proof of concept for simulating the appengine dispatch.xml while in development mode

gistfile1.java

package com.yumyumlabs.web.filter;

import com.google.appengine.api.labs.modules.ModulesService;
import com.google.appengine.api.labs.modules.ModulesServiceFactory;
import com.google.appengine.api.utils.SystemProperty;

import javax.servlet.*;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import java.io.ByteArrayOutputStream;
import java.io.IOException;
import java.io.InputStream;
import java.io.OutputStreamWriter;
import java.net.HttpURLConnection;
import java.net.URL;
import java.net.URLEncoder;
import java.util.Enumeration;
import java.util.HashMap;
import java.util.Map;
import java.util.logging.Logger;

public class AppengineDispatchFilter implements Filter {
    final static Logger logger = Logger.getLogger(AppengineDispatchFilter.class.getName());

    private final static boolean IS_PROD = SystemProperty.environment.value() == SystemProperty.Environment.Value.Production;

    final static Map<String, String> urlModuleMap = new HashMap<>();

    @Override
    public void init(FilterConfig filterConfig) throws ServletException {
        if (!IS_PROD) {
            logger.info("fake dispatcher initialized with dispatch.xml config:");
            mapConfigurationDispatch();
        }
    }

    // TODO: instead of changing this by hand, read from dispatch.xml and figure it out
    // TODO: https://code.google.com/p/googleappengine/source/browse/trunk/java/src/main/com/google/apphosting/utils/config/DispatchXmlReader.java
    private void mapConfigurationDispatch() {
        urlModuleMap.put("*/admin*", "webadmin");
        urlModuleMap.put("*/api/recipe/list-by-path*", "searcher");
        urlModuleMap.put("*/api/recipe/search*", "searcher");
        urlModuleMap.put("*/c/i/*", "content");

        // TODO: this shouldn't be here if TaskOptions header host was working.
        urlModuleMap.put("*/tasks*", "tasks");
    }

    @Override
    public void doFilter(ServletRequest request, ServletResponse response, FilterChain chain) throws IOException, ServletException {
        if (IS_PROD) {
            chain.doFilter(request, response);
        }
        else {
            HttpServletResponse resp = (HttpServletResponse) response;
            HttpServletRequest req = ((HttpServletRequest) request);
            String uri = req.getRequestURI().toString();
            String queryString = req.getQueryString();
            String method = req.getMethod();

            String module = null;
            for (String originalRule : urlModuleMap.keySet()) {
                String rule = originalRule;
                // we're not supporting this type of rule
                if (rule.startsWith("*")) {
                    rule = rule.substring(1, rule.length());
                }

                if (rule.endsWith("*")) {
                    rule = rule.substring(0, rule.length() - 1);
                    if (uri.startsWith(rule)) {
                        module = urlModuleMap.get(originalRule);
                        logger.info("Found rule[" + originalRule + "] module[" + module + "] on uri[" + uri + "]");
                        break;
                    }
                }

                if (rule.equals(uri)) {
                    module = urlModuleMap.get(originalRule);
                    logger.info("Found exact match[" + originalRule + "] module[" + module + "]");
                    break;
                }
            }

            if (module != null) {
                ModulesService modulesApi = ModulesServiceFactory.getModulesService();
                String moduleHost = modulesApi.getModuleHostname(module, modulesApi.getDefaultVersion(module));

                String url = "http://" + moduleHost + uri;

                if (queryString != null) {
                    url += "?" + queryString;
                }

                URL u = new URL(url);
                HttpURLConnection connection = (HttpURLConnection) u.openConnection();
                connection.setRequestMethod(method);
                connection.setConnectTimeout(30000);
                connection.setReadTimeout(30000);
                connection.setDoOutput(true);


                // Simply passing headers through. TODO: improve this for multi value
                StringBuilder headerDbg = new StringBuilder();
                Enumeration headers = req.getHeaderNames();
                while (headers != null && headers.hasMoreElements()) {
                    Object name = headers.nextElement();
                    Enumeration values = req.getHeaders(name.toString());
                    while (values != null && values.hasMoreElements()) {
                        Object value = values.nextElement();
                        if (value != null) {
                            String key = name.toString();
                            if ("host".equals(key.toLowerCase())) {
                                logger.info("dispatch OldHost[" + value.toString() + "] NewHost[" + moduleHost + "]");
                            }
                            else {
                                if (headerDbg.length() > 0)
                                    headerDbg.append("&");

                                headerDbg.append(key).append("={").append(value.toString()).append("}");
                            }

                            connection.setRequestProperty(key, value.toString());
                        }
                    }
                }

                logger.info("dispatching[" + module + "] " + method + " to " + url + " with headrs:[" + headerDbg.toString() + "]");

                if ("GET".equals(method)) {
                    connection.setInstanceFollowRedirects(true); // TODO: handle redirects passing through the redirect header to this request.
                }
                else if ("POST".equals(method)) {
                    connection.setDoInput(true);

                    OutputStreamWriter writer = new OutputStreamWriter(connection.getOutputStream());

                    Enumeration<String> params = req.getParameterNames();
                    int i = 0;
                    while (params != null && params.hasMoreElements()) {
                        String param = params.nextElement();

                        String[] values = req.getParameterValues(param);
                        if (values != null) {
                            for (String value : values) {
                                if (i++ > 0) {
                                    writer.write("&");
                                }

                                param = URLEncoder.encode(param, "utf-8");
                                value = URLEncoder.encode(value, "utf-8");

                                writer.write(param);
                                writer.write("=");
                                writer.write(value);
                            }
                        }
                    }
                    writer.flush();
                    writer.close();
                }

                connection.connect();
                int status = connection.getResponseCode();
                resp.setStatus(status);

                if (status != HttpURLConnection.HTTP_OK) {
                    logger.info("dispatch of " + url + " with status " + status);
                }

                ByteArrayOutputStream out = new ByteArrayOutputStream();
                byte[] tmp = new byte[4096];
                InputStream in = connection.getInputStream();
                int len = 0;
                while ((len = in.read(tmp)) > 0) {
                    out.write(tmp, 0, len);
                }
                in.close();
                out.close();


                byte[] content = out.toByteArray();
                response.getOutputStream().write(content);
            }
            else {
                chain.doFilter(request, response);
            }
        }
    }

    @Override
    public void destroy() {
        if (!IS_PROD)
            logger.info("destroy");
    }
}

in web.xml:

<filter>
    <filter-name>appengineDispatch</filter-name>
    <filter-class>com.yumyumlabs.web.filter.AppengineDispatchFilter</filter-class>
</filter>

<filter-mapping>
    <filter-name>appengineDispatch</filter-name>
    <url-pattern>/*</url-pattern>
</filter-mapping>

Here is an updated version that supports parsing the dispatch.xml file. It also should work with POST methods with non-form style payloads (e.g. a JSON payload) :

https://gist.github.com/codedance/9340965