Implementing a thread pool inside a Service

Question

I\'m currently working on implementing a Service that when requested will perform some work on several parallel threads.

My implementation is based on the <

Answer 1

I think you are trying to implement a service, which introduces many problems, but solves none. Effectively you reduce the calling code by one line - the creation of the Executor - but take away the ability to fine-control it. There isn't any gain in scheduling many tasks, as this is already solved by the OS' thread scheduler. In addition a malicious caller can break several other programs by just adding enough while(true) sleep(100); loops.

On your questions:

1. You cannot make sure that all threads are properly interrupted ever, because there is no way to interrupt a thread that is not properly watching the interrupt flag. A while(true) ; cannot be interrupted except by System.exit(). You could theoretically stop a thread, but this feature is deprecated for a reason, because it can leave the actual task in an incomplete/unfinished state (i.E. leaving TCP-connections half-open).

2. No, you are not correctly implementing that. For once tasks left int he queue would simply vanish in the void, and then an Excecutor cannot be re-used once shutdown is called. So you at least need to create a new Excecutor instance on service start, and you really should figure out what to do with the remaining tasks.

3. No, because of 2.

4. The pros/cons of list types depend on your use-case. An ArrayList has a higher cost when growing/shrinking but a lower cost when indexing a specific element (indexOf), while a linked-list is about the opposite. Since your queue always adds to the tail, doesn't care about any other element but the first, and it is growing/shrinking frequently, a linked-list is the best choice.

5. You should not stop the task this way at all, because the order of execution with threads is undefined. In worst case your calling program is interrupted each time till the service is done executing, which will cause the service to constantly start & stop for no particular reason, while wasting a lot of processing time for nothing. Why would you even want to stop the service? If it has nothing to do, it won't do anything but to use a few bytes of memory.

Answer 2

1. No. shutdownNow attempts to interrupt the currently actively tasks. There are no guarantees that it will able to do it
2. Yes it is. From the documentation:

   Once neither of these situations hold, the service's onDestroy() method is called and the service is effectively terminated. All cleanup (stopping threads, unregistering receivers) should be complete upon returning from onDestroy().

3. there is no reason to have those members declared as static. static member are associated withe class rather than with any object, and the common use is to share the same static member between different (services in your case) instances.
4. You should read carefully the documentation about all the possible implementations of the BlockingQueue<E> interface, but I doubt that, for normal cases use, you will see differences from the performances perspective.