This closes #2514

artemis-commons/src/main/java/org/apache/activemq/artemis/utils/ActiveMQThreadPoolExecutor.java

@@ -17,10 +17,10 @@
 package org.apache.activemq.artemis.utils;
 
 import java.util.concurrent.LinkedBlockingQueue;
-import java.util.concurrent.RejectedExecutionHandler;
 import java.util.concurrent.ThreadFactory;
 import java.util.concurrent.ThreadPoolExecutor;
 import java.util.concurrent.TimeUnit;
+import java.util.concurrent.atomic.AtomicIntegerFieldUpdater;
 
 /*
  * ActiveMQThreadPoolExecutor: a special ThreadPoolExecutor that combines
@@ -29,33 +29,125 @@ import java.util.concurrent.TimeUnit;
  * and will be removed after idling for a specified keep time.
  * But in contrast to a standard cached executor, tasks are queued if the
  * maximum pool size if reached, instead of rejected.
+ *
+ * This is achieved by using a specialized blocking queue, which checks the
+ * state of the associated executor in the offer method to decide whether to
+ * queue a task or have the executor create another thread.
+ *
+ * Since the thread pool's execute method is reentrant, more than one caller
+ * could try to offer a task into the queue. There is a small chance that
+ * (a few) more threads are created as it should be limited by max pool size.
+ * To allow for such a case not to reject a task, the underlying thread pool
+ * executor is not limited. Only the offer method checks the configured limit.
  */
 public class ActiveMQThreadPoolExecutor extends ThreadPoolExecutor {
 
-   // Handler executed when a task is submitted and a new thread cannot be created (because maxSize was reached)
-   // It queues the task on the executors's queue (using the add() method, see ThreadPoolQueue class below)
-   private static final RejectedExecutionHandler QUEUE_EXECUTION_HANDLER = (r, e) -> {
-      if (!e.isShutdown()) {
-         e.getQueue().add(r);
-      }
-   };
-
-   // A specialized LinkedBlockingQueue that takes new elements by calling add() but not offer()
-   // This is to force the ThreadPoolExecutor to always create new threads and never queue
+   @SuppressWarnings("serial")
    private static class ThreadPoolQueue extends LinkedBlockingQueue<Runnable> {
 
+      private ActiveMQThreadPoolExecutor executor = null;
+
+      // keep track of the difference between the number of idle threads and
+      // the number of queued tasks. If the delta is > 0, we have more
+      // idle threads than queued tasks and can add more tasks into the queue.
+      // The delta is incremented if a thread becomes idle or if a task is taken from the queue.
+      // The delta is decremented if a thread leaves idle state or if a task is added to the queue.
+      private static final AtomicIntegerFieldUpdater<ThreadPoolQueue> DELTA_UPDATER = AtomicIntegerFieldUpdater.newUpdater(ThreadPoolQueue.class, "threadTaskDelta");
+      private volatile int threadTaskDelta = 0;
+
+      public void setExecutor(ActiveMQThreadPoolExecutor executor) {
+         this.executor = executor;
+      }
+
       @Override
       public boolean offer(Runnable runnable) {
-         return false;
+         boolean retval = false;
+
+         if (threadTaskDelta > 0 || (executor.getPoolSize() >= executor.getMaximumPoolSize())) {
+            // A new task will be added to the queue if the maximum number of threads has been reached
+            // or if the delta is > 0, which means that there are enough idle threads.
+
+            retval = super.offer(runnable);
+
+            // Only decrement the delta if the task has actually been added to the queue
+            if (retval)
+               DELTA_UPDATER.decrementAndGet(this);
+         }
+
+         return retval;
       }
 
       @Override
-      public boolean add(Runnable runnable) {
-         return super.offer( runnable );
+      public Runnable take() throws InterruptedException {
+         // Increment the delta as a thread becomes idle
+         // by waiting for a task to take from the queue
+         DELTA_UPDATER.incrementAndGet(this);
+
+
+         Runnable runnable = null;
+
+         try {
+            runnable = super.take();
+            return runnable;
+         } finally {
+            // Now the thread is no longer idle waiting for a task
+            // If it had taken a task, the delta remains the same
+            // (decremented by the thread and incremented by the taken task)
+            // Only if no task had been taken, we have to decrement the delta.
+            if (runnable == null) {
+               DELTA_UPDATER.decrementAndGet(this);
+            }
          }
       }
 
+      @Override
+      public Runnable poll(long arg0, TimeUnit arg2) throws InterruptedException {
+         // Increment the delta as a thread becomes idle
+         // by waiting for a task to poll from the queue
+         DELTA_UPDATER.incrementAndGet(this);
+
+         Runnable runnable = null;
+
+         try {
+            runnable = super.poll(arg0, arg2);
+         } finally {
+            // Now the thread is no longer idle waiting for a task
+            // If it had taken a task, the delta remains the same
+            // (decremented by the thread and incremented by the taken task)
+            if (runnable == null) {
+               DELTA_UPDATER.decrementAndGet(this);
+            }
+         }
+
+         return runnable;
+      }
+   }
+
+   private int maxPoolSize;
+
    public ActiveMQThreadPoolExecutor(int coreSize, int maxSize, long keep, TimeUnit keepUnits, ThreadFactory factory) {
-      super( coreSize, maxSize, keep, keepUnits, new ThreadPoolQueue(), factory, QUEUE_EXECUTION_HANDLER );
+      this(coreSize, maxSize, keep, keepUnits, new ThreadPoolQueue(), factory);
+   }
+
+   // private constructor is needed to inject 'this' into the ThreadPoolQueue instance
+   private ActiveMQThreadPoolExecutor(int coreSize,
+                                      int maxSize,
+                                      long keep,
+                                      TimeUnit keepUnits,
+                                      ThreadPoolQueue myQueue,
+                                      ThreadFactory factory) {
+      super(coreSize, Integer.MAX_VALUE, keep, keepUnits, myQueue, factory);
+      maxPoolSize = maxSize;
+      myQueue.setExecutor(this);
+   }
+
+   @Override
+   public int getMaximumPoolSize() {
+      return maxPoolSize;
+   }
+
+   @Override
+   public void setMaximumPoolSize(int maxSize) {
+      maxPoolSize = maxSize;
    }
 }