GWT study

    In the module XML file, you specify your application's entry point class. In order to compile, a GWT module must specify an entry point. If a GWT module has no entry point, then it can only be inherited by other modules. It is possible to include other modules that have entry points specified in their module XML files. If so, then your module would have multiple entry points. Each entry point is executed in sequence.

By default, StockWatcher uses two style sheets: the default GWT style sheet, standard.css (which is referenced via the inherited theme), and the application style sheet, StockWatcher.css which was generated by webAppCreator.

The host page references the path of JavaScript source code (generated by GWT) responsible for the dynamic elements on the page. The contents of the entire body element can be generated dynamically, for example, as it is with starter application.

The StockWatcher class implements the GWT interface EntryPoint. It contains the method onModuleLoad. Because the StockWatcher class is specified as the entry point class in StockWatcher's module definition, when you launch StockWatcher the onModuleLoad method is called.

The StockWatcher class inherits functionality via other GWT modules you included in StockWatcher's module definition (StockWatcher.gwt.xml). For example, when building the user interface, you'll be able to include types and resources from the package com.google.gwt.user.client.ui because it is part of the GWT core functionality included in the GWT module com.google.gwt.user.User.

A Root panel is the container for the dynamic elements of your application. It is at the top of any GWT user interface hierarchy. There are two ways you can use a Root panel, either to generate the entire body of the page or to generate specific elements embedded in the body.
The Root panel works by wrapping an element in the HTML host page. By default (that is, if you don't add any placeholders in the host page) the Root panel wraps the body element. However, you can wrap any element if you name it and then, when you call the Root panel, pass the name as a parameter.
RootPanel.get()             // Default. Wraps the HTML body element.
RootPanel.get("stockList")  // Wraps any HTML element with an id of "stockList"

For StockWatcher, you'll follow the preferred strategy. Rather than put links to the style sheets in the HTML host page, you'll use the module XML file. Then, when you compile StockWatcher, the GWT compiler will bundle all the static resources required to run your application including the style sheets. This mechanism is called Automatic Resource Inclusion.

In GWT, each class of widget has an associated style name (like gwt-Button) that binds it to a CSS style rule. This is the widget's primary style. Default values are defined for the primary style in the theme style sheet.
Type of Element            HTML Tag         CSS Selector
Buttons in static HTML and GWT-generated buttons    <button>         button
Only GWT-generated buttons          <button class="gwt-Button">    button.gwt-Button
Only my special GWT-generated button       <button class="gwt-Button my-button">  button.my-button

Dependent styles are powerful because they are automatically updated whenever the primary style name changes. In contrast, secondary style names that are not dependent style names are not automatically updated when the primary style name changes.
To do this, you'll use the addStyleDependentName method instead of the addStyleName method

If you launched the development mode server, you can run your application in production mode (after compiling it) by removing the gwt.codesvr parameter from the URL before loading the application.

The GWT RPC framework makes it easy for the client and server components of your web application to exchange Java objects over HTTP. The server-side code that gets invoked from the client is often referred to as a service. The implementation of a GWT RPC service is based on the well-known Java servlet architecture. Within the client code, you'll use an automatically-generated proxy class to make calls to the service. GWT will handle serialization of the Java objects passing back and forth—the arguments in the method calls and the return value.

In order to define your RPC interface, you need to write three components:

Define an interface (StockPriceService) for your service that extends RemoteService and lists all your RPC methods.
Create a class (StockPriceServiceImpl) that extends RemoteServiceServlet and implements the interface you created above.
Define an asynchronous interface (StockPriceServiceAsync) to your service to be called from the client-side code.

Every service implementation is ultimately a servlet, but rather than extending HttpServlet, it extends RemoteServiceServlet instead. RemoteServiceServlet automatically handles serialization of the data being passed between the client and the server and invoking the intended method in your service implementation.

Notice the @RemoteServiceRelativePath annotation. This associates the service with a default path relative to the module base URL.
In the <servlet-mapping> element, the url-pattern can be in the form of an absolute directory path (for example, /spellcheck or /common/login). If you specify a default service path with a @RemoteServiceRelativePath annotation on the service interface (as you did with StockPriceService), then make sure the url-pattern matches the annotation value.

To add an AsyncCallback parameter to all of our service methods, you must define a new interface as follows:

It must have the same name as the service interface, appended with Async (for example, StockPriceServiceAsync).
It must be located in the same package as the service interface.
Each method must have the same name and signature as in the service interface with an important difference: the method has no return type and the last parameter is an AsyncCallback object.

Often, you will need to integrate GWT with existing handwritten JavaScript or with a third-party JavaScript library. Occasionally you may need to access low-level browser functionality not exposed by the GWT class API's. The JavaScript Native Interface (JSNI) feature of GWT can solve both of these problems by allowing you to integrate JavaScript directly into your application's Java source code.

JSNI methods are declared native and contain JavaScript code in a specially formatted comment block between the end of the parameter list and the trailing semicolon. A JSNI comment block begins with the exact token /*-{ and ends with the exact token }-*/. JSNI methods are called just like any normal Java method. They can be static or instance methods.

generated GWT files. At compile time, the GWT compiler performs some syntax checks on the JavaScript inside the method, then generates interface code for converting method arguments and return values properly.

As of the GWT 1.5 release, the Java varargs construct is supported. The GWT compiler will translate varargs calls between 2 pieces of Java code. However, calling a varargs JavaScript method from Java will result in the callee receiving the arguments in an array.

When accessing the browser's window and document objects from JSNI, you must reference them as $wnd and $doc, respectively. Your compiled script runs in a nested frame, and $wnd and $doc are automatically initialized to correctly refer to the host page's window and document.

Calling Java methods from JavaScript is somewhat similar to calling Java methods from C code in JNI. In particular, JSNI borrows the JNI mangled method signature approach to distinguish among overloaded methods. JavaScript calls into Java methods are of the following form:
[instance-expr.]@class-name::method-name(param-signature)(arguments)
instance-expr. : must be present when calling an instance method and must be absent when calling a static method
class-name : is the fully-qualified name of the class in which the method is declared (or a subclass thereof)
param-signature : is the internal Java method signature as specified here but without the trailing signature of the method return type since it is not needed to choose the overload
arguments : is the actual argument list to pass to the called method

Calling Java constructors from JavaScript is identical to the above use case, except that the method name is alway new.

someTopLevelInstance.new InstanceInner(123) becomes @pkg.TopLevel.InstanceInner::new(Lpkg/TopLevel;I)(someTopLevelInstance, 123)
The enclosing instance of a non-static class is implicitly defined as the first parameter for constructors of a non-static class. Regardless of how deeply-nested a non-static class is, it only needs a reference to an instance of its immediately-enclosing type.

Static and instance fields can be accessed from handwritten JavaScript. Field references are of the form
[instance-expr.]@class-name::field-name

public class JSNIExample {
 
  String myInstanceField;
  static int myStaticField;
 
  void instanceFoo(String s) {
    // use s
  }
 
  static void staticFoo(String s) {
    // use s
  }
 
  public native void bar(JSNIExample x, String s) /*-{
    // Call instance method instanceFoo() on this
    [e-mail]::instanceFoo(Ljava/lang/String;)(s );
 
    // Call instance method instanceFoo() on x
    [e-mail]::instanceFoo(Ljava/lang/String;)(s );
 
    // Call static method staticFoo()
    @com.google.gwt.examples.JSNIExample::staticFoo(Ljava/lang/String;)(s);
 
    // Read instance field on this
    var val = [e-mail]::myInstanceField ;
 
    // Write instance field on x
    [e-mail]::myInstanceField = val + " and stuff";
 
    // Read static field (no qualifier)
    @com.google.gwt.examples.JSNIExample::myStaticField = val + " and stuff";
  }-*/;
}

Sometimes you need to access a method or constructor defined in GWT from outside JavaScript code. This code might be hand-written and included in another java script file, or it could be a part of a third party library. In this case, the GWT compiler will not get a chance to build an interface between your JavaScript code and the GWT generated JavaScript directly.
A way to make this kind of relationship work is to assign the method via JSNI to an external, globally visible JavaScript name that can be referenced by your hand-crafted JavaScript code.

package mypackage;
 
public MyUtilityClass
{
    public static int computeLoanInterest(int amt, float interestRate,
                                          int term) { ... }
    public static native void exportStaticMethod() /*-{
       $wnd.computeLoanInterest =
          $entry(@mypackage.MyUtilityClass::computeLoanInterest(IFI));
    }-*/;
}
Notice that the reference to the exported method has been wrapped in a call to the $entry function. This implicitly-defined function ensures that the Java-derived method is executed with the uncaught exception handler installed and pumps a number of other utility services. The $entry function is reentrant-safe and should be used anywhere that GWT-derived JavaScript may be called into from a non-GWT context.
On application initialization, call MyUtilityClass.exportStaticMethod() (e.g. from your GWT Entry Point). This will assign the function to a variable in the window object called computeLoanInterest.

Static String Internationalization
Static string initialization requires very little overhead at runtime and therefore is a very efficient technique for translating both constant and parameterized strings. It is also the simplest technique to implement. Static string internationalization uses standard Java properties files to store translated strings and parameterized messages, then implements strongly-typed Java interfaces to retrieve their values.

Dynamic String Internationalization
Dynamic string internationalization is slower than static string internationalization, but is very flexible. Applications using this technique look up localized strings in the module's host page; therefore, they do not need to be recompiled when you add a new locale. If you need to integrate a GWT application with an existing server-side localization system, dynamic string internationalization is the option to consider.

Localizable Interface
The most powerful technique is to implement the Localizable interface. Implementing Localizable allows you to go beyond simple string substitution and create localized versions of custom types. It's an advanced internationalization technique that you probably won't have to use very often.

First create the Java interface (StockWatcherConstants) that accesses the properties files which hold each translation. The interface uses annotations to specify the default translation. This interface implements the GWT Constants interface. This interface is bound automatically to any StockWatcherConstants*.properties files you create because of its name.

First create the Java interface (StockWatcherMessages). It accesses the properties files which hold the translations of each parameterized message. This interface implements the GWT Messages interface. Unlike the StockWatcherConstants interface, the methods in this interface contain parameters. When you call these methods, the arguments you pass will replace the placeholders you left in the strings in the properties files. This interface is bound automatically to any StockWatcherMessages*.properties files you create because of its name.