TokenProcessor 同步令牌源码，极具参考价值

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/\*  
  \* $Header: /home/cvs/jakarta-struts/src/share/org/apache/struts/util/**TokenProcessor**.java,v 1.2 2003/04/25 03:15:49 dgraham Exp $  
  \* $Revision: 1.2 $  
  \* $Date: 2003/04/25 03:15:49 $  
  \*  
  \* ====================================================================  
  \*  
  \* The Apache Software License, Version 1.1  
  \*  
  \* Copyright (c) 2003 The Apache Software Foundation. All rights  
  \* reserved.  
  \*  
  \* Redistribution and use in source and binary forms, with or without  
  \* modification, are permitted provided that the following conditions  
  \* are met:  
  \*  
  \* 1. Redistributions of source code must retain the above copyright  
  \* notice, this list of conditions and the following disclaimer.  
  \*  
  \* 2. Redistributions in binary form must reproduce the above copyright  
  \* notice, this list of conditions and the following disclaimer in  
  \* the documentation and/or other materials provided with the  
  \* distribution.  
  \*  
  \* 3. The end-user documentation included with the redistribution, if  
  \* any, must include the following acknowlegement:  
  \* "This product includes software developed by the  
  \* Apache Software Foundation (http://www.apache.org/)."  
  \* Alternately, this acknowlegement may appear in the software itself,  
  \* if and wherever such third-party acknowlegements normally appear.  
  \*  
  \* 4. The names "The Jakarta Project", "Struts", and "Apache Software  
  \* Foundation" must not be used to endorse or promote products derived  
  \* from this software without prior written permission. For written  
  \* permission, please contact apache@apache.org.  
  \*  
  \* 5. Products derived from this software may not be called "Apache"  
  \* nor may "Apache" appear in their names without prior written  
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  \* THIS SOFTWARE IS PROVIDED \`\`AS IS'' AND ANY EXPRESSED OR IMPLIED  
  \* WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES  
  \* OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE  
  \* DISCLAIMED. IN NO EVENT SHALL THE APACHE SOFTWARE FOUNDATION OR  
  \* ITS CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,  
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  \* LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF  
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  \* ====================================================================  
  \*  
  \* This software consists of voluntary contributions made by many  
  \* individuals on behalf of the Apache Software Foundation. For more  
  \* information on the Apache Software Foundation, please see  
  \* <http://www.apache.org/>.  
  \*  
  \*/  
   
 package org.apache.struts.util;  
   
 import java.security.MessageDigest;  
 import java.security.NoSuchAlgorithmException;  
   
 import javax.servlet.http.HttpServletRequest;  
 import javax.servlet.http.HttpSession;  
   
 import org.apache.struts.Globals;  
 import org.apache.struts.taglib.html.Constants;  
   
 /\*\*  
  \* **TokenProcessor** is responsible for handling all token related functionality. The  
  \* methods in this class are synchronized to protect token processing from multiple  
  \* threads. Servlet containers are allowed to return a different HttpSession object  
  \* for two threads accessing the same session so it is not possible to synchronize  
  \* on the session.  
  \*  
  \* @since Struts 1.1  
  \* @author Craig R. McClanahan  
  \* @author David Graham  
  \*/  
 public class **TokenProcessor** \{  
   
 /\*\*  
  \* The singleton instance of this class.  
  \*/  
  private static **TokenProcessor** instance = new **TokenProcessor**();  
   
 /\*\*  
  \* Retrieves the singleton instance of this class.  
  \*/  
  public static **TokenProcessor** getInstance() \{  
  return instance;  
  \}  
   
 /\*\*  
  \* Protected constructor for **TokenProcessor**. Use **TokenProcessor**.getInstance()  
  \* to obtain a reference to the processor.  
  \*/  
  protected **TokenProcessor**() \{  
  super();  
  \}  
   
 /\*\*  
  \* Return <code>true</code> if there is a transaction token stored in  
  \* the user's current session, and the value submitted as a request  
  \* parameter with this action matches it. Returns <code>false</code>  
  \* under any of the following circumstances:  
  \* <ul>  
  \* <li>No session associated with this request</li>  
  \* <li>No transaction token saved in the session</li>  
  \* <li>No transaction token included as a request parameter</li>  
  \* <li>The included transaction token value does not match the  
  \* transaction token in the user's session</li>  
  \* </ul>  
  \*  
  \* @param request The servlet request we are processing  
  \*/  
  public synchronized boolean isTokenValid(HttpServletRequest request) \{  
  return this.isTokenValid(request, false);  
  \}  
   
 /\*\*  
  \* Return <code>true</code> if there is a transaction token stored in  
  \* the user's current session, and the value submitted as a request  
  \* parameter with this action matches it. Returns <code>false</code>  
  \* <ul>  
  \* <li>No session associated with this request</li>  
  \* <li>No transaction token saved in the session</li>  
  \* <li>No transaction token included as a request parameter</li>  
  \* <li>The included transaction token value does not match the  
  \* transaction token in the user's session</li>  
  \* </ul>  
  \*  
  \* @param request The servlet request we are processing  
  \* @param reset Should we reset the token after checking it?  
  \*/  
  public synchronized boolean isTokenValid(  
  HttpServletRequest request,  
  boolean reset) \{  
   
 // Retrieve the current session for this request  
  HttpSession session = request.getSession(false);  
  if (session == null) \{  
  return false;  
  \}  
   
 // Retrieve the transaction token from this session, and  
 // reset it if requested  
  String saved = (String) session.getAttribute(Globals.TRANSACTION\_TOKEN\_KEY);   if (saved == null) \{  
  return false;  
  \}  
   
  if (reset) \{  
  this.resetToken(request);  
  \}  
   
 // Retrieve the transaction token included in this request  
  String token = request.getParameter(Constants.TOKEN\_KEY);  
  if (token == null) \{  
  return false;  
  \}  
   
  return saved.equals(token);  
  \}  
   
 /\*\*  
  \* Reset the saved transaction token in the user's session. This  
  \* indicates that transactional token checking will not be needed  
  \* on the next request that is submitted.  
  \*  
  \* @param request The servlet request we are processing  
  \*/  
  public synchronized void resetToken(HttpServletRequest request) \{  
   
  HttpSession session = request.getSession(false);  
  if (session == null) \{  
  return;  
  \}  
  session.removeAttribute(Globals.TRANSACTION\_TOKEN\_KEY);  
  \}  
   
 /\*\*  
  \* Save a new transaction token in the user's current session, creating  
  \* a new session if necessary.  
  \*  
  \* @param request The servlet request we are processing  
  \*/  
  public synchronized void saveToken(HttpServletRequest request) \{  
   
  HttpSession session = request.getSession();  
  String token = generateToken(request);  
  if (token != null) \{  
  session.setAttribute(Globals.TRANSACTION\_TOKEN\_KEY, token);  
  \}  
   
  \}  
   
 /\*\*  
  \* Generate a new transaction token, to be used for enforcing a single  
  \* request for a particular transaction.  
  \*  
  \* @param request The request we are processing  
  \*/  
  public String generateToken(HttpServletRequest request) \{  
   
  HttpSession session = request.getSession();  
  try \{  
  byte id\[\] = session.getId().getBytes();  
  byte now\[\] = new Long(System.currentTimeMillis()).toString().getBytes();  
  MessageDigest md = MessageDigest.getInstance("MD5");  
  md.update(id);  
  md.update(now);  
  return this.toHex(md.digest());  
   
  \} catch (IllegalStateException e) \{  
  return null;  
  \} catch (NoSuchAlgorithmException e) \{  
  return null;  
  \}  
   
  \}  
   
 /\*\*  
  \* Convert a byte array to a String of hexadecimal digits and return it.  
  \*<p>  
  \*<strong>WARNING</strong>: This method is not part of **TokenProcessor**'s  
  \*public API. It's provided for backward compatibility only.  
  \*</p>  
  \* @param buffer The byte array to be converted  
  \*/  
  public String toHex(byte buffer\[\]) \{  
  StringBuffer sb = new StringBuffer();  
  String s = null;  
  for (int i = 0; i < buffer.length; i++) \{  
  s = Integer.toHexString((int) buffer\[i\] & 0xff);  
  if (s.length() < 2) \{  
  sb.append('0');  
  \}  
  sb.append(s);  
  \}  
  return sb.toString();  
  \}  
   
 \}