HTTP Status 500 - java.lang.AbstractMethodError: in tomcat 7

I was once asked to upgrade all my Apache tomcat from 6 to 7. While upgrading the tomcat, one of my application started throwing HTTP Status 500 - java.lang.AbstractMethodError: in tomcat 7 while worked perfectly fine with tomcat 6. After a lot of googling, I finally found that the issue is in tomcat 7, the servlet Api has been upgraded to 3.0, while the application was compiled with a lower version of servlet api. Therefore, it was throwing this error. In order to get rid of this error, you need to re-compile the application with the higher version of servlet api. Hope this post helps.