SECURITY: cachedTheadPool() may cause OutOfMemoryError when there are too many threads.

**BUG REPORT**

1. Please describe the issue you observed:

When the tasks are too many, using cached thread pool may risk in causing OutOfMemoryError, while fixed thread pool can be freely configured (thread number) to avoid the above error!

Besides, change cached thread pool to fixed thread pool has no performance loss if the thread number is properly set.

