Google Web Toolkit
by Bruce Perry
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Pages: 1, 2, 3, 4, 5
Entry Point
The MyForm.java class implements the GWT API interface EntryPoint. As a result, the class must implement the onModuleLoad() method, which the browser's JavaScript engine calls when the browser loads your Ajax application.
In other words, the GWT compiler compiles this class into JavaScript code. The MyForm.java class sets up the form widgets for the browser view. The class also determines the response to the users clicking the OK, Submit button. The comments in the code describe exactly what's going on, so I won't repeat these comments in the article's text.
package com.parkerriver.gwt.testapp.client;
import com.google.gwt.core.client.EntryPoint;
import com.google.gwt.core.client.GWT;
import com.google.gwt.user.client.DOM;
import com.google.gwt.user.client.Window;
import com.google.gwt.user.client.Element;
import com.google.gwt.user.client.rpc.ServiceDefTarget;
import com.google.gwt.user.client.rpc.AsyncCallback;
import com.google.gwt.user.client.ui.*;
import java.util.Iterator;
public class MyForm implements EntryPoint {
//The id pf the HTMl div element that provides
//status info
private String statusId = "status";
//A Grid object; actually, an HTML table
private Grid grid = new Grid(5, 2);
//Other user interface objects
private Label nmLab = new Label();
private Label ageLab = new Label();
private Label homeLab = new Label();
private TextBox nmTxt = new TextBox();
private TextBox ageTxt = new TextBox();
private TextBox homeTxt = new TextBox();
private Button okBut = new Button();
private TextArea tarea = new TextArea();
/* This method is called when the browser loads
the application. The method sets up 3 Labels and TextBoxes; as
well as a Button and the TextArea that will display the server's
response. */
public void onModuleLoad() {
//set up labels and text fields
nmLab.setText("Full Name:");
nmTxt.setMaxLength(25);
ageLab.setText("Age:");
ageTxt.setVisibleLength(3);
ageTxt.setMaxLength(3);
homeLab.setText("Home country:");
homeTxt.setMaxLength(25);
//Place these widgets within the Grid
grid.setWidget(0,0,nmLab);
grid.setWidget(0,1,nmTxt);
grid.setWidget(1,0,ageLab);
grid.setWidget(1,1,ageTxt);
grid.setWidget(2,0,homeLab);
grid.setWidget(2,1,homeTxt);
//set up button and textarea
tarea.setCharacterWidth(40);
tarea.setVisibleLines(25);
okBut.setText("OK, Submit");
//Give the button its behavior by adding a listener object;
//literally, a ClickListener object that has an
//onClick event handler.
okBut.addClickListener(new ClickListener() {
public void onClick(Widget sender) {
//Notify the user of the remote
//procedure call status; see method below
showRpcStatus(true);
//Create an instance of the client-side stub for our
//server-side service.
ShowRespServiceAsync respService =
(ShowRespServiceAsync) GWT
.create(ShowRespService.class);
ServiceDefTarget endpoint = (ServiceDefTarget) respService;
//The implementation of our service is an instance
//of RemoteServiceServlet, so provide the server path
//to the servlet; this path appears in web.xml
endpoint.setServiceEntryPoint("/parkerriver/s/showresp");
//This interface handles the response from the server.
//It will display the server's response in a TextArea.
//The display will be in a red color if the message
//represents an error.
AsyncCallback callback = new AsyncCallback() {
public void onSuccess(Object result) {
//If there is one, remove the 'warning' CSS style
//relating to any error text's appearance
if(tarea.getStyleName().
equalsIgnoreCase("warning")){
tarea.removeStyleName("warning");
}
//Textarea displays the server's return value
tarea.setText((String)result);
}
public void onFailure(Throwable caught) {
//Textarea displays any exception messages
tarea.setStyleName("warning");
tarea.setText(
"Server request raised an error; Java exception : "+
caught == null ? "An unknown exception" :
caught.getMessage());
}
};
//Call the service method, validating the form
//values first.
try{
respService.displayResponse(
getPanelTextContent(grid,true),
callback);
} catch (Exception e) {
tarea.setStyleName("warning");
tarea.setText("Server request raised an error: "+
e.getMessage());
} finally {
//Remove the status message when we are finished
//making the RPC call
showRpcStatus(false);
}
}
});
//Now add these widgets to the grid
grid.setWidget(3,0,okBut);
grid.setWidget(3,1,tarea);
//set the vertical alignment for the OK button's cell
grid.getCellFormatter().setVerticalAlignment(3,0,
HasVerticalAlignment.ALIGN_TOP);
//set the vertical alignment for the TextBoxes,
//in order to line them up properly
grid.getCellFormatter().setVerticalAlignment(0,1,
HasVerticalAlignment.ALIGN_BOTTOM);
grid.getCellFormatter().setVerticalAlignment(1,1,
HasVerticalAlignment.ALIGN_BOTTOM);
grid.getCellFormatter().setVerticalAlignment(2,1,
HasVerticalAlignment.ALIGN_BOTTOM);
//Add the grid, actually an HTML table, to a div element
//in the browser's HTML with id value "gridholder"
RootPanel.get("gridholder").add(grid);
}
/*Initiate a simple check for blank fields, then return the
submitted values as a single string.
HasWidgets is an interface that Grid and other panel type
objects implement. Therefore, we can pass the grid into
this method; iterate over its contained TextBoxes, and
validate the TextBox's content.
*/
private String getPanelTextContent(HasWidgets panelType,
boolean validateContent) {
StringBuffer buf = new StringBuffer("");
String tmp = null;
if(panelType != null) {
//for the sake of brevity, clipped...
}
//return the TextBoxes content each
//separated by a space
return buf.toString();
}
/* Overly simplified validation! */
private boolean validateText(String _content){
return _content.length() > 0;
}
private int getTextboxCount(HasWidgets pType){
//Not shown: returns the number of TextBox widgets
//in the panel
}
/* Show the user a status message if the
response takes long to arrive.*/
private void showRpcStatus(boolean _on){
//Use the GWT DOM API for JavaScript DOM
//programming
Element el = DOM.getElementById(statusId);
if(el != null) {
if(_on) {
DOM.setStyleAttribute(el,"font-size","1.2em");
DOM.setStyleAttribute(el,"color","green");
DOM.setInnerHTML(el, "Fetching server info...");
} else{
DOM.setInnerHTML(el, "");
}
}
}
}Most of this code deals with the GWT API. It is interesting to note that if you have to implement JavaScript DOM programming, as in the showRpcStatus() method, you can accomplish this task in Java by using the com.google.gwt.user.client.DOM class.
