Where to find source code for java.lang native methods?

You can look at the source code for OpenJDK (licensed under GPLv2 with Classpath Exception). Probably the best way to study the JDK implementation internals, unless you want to be bound by the Java Research Licence, in which case you can access the actual JDK 6 source.

For JDK6 you can download the source from java.net. For java.lang the story begins at j2se/src/share/native/java/lang/, and then search... JDK7 rearranges the directory structure a little.

Some methods, such as Object.hashCode, may be implemented by hotspot instead or in addition to through JNI/Java.

JDK6 is freely licensed through the Java Research License (JRL) and Java Internal Use License (JIUL). JDK7 and OpenJDK6 is licensed under GPL 2 with CLASSPATH exception (roughly speaking you can link to it without catching the GNU virus). I am not a lawyer.

(BTW: The real lawyers would like to point out that I am still an employee of Sun Microsystems. Sun UK is no more. It is now Oracle.)

Update (Adding location for Thread.c): http://hg.openjdk.java.net/jdk7/jdk7/jdk/file/9b8c96f96a0f/src/share/native/java/lang/Thread.c