Question

I have Ant build and execute a java program. This program tries to do something that sometimes hangs, so we execute it in a thread.

actionThread.start();
try {
    actionThread.join(10000);
} catch (InterruptedException e) {
    System.out.println("InterruptedException: "+e.getMessage());
} 
if (actionThread.isAlive()) {
    actionThread.interrupt();
    System.out.println("Thread timed out and never died");
}

The ant call looks like this:

<java fork="true" failonerror="yes" classname="myPackage.myPathName" classpath="build">  
    <arg line=""/>
    <classpath>
        <pathelement location="bin" />
        <fileset dir="lib">
            <include name="**/*.jar"/>
        </fileset>
    </classpath>
</java>

And when this runs I see the “Thread timed out and never died” statement, and I also see the main program finish execution, but then Ant just hangs. Presumably it is waiting for the child threads to finish, but they never will.

How can I have Ant be done once it is done executing main() and just kill or ignore dead threads?

Answer 1

You can use public final void setDaemon(boolean on) method of the Thread class i.e actionThread.setDaemon(true). That way you will ensure that JVM exits once the main Thread is finished.

Java Doc says:

Marks this thread as either a daemon
thread or a user thread. The Java
Virtual Machine exits when the only
threads running are all daemon
threads.

This method must be called before the
thread is started.

UPDATE

---

System.exit() Vs Daemon thread

For a bigger piece of code you cannot always be sure that all the active threads are the daemon threads. If a new thread is created by the daemon thread and if its setDaemon(true) is not set then it will inherit it from the parent. Although there can be scenarios where the newly created thread is set as a non-daemon (and then we will face your current problem).

I personally think if you are done then you can call System.exit(0);

As per Java doc for System.exit():

Terminates the currently running Java
virtual machine by initiating its
shutdown sequence. This method never
returns normally…In the first phase
all registered shutdown hooks, if any,
are started in some unspecified order
and allowed to run concurrently until
they finish. In the second phase all
uninvoked finalizers are run if
finalization-on-exit has been enabled.
Once this is done the virtual machine
halts.

There are other SO post where they have discussed this:

When should we call System.exit in Java

System.exit() can be used to run
shutdown hooks before the program
quits. This is a convenient way to
handle shutdown in bigger programs,
where all parts of the program can’t
(and shouldn’t) be aware of each
other.

From which thread should System.exit() be called in a Swing-app?

The GC would take a while before
things are rounded off and the app
exits. There is nothing wrong with
calling System.exit, once you dealt
with closing what you ought to.

As a third solution (1st is System.exit() , 2nd setDaemon()) you can also check for interrupted threads before doing any processing or decide ‘what to do’ in the handling of InterruptedException.

Hope this will help.