API for determining the hash of a module

[Ethan McCue]

Hi all,

Not sure if this is best for core-libs or langtools, but best I can tell
there is no way to determine the hash of a module programmatically.

Hashes of dependent modules are recorded in JMOD files

jmod describe jdks/aarch64_windows/jmods/java.base.jmod

java.base at 23.0.1
....
platform linux-ppc64le
hashes java.datatransfer SHA-256
e9cac9cb3758d80a42e814cc0a03e5f34b511491b5f5542da1ccff6fbc4c8ac8
hashes java.desktop SHA-256
342b94944765b78932de60aaf9e47811f23d60461c7014557e84ca1d6f6c135d
hashes java.instrument SHA-256
e238b7968a4a2cf233781def3a5c18e5ba828a077de47adb0524ac07d16289d4
hashes java.logging SHA-256
0702fb1772a4afc72ed95c834075ee608eff8ba8c501efc6520b457ac9ae2560
hashes java.management SHA-256
e61678895158866416322cb814b2425334374879cfb42d2201f8959a20a6568a
...

But simply SHA-256ing the JMODs of these modules will not yield these
hashes.

Best I can gather, this functionality is only in a private API.

https://github.com/openjdk/jdk/blob/master/src/java.base/share/classes/jdk/internal/module/ModuleHashes.java#L111

If I try to replicate it with my own ModuleReferences I quickly run into
"jmods cannot be used at run time" errors.

https://github.com/openjdk/jdk/blob/a6277bb521e07e569cd75a4641b2a05a26f47b0a/src/java.base/share/classes/jdk/internal/module/ModulePath.java#L137-L141

Is there an API I am missing or at least documentation of how exactly these
hashes are computed in a way I can replicate?
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