Setting up ExecutorService to manage a group in a bulk mailer

Question

I'm trying to write a bulk mailer for a scheduling application which deals with groups of people. What I'm trying to do is to takes a collection and split into the individuals and then use newCachedThreadPool() to build and send the message in a separate thead so that the mailer doesn't affect the rest of the application. I've got the body being built successfully but instead of getting it to send individually, it is sending multiple times to the same address, so if I have five addresses, it can send five copies of the same mail to the last address five times and ignore the first four. I'm also trying to ensure that it has only one address in the TO field as well (which was what was happening in the non-concurrent version) but the pool appears to want to add all the addresses into one to field when it does send.
  private Calendar cal1;
  private String sender;
  private String recip;
  private String subject1;
  private ArrayList<String> recip1;
  class sendMail implements Runnable {
       public sendMail (Calendar cal1,
                          String sender,
                          ArrayList<String> recip1,
                          String subject1) {
          public void run() {
               try {
                      Iterator it = recip1.iterator();
                      while (it.hasNext()){ 
                                String to = it.next().toString();
                                     msg.setFrom(new InternetAddress(sender));
                                     msg.addRecipient(javax.mail.Message.RecipientType.TO, new InternetAddress(to));
                                    msg.setSubject(subject1);
                                    msg.setSentDate(new Date());
                                    msg.setContent(content, "text/calendar");
                                   msg.setText("Test calendaring mail");
                       Transport tr = sess.getTransport(config.getProtocol());
                       tr.connect();
                       tr.sendMessage(msg, msg.getRecipients(javax.mail.Message.RecipientType.TO));
                } catch (Exception e) {
                        e.printStackTrace();
  public boolean mailEntity(Calendar cal,
                            String originator,
                            Collection<String>recipients,
                            String subject) throws CalFacadeException {
       cal1 = cal;
       sender = originator;
       subject1 = subject;
         recip1 = new ArrayList<String>();
       pool = Executors.newCachedThreadPool();
       if (debug) {
         debugMsg("mailEntity called with " + cal);
      getConfig();
      if (config.getDisabled()) {
         return false;
      try {
          String domain = "bar.com";
           Iterator it = recipients.iterator();
             while (it.hasNext()) {
                  String recip = it.next().toString();
                  debugMsg("String is " + recip);
                  String[] splitting = recip.split("@");
                  String split = splitting[1];
                  debugMsg("Split string is " + split);
                  if (split.equals(domain)) {
                  log.info("Recipient is " + recip);
                     try {
                        conn = DriverManager.getConnection("connection");
                        stmt = conn.createStatement();
                        rs = stmt.executeQuery ("SELECT DISTINCT username FROM listname");
                        while (rs.next()) {
                         String groupmember = rs.getString(1);
                         recip1.add(groupmember);
                            if (originator == null) {
                               sender = config.getFrom();
                            if (subject == null) {
                                  subject1 = config.getSubject();
                          pool.execute(new sendMail(cal1, sender, recip1, subject1));
                    rs.close();
                    stmt.close();
                 }catch (Exception e) {
                    e.printStackTrace();
                  } finally {
                    if (conn != null) {
                        conn.close();
                } else {
                     recip1.add(recip);
                     if (originator == null) {
                        originator = config.getFrom();
                     if (subject == null) {
                            subject1 = config.getSubject();
                     pool.execute(new sendMail(cal1, sender, recip1, subject1));
           }//end while
               return true;
                } catch (Throwable t) {
                  if (debug) {
                      t.printStackTrace();
             throw new CalFacadeException(t);
  }I'd be grateful for any pointers.

Answer 1

Hi BaroqueThoughts,
I've pasted your code into an IDE and it looks like that the sendMail class is a inner class, with the listed variables as fields of the parent class
  private Calendar cal1;
  private String sender;
  private String recip;
  private String subject1;
  private ArrayList<String> recip1;
  class sendMail implements Runnable I notice in the run() method the field recip1 is used, the scope of this variable will be of the parent class not the inner class (sendMail). This means that all instances of sendMail will access the same list, which being an ArrayList alone is not really suited for multithreaded usage.
public sendMail (Calendar cal1, String sender,
ArrayList<String> recip1, String subject1) {
}The above constructor does nothing with the passed in values, none of the variables are stored in a field of the class. By coincidence there appear to be variables with the same names and types declared in the parent class (which the inner class can access) so there's no compilation problems.
pool.execute(new sendMail(cal1, sender, recip1, subject1));If you hadn't guessed by now that means the variables your entering into the sendMail() constructor during the mailEntity() are not being used, the variables held in the parent class are the ones being used.
In short, I'd say that the problems your experiencing are most likely due to concurrent accessing of fields.
Pointer: Refactor the Runnable class into it's own file and so making it a public class, this will should all the fields that you will need to control access to or relocate (e.g. synchronize / volatile / use different API class/ factor out into runnable class).