I'm trying to call a servlet plugin(hosted in SSO) using a java client , htt

If you're using Apache HttpClient or similar Java client, you can first retrieve that login page (which you already done), and then do a POST with your login credentials, as if you're submitting the login form using browser. After that, your Java HTTP client should receive cookies representing authenticated session with the SSO system. Then reuse that HTTP client to invoke the target Servlet.

I don't want manual intervention of posting the form with user credentials . Is there any way in Apache Http Client (version 4.3 am using) to call a custom servlet with user credentials to authenticate the session automatically by setting the user/pwd

No manual intervention, I meant you can gather information by looking at the HTML/JavaScript code in the SSO login page, and see what needs to be included in the POST request (HTML form) to the target URL when a user is logging in. You can use HTTP debugging tools available in Chrome or Firefox for this.

Then you can use HttpClient in your code to send that same POST request (with credentials you received from process) to the same target URL, so that the HttpClient will receive the cookies. After that, when you use the same HttpClient to invoke the target Servlet, it will get through within getting the login screen again.