我自己写的String类对象的过滤器Fiter不起作用，求大神指点

string 过滤器 --------------------编程问答-------------------- 上代码如下：

package com.minshu.servlet;

import java.io.IOException;
import java.io.PrintWriter;

import javax.servlet.ServletException;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;

public class SurveyServlet extends HttpServlet {

/**
*
*/
private static final long serialVersionUID = -906581454692189344L;

public void doGet(HttpServletRequest request, HttpServletResponse response)
throws ServletException, IOException {

doPost(request, response);
}

public void doPost(HttpServletRequest request, HttpServletResponse response)
throws ServletException, IOException {

response.setContentType("text/html;charset=utf-8");
PrintWriter out = response.getWriter();
out
.println("<!DOCTYPE HTML PUBLIC \"-//W3C//DTD HTML 4.01 Transitional//EN\">");
out.println("<HTML>");
out.println(" <HEAD><TITLE>A Servlet</TITLE></HEAD>");
out.println(" <BODY>");

request.setCharacterEncoding("utf-8");

out.println("姓名："+ request.getParameter("name")+" ");
out.println("EMail:"+ request.getParameter("email")+" ");
out.println("age"+ request.getParameter("age")+" ");
out.println("编程时间："+ request.getParameter("code")+" ");

out.println("操作系统:");
String os[]= request.getParameterValues("os");
out.println("<ul>");
for(int i=0;i<os.length;i++){
out.println("<li>"+os[i]+"</li>");
}
out.println("</ul> ");

out.println("编程语言:");
String language[]= request.getParameterValues("language");
out.println("<ul>");
for(int i=0;i<language.length;i++){
out.println("<li>"+language[i]+"</li>");
}
out.println("</ul> ");

out.println(filterHtml((String)request.getParameter("comment"))+" ");

out.println(" </BODY>");
out.println("</HTML>");
out.flush();
out.close();
}

public String filterHtml(String value) {
value=value.replaceAll("$", "$amp;");

value=value.replaceAll("<", "$lt;");
value=value.replaceAll(">", "$gt;");

value=value.replaceAll(" ", "$nbsp;");
value=value.replaceAll("'", "$acute");
value=value.replaceAll("\n", " ");
value=value.replaceAll("\"", "$quot;");

return value;

}

} --------------------编程问答-------------------- 加入out.println(filterHtml((String)request.getParameter("comment"))+" ");后，报错

--------------------编程问答-------------------- 求大神指点，我的 public String filterHtml(String value) {
value=value.replaceAll("$", "$amp;");

value=value.replaceAll("<", "$lt;");
value=value.replaceAll(">", "$gt;");

value=value.replaceAll(" ", "$nbsp;");
value=value.replaceAll("'", "$acute");
value=value.replaceAll("\n", " ");
value=value.replaceAll("\"", "$quot;");

return value;

}

到底错在那里了？ --------------------编程问答-------------------- value=value.replaceAll("$", "$amp --------------------编程问答-------------------- $括起来试一下 [$] --------------------编程问答--------------------

改成\\$ --------------------编程问答-------------------- replaceAll的第一个参数，是一个正则表达式，所以，有些字符是需要转义的
而有的，比如，\n，你可以试试用"$"来代替
除了这些之外，<,>等等的替换，你可以搜一下公共的API --------------------编程问答-------------------- 是不是你的文档是编码是iso-8859-1的格式；建议均改成utf-8，另外如果是tomcat的话，也可通过指定编码格式来完成。可参见服务器端获取get请求数据乱码通过设置Tomcat的server.xml文件中Connector元素的URIEncoding属性为一个支持中文的编码即可解决。 --------------------编程问答-------------------- value=value.replaceAll("$", "$amp;");

value=value.replaceAll("<", "$lt;");
value=value.replaceAll(">", "$gt;");

value=value.replaceAll(" ", "$nbsp;");
value=value.replaceAll("'", "$acute");
value=value.replaceAll("\n", " ");
这些都不能这么写，看replaceAll中的方法：
public Matcher appendReplacement(StringBuffer sb, String replacement) {

 // If no match, return error
 if (first < 0)
 throw new IllegalStateException("No match available");

 // Process substitution string to replace group references with groups
 int cursor = 0;
 String s = replacement;
 StringBuffer result = new StringBuffer();

 while (cursor < replacement.length()) {
 char nextChar = replacement.charAt(cursor);
 if (nextChar == '\\') {
 cursor++;
 nextChar = replacement.charAt(cursor);
 result.append(nextChar);
 cursor++;
 } else if (nextChar == '$') {
 // Skip past $
 cursor++;

 // The first number is always a group
 int refNum = (int)replacement.charAt(cursor) - '0';
 if ((refNum < 0)||(refNum > 9))
 throw new IllegalArgumentException(
 "Illegal group reference");这里抛出异常了。replaceAll的第二个参数不能以$开头
 cursor++; --------------------编程问答-------------------- replaceAll的第一个参数，是一个正则表达式，所以，有些字符是需要转义的
而有的，比如，\n，你可以试试用"$"来代替
除了这些之外，<,>等等的替换，你可以搜一下公共的API

补充：Java ,  Eclipse