content: /*
   * 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 WARRANTIESOR 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.aries.blueprint.utils.threading.impl;
  
  import java.util.Queue;
  import java.util.concurrent.Callable;
  import java.util.concurrent.CancellationException;
  import java.util.concurrent.LinkedBlockingQueue;
  import java.util.concurrent.atomic.AtomicReference;
  
  public class DiscardableCallable<V> implements Callable<V>, Runnable, Discardable<Runnable> {
      private AtomicReference<Callable<V>> c = new AtomicReference<Callable<V>>();
      private Queue<Discardable<Runnable>> _removeFromListOnCall;
  
      public DiscardableCallable(Callable<V> call, Queue<Discardable<Runnable>> _unprocessedWork) {
          c.set(call);
          _removeFromListOnCall = _unprocessedWork;
          _removeFromListOnCall.add(this);
      }
  
      private DiscardableCallable(Callable<V> call) {
          c.set(call);
          _removeFromListOnCall = new LinkedBlockingQueue<Discardable<Runnable>>();
      }
  
      public Runnable discard() {
          _removeFromListOnCall.remove(this);
          return new DiscardableCallable<V>(c.getAndSet(null));
      }
  
      public V call() throws Exception {
          _removeFromListOnCall.remove(this);
          Callable<V> call = c.get();
          if (call != null) {
              return call.call();
          }
          throw new CancellationException();
      }
  
      public void run() {
          try {
              call();
          } catch (Exception e) {
          }
      }
  }