I am calling webhook when certain alert triggers in splunk, I want to exact payload by looking at the alert result so that I can better program the JAVA rest API.
Using the built-in webhook action, no. This is explicitly stated in the dev guide: http://dev.splunk.com/view/dev-guide/SP-CAAAE7A
At this time, the JSON payload format can't be changed.
That said, the custom alert action framework / Modular alert framework, means that anyone can create their own custom alert action, and do whatever you want with results, including invoke a webhook with a customized payload. Splunkbase has a number of custom alert actions, some of which invoke webhooks for different platforms and if none of these are suitable, you can always develop your own following the guide and resources in the docs: http://docs.splunk.com/Documentation/Splunk/7.0.2/AdvancedDev/ModAlertsIntro
The Splunk Add-on Builder may also help in provide some utilities for building your custom alert action.