As the name suggests a thread pool is an Executor in Java. It's even an ExecutorService but that's irrelevant for the understanding of the fundamental behavior. The only important operational method of a thread pool is void execute(Runnable task). You pass in your task and the pool will eventually execute it on one of its worker threads. A thread pool is made up of the following components:
- The corePoolSize defines kind of a minimum number of worker threads to keep in the internal worker pool. The reason it's not called minPoolSize is probably that directly after the creation of the thread pool the internal worker pool starts with zero worker threads. Initial workers are then created as needed but they're only ever removed from the worker pool if there are more of them than corePoolSize.
- The maxPoolSize defines a strict upper bound for the number of work threads in the internal worker pool.
- The keepAliveTime defines the time that an idle worker thread may stay in the internal worker pool if there are more than corePoolSize workers in the pool.
There are only three different outcomes, the task can be enqueued in the work queue, a new worker thread can be created, or the task can be rejected. Three conditions are checked to determine the outcome at a specific point in time. The first condition is only relevant in the warm-up phase of the pool, but then it becomes interesting:
That means that, with an unbounded work queue, no more than corePoolSize workers will ever be created; maxPoolSize becomes completely irrelevant. Now we have seen one pool configuration that only ever creates new workers (the default) and one that only ever enqueues tasks. Between these two evils is probably the a thread pool with both a bounded worker pool and a bounded work queue, but obviously such a thread pool will reject tasks when hammered enough.
That's all not what I wanted, but wait:
Subclassing LinkedBlockingQueue would do that trick but there's a small problem remaining now: The three conditions (see above) are checked in the execute() method of the thread pool without any synchronization. That means, if my work queue does not accept a task because there are still less than maxPoolSize workers allocated the third condition is not necessarily true a nanosecond later. The task would be completely rejected from the pool rather than be enqueued. The solution to this problem is a custom rejection handler that takes the rejected task and puts it back at the beginning of the work queue. And now it becomes clear why subclassing LinkedBlockingDequeue is a better alternative: It provides the needed addFirst(Runnable task) method.
If you try to implement these ideas you'll likely discover a few technical complications, such as the LinkedBlockingDeque class not being available in Java 1.5. If you're interested in my concrete solution please have a look at the source code of my good thread pool. Enjoy...