TokenProcessor 同步令牌源码，极具参考价值

/*
* $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
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* Copyright (c) 2003 The Apache Software Foundation. All rights
* reserved.
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* any, must include the following acknowlegement:
* "This product includes software developed by the
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* if and wherever such third-party acknowlegements normally appear.
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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();
}

}