Customizing Liferay Classes using ext plugin in 6.1

Liferay is highly flexible that developers can override any Liferay class by just replacing them in Ext Plugin. But overriding is not a very good action considering "Open for extension closed for Modification" Principle. 
so in this blog post I am going to show simple steps how to extend a Liferay class. 

Note: use ext plugin  class extension or replacement as a last resort if and only if the same cannot be achieved through hooks or other ways.

Step 1: Create you class by extending the Liferay class.
Step 2: Create a file called ext-spring in folder docroot/WEB-INF/ext-impl/src/META-INF
Step 3: From Liferay source code get the file portal-impl/src/META-INF/portal-spring.xml
Step 4: just copy the bean class you are extending from portal-spring.xml and paste to your ext-spring.xml, change the class name to your new extended class. (please ensure you have the <beans> outer tag too in ext-spring as in portal-spring)
step 5: deploy your ext, now Liferay will call your class instead of its own implementation.