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.
