Examining The Sample App

When we created our project in Eclipse, we got a sample GWT/App Engine application for free. It doesn't do much, but it's a good way of seeing the sorts of things we'll have to deal with in our own application.

App Engine works by means of Java Servlets. These are Java classes that are associated on the server with a specific URL. When a web browser asks the server for that URL, the Java class is run and it returns data back to the browser. That data may be a complete web page, or it may be smaller amounts of data.

GWT will call servlets in the server by asking for specific URLs. GWT can then parse the data sent back to figure out what to change on the current page.

In the Eclipse project, look under war/WEB-INF and open web.xml. Expand web-app and then servlet, and you'll see that the servlet named greetServlet maps to the Java class omega.server.GreetingServiceImpl. Expand servlet-mapping and you'll see that the servlet named greetServlet also maps to the URL /omega/greet (yours will have project names different from omega).

For servlet based projects, the web.xml file manages the mapping between URLs and Java classes.

You should also see an HTML file at the same level as WEB-INF. That's the HTML file that provides the container for the GWT app. If you look at it, you'll see that there are a couple of empty divs for the name field and the send button. The GWT code in Omega.java fills those in wtih an edit field and a button, and calls the server when the button is pressed.

Now look at Omega.java (again, the name of yours will depend on your project name). In the onModuleLoad method, you'll see what looks like Java Swing initialization. Note though the RootPanel.get lines. Those are actually adding user interface elements to the HTML page itself, in the divs with those specific ids.

Scroll farther down to the MyHandler nested class. This handles the click on the button, or the pressing of the Enter key. Both call sendNameToServer, which is just below them in the file. The line that starts greetingService.greet makes a call to the server. The onFailure method is run if that call fails for some reason. The onSuccess method is run if the call succeeds. On success a dialog box is shown that contains the HTML sent back by the server.

Now look at GreetingServiceImpl.java in the server folder. This is the class that gets called by the client. You can see it just returns some HTML with info about what browser they're running.

That's a quick tour through the sample app. Studying this will give you a better handle on what we'll be doing as we add our own functionality.