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Follow UsBANGALORE, INDIA: A few months back speculations were high amongst the developer community about the fate of Java after the suspected takeover of Sun Microsystems by IBM, that was not to be, and instead it was Oracle that was successful at it. Amidst speculations, came the announcement that Google has made Java available as the programming language for the Google App Engine.
Since the launch of this App Engine, Google has only provided support to Python as a programming language, but Java had been the most requested feature for App Engine by the developer community. By moving beyond Python and embracing Java, Google has opened up avenues to develop Java based web applications that would run on Google's Cloud offering.
Providing Java support to the Google App Engine also paves the way for other programming languages that run on Java virtual machines, for instance languages like JavaScript, Groovy or JRuby. In this article, we look at the App Engine Java overview and how you can develop and deploy an application on the Cloud.
The Google App Engine Java environment provides a Java 6 JVM, a Java Servlets interface, and also supports standard interfaces for data store and services like Java Data Objects, Java Persistence API, JavaMail, Jcache, etc. The App Engine uses the Java Servlet standard for web applications, whereby the user or developer provides the application's servlet classes, JSP pages and other files along with the deployment descriptor (web.xml) in a standard WAR directory structure.
Getting started
Since the applications on App Engine would run on Java 6 virtual machine, it is ideally required to use Java 6 for developing and testing the application. Though the App Engine SDK is compatible with Java 5 for development purpose, for developing apps for App Engine, you should have JDK 5 or JDK 6 installed on your system. And you can use Eclipse as a development environment. App Engine plugin is available for Eclipse versions 3.3 and 3.4, and through this plugin you can install the App Engine SDK from Eclipse using Software Updates.
To do so, go to Help > Software updates and select the Available Software tab and there click on Add Site button and provide this as URL http://dl.google.com/ eclipse/plugin/3.4 and click OK. The available software list will get refreshed and a new entity appears named Google Update Site. Select this and its two options of SDKs and Plugins to install the Google App Engine SDK and integrate it with Eclipse. Now you can start developing apps for the Google's Cloud.
| While creating a new web application for App Engine, ensure that the highlighted option is unchecked and the 'Use Google App Engine' remains checked. |
Demo application
We create a small demo app that will display a message and show the system time (Google's cloud time). For that go to File > New and then select Web Application Project. In the project wizard window, give project name as PcqDemoApp and package as com.pcq. Under Google SDKs option, make sure you uncheck the ?Use Google Web Toolkit? option and ensure that the ?Use Google App Engine? option is checked. Once you click Finish, the whole project complying to Java Servlet standard gets created with descriptor files along with a hellopcqServlet.java, which acts as a starting point for the project. Open this servlet file and modify the code as shown in the snippet below:
public class PcqDemoAppServlet extends HttpServlet {
public void doGet(HttpServletRequest req, HttpServletResponse resp) throws IOException {
Calendar time= Calendar.getInstance();
resp.setContentType("text/plain");
resp.getWriter().println("This is PCQ Demo");
resp.getWriter().println("The Time is : "+
(time.get(Calendar.MONTH) + 1)
+ "-" + time.get(Calendar.DATE)
+ "-" + time.get(Calendar.YEAR)
+ " " + time.get(Calendar.HOUR_OF_DAY)
+ ":" + time.get(Calendar.MINUTE)
+ ":" + time.get(Calendar.SECOND));
}
}
As we have used a Calendar class, add the necessary java.util import for this class to the code.
Running the application
To test and run the project, go to Run > Run Configuration and then from the Run configuration wizard select the Web Application option from the list and select the project and give a name to it, click on Run. Most of the times, development machines have some web server installed and running on the default port 8080. The Run Configuration wizard gives you an option through which you can run the application using any free port. After successfully running the application, the WAR deployables get created and now we can deploy the application on the cloud.
| To deploy the app, pass the Application ID which you had created while registering on App Engine site and select the Java project that you have developed | The embedded web-server uses port 8080 which is generally not free. There is an option to automatically select a free port to run the application |
To deploy the application, you first need to register with the App Engine site, appengine.google.com, and when you have registered, create an application and you need to provide the same application name as ID to your Java web application. Once you are through with this step, from Eclipse, right-click on the project name and then select Google > Deploy to App Engine.
On the wizard, click on App Engine Project Settings and in the window give the Application ID which you had created on App Engine website, in our case ?hellopcq?. Click OK and then on the deployment wizard window give your login credentials and the project you want to deploy. Once the deployment has been completed successfully, you can access the deployed application on the web by going to URL, http://hello pcq.appspot.com. This way Java developers can now create applications for the cloud computing platform using Google's App Engine.