Thread Pool and Scheduler

[Mick]

Questions regarding Threading. The structure of my code is...

The UI thread calls a Routine R0 which *uses a background thread* to call two Routines R1 and R2 which each need to retrieve information from the internet. R1 and R2 are called in succession. R1 must complete before R2 is called. This sequence is done in a loop, with each iteration selecting the next item from an ArrayList to operate on.

*Note* I believe that R0 does not need to use a background thread as it is R1 and R2 which go to the internet. But it works in simple testing so I'll leave it as is for now.

R1 and R2 each extend Thread, so each begins with a public void run() {...}. Also each needs to store a result on the UI thread so each uses a runOnUiThread(new Runnable(...)) at the end of its processing.

All of the code works perfectly (with a bit of cheating, a finite loop after calls to R1 and R2 waiting for the result) when tested in a simple environment where there is no possibility of a conflict. When installed in the App it works perfectly for the first iteration then the App "hangs" - it does not finish the next
iteration, it literally just sits there; no processing is going on as the LogCat has paused; and the program does respond to other menu selections correctly - so it has not "crashed";

I have not been this deep into Threads so I have been reading and getting confused by several blogs posts etc on the topic of Threads, so I have some questions...

Do I need to use a thread pool?
Will I need to use a Scheduler?
Does anyone have a favorite, good basic introduction to this topic? Especially one with an introductory level example?

Hopefully, next time I will have some code to show

Thank you,
Mick

[Norm]

then the App "hangs"

Sounds like the threads may be hung waiting on some event that never happens.
I use print statements for debugging. Try adding lots of print statements so you can see where the code is executing and not executing(ie is hung). It can be an iterative process to get the print statements in the right place.

[Mick]

Hi Norm,
Ooops, I neglected to mention that this is an Android App

Each of tasks R1 and R2 accesses the internet, which is a relatively long running process. But each starts that access then returns immediately. I must delay the start of R2 until after that "long running" R1 process finishes. I believe that is why R1 and R2 need to be in different threads, and need to be run by a scheduler. Unless there is another option.

As I mentioned the code works perfectly when the components are run independently of each other. But in the App, R1 and R2 are in a loop processing data from an ArrayList. Each uses a single-statement runOnUiThread to store the result. If I put a finite delay loop after each, waiting for the new value to appear in a global String result field, it all works. But such a delay loop is problematic in Android. I need to find a better technique than this...

result = " ";
while (result.equals(" "));

I have more than 50 years experience in technical computer programming so I am not new to concurrent processing, but I am new to this in Android; and it is a daunting topic. This is why I am looking for a good starting point, especially something with an example.
Mick

[Norm]

Also posted here: https://coderanch.com/t/770327/mobil...Pool-Scheduler