they typically distribute binaries packed in jars. And their code cannot be viewed unless it is an open source code.

That's not true at all. A jar is just a collection of java class files and java class files can be de-compiled into source code. Maybe it's not as easy to read as the original but it's not that difficult either. Just like perl, the only thing controlling what you do with the it is the license it's released under.

update: BTW, I had to go the decompiler route for one of my own jars when I lost the code - bad dev ... no cookie.