Anyone know good and recommended free and opensource *java based* Billing or Accounting or ERP systems?

ERP stands for Enterprise Resource Planning. People generally say that it a software,but it a concept which is introduced to integrate all the business activities and departments together. And people develop their own Business application on this generalized concept.

Have a look on the below link that list several systems that may answer your requirement:

http://java-source.net/open-source/erp-crm

Dude its not a programming podium to tell which erp software is developed using which programming language "JAVA IS A PROGRAMMING LANGUAGE" not an ERP.you should tell your business requirement so that we can suggest you better erp software to map you business in it.if you are a software developer then better to ask this question on

https://www.codeproject.com/

https://stackoverflow.com/

http://www.c-sharpcorner.com/

https://www.devexpress.com/

Apache Ofbiz is very good Java ERP open source system

https://ofbiz.apache.org/

look for it on www.sourceforge.net

There are several of them here below is one:

Apache OffBiz

ERP stands for Enterprise Resource Planning,JAVA IS A PROGRAMMING LANGUAGE can use programing language as Java ,C# and python  to developing erb software that achieved requirments needed.

You may use tridens or any other application recommended by google

Nothing is free in this world if  you just google your requirements you may get the source for your requiement and approximate cost. JD edwards is oracle based ERP system.