使用HttpServletRequestWrapper在filter修改request参数
处理过程：
（1）页面提交请求“/sa?userid=123456789&username=rensanning”
（2）通过Filter处理加入新参数“name=newname”
（3）sa的Servlet中forward到“/sb?myname=rensanning&mycountry=china”
（4）sb的Servlet中生成页面输出所有参数：userid、username、myname、mycountry、name

第一步：
最简单，在页面上先输出一个href为改地址的链接即可。

<a href="<%=request.getContextPath()%>/sa?userid=123456789&username=rensanning">Click me!!!</a>

第二步：
新作Filter类：ParameterFilter，配置web.xml

<filter-mapping><filter-name>ParameterFilter</filter-name><url-pattern>/*</url-pattern></filter-mapping>

在doFilter方法中包装request，设置新参数值。

Map<String,String[]> m = new HashMap<String,String[]>(req.getParameterMap());m.put("name", new String[]{"newname"});req = new ParameterRequestWrapper((HttpServletRequest)req, m);chain.doFilter(req, res);

包装类代码如下：

class ParameterRequestWrapper extends HttpServletRequestWrapper {private Map<String, String[]> params;public ParameterRequestWrapper(HttpServletRequest request,Map<String, String[]> newParams) {super(request);this.params = newParams;}@Overridepublic String getParameter(String name) {String result = "";Object v = params.get(name);if (v == null) {result = null;} else if (v instanceof String[]) {String[] strArr = (String[]) v;if (strArr.length > 0) {result =  strArr[0];} else {result = null;}} else if (v instanceof String) {result = (String) v;} else {result =  v.toString();}return result;}@Overridepublic Map getParameterMap() {return params;}@Overridepublic Enumeration getParameterNames() {return new Vector(params.keySet()).elements();}@Overridepublic String[] getParameterValues(String name) {String[] result = null;Object v = params.get(name);if (v == null) {result =  null;} else if (v instanceof String[]) {result =  (String[]) v;} else if (v instanceof String) {result =  new String[] { (String) v };} else {result =  new String[] { v.toString() };}return result;}}

第三步：
sa的Servlet中，doGet()方法做forward跳转。

RequestDispatcher rd = getServletContext().getRequestDispatcher("/sb?myname=rensanning&mycountry=china");rd.forward(request, response);

第四步：
sb的Servlet中生成页面输出所有QueryString和ParameterMap的参数。（代码略）

问题：
（1）Filter也只执行了一次

由于RequestDispatcher.forward是Servlet之间的跳转，所以默认不走Filter。在Servlet2.4规范中定义了可以在web.xml中配置：

<filter-mapping><filter-name>ParameterFilter</filter-name><url-pattern>/*</url-pattern><dispatcher>REQUEST</dispatcher><dispatcher>FORWARD</dispatcher></filter-mapping>

（2）结果的QueryString中存在forward时URL的两个参数（myname、mycountry），而ParameterMap中没有。

forward时QueryString中的参数未被放入ParameterMap？？？

如下修改wrapper类：

class ParameterRequestWrapper extends HttpServletRequestWrapper {private Map<String, String[]> params;public ParameterRequestWrapper(HttpServletRequest request,Map<String, String[]> newParams) {super(request);this.params = newParams;// RequestDispatcher.forward parameterrenewParameterMap(request);}@Overridepublic String getParameter(String name) {String result = "";Object v = params.get(name);if (v == null) {result = null;} else if (v instanceof String[]) {String[] strArr = (String[]) v;if (strArr.length > 0) {result =  strArr[0];} else {result = null;}} else if (v instanceof String) {result = (String) v;} else {result =  v.toString();}return result;}@Overridepublic Map getParameterMap() {return params;}@Overridepublic Enumeration getParameterNames() {return new Vector(params.keySet()).elements();}@Overridepublic String[] getParameterValues(String name) {String[] result = null;Object v = params.get(name);if (v == null) {result =  null;} else if (v instanceof String[]) {result =  (String[]) v;} else if (v instanceof String) {result =  new String[] { (String) v };} else {result =  new String[] { v.toString() };}return result;}private void renewParameterMap(HttpServletRequest req) {String queryString = req.getQueryString();if (queryString != null && queryString.trim().length() > 0) {String[] params = queryString.split("&");for (int i = 0; i < params.length; i++) {int splitIndex = params[i].indexOf("=");if (splitIndex == -1) {continue;}String key = params[i].substring(0, splitIndex);if (!this.params.containsKey(key)) {if (splitIndex < params[i].length()) {String value = params[i].substring(splitIndex + 1);this.params.put(key, new String[] { value });}}}}}}

再次运行代码：
（1）Filter进入两次：
引用URL:http://localhost:8080/t/sa
***-----From QueryString-----***
userid=123456789
username=rensanning
***-----From ParameterMap-----***
username=rensanning,
userid=123456789,

引用URL:http://localhost:8080/t/sb
***-----From QueryString-----***
myname=rensanning
mycountry=china
***-----From ParameterMap-----***
username=rensanning,
name=newname,
userid=123456789,

（2）最终页面结果：
引用
***-----From QueryString-----***

myname=rensanning

mycountry=china




***-----From ParameterMap-----***

username=rensanning,

name=newname,

userid=123456789,

mycountry=china,

myname=rensanning,