Servlet filter for forcing HTTPS when behind a SSL termination proxy that sends X-Forwarded-Proto

HttpsFilter.java

import javax.servlet.*;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import java.io.IOException;
import java.net.URI;
import java.net.URISyntaxException;

import static java.lang.String.format;

public class HttpsFilter implements Filter {
	private boolean enabled;

	@Override
	public void init(FilterConfig filterConfig) throws ServletException {
		this.enabled = "production".equals(System.getenv("ENV"));
	}

	@Override
	public void destroy() {
	}

	@Override
	public void doFilter(ServletRequest request, ServletResponse response, FilterChain chain) throws IOException, ServletException {
		HttpServletRequest  httpRequest  = (HttpServletRequest)  request;
		HttpServletResponse httpResponse = (HttpServletResponse) response;

		if (!enabled) {
			chain.doFilter(request, response);
			return;
		}

		String xfp = httpRequest.getHeader("X-Forwarded-Proto");

		if ("https".equals(xfp)) {
			httpResponse.setHeader("Strict-Transport-Security", "max-age=60");

			chain.doFilter(request, response);
		}
		else if ("http".equals(xfp)) {
			try {
				URI uri1 = new URI(httpRequest.getRequestURL().toString());

				if (uri1.getPort() >= 0) {
					throw new ServletException(format("Only standard ports are supported (given %s)", uri1.getPort()));
				}

				URI uri2 = new URI("https",
				                   uri1.getUserInfo(),
				                   uri1.getHost(),
				                   /* port: */ -1,
				                   uri1.getPath(),
				                   httpRequest.getQueryString(),
				                   /* fragment: */ null);

				httpResponse.sendRedirect(uri2.toString());
			}
			catch (URISyntaxException e) {
				throw new ServletException("Something went wrong with the URIs", e);
			}
		}
		else {
			throw new ServletException(format("Unsupported value for X-Forwarded-Proto: %s", xfp));
		}
	}
}

Server filters are a PITA compared to Rack/Ring middleware. :(