Adding an HTTP Header to the request in a servlet filter

Question

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?

Answer 1

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);

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