Assuming that you are deploying two separate EAR files onto the same application server instance...
1. How is communication b/w two J2EE applications achievedwhich are running in the same app server?
The same as if they were in separate app servers (see below)
2.Do diff .ear files in the same app server(weblogic)run in the same JVM?-ram
Yes. All EAR files deployed to the same application server instance run in the same JVM. However each is given its own classloader, which is what makes them separate applications. Therefore an EJB in EAR_1 would need to use remote interfaces to access an EJB in EAR_2
for more details see:http://e-docs.bea.com/wls/docs81/programming/classloading.html