InternetAddress(mailingList[i]);
                  }
                  
                  Properties props = new Properties();
                  props.put("mail.host", "mailserver");
                  Session mailConnection = Session.getInstance(props, null);
                  
                  Message msg = new MimeMessage(mailConnection);      
                  //Create the message part
                  BodyPart messageBodyPart = new MimeBodyPart();

                  //Fill the message
                  messageBodyPart.setText(content);

                  Multipart multipart = new MimeMultipart();
                  multipart.addBodyPart(messageBodyPart);

                  //Part two is attachment
                  messageBodyPart = new MimeBodyPart();
                  DataSource source = new FileDataSource(attachment);
                  messageBodyPart.setDataHandler(new DataHandler(source));
                  messageBodyPart.setFileName(attachment.getName());
                  multipart.addBodyPart(messageBodyPart);

                  // Put parts in message
                  msg.setContent(multipart);

                  Address sender = new InternetAddress ("notifications@camenergy.com", "system");
                  msg.setFrom(sender);
                  msg.setRecipients(Message.RecipientType.TO, addresses_to);
                  msg.setSubject(title);
                  
                  Transport.send(msg);
                              
                  System.out.println("email sent.");

            } catch (Exception e) {
                  e.printStackTrace();
                  System.out.println("fail to send email.");
            }
      }
      

I am not sure I understand you.
By "in the javax.mail package" you mean classes that import javax.mail packages?
"Rename the package".. the package that contains above mentioned classes?

No I mean your application classes, make sure that you are not using that package name.
Also check that u don't have two copies of mail.jar available in classpath.

I resolved the problem by moving the two email related functions away from Data class so they are no longer involved in the object creation process. It worked although I still don't know what happened.  Anyway, thank you guys for trying to help.

:-)

> Sign all classes and jars with the same cert if they need to be signed

that had nothing to do with your problem :)