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   /*
    * Copyright 2012 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:
    *
    *   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.jboss.netty.handler.codec.http;
  
  import java.text.ParsePosition;
  import java.text.SimpleDateFormat;
  import java.util.Date;
  import java.util.Locale;
  import java.util.TimeZone;
  
  /**
   * This DateFormat decodes 3 formats of {@link Date}, but only encodes the one,
   * the first:
   * <ul>
   * <li>Sun, 06 Nov 1994 08:49:37 GMT: standard specification, the only one with
   * valid generation</li>
   * <li>Sun, 06 Nov 1994 08:49:37 GMT: obsolete specification</li>
   * <li>Sun Nov 6 08:49:37 1994: obsolete specification</li>
   * </ul>
   */
  public final class HttpHeaderDateFormat extends SimpleDateFormat {
      private static final long serialVersionUID = -925286159755905325L;
  
      private final SimpleDateFormat format1 = new HttpHeaderDateFormatObsolete1();
      private final SimpleDateFormat format2 = new HttpHeaderDateFormatObsolete2();
  
      private static final ThreadLocal<HttpHeaderDateFormat> FORMAT_THREAD_LOCAL =
              new ThreadLocal<HttpHeaderDateFormat>() {
          @Override
          protected HttpHeaderDateFormat initialValue() {
              return new HttpHeaderDateFormat();
          }
      };
  
      public static HttpHeaderDateFormat get() {
          return FORMAT_THREAD_LOCAL.get();
      }
  
      /**
       * Standard date format<p>
       * Sun, 06 Nov 1994 08:49:37 GMT -> E, d MMM yyyy HH:mm:ss z
       */
      private HttpHeaderDateFormat() {
          super("E, dd MMM yyyy HH:mm:ss z", Locale.ENGLISH);
          setTimeZone(TimeZone.getTimeZone("GMT"));
      }
  
      @Override
      public Date parse(String text, ParsePosition pos) {
          Date date = super.parse(text, pos);
          if (date == null) {
              date = format1.parse(text, pos);
          }
          if (date == null) {
              date = format2.parse(text, pos);
          }
          return date;
      }
  
      /**
       * First obsolete format<p>
       * Sunday, 06-Nov-94 08:49:37 GMT -> E, d-MMM-y HH:mm:ss z
       */
      private static final class HttpHeaderDateFormatObsolete1 extends SimpleDateFormat {
          private static final long serialVersionUID = -3178072504225114298L;
  
          HttpHeaderDateFormatObsolete1() {
              super("E, dd-MMM-yy HH:mm:ss z", Locale.ENGLISH);
              setTimeZone(TimeZone.getTimeZone("GMT"));
          }
      }
  
      /**
       * Second obsolete format
       * <p>
       * Sun Nov 6 08:49:37 1994 -> EEE, MMM d HH:mm:ss yyyy
       */
      private static final class HttpHeaderDateFormatObsolete2 extends SimpleDateFormat {
          private static final long serialVersionUID = 3010674519968303714L;
  
          HttpHeaderDateFormatObsolete2() {
              super("E MMM d HH:mm:ss yyyy", Locale.ENGLISH);
              setTimeZone(TimeZone.getTimeZone("GMT"));
          }
      }
  }