Twitter supports oauth and there are many libraries for it, for Java(twitter4J)(oauth-signpost), most of the time you see examples of a desktop version, so it can take time to implement it on a web application (it did take time for my case).
Step 1. Get a Twitter account and create a Twitter application from http://twitter.com/apps (don’t forget to check in browser and enter a valid callback url. Callback URL can not be localhost 
Step 2. Download twitter4J add the library (in my case twitter4j-core-2.1.1-SNAPSHOT.jar) to project and update classpath
Step 3. Create a web application (I assume you have the Google plugin for Eclipse )
Step 4. Add the following servlets and update your web.xml. First servlet will handle login part. It will create a RequestToken which will create the magical aggrements between the consumer(your application) and provider(twitter). First servlet save the token and secret token since we will need them later.
String token = requestToken.getToken(); String tokenSecret = requestToken.getTokenSecret();
Step 5. You forget to update web.xml didn’t you, update the web.xml
Step 6. Enable session by adding
<sessions-enabled>true</sessions-enabled>
to appengine-web.xml.
You can see my version from http://90.latest.denemekaya.appspot.com/login
ps. don’t forget to set the callback in Twittter to the url of the servlet for callback.
Links:
- http://www.pakzilla.com/2009/10/03/tutorial-java-based-twitter-app-on-google-app-engine/
- Explain the same procedure with Grails — http://blogs.bytecode.com.au/glen/2009/12/08/log-into-your-grails-app-using-your-twitter-credentials.html
