Hi Sharath,
Please do couple of quick checks:
1. While adding external library DC as used component in J2EE library DC, did you choose both build time and runtime dependencies? If yes, build the DC and verify that the generated SDA file contains the jar. Check that the jar file exists in gen/default/deploy folder.
2. If check 1 is successful, deploy the J2EE library DC before deploying the Webdynpro DC.
Regards,
Kunal