there are several sites out providing Java source code.
Unluckily, these does not refer to the Java native binaries.
In short: The source code of native functions is not shown.
An example is StrictMath.floor.
The only thing provided is the raw Java declaration
public static native double floor(double a);
What I am interested in is the actual c/c++ code. How did they actually implement it?
The OpenJDK project contains the full source. The source for jdk 6 is roughly equivalent to what is in the Sun/Oracle jdk. The source for jdk 7 should be almost exactly equivalent.