[Java.Interop] Allow JniRuntime init from JavaVM* and JNIEnv*

[jonpryor]

Context: #1153

JNI supports two modes of operation:

1. Native code creates the JVM, e.g. via JNI_CreateJavaVM()

2. The JVM already exists, and when Java code calls System.loadLibrary(), the JVM calls the JNI_OnLoad() function on the specified library.

Java.Interop samples and unit tests rely on the first approach, e.g. TestJVM subclasses JreRuntime, which is responsible for calling JNI_CreateJavaVM() so that Java code can be used.

PR #1153 is exploring the use of .NET Native AOT to produce a native library which is used with Java-originated initialization.

In order to make Java-originated initialization work, we need to be able to initialize JniRuntime and JreRuntime around existing JVM-provided pointers:

• The JavaVM* provided to JNI_OnLoad(), which can be used to set JniRuntime.CreationOptions.InvocationPointer:

  [UnmanagedCallersOnly(EntryPoint="JNI_OnLoad")]
  int JNI_OnLoad(IntPtr vm, IntPtr reserved)
  {
      var options = new JreRuntimeOptions {
          InvocationPointer = vm,
      };
      var runtime = options.CreateJreVM ();
      return runtime.JniVersion;
      return JNI_VERSION_1_6;
  }
  

• The JNIEnv* value provided to Java native methods when they are invoked, which can be used to set JniRuntime.CreationOptions.EnvironmentPointer:

  [UnmanagedCallersOnly(EntryPoint="Java_example_Whatever_init")]
  void Whatever_init(IntPtr jnienv, IntPtr Whatever_class)
  {
      var options = new JreRuntimeOptions {
          EnvironmentPointer = jnienv,
      };
      var runtime = options.CreateJreVM ();
  }
  

Update JniRuntime and JreRuntime to support these Java-originated initialization strategies. In particular, don't require that JreRuntimeOptions.JvmLibraryPath be set, avoiding:

System.InvalidOperationException: Member `JreRuntimeOptions.JvmLibraryPath` must be set.
   at Java.Interop.JreRuntime.CreateJreVM(JreRuntimeOptions builder)
   at Java.Interop.JreRuntime..ctor(JreRuntimeOptions builder)
   at Java.Interop.JreRuntimeOptions.CreateJreVM()

[jonathanpeppers]

Is it worth doing FileNotFoundException?

[jonpryor]

Updated.

[jonathanpeppers]

What is Collection<string>? I've never heard of this? It would be an API break to change it, so probably can't be a List<string>.

[jonpryor]

Collection<T>. It's existed since .NET Framework 2.0, and is the recommended collection type in the .NET Framework Design Guidelines:

✔️ DO use Collection or a subclass of Collection<T> for properties or return values representing read/write collections.

This isn't an API break because the type hasn't changed at all, and -- again -- Collection<T> is in fact the preferred collection type here.

[jonathanpeppers]

TIL, List vs Collection: https://stackoverflow.com/a/271842

[jonpryor]

Additionally, we've never "shipped" Java.Runtime.Environment.dll, so I'm fine witih making API breaks here if warranted.

[jonathanpeppers]

This file is old, but could we start adding new tests with the pattern:

public async Task UseInvocationPointerOnNewThread ()
{
    await Task.Run(() => ...);
}

[jonpryor]

@jonathanpeppers: what's the benefit to using the Task-based version? Other than ability to use async/await?

[jonathanpeppers]

I cringe when I see Thread.Join() that is the only reason, lol.