ThreadPoolExecutor : Tasks are getting queued up and not submitted

Question

We have a scenario where tasks submitted to ThreadPoolExecutor are long running. When the thread pool is started we start it with core pool size = 5, max pool size = 20 and queu

Answer 1

The Javadocs for ThreadPoolExecutor states:

Any BlockingQueue may be used to transfer and hold submitted tasks. The use of this queue interacts with pool sizing:

• If fewer than corePoolSize threads are running, the Executor always prefers adding a new thread rather than queuing.
• If corePoolSize or more threads are running, the Executor always prefers queuing a request rather than adding a new thread.
• If a request cannot be queued, a new thread is created unless this would exceed maximumPoolSize, in which case, the task will be rejected.

Unless you exceed your queue size after 5 threads are "hanging", you're not going to get more threads.

The real answer is: fix the problem that's causing your threads to hang. Otherwise you're going to have to implement some scheme that uses the Futures returned by submit() to cancel threads if they are running too long.