GWT deferred binding

GWT offers javascript files 开发者_StackOverflowmainly for Fifefox, chrome....., but what happens if another browser is being used such as K Melon or another browser which GWT does not specifically generates the javascript for that browser!!

I have tested for K Melon, it works correctly but i just want to know, how GWT knows which Javascript files to load

This is probably way more information than you wanted.

As far as mechanics go, GWT grabs the user agent from the browser, then uses a mechanism called a "property provider" to classify the browser. So, a property provider sets a property named "user.agent" to one of several predefined classes of browsers.

Here is an XML file which defines constants for all classes of browsers GWT currently knows about:

http://code.google.com/p/google-web-toolkit/source/browse/trunk/user/src/com/google/gwt/user/UserAgent.gwt.xml?

At compile time, your app is compiled separately for each class of browser you want to support, and a different batch of JS gets loaded for each type of browser. The beauty of this is that the end user downloads the minimum amount of code needed to run the app, and won't be bloated by having conditional code for IE when you are running Chrome or Firefox.

Its a bit complex, but you can peek at the code that writes out some JavaScript to examine the User Agent string here to determine which class of browser is currently running:

http://code.google.com/p/google-web-toolkit/source/browse/trunk/user/src/com/google/gwt/user/rebind/UserAgentPropertyGenerator.java

Once it picks a value for the user.agent property, the various parts of GWT conditionally substitute in code where browser specific logic is needed. Most browsers return a long list of "compatible" user agents. GWT tries to pick the best match from those. You can see that there is a default case - if the user agent string isn't recognized at all it sets the user.agent value to 'unknown' and fires up a warning to alert that the developer needs to manually pick one.

To dive a little deeper, here's the mechanism the compiler uses to pull in browser specific code based on the user.agent property:

(from Window.gwt.xml)

<module>
  <inherits name="com.google.gwt.core.Core"/>
  <inherits name="com.google.gwt.user.UserAgent"/>

   <replace-with class="com.google.gwt.user.client.impl.WindowImplIE">
      <when-type-is class="com.google.gwt.user.client.impl.WindowImpl"/>
      <any>
        <when-property-is name="user.agent" value="ie6"/>
        <when-property-is name="user.agent" value="ie8"/>
        <when-property-is name="user.agent" value="ie9"/>
      </any>
   </replace-with>
</module>

You'll notice that if 'webkit' is seen in the user agent, GWT returns the string 'safari' to identify the class of browser. There is no separate class of browser for Chrome. Chrome and Safari are very similar, and pretty closely written to specs, so there's not a lot of special casing for them. But that's not to say that there isn't Chrome specific code in some places. Sometimes a difference in browser will be detected at runtime. Its only done this way when there are just small browser differences. Here's an example of the way a minor difference between Chrome and Safari is handled:

public class HyperlinkImplSafari extends HyperlinkImpl {

  private static boolean shiftIsModifier = onChrome();

  private static native boolean onChrome() /*-{
    return navigator.userAgent.indexOf("Chrome") != -1;
  }-*/;

  @Override
  public boolean handleAsClick(Event event) {
    //...
    boolean modifiers = alt || ctrl || meta;
    if (shiftIsModifier) {
      modifiers |= shift;
    }

    return !modifiers && !middle && !right;
  }
}

Check this post out - Google GWT cross-browser support: is it BS?

Even with the big names in browser industry, my experience with GWT apps hasn't been 100% compatibility, but really comes close.

For other browsers, it will look for the engine (most of them are offshoots of webkit or variants of mozilla's offering), if that is recognized, GWT generates the code for it more or less.

Anyways, the user base for such browsers itself is too tiny. In practice, it mostly suffices to test compatibility between IE6 (sigh), IE7, IE8 (yes they behave differently!), Firefox, and nowadays, chrome, safari and IE9.