I believe there are a lot of similar questions even on stackoverflow. But most of them are dated 1-2 years ago. Sorry about same question again.
Could you suggest easy to run and tweak java based opensource E-Commerce solution. Before i used Magento, but it was really hard to change code at start. I spend almost 2 week to realize how Magento works. Performance was not good enough too.
Now i'm looking for simple java solution, which can be customized or rewritten easy.
I'm on the hunt for more or less the same.
Up until now, KonaKart has been my prime candidate. But their entire stack isn't opensource, this is limited to (snippet from http://www.konakart.com/product/customization):
The parts of KonaKart that customers are most likely to want to customize are all open >source. These include the Struts action classes and forms, the JSPs, the velocity templates, >the BIRT reports, the payment modules, order total modules, shipping modules and the GWT One >Page Checkout code. They are shipped under the GNU Lesser General Public License
An alternative is http://www.shopizer.com/ which offer the entire codebase under the LGPL license.
Since I'm purely in the very early stages of spotting a suitable system, I can't say much about performance of either system.