doccheck reports, and grep confirms, a reference to a page for version 1.2 of the JNI spec on java.sun.com. While the domain name still exists, it redirects to a general top level page for Java.

*** http://java.sun.com/products/jdk/1.2/docs/guide/jni/index.html
build/linux-x86_64-server-release/images/docs/specs/jdwp/jdwp-protocol.html:Returns the JNI signature of a reference type. JNI signature formats are described in the <a href="http://java.sun.com/products/jdk/1.2/docs/guide/jni/index.html">Java Native Inteface Specification</a><p>For primitive classes the returned signature is the signature of the corresponding primitive type; for example, "I" is returned as the signature of the class represented by java.lang.Integer.TYPE.

The reference seems to come from here:

open/make/data/jdwp/jdwp.spec: "<a href=\"http://java.sun.com/products/jdk/1.2/docs/guide/jni/index.html\">Java Native Inteface Specification</a>"

(I also note the spelling typo in the link!)

The JNI Spec is now included in the main specs/ directory, so this link can possibly be replaced to a simple relative link such as "../jni/index.html".