Web 2.0 Development with the Google Web Toolkit
to the example.gwt.xml file. We'll rewrite the main code and add a couple packages to do calls to servers that provide JSON output (see The Same Origin Policy sidebar). For this, add two classes to the client: JSONRequest and JSONRequestHandler; their code is shown in Listings 2 and 3.
The Same Origin Policy
The Same Origin Policy (SOP) is a security restriction, which basically prevents a page loaded from a certain origin to access a page from a different origin. By origin, we mean the trio: protocol + host + port. In http://www.mysite.com:80/some/path/to/a/page, the protocol is http, the host is www.myhost.com, and the port is 80. The SOP would allow access to any document coming from http://www.mysite.com:80, but disallow going to https://www.mysite.com:80/something (different protocol), http://dev.mysite.com:80/something (different host) or http://www.mysite.com:81/something (different port).
Of course, for GWT, this is a bit of a bother, because it means that a client application cannot simply connect to any other server or Web service to get data from it. There are (at least) two ways around this: a special, simpler way that allows getting JSON data only or a more complex solution that implies coding a server-side proxy. Your client calls the proxy, and the proxy calls the service. Both solutions are explained in the Google Web Toolkit Applications book (see Resources). In this article, we use the JSON method, and you can find the source code at www.gwtsite.com/code/webservices.
The simple JSON method requires a special callback routine, and this could be a showstopper. However, many sites implement this, including Amazon, Digg, Flickr, GeoNames, Google, Yahoo! and YouTube, and the method is catching on, so it's quite likely you will be able to find an appropriate service.
Listing 2. Source Code for the JSONRequest Class
Listing 3. Source Code for the JSONRequestHandler Class
You can find the code for this listing and the previous one at www.gwtsite.com/code/webservices.
Let's opt to create the screen completely with GWT code. The button will send a request to a server (in this case, Yahoo! News) that provides an API with JSON results. When the answer comes in, we will display the received code in a text area. The complete code is shown in Listing 4, and Figure 3 shows the running program.
Practical books for the most technical people on the planet. Newly available books include:
- Agile Product Development by Ted Schmidt
- Improve Business Processes with an Enterprise Job Scheduler by Mike Diehl
- Finding Your Way: Mapping Your Network to Improve Manageability by Bill Childers
- DIY Commerce Site by Reven Lerner
Plus many more.
- Building a Multisourced Infrastructure Using OpenVPN
- Happy GPL Birthday VLC!
- Unikernels, Docker, and Why You Should Care
- diff -u: What's New in Kernel Development
- What's New in 3D Printing, Part III: the Software
- Giving Silos Their Due
- Controversy at the Linux Foundation
- Don't Burn Your Android Yet
- Non-Linux FOSS: Snk
- Firefox OS