Question

I have a servlet program for counting numbers, I want to control it through an html interface.
by pressing the start button the program must start running and by pressing pause button the servlet program must be paused and by clicking on the restart button it must restart again. by the way i used thread. My problem is that each time I should click one button and send its value to the servlet, and when I’m getting the buttons values inside the servlet a NullPointerException is occur… any help ??

Answer 1

I wouldn’t use a Thread for that purpose and in general is not usually a good idea to create threads in servlets.

Say we count one number per millisecond meaning: it will give me the time between one click and another in milliseconds.

One work around could be:
Click on start = save the start time in the session.
click on stop = to get the count we do currentTime-StartTime (saved in session)

Now if you really must use Threads be sure then to create it using another class.
One suggestion might be create a ThreadManager class and store it in the session (use a listener for this) and then start it in that session object.
Even better store the ThreadManager inside the servletContext and have a way to create your thread per session.
To create Threads favor the Executor classes instead of the Thread classes.
Also make sure you stop your threads since having threads created by us inside a web container may prevent it from stoping entirely.

If you provide some code I can help you further.
Good luck, have fun.