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This artifact provides a single jar that contains all classes required to use remote EJB and JMS, including all dependencies. It is intended for use by those not using maven, maven users should just import the EJB and JMS BOM's instead (shaded JAR's cause lots of problems with maven, as it is very easy to inadvertently end up with different versions on classes on the class path).

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/**
 * Licensed to the Apache Software Foundation (ASF) under one or more
 * contributor license agreements. See the NOTICE file distributed with
 * this work for additional information regarding copyright ownership.
 * The ASF licenses this file to You under the Apache License, Version 2.0
 * (the "License"); you may not use this file except in compliance with
 * the License. You may obtain a copy of the License at
 *
 *     http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */
package org.apache.activemq.artemis.utils;

import java.util.concurrent.ConcurrentLinkedQueue;
import java.util.concurrent.Executor;

import org.apache.activemq.artemis.api.core.ActiveMQInterruptedException;
import org.apache.activemq.artemis.core.client.ActiveMQClientLogger;


/**
 * A factory for producing executors that run all tasks in order, which delegate to a single common executor instance.
 */
public final class OrderedExecutorFactory implements ExecutorFactory
{
   private final Executor parent;

   /**
    * Construct a new instance delegating to the given parent executor.
    *
    * @param parent the parent executor
    */
   public OrderedExecutorFactory(final Executor parent)
   {
      this.parent = parent;
   }

   /**
    * Get an executor that always executes tasks in order.
    *
    * @return an ordered executor
    */
   public Executor getExecutor()
   {
      return new OrderedExecutor(parent);
   }

   /**
    * An executor that always runs all tasks in order, using a delegate executor to run the tasks.
    * 
    * More specifically, any call B to the {@link #execute(Runnable)} method that happens-after another call A to the
    * same method, will result in B's task running after A's.
    */
   private static final class OrderedExecutor implements Executor
   {
      private final ConcurrentLinkedQueue tasks = new ConcurrentLinkedQueue();

      // @protected by tasks
      private boolean running;

      private final Executor parent;

      private final Runnable runner;

      /**
       * Construct a new instance.
       *
       * @param parent the parent executor
       */
      public OrderedExecutor(final Executor parent)
      {
         this.parent = parent;
         runner = new Runnable()
         {
            public void run()
            {
               for (;;)
               {
                  // Optimization, first try without any locks
                  Runnable task = tasks.poll();
                  if (task == null)
                  {
                     synchronized (tasks)
                     {
                        // if it's null we need to retry now holding the lock on tasks
                        // this is because running=false and tasks.empty must be an atomic operation
                        // so we have to retry before setting the tasks to false
                        // this is a different approach to the anti-pattern on synchronize-retry,
                        // as this is just guaranteeing the running=false and tasks.empty being an atomic operation
                        task = tasks.poll();
                        if (task == null)
                        {
                           running = false;
                           return;
                        }
                     }
                  }
                  try
                  {
                     task.run();
                  }
                  catch (ActiveMQInterruptedException e)
                  {
                     // This could happen during shutdowns. Nothing to be concerned about here
                     ActiveMQClientLogger.LOGGER.debug("Interrupted Thread", e);
                  }
                  catch (Throwable t)
                  {
                     ActiveMQClientLogger.LOGGER.caughtunexpectedThrowable(t);
                  }
               }
            }
         };
      }

      /**
       * Run a task.
       *
       * @param command the task to run.
       */
      public void execute(final Runnable command)
      {
         synchronized (tasks)
         {
            tasks.add(command);
            if (!running)
            {
               running = true;
               parent.execute(runner);
            }
         }
      }

      public String toString()
      {
         return "OrderedExecutor(running=" + running + ", tasks=" + tasks + ")";
      }
   }
}