Is it possible to add to the available parameters of a request (HttpServletRequest)

Question

I want to intercept a request in a filter/servlet and add a few parameters to it. However, the request does not expose a \'setParameter\' method and the parameter map when m

Answer 1

You can wrap HttpServletRequest into new HttpServletRequestWrapper object and overwrite some methods.

The following code is from http://www.ocpsoft.org/opensource/how-to-safely-add-modify-servlet-request-parameter-values/ .

To add parameter in filter:

public class MyFilter implements Filter {
...
@Override
public void doFilter(ServletRequest request, ServletResponse response, FilterChain chain) throws IOException, ServletException {
    if (request instanceof HttpServletRequest) {
        HttpServletRequest httprequest = (HttpServletRequest) request;
        Map extraParams = new HashMap();
        extraParams.put("myparamname", String[] { "myparamvalue" });
        request = new WrappedRequestWithParameter(httprequest, extraParams);
    }
    chain.doFilter(request, response);
}
...

class WrappedRequestWithParameter extends HttpServletRequestWrapper {
    private final Map modifiableParameters;
    private Map allParameters = null;

    public WrappedRequestWithParameter(final HttpServletRequest request, final Map additionalParams) {
        super(request);
        modifiableParameters = new TreeMap();
        modifiableParameters.putAll(additionalParams);
    }

    @Override
    public String getParameter(final String name) {
        String[] strings = getParameterMap().get(name);
        if (strings != null) {
            return strings[0];
        }
        return super.getParameter(name);
    }

    @Override
    public Map getParameterMap() {
        if (allParameters == null) {
            allParameters = new TreeMap();
            allParameters.putAll(super.getParameterMap());
            allParameters.putAll(modifiableParameters);
        }
        // Return an unmodifiable collection because we need to uphold the interface contract.
        return Collections.unmodifiableMap(allParameters);
    }

    @Override
    public Enumeration getParameterNames() {
        return Collections.enumeration(getParameterMap().keySet());
    }

    @Override
    public String[] getParameterValues(final String name) {
        return getParameterMap().get(name);
    }
}
}