Hello, I am a long time java web developer but I am new to portlets technology.
Currently I am in the stage of finding my options.
I have tried Liferay and although it is very cool, it is a bit heavy for my needs.
Can you recommend a more lightweight portlet container?
You could have a look at apaches' Jetspeed portal. Although I don't really think it classifies as 'lightweight'.
I think that the JSR-168/286 reference implementation Apache Pluto is probably going to be as lightweight as you are going to get.
Most, if not all, portlet products are targeted firmly at the enterprise market which means they tend to come shipped as part of a 'heavyweight' enterprise portal product, even though the portlet (producer) component should generally be fairly lightweight.
Portals are quite heavyweight by nature, but I've found the Pluto portlet container useful for testing individual portlets as it can be set up on Tomcat or Jetty in a few minutes and requires minimal initial configuration. Pluto is used as the portlet container by Jetspeed.