how do i pass data from action phase ( processAction ( ) ) to render phase ( render ( ) ) ?

javax.portlet.actionScopedRequestAttributes which is set to false by default, but if you set its value to true then you can set request attribute in the processAction (  ) method and you should be able to access those request attributes in the render (  ) method.

One drawback of this approach is the request attributes are stored in PortletSession under the hood and it has performance implications of storing data in portlet session,

but positive side this approach is better than storing and removing objects in session by yourself.
Source code