Last week, I started working on a new project using GWT. On my last project, we used GWT HTTP Calls and my new project is using RPC. We'll likely migrate to a JSON backend eventually, but in the meantime, I wanted to be able to develop in hosted mode (localhost:8888) and call services on another host (localhost:8080), where the services are running in a JSF/Spring webapp.

At first, I thought it'd be easy thanks to the handy-dandy ProxyServlet I mentioned in Implementing OAuth with GWT. However, when I tried to hook it in and use it, I saw the following error in my server-side logs.

java.lang.NullPointerException
        at javax.servlet.GenericServlet.getServletName(GenericServlet.java:322)
        at javax.servlet.GenericServlet.log(GenericServlet.java:277)
        at com.google.gwt.user.server.rpc.RemoteServiceServlet.doGetSerializationPolicy(RemoteServiceServlet.java:219)
        at com.google.gwt.user.server.rpc.RemoteServiceServlet.getSerializationPolicy(RemoteServiceServlet.java:117)
        at com.google.gwt.user.server.rpc.impl.ServerSerializationStreamReader.prepareToRead(ServerSerializationStreamReader.java:429)
        at com.google.gwt.user.server.rpc.RPC.decodeRequest(RPC.java:234)

Looking at RemoteServiceServlet.java:219, there's a logging call that fails for some reason (at least in my application).

/*
 * Check that the module path must be in the same web app as the servlet
 * itself. If you need to implement a scheme different than this, override
 * this method.
 */
if (modulePath == null || !modulePath.startsWith(contextPath)) {
  String message = "ERROR: The module path requested, "
      + modulePath
      + ", is not in the same web application as this servlet, "
      + contextPath
      + ".  Your module may not be properly configured or your client and server code maybe out of date.";
  log(message, null);
}

In the above code, you might notice that GWT is checking to make sure the client is hosted in the same application as the server. After I figured this out, it was pretty easy to modify my ProxyServlet to trick GWT RPC into thinking the client was in the same web application. In the ProxyServlet's handleContentPost method, I added the following code to replace "localhost:8888/" with "localhost:8080/services/" (in the content of the post to the server). 

if (contentType.startsWith("text/x-gwt-rpc")) {
    String clientHost = httpServletRequest.getLocalName();
    if (clientHost.equals("127.0.0.1")) {
        clientHost = "localhost";
    }

    int clientPort = httpServletRequest.getLocalPort();
    String clientUrl = clientHost + ((clientPort != 80) ? ":" + 
                       clientPort : "");
    String serverUrl = stringProxyHost + ((intProxyPort != 80) ? ":" + 
                       intProxyPort : "") + httpServletRequest.getServletPath();
    postContent = postContent.replace(clientUrl , serverUrl);
}

After manipulating the posted content, I was successfully able to use GWT RPC cross-domain.

Woo hoo!

For your convenience, the full handleContentPost() method is listed below.

private void handleContentPost(PostMethod postMethodProxyRequest, 
                               HttpServletRequest httpServletRequest) 
            throws IOException, ServletException {
    StringBuilder content = new StringBuilder();
    BufferedReader reader = httpServletRequest.getReader();
    for (;;) {
        String line = reader.readLine();
        if (line == null) break;
        content.append(line);
    }

    String contentType = httpServletRequest.getContentType();
    String postContent = content.toString();

    if (contentType.startsWith("text/x-gwt-rpc")) {
        String clientHost = httpServletRequest.getLocalName();
        if (clientHost.equals("127.0.0.1")) {
            clientHost = "localhost";
        }

        int clientPort = httpServletRequest.getLocalPort();
        String clientUrl = clientHost + ((clientPort != 80) ? ":" + 
                           clientPort : "");
        String serverUrl = stringProxyHost + ((intProxyPort != 80) ? ":" + 
                           intProxyPort : "") + httpServletRequest.getServletPath();
        postContent = postContent.replace(clientUrl , serverUrl);
    }

    String encoding = httpServletRequest.getCharacterEncoding();
    debug("POST Content Type: " + contentType + " Encoding: " + encoding,
          "Content: " + postContent);
    StringRequestEntity entity;
    try {
        entity = new StringRequestEntity(postContent, contentType, encoding);
    } catch (UnsupportedEncodingException e) {
        throw new ServletException(e);
    }
    // Set the proxy request POST data
    postMethodProxyRequest.setRequestEntity(entity);
}

From http://raibledesigns.com/rd/entry/how_to_do_cross_domain