[java] Adding an HTTP Header to the request in a servlet filter

I'm integrating with an existing servlet that pulls some properties out of the HTTP header. Basically, I'm implementing an interface that doesn't have access to the actual request, it just has access to a map of k->v for the HTTP headers.

I need to pass in a request parameter. The plan is to use a servlet filter to go from parameter to header value but of course the HttpServletRequest object doesn't have an addHeader() method.

Any ideas?

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.

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>