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   /*
    * Copyright 2013 The Netty Project
    *
    * The Netty Project 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:
    *
    *   https://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 io.netty.util.concurrent;
  
  import io.netty.util.internal.ObjectUtil;
  import io.netty.util.internal.logging.InternalLogger;
  import io.netty.util.internal.logging.InternalLoggerFactory;
  
  import java.util.ArrayDeque;
  import java.util.Queue;
  import java.util.concurrent.TimeUnit;
  
  /**
   * Executes {@link Runnable} objects in the caller's thread. If the {@link #execute(Runnable)} is reentrant it will be
   * queued until the original {@link Runnable} finishes execution.
   * <p>
   * All {@link Throwable} objects thrown from {@link #execute(Runnable)} will be swallowed and logged. This is to ensure
   * that all queued {@link Runnable} objects have the chance to be run.
   */
  public final class ImmediateEventExecutor extends AbstractEventExecutor {
      private static final InternalLogger logger = InternalLoggerFactory.getInstance(ImmediateEventExecutor.class);
      public static final ImmediateEventExecutor INSTANCE = new ImmediateEventExecutor();
      /**
       * A Runnable will be queued if we are executing a Runnable. This is to prevent a {@link StackOverflowError}.
       */
      private static final FastThreadLocal<Queue<Runnable>> DELAYED_RUNNABLES = new FastThreadLocal<Queue<Runnable>>() {
          @Override
          protected Queue<Runnable> initialValue() throws Exception {
              return new ArrayDeque<Runnable>();
          }
      };
      /**
       * Set to {@code true} if we are executing a runnable.
       */
      private static final FastThreadLocal<Boolean> RUNNING = new FastThreadLocal<Boolean>() {
          @Override
          protected Boolean initialValue() throws Exception {
              return false;
          }
      };
  
      private final Future<?> terminationFuture = new FailedFuture<Object>(
              GlobalEventExecutor.INSTANCE, new UnsupportedOperationException());
  
      private ImmediateEventExecutor() { }
  
      @Override
      public boolean inEventLoop() {
          return true;
      }
  
      @Override
      public boolean inEventLoop(Thread thread) {
          return true;
      }
  
      @Override
      public Future<?> shutdownGracefully(long quietPeriod, long timeout, TimeUnit unit) {
          return terminationFuture();
      }
  
      @Override
      public Future<?> terminationFuture() {
          return terminationFuture;
      }
  
      @Override
      @Deprecated
      public void shutdown() { }
  
      @Override
      public boolean isShuttingDown() {
          return false;
      }
  
      @Override
      public boolean isShutdown() {
          return false;
      }
  
      @Override
      public boolean isTerminated() {
          return false;
      }
  
      @Override
     public boolean awaitTermination(long timeout, TimeUnit unit) {
         return false;
     }
 
     @Override
     public void execute(Runnable command) {
         ObjectUtil.checkNotNull(command, "command");
         if (!RUNNING.get()) {
             RUNNING.set(true);
             try {
                 command.run();
             } catch (Throwable cause) {
                 logger.info("Throwable caught while executing Runnable {}", command, cause);
             } finally {
                 Queue<Runnable> delayedRunnables = DELAYED_RUNNABLES.get();
                 Runnable runnable;
                 while ((runnable = delayedRunnables.poll()) != null) {
                     try {
                         runnable.run();
                     } catch (Throwable cause) {
                         logger.info("Throwable caught while executing Runnable {}", runnable, cause);
                     }
                 }
                 RUNNING.set(false);
             }
         } else {
             DELAYED_RUNNABLES.get().add(command);
         }
     }
 
     @Override
     public <V> Promise<V> newPromise() {
         return new ImmediatePromise<V>(this);
     }
 
     @Override
     public <V> ProgressivePromise<V> newProgressivePromise() {
         return new ImmediateProgressivePromise<V>(this);
     }
 
     static class ImmediatePromise<V> extends DefaultPromise<V> {
         ImmediatePromise(EventExecutor executor) {
             super(executor);
         }
 
         @Override
         protected void checkDeadLock() {
             // No check
         }
     }
 
     static class ImmediateProgressivePromise<V> extends DefaultProgressivePromise<V> {
         ImmediateProgressivePromise(EventExecutor executor) {
             super(executor);
         }
 
         @Override
         protected void checkDeadLock() {
             // No check
         }
     }
 }