Cancel Function for Ongoing Process Question

I have an application that downloads a series of items from a web site -
with J2SE.  I want to implement a cancel function.   I'm gathering this
means I need to use threads in order to interupt the download - is this
correct or am I complicating this?

One approach I've used with other languages - like VB6 - is to poll the
GUI - but I gather thats not very good practice in Object Oriented,
Event driven applications.   I also can't figure out how to poll a
button for its state anyways - is this possible?

If threads are the correct approach - is there a simple pattern or
example that explains how to do this - without getting into J2EE or
client - server complications - (beyond my expertise at this point)?

Thanks !

- Ben Hoffmann

Ben Hoffmann wrote:
> I have an application that downloads a series of items from a web site -
> with J2SE.  I want to implement a cancel function.   I'm gathering this
> means I need to use threads in order to interupt the download - is this
> correct or am I complicating this?

In theory, there are two possibilities here: use multiple threads, or
use asynchronous (non-blocking) I/O functions.  However, because you are
using a graphical user interface, you are required to perform blocking
operations outside of the event dispatch thread.  That means that
although you still have the choice of blocking or non-blocking I/O, you
will need to create a new thread in either case.

> If threads are the correct approach - is there a simple pattern or
> example that explains how to do this - without getting into J2EE or
> client - server complications - (beyond my expertise at this point)?

The basic idea behind it is as follows:

// in some accessible location:

Thread t;

// in an event handler:

t = new Thread(new Runnable() {
public void run()
{
try
{
// code to read from socket
}
catch (InterruptedIOException e) { }
catch (IOException e)
{
handleException(e);
}
}
});

t.start();

And later, to cancel:

t.interrupt();

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