Enable two-phase rendering for a portlet
By default, two-phase rendering is turned off. To enable two-phase rendering for a portlet, update the portlet.xml deployment descriptor for the portlet.
Edit the portlet.xml file for the portlet.
Add the following entry to the file:
<portlet> ... <container-runtime-option> <name>javax.portlet.renderHeaders</name> <value>true</value> </container-runtime-option> </portlet>
Parent
Use two-phase rendering with JSR 286 portlets