I have chosen Ext-JS to develop the UI for my next application. I am wondering what is the best way for Ext-JS to interact with my server.
Should I use plain JSPs which return a JSON response? Should I use DWR or Jabsorb which provides direct remoting with backend java code. Any other solution?
I am more concerned about performance and nothing else.
You can certainly use plain JSPs or servlets, manually handling your requests and forming JSON responses. You'll make life easier on yourself using DWR or something like it to handle the plumbing for you, and the performance shouldn't be noticeably different.
Current versions of Ext JS use the Ext.Direct stack for integrating supported back ends into the UI data layer, making it much simpler to bind components like grids and forms to your back end services. Here is a grid sample that demonstrates such remoting. Here is the Ext.Direct forums -- you should have a look at what others are doing. Look at this post to see what Ext.Direct providers are currently available for Java.
Again, you can certainly make your calls manually from Ext using Ext.Ajax directly (or using the standard Store methods for making calls), but the Ext.Direct stack can make it much easier to integrate everything together.