JavaWeb中HttpSession中表单的重复提交示例

package com.lsy.javaweb;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpSession;
import java.security.MessageDigest;
import java.security.NoSuchAlgorithmException;
public class TokenProcessor {
  private static final String TOKEN_KEY = "TOKEN_KEY";
  private static final String TRANSACTION_TOKEN_KEY = "TRANSACTION_TOKEN_KEY";
  /**
   * The singleton instance of this class.
   */
  private static TokenProcessor instance = new TokenProcessor();
  /**
   * The timestamp used most recently to generate a token value.
   */
  private long previous;
  /**
   * Protected constructor for TokenProcessor. Use
   * TokenProcessor.getInstance() to obtain a reference to the processor.
   */
  protected TokenProcessor() {
    super();
  }
  /**
   * Retrieves the singleton instance of this class.
   */
  public static TokenProcessor getInstance() {
    return instance;
  }
  /**
   * <p>
   * 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:
   * </p>
   *
   * <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(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(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(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 String saveToken(HttpServletRequest request) {
    HttpSession session = request.getSession();
    String token = generateToken(request);
    if (token != null) {
      session.setAttribute(TRANSACTION_TOKEN_KEY, token);
    }
    return 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 synchronized String generateToken(HttpServletRequest request) {
    HttpSession session = request.getSession();
    return generateToken(session.getId());
  }
  /**
   * Generate a new transaction token, to be used for enforcing a single
   * request for a particular transaction.
   *
   * @param id
   *      a unique Identifier for the session or other context in which
   *      this token is to be used.
   */
  public synchronized String generateToken(String id) {
    try {
      long current = System.currentTimeMillis();
      if (current == previous) {
        current++;
      }
      previous = current;
      byte[] now = new Long(current).toString().getBytes();
      MessageDigest md = MessageDigest.getInstance("MD5");
      md.update(id.getBytes());
      md.update(now);
      return toHex(md.digest());
    } catch (NoSuchAlgorithmException e) {
      return null;
    }
  }
  /**
   * Convert a byte array to a String of hexadecimal digits and return it.
   *
   * @param buffer
   *      The byte array to be converted
   */
  private String toHex(byte[] buffer) {
    StringBuffer sb = new StringBuffer(buffer.length * 2);
    for (int i = 0; i < buffer.length; i++) {
      sb.append(Character.forDigit((buffer[i] & 0xf0) >> 4, 16));
      sb.append(Character.forDigit(buffer[i] & 0x0f, 16));
    }
    return sb.toString();
  }
}