Adding an HTTP Header to the request in a servlet filter

Extend HttpServletRequestWrapper, override the header getters to return the parameters as well:

public class AddParamsToHeader extends HttpServletRequestWrapper {    public AddParamsToHeader(HttpServletRequest request) {        super(request);    }    public String getHeader(String name) {        String header = super.getHeader(name);        return (header != null) ? header : super.getParameter(name); // Note: you can't use getParameterValues() here.    }    public Enumeration getHeaderNames() {        List<String> names = Collections.list(super.getHeaderNames());        names.addAll(Collections.list(super.getParameterNames()));        return Collections.enumeration(names);    }}

..and wrap the original request with it:

chain.doFilter(new AddParamsToHeader((HttpServletRequest) request), response);

That said, I personally find this a bad idea. Rather give it direct access to the parameters or pass the parameters to it.

as https://stackoverflow.com/users/89391/miku pointed out this would be a complete ServletFilter example that uses the code that also works for Jersey to add the remote_addr header.

package com.bitplan.smartCRM.web;import java.io.IOException;import java.util.Collections;import java.util.Enumeration;import java.util.HashMap;import java.util.List;import java.util.Map;import javax.servlet.Filter;import javax.servlet.FilterChain;import javax.servlet.FilterConfig;import javax.servlet.ServletException;import javax.servlet.ServletRequest;import javax.servlet.ServletResponse;import javax.servlet.http.HttpServletRequest;import javax.servlet.http.HttpServletRequestWrapper;/** *  * @author wf *  */public class RemoteAddrFilter implements Filter {    @Override    public void destroy() {    }    @Override    public void doFilter(ServletRequest request, ServletResponse response,            FilterChain chain) throws IOException, ServletException {        HttpServletRequest req = (HttpServletRequest) request;        HeaderMapRequestWrapper requestWrapper = new HeaderMapRequestWrapper(req);        String remote_addr = request.getRemoteAddr();        requestWrapper.addHeader("remote_addr", remote_addr);        chain.doFilter(requestWrapper, response); // Goes to default servlet.    }    @Override    public void init(FilterConfig filterConfig) throws ServletException {    }    // https://stackoverflow.com/questions/2811769/adding-an-http-header-to-the-request-in-a-servlet-filter    // http://sandeepmore.com/blog/2010/06/12/modifying-http-headers-using-java/    // http://bijubnair.blogspot.de/2008/12/adding-header-information-to-existing.html    /**     * allow adding additional header entries to a request     *      * @author wf     *      */    public class HeaderMapRequestWrapper extends HttpServletRequestWrapper {        /**         * construct a wrapper for this request         *          * @param request         */        public HeaderMapRequestWrapper(HttpServletRequest request) {            super(request);        }        private Map<String, String> headerMap = new HashMap<String, String>();        /**         * add a header with given name and value         *          * @param name         * @param value         */        public void addHeader(String name, String value) {            headerMap.put(name, value);        }        @Override        public String getHeader(String name) {            String headerValue = super.getHeader(name);            if (headerMap.containsKey(name)) {                headerValue = headerMap.get(name);            }            return headerValue;        }        /**         * get the Header names         */        @Override        public Enumeration<String> getHeaderNames() {            List<String> names = Collections.list(super.getHeaderNames());            for (String name : headerMap.keySet()) {                names.add(name);            }            return Collections.enumeration(names);        }        @Override        public Enumeration<String> getHeaders(String name) {            List<String> values = Collections.list(super.getHeaders(name));            if (headerMap.containsKey(name)) {                values.add(headerMap.get(name));            }            return Collections.enumeration(values);        }    }}

web.xml snippet:

<!--  first filter adds remote addr header --><filter>    <filter-name>remoteAddrfilter</filter-name>    <filter-class>com.bitplan.smartCRM.web.RemoteAddrFilter</filter-class></filter><filter-mapping>    <filter-name>remoteAddrfilter</filter-name>    <url-pattern>/*</url-pattern></filter-mapping>

You'll have to use an HttpServletRequestWrapper:

public void doFilter(final ServletRequest request, final ServletResponse response, final FilterChain chain) throws IOException, ServletException {    final HttpServletRequest httpRequest = (HttpServletRequest) request;    HttpServletRequestWrapper wrapper = new HttpServletRequestWrapper(httpRequest) {        @Override        public String getHeader(String name) {            final String value = request.getParameter(name);            if (value != null) {                return value;            }            return super.getHeader(name);        }    };    chain.doFilter(wrapper, response);}

Depending on what you want to do you may need to implement other methods of the wrapper like getHeaderNames for instance. Just be aware that this is trusting the client and allowing them to manipulate any HTTP header. You may want to sandbox it and only allow certain header values to be modified this way.