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[Android] Put Java JNI function in a separate static library #108513

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[Android] Put Java JNI function in a separate static library #108513

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Commits on Nov 25, 2024

  1. [Android] Put Java JNI function in a separate static library 

    In dotnet/android#9006 we are working on linking
the .NET for Android runtime dynamically at the application build time.
Linking involves all the BCL native libraries, including
`System.Security.Cryptography.Native.Android` and one of the goals is to
hide all the exported symbols used as p/invokes by the managed BCL
libraries.  This is because p/invoke calls are handled internally and,
with dynamic linking of the runtime, there is no longer any reason to
use `dlopen` and `dlsym` to look them up, they are all resolved
internally, at the link time.

Symbol hiding works fine thanks to the `--exclude-libs` `clang` flag,
which makes all the exported symbols in the indicated `.a` archives to
not be exported by the linker.  However,
`System.Security.Cryptography.Native.Android` is special in the sense
that it contains one symbol which must not be hidden,
`Java_net_dot_android_crypto_DotnetProxyTrustManager_verifyRemoteCertificate`.

The above function is a Java `native` method implementation, and it
requires that not only its name follows the Java JNI naming rules, but
that is also available for the JVM to look up using `dlsym`.

I tried using the `--export-dynamic-symbol` clang flag to
export **just** this function, but it doesn't appear to work no matter
where I put the flag in relation to reference to the `.a` archives.

Instead, the problem can be dealt with by putting the JNI function in a
separate static library, so that I can link it without changing symbol
visibility, while making all the
`System.Security.Cryptography.Native.Android` symbols invisible.
    @grendello
    grendello committed Nov 25, 2024
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    @grendello
    grendello committed Nov 25, 2024
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