Getting problem with the new java.util.concurrent package

Question

i m using the new concurrency package of jdk 1.5 in my code. see the line:
Executor threadPool = Executors.newCachedThreadPool();
about this, api doc says Creates a thread pool that creates new threads as needed, but will reuse previously constructed threads when they are available
but i dont find this method to create new threads whenever required. please tell me about this.
in my code, in some rare cases many threads r created, and very difficult to detect the number of active threads.
as the previous one failed i tried with another one newFixedThreadPool(int). but here i got another problem. it works successfully but i dont understand what should be the size of the pool. i tried with 1000, it works well, but in a very rare case, it failed also. then i tried with 10000. but i m still confused, i dont know whether it would work always or not. with traditional threads my code never fails.
please tell me, what should be the size of the pool and also let me know what is the problem with the first one (newCachedThreadPool)

Answer 1

The thread pool should not be more than 100 threads. Ideally you should only have a small number of threads per CPU. If your threads are not long running it is possible that you could add 10,000 runnables but on start a small number of threads. Note, adding a thread take a non trivial amount of time so while the thread is added, all the others could finish. This could mean the number of threads could be 140 or smaller.
Have you looked at the code to see what it does?