请教书中例子一些技术性的问题

import javax.servlet.*;
import java.io.IOException;

/**
* <p>Filter that sets the character encoding to be used in parsing the
* incoming request, either unconditionally or only if the client did not
* specify a character encoding. Configuration of this filter is based on
* the following initialization parameters:</p>
* <ul>
* <li><strong>encoding</strong> - The character encoding to be configured
* for this request, either conditionally or unconditionally based on
* the <code>ignore</code> initialization parameter. This parameter
* is required, so there is no default.</li>
* <li><strong>ignore</strong> - If set to "true", any character encoding
* specified by the client is ignored, and the value returned by the
* <code>selectEncoding()</code> method is set. If set to "false,
* <code>selectEncoding()</code> is called <strong>only</strong> if the
* client has not already specified an encoding. By default, this
* parameter is set to "true".</li>
* </ul>
*
* <p>Although this filter can be used unchanged, it is also easy to
* subclass it and make the <code>selectEncoding()</code> method more
* intelligent about what encoding to choose, based on characteristics of
* the incoming request (such as the values of the <code>Accept-Language</code>
* and <code>User-Agent</code> headers, or a value stashed in the current
* user's session.</p>
*
* @author <a href="mailto:jwtronics@yahoo.com">John Wong</a>
*
* @version $Id: SetCharacterEncodingFilter.java,v 1.1 2002/04/10 13:59:27 johnwong Exp $
*/

public class SetCharacterEncodingFilter
    implements Filter {

// ----------------------------------------------------- Instance Variables

  /**
   * The default character encoding to set for requests that pass through
   * this filter.
   */
  protected String encoding = null;

  /**
   * The filter configuration object we are associated with. If this value
   * is null, this filter instance is not currently configured.
   */
  protected FilterConfig filterConfig = null;

  /**
   * Should a character encoding specified by the client be ignored?
   */
  protected boolean ignore = true;

// --------------------------------------------------------- Public Methods

  /**
   * Take this filter out of service.
   */
  public void destroy() {

    this.encoding = null;
    this.filterConfig = null;

  }

  /**
   * Select and set (if specified) the character encoding to be used to
   * interpret request parameters for this request.
   *
   * @param request The servlet request we are processing
   * @param result The servlet response we are creating
   * @param chain The filter chain we are processing
   *
   * @exception IOException if an input/output error occurs
   * @exception ServletException if a servlet error occurs
   */
  public void doFilter(ServletRequest request, ServletResponse response,
                       FilterChain chain) throws IOException, ServletException {

// Conditionally select and set the character encoding to be used
    if (ignore || (request.getCharacterEncoding() == null)) {
      String encoding = selectEncoding(request);
      if (encoding != null)
        request.setCharacterEncoding(encoding);
    }

// Pass control on to the next filter
    chain.doFilter(request, response);

  }

  /**
   * Place this filter into service.
   *
   * @param filterConfig The filter configuration object
   */
  public void init(FilterConfig filterConfig) throws ServletException {

    this.filterConfig = filterConfig;
    this.encoding = filterConfig.getInitParameter("encoding");
    PropsUtil.ENCODING = this.encoding;
    String value = filterConfig.getInitParameter("ignore");
    if (value == null)
      this.ignore = true;
    else if (value.equalsIgnoreCase("true"))
      this.ignore = true;
    else if (value.equalsIgnoreCase("yes"))
      this.ignore = true;
    else
      this.ignore = false;

  }

// ------------------------------------------------------ Protected Methods

  /**
   * Select an appropriate character encoding to be used, based on the
   * characteristics of the current request and/or filter initialization
   * parameters. If no character encoding should be set, return
   * <code>null</code>.
   * <p>
   * The default implementation unconditionally returns the value configured
   * by the <strong>encoding</strong> initialization parameter for this
   * filter.
   *
   * @param request The servlet request we are processing
   */
  protected String selectEncoding(ServletRequest request) {

    return (this.encoding);

  }

}
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