Wait for mail sending in another thread to complete

Question

I've got a problem I can't really figure out. I have my main thread, and in it I want to

1. Send an email with an attachment
2. Delete the files that were attached

in that order. My problem is that I am using an email helper that I don't have control over, and it generates another thread that does the sending. Because of this, my files are being deleted before they are done being attached, and I am getting a FNF error in the mailer. I am looking to find a way to make my main thread wait until the files are done being attached. I don't know how long that will take. I don't have control over the other thread creation, so I can't use join(). Is there something I can use with Transport maybe, or a way to wait for all threads made in a method/class to stop?

The layout of my program is

//do stuff
MailHelper.sendMail(...); //thread is created somewhere in this method
deleteFiles(); //this happens before sendMail is finished!

I need to use Java 6. Worst-case I can have my main thread sleep for a few seconds, but that's unideal. Any help is appreciated

Answer 1

This is an interesting question! Basically you want to wait for all child threads to complete, but have no control over them.

Here is a demonstration of the technique using ThreadGroup:

Assuming you have a MailHelper class like this:

public class MailHelper {

    public void sendMail(){
        Thread t = new Thread(new Runnable() {

            @Override
            public void run() {
                System.out.println("MailHelper: Sending mail for 6s");
                for(int i = 0; i < 6; i++){
                    try {
                        Thread.sleep(1000);
                    } catch (InterruptedException e) {
                        e.printStackTrace();
                    }
                    System.out.println(".");
                }
                System.out.println("MailHelper: Sent mail!");
            }
        });
        t.start();
    }

}

then our Main class demonstrates how use it:

public class Main {

    public static void main(String[] args) throws InterruptedException {
        final MailHelper mh = new MailHelper();

        ThreadGroup mailThreadGroup = new ThreadGroup("mailGroup");
        Thread callSendmailThread = new Thread(mailThreadGroup, new Runnable() {

            @Override
            public void run() {
                System.out.println("Calling sendMail().");
                mh.sendMail();
                System.out.println("sendMail() returned.");
            }
        });
        callSendmailThread.start();
        callSendmailThread.join();
        System.out.println("callSendmailThread joined. Waiting for rest of ThreadGroup to finish.");

        // We cannot rely on ThreadGroup.activeCount() to give us an accurate number, and it could be zero!
        Thread[] mailThreads = new Thread[mailThreadGroup.activeCount() + 1];
        //Therefore retry enumerate until our array was large enough to hold all
        while ( mailThreadGroup.enumerate( mailThreads, true ) == mailThreads.length ) {
            mailThreads = new Thread[ mailThreads.length * 2 ];
        }

        for(Thread t : mailThreads){
            if(t != null && t.isAlive()){
                System.out.println("Joining thread " + t.getName());
                t.join();
                System.out.println("Thread " + t.getName() + " joined.");
            }
        }
        mailThreadGroup.destroy();
        System.out.println("Done!");

    }
}

The output:

Calling sendMail().
sendMail() returned.
callSendmailThread joined. Waiting for rest of ThreadGroup to finish.
Joining thread Thread-1
MailHelper: Sending mail for 6s
.
.
.
.
.
.
MailHelper: Sent mail!
Thread Thread-1 joined.
Done!

Note that you must ensure that the Thread-1 is actually started by the time you enumerate the ThreadGroup, thus joining the callSendMailThread is absolutely necessary. Otherwise you'd get a race condition.

Also note that the quirky behaviour of ThreadGroup.enumerate() must be accounted for, by retrying to enumerate all the items several times.

Answer 2

The easy way to solve the issue logically is to track if the mail is send sucessfully. It can be done by any of the below

1) Set a global variable with some value after mail is send from other thread and reset it value once the attachement is deleted.
2) Instead of variable you can also try creating a file.


Thank you,
Mukeshkoshym