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  1. PO
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    <p>Note: I now prefer and recommend <a href="https://stackoverflow.com/a/24493856/394431">my other answer</a>.</p> <p>Here's a version which feels to me much more straightforward: Increase the corePoolSize (up to the limit of maximumPoolSize) whenever a new task is executed, then decrease the corePoolSize (down to the limit of the user specified "core pool size") whenever a task completes.</p> <p>To put it another way, keep track of the number of running or enqueued tasks, and ensure that the corePoolSize is equal to the number of tasks as long as it is between the user specified "core pool size" and the maximumPoolSize.</p> <pre><code>public class GrowBeforeQueueThreadPoolExecutor extends ThreadPoolExecutor { private int userSpecifiedCorePoolSize; private int taskCount; public GrowBeforeQueueThreadPoolExecutor(int corePoolSize, int maximumPoolSize, long keepAliveTime, TimeUnit unit, BlockingQueue&lt;Runnable&gt; workQueue) { super(corePoolSize, maximumPoolSize, keepAliveTime, unit, workQueue); userSpecifiedCorePoolSize = corePoolSize; } @Override public void execute(Runnable runnable) { synchronized (this) { taskCount++; setCorePoolSizeToTaskCountWithinBounds(); } super.execute(runnable); } @Override protected void afterExecute(Runnable runnable, Throwable throwable) { super.afterExecute(runnable, throwable); synchronized (this) { taskCount--; setCorePoolSizeToTaskCountWithinBounds(); } } private void setCorePoolSizeToTaskCountWithinBounds() { int threads = taskCount; if (threads &lt; userSpecifiedCorePoolSize) threads = userSpecifiedCorePoolSize; if (threads &gt; getMaximumPoolSize()) threads = getMaximumPoolSize(); setCorePoolSize(threads); } } </code></pre> <p>As written the class doesn't support changing the user specified corePoolSize or maximumPoolSize after construction, and doesn't support manipulating the work queue directly or via <code>remove()</code> or <code>purge()</code>.</p>
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    1. POHow to get the ThreadPoolExecutor to increase threads to max before queueing?
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    1. USRobert Tupelo-Schneck
    1. USRobert Tupelo-Schneck
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    1. COI like it except for the `synchronized` blocks. Can you call through to the queue to get the number of tasks. Or maybe use an `AtomicInteger`?
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      1. USGray
    2. COI wanted to avoid them, but the problem is this. If there are a number of `execute()` calls in separate threads, each is going to (1) figure out how many threads are needed, (2) `setCorePoolSize` to that number, and (3) call `super.execute()`. If steps (1) and (2) are not synchronized, I'm not sure how to prevent an unfortunate ordering where you set the core pool size to a lower number after a higher number. With direct access to the superclass field this could be done using compare-and-set instead, but I don't see a clean way to do it in a subclass without synchronization.
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      1. USRobert Tupelo-Schneck
    3. COI think the penalties for that race condition are relatively low as long as the `taskCount` field is valid (i.e. a `AtomicInteger`). If two threads recalc the pool-size immediately after each other, it should get proper values. If the 2nd one shrinks the core threads then it must have seen a drop in the queue or something.
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      1. USGray
 

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