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Gmail Contextual Gadgets with GWT

Today we’ll create Gmail Contextual gadget with the help of GWT.
According to Google definition, Contextual gadget is “a gadget that is triggered by clues in Gmail, such as the contents of Subject lines and email messages. For example, Gmail already provides a YouTube contextual gadget. If the body of an email contains a link to a YouTube video, a clickable thumbnail view of the video appears at the bottom of the email.”

Gmail Contextual gadgets can work only with Google Apps for domain, you can’t set it up with your * account, because gadgets are available via Google Apps Marketplace only.

There are very good specs from Google about creating gadgets with pure JavaScript I recommend to look through it, to understand, what we are doing.
GWT version has some differences with native JavaScript, we will take look at this.


<?xml version="1.0" encoding="UTF-8" ?>
<ApplicationManifest xmlns="">
    <!-- URL explaining how customers get support. -->
        <Link rel="support" href="" />
    <!-- URL that is displayed to admins during the deletion process,
     to specify policies such as data retention, how to claim accounts, etc. -->
        <Link rel="deletion-policy" href="" />

    <!-- Name and description pulled from message bundles -->
    <Name>Demo Contextual Gadget</Name>
    <Description>Simple GWT gadget</Description>

    <!-- EXTRACTOR -->
    <Extension id="body" type="contextExtractor">
        <Name>Email Body</Name>
        <Param name="email_body" value=".*" />
        <Triggers ref="CtxDemoGadget" />
        <Scope ref="bodyScope" />
        <Container name="mail" />

    <!-- GADGET -->
    <Extension id="CtxDemoGadget" type="gadget">
        <Name>Demo Gmail Contextual Gadget</Name>
        <!-- Public path to your main class -->
        <Container name="mail" />

    <!-- SCOPE -->
    <Scope id="bodyScope">

I take only Body Extractor, you can add any other.
Now, we need to define extractors for GWT application, create Extractors.xml in the same folder, where placed your main class with following lines:

<?xml version="1.0" encoding="UTF-8"?>
<Require feature="google.contentmatch">
   <Param name="extractors">

To add multiply extractors use:

<Param name="extractors">,,

GWT application code

package com.mozgoweb.demo.client;


 * Main entry point.
 * @author Denis Lunev <>
 * Twitter functionality based on "Cross-Site Scripting with GWT and Twitter"
 * by Alex Moffat
@ModulePrefs(title = "DemoGadget", author = "Denis Lunev", author_email = "")
@GadgetFeature.FeatureName(value = "google.contentmatch")
@InjectModulePrefs(files = {"Extractors.xml"})

public class CtxGadgetEntryPoint extends Gadget<UserPreferences> {
    public CtxGadgetEntryPoint() {}

     For gadget use init() instead of onModuleLoad 
    protected void init(UserPreferences preferences) {
        final JsArray<JavaScriptObject> matches = jsGetContentMatches();
        if (matches != null) {
            final int numMatches = matches.length();
            result = new JSONObject[numMatches];
            for (int i = 0; i < numMatches; i++) {
                //Do something with email body
	Native JS method to invoke extractors
    native JsArray<JavaScriptObject> jsGetContentMatches() /*-{
        return google.contentmatch.getContentMatches();

Then build the project and open xml file with main class, in my case it’s build/web/com.mozgoweb.demo.gadget/CtxGadgetEntryPoint.gadget.xml
Make sure, that your extractors are presented and add view=”card” to Content tag:

...<Content type="html" view="card">...

Card view is required for Contextual gadgets.
Now you need to upload builded gadgets (all files from folder, where located CtxGadgetEntryPoint.gadget.xml) to FTP or any other web storage, which we specified in Manifest.

Apps Marketplace

Login to Marketplace:
“My Vendor Profile” -> “Create a new listing”;
Check “My product may be directly installed into Google Apps domains”;
Fill “Application Manifest” with content from our Manifest.xml.

So, you’ve created application, now we need to connect it to domain.
Hit “Add it now” button and write your domain, then login with your domain admin credentials and activate application.
Thats all! Open any email and see how it works. Remember, that gadgets have very solid cache and it can take some time for your changes to be accepted, even with ?nogadgetcache=1 parameter.

My sample application searches words, starting with “@” in email body and shows twitter info for these users.

I used Twitter + GWT example by Alex Moffat for this Demo Gadget.
You can find source code here.

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