We used to provide an hprof document. And old copy can be found here:

http://hg.openjdk.java.net/jdk8/jdk8/jdk/raw-file/tip/src/share/demo/jvmti/hprof/manual.html

The primary purpose was to document how to use the "HPROF Heap and CPU Profiling Agent", a jvmti agent for heap and cpu profiling. Eventually support for the hprof agent was removed, and this entire document along with it. However producing hprof files is still supported by hotspot (via the "dumpheap" jcmd and "jmap -dump") and SA (via "jhsdb jmap --binaryheap" and via the clhsdb "dumpheap" commands). There are also numerous 3rd party tools that still read hprof binary files.

There is a section of the hprof documentation that describes the format of an hprof file. I'm suggesting with this RFE that we revive and maintain it.

http://hg.openjdk.java.net/jdk8/jdk8/jdk/raw-file/tip/src/share/demo/jvmti/hprof/manual.html#mozTocId848088

I'm not sure how out of date it is, but at the very least it is a starting point.

There is also the following hprof doc, which seems to be almost identical to manual.html, but has some differences:

https://docs.oracle.com/javase/8/docs/technotes/samples/hprof.html