You're kidding, right? Where is the source code for java compiler and jvm stuff? Can you send a pull request? Can you see the commits? Can you see design meetings about the language? Java is not more open than .NET in any way.
Java has developed in a somewhat collaborate/committee fashion which has been open to the public since before the JVM itself was open sourced. It is probably quite hard for random individuals to contribute directly to that, but yes - you can see what's going on it the JCP process, https://jcp.org/en/jsr/overview