I am studying java nio and found that implementation classes are under sun.nio , which do not seem to provide sources in jdk1.6/src.zip .
sun.nio
jdk1.6/src.zip
Are there any open source packages? And where to find sources from them?
You can download java code and native code that java.nio implements at http://download.java.net/openjdk/jdk7/ (see top_dir / j2se / src / share / classes / java / nio / after extracting the source zip)
Note that you will also be able to see platform-specific code, such as code for Solaris, Linux, and Windows.
It is available under a community source license somewhere, but you'd better know the Berkeley Sockets API on which it is based.