使用HttpServletRequestWrapper在filter修改request参数

javax.servlet.ServletRequest中的

Map<String, String[]> parameterMap = request.getParameterMap();不能被修改，原因如下：

org/apache/tomcat/embed/tomcat-embed-core/8.0.32/tomcat-embed-core-8.0.32-sources.jar!/org/apache/catalina/util/ParameterMap.java

    /**

     * Associate the specified value with the specified key in this map.  If

     * the map previously contained a mapping for this key, the old value is

     * replaced.

     *

     * @param key Key with which the specified value is to be associated

     * @param value Value to be associated with the specified key

     *

     * @return The previous value associated with the specified key, or

     *  <code>null</code> if there was no mapping for key

     *

     * @exception IllegalStateException if this map is currently locked

     */

    @Override

    public V put(K key, V value) {

        if (locked)

            throw new IllegalStateException

                (sm.getString("parameterMap.locked"));

        return (super.put(key, value));

    }

https://my.oschina.net/cwalet/blog/35431

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;
}
@Override
public 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;
}
@Override
public Map getParameterMap() {
return params;
}
@Override
public Enumeration getParameterNames() {
return new Vector(params.keySet()).elements();
}
@Override
public 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 parameter
renewParameterMap(request);
}
@Override
public 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;
}
@Override
public Map getParameterMap() {
return params;
}
@Override
public Enumeration getParameterNames() {
return new Vector(params.keySet()).elements();
}
@Override
public 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 });
}
}
}
}
}
}

import javax.servlet.ServletRequest;

import javax.servlet.http.HttpServletRequest;

import javax.servlet.http.HttpServletRequestWrapper;

import java.util.Enumeration;

import java.util.Map;

import java.util.Vector;

/**

 * Created by MyWorld on 2016/9/25.

 */

public class ParameterRequestWrapper extends HttpServletRequestWrapper {

    /**

     * Constructs a request object wrapping the given request.

     *

     * @param request The request to wrap

     * @throws IllegalArgumentException if the request is null

     * http://www.cnblogs.com/softidea/p/5903873.html

     *

     */

    private Map<String, String[]> params;

    public ParameterRequestWrapper(HttpServletRequest request, Map<String, String[]> params) {

        super(request);

        this.params = params;

        addHeaderInfoToParams(request);

    }

    private void addHeaderInfoToParams(HttpServletRequest request) {

        request.getHeader("_pid");

    }

    @Override

    public String getParameter(String name) {

        String[] values = getParameterValues(name);

        if (values == null || values.length == 0) {

            return null;

        } else {

            return values[0];

        }

    }

    @Override

    public Map<String, String[]> getParameterMap() {

        return params;

    }

    @Override

    public Enumeration<String> getParameterNames() {

        return new Vector(params.keySet()).elements();

    }

    @Override

    public String[] getParameterValues(String name) {

        String[] values = params.get(name);

        if (values == null) {

            return null;

        } else {

            return values;

        }

    }

}

import javax.servlet.*;

import javax.servlet.http.HttpServletRequest;

import java.io.IOException;

import java.util.HashMap;

import java.util.Map;

/**

 * Created by MyWorld on 2016/9/25.

 */

public class AddExtraToParamsFilter implements Filter {

    @Override

    public void init(FilterConfig filterConfig) throws ServletException {

    }

    @Override

    public void doFilter(ServletRequest request, ServletResponse response, FilterChain chain) throws IOException, ServletException {

        Map<String, String[]> params = new HashMap<>(request.getParameterMap());

        HttpServletRequest httpServletRequest = (HttpServletRequest) request;

        String pid = httpServletRequest.getHeader("_pid");

        if (pid != null) {

            params.put("pid", new String[]{pid});

        }

        request = new ParameterRequestWrapper(httpServletRequest, params);

        chain.doFilter(request, response);

    }

    @Override

    public void destroy() {

    }

}

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,

ForwardParameter.rar (4.8 KB)

http://rensanning.iteye.com/blog/1706208

巴特西

使用HttpServletRequestWrapper在filter修改request参数

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