How much of the Java SE api is written in Java?

Question

How much of the Java SE api is actually written in Java itself?

Answer 1

The easiest way to find out is just to look at the source code (that's from Sun's J2SE website; there's also the OpenJDK source for 6 and 7). It's pretty much as you'd expect - things which absolutely have to be written in native code, e.g. "open a socket or a file" are native, but almost everything else is written in Java (in Sun's implementation, anyway).

Answer 2

Download OpenJDK and find out!

Most of it is Java, but there are some low-level things that must be native code (graphics and file operations come to mind).