Inject nocache.js into the main index.html

[Craig Mitchell]

I tried to inject the non-dev nocache.js into my main index.html file.  Like this:

String gwtNoCacheJs = loadFileFromServlet("/dt/dt.nocache.js");

if (gwtNoCacheJs.contains("superdevmode")) {
  gwtNoCacheJs = "<script type='text/javascript' src='/dt/dt.nocache.js'></script>";
}
else {
  gwtNoCacheJs = "<script type='text/javascript'>\n" + gwtNoCacheJs + "\n</script>";
}

String html = loadFile("index.html").replace("XXX", gwtNoCacheJs);

However, it doesn't work, as nocache.js wants to load files from the same sub directory it's located in, and not the root directory.

Has anyone done this?  It probably isn't work the effort, as it'll only save one network call, but I was curious if it's possible.

[Colin Alworth]

I believe that https://github.com/gwtproject/gwt/blob/main/dev/core/src/com/google/gwt/core/ext/linker/impl/computeScriptBase.js is what you're going to want to read, or possibly replace on your classpath. Alternatively, subclass the CrossSiteIframeLinker to override getJsComputeScriptBase(LinkerContext) to provide your own file.

From that file:

/**
 * Determine our own script's URL by trying various things
 *
 * First - use the baseUrl meta tag if it exists
 * Second - look for a script tag with the src set to MODULE_NAME.nocache.js and
 *   if it's found, use it to determine the baseUrl
 * Third - if the page is not already loaded, try to use some document.write
 *   magic to install a temporary tag and use that to determine the baseUrl.
 *
 * This is included into the selection scripts
 * wherever COMPUTE_SCRIPT_BASE appears with underlines
 * on each side.
 */

The "Second" step is where you appear to be getting stuck - since there is no script tag with a src attr, the rest of the loading code doesn't know what to do. So, add a meta tag for baseUrl so the script knows where the other resources are loaded from.

Note that I haven't messed with this in years, and might have missed a point or two.

[Craig Mitchell]

Thanks Colin.  Dug around a little and figured out all I needed was:

<meta name="gwt:property" content="baseUrl=/dt/" />

("dt" is my GWT module short name)

Now everything works, including superdevmode.  Ie: No need for the gwtNoCacheJs.contains("superdevmode") check anymore.

I think my page does load a bit faster now (on subsequent times, not the first visit).  https://drift.team/ if anyone is curious what the end result root html source looks like.

[Craig Mitchell]

I should point out, this is only useful if you have a single page app.  If you load different html pages, don't do this, as you'll be reloading the nocache.js for each page.

[Jens]

Yeah, using the gwt property is the easiest solution. I also use it. But be aware that you now have to update your webserver configuration to tell the browser to not cache the index.html anymore. Otherwise the embedded GWT code will be outdated if you deploy an updated version of your GWT app and all the hashes of *.cache.js files have changed.

-- J.

[Craig Mitchell]

Good point Jens.  Here's some handy code that works well for me:

In the HttpServlet that servers up the index.html file:

@Override
protected void doGet(HttpServletRequest request, HttpServletResponse response) throws IOException {
  String serverVersion =  [LOAD YOUR SERVER VERSION]
  String quotedETag = "\"" + serverVersion + "\"";

  response.setHeader("ETag", quotedETag);
  response.setHeader("Cache-Control", "no-cache, must-revalidate");

  String ifNoneMatch = request.getHeader("If-None-Match");

  // The browser has the latest version
  if (quotedETag.equals(ifNoneMatch)) {
    response.setStatus(HttpServletResponse.SC_NOT_MODIFIED); // 304 Not Modified
  }
  // The browser needs the latest version
  else {

    String html = [LOAD YOUR HTML]

    response.setContentType("text/html");

    response.setCharacterEncoding("UTF-8");

    response.getWriter().append(html);

  }

}

You need a serverVersion somehow.  If using Google App Engine, you can do  System.getenv("GAE_VERSION");  otherwise, it's up to you how you get it and make it unique for each deploy.