[Hello-Java.Base] MonoVM + peer collection

[jonpryor]

Context: a5b229d

Now that MonoVM can be used on the desktop (a5b229d), let's show it in action!

Update samples/Hello-Java.Base to call GC.Collect() and GC.WaitForPendingFinalizers() to help cause the MyJLO instance to be collected before app exit.

Which is fine, but what we also need are GREF logs. Unfortunately, we can't use JreRuntimeOptions.JniGlobalReferenceWriter for that purpose, because libjava-interop.dylib is native code and we want both managed and native code to write GREF logs to the same place.

Instead, set the JAVA_INTEROP_GREF_LOG environment variable to a file path:

% JAVA_INTEROP_GREF_LOG=g.txt  ./samples/Hello-Java.Base/bin/Release/osx-x64/publish/Hello-Java.Base
MonoVM support enabled
# jonp: LoadJvmLibrary(/Library/Java/JavaVirtualMachines/microsoft-17.jdk/Contents/Home/lib/libjli.dylib)=1164832560
# jonp: JNI_CreateJavaVM=4554903296; JNI_GetCreatedJavaVMs=4554903376
# jonp: executing JNI_CreateJavaVM=10f7e4f00
# jonp: r=0 javavm=111647690 jnienv=7fbfb7afbaa8
WARNING in native method: JNI call made without checking exceptions when required to from CallStaticObjectMethodV
binding? net.dot.jni.sample.MyJLO@575bab75
WARNING in native method: JNI call made without checking exceptions when required to from CallVoidMethodV

Then read g.txt. Selected output includes:

+g+ grefc 5 gwrefc 0 obj-handle 0x7fbfb6104920/L -> new-handle 0x7fbfb57274c0/G from thread '(null)'(4)
   at Java.Interop.MonoRuntimeObjectReferenceManager.CreateGlobalReference(JniObjectReference reference)
   at Java.Interop.JniObjectReference.NewGlobalRef()
   at Java.Interop.JniRuntime.JniValueManager.ConstructPeer(IJavaPeerable peer, JniObjectReference& reference, JniObjectReferenceOptions options)
   at Java.Interop.JavaObject.Construct(JniObjectReference& reference, JniObjectReferenceOptions options)
   at Java.Lang.Object..ctor()
   at Hello.MyJLO..ctor()
   at Hello.App.<>c.<CreateJLO>b__3_0()
   at System.Threading.Thread.StartCallback()

Created PeerReference=0x7fbfb57274c0/G IdentityHashCode=0x575bab75 Instance=0xef12a08e Instance.Type=Hello.MyJLO, Java.Type=net/dot/jni/sample/MyJLO

*take_weak obj=0x10fca0478; handle=0x7fbfb57274c0
+w+ grefc 7 gwrefc 1 obj-handle 0x7fbfb57274c0/G -> new-handle 0x7fbf9570a801/W from thread '(null)'(1)
take_weak_global_ref_jni
-g- grefc 6 gwrefc 1 handle 0x7fbfb57274c0/G from thread '(null)'(1)
take_weak_global_ref_jni
*try_take_global obj=0x10fca0478 -> wref=0x7fbf9570a801 handle=0x0
-w- grefc 6 gwrefc 0 handle 0x7fbf9570a801/W from thread '(null)'(1)
take_global_ref_jni
Finalizing PeerReference=0x0/G IdentityHashCode=0x575bab75 Instance=0xef12a08e Instance.Type=Hello.MyJLO

Which shows the creation of JNI handle 0x7fbfb57274c0/G, which is associated with Hello.MyJLO, and then we go through the GC process:

• Create a JNI Weak Global Reference 0x7fbf9570a801/W (+w+ …)
• Delete the JNI Global Reference 0x7fbfb57274c0/G (-g- …)
• Perform a Java-side GC (not mentioned)
• Try to create a JNI Global Reference from the Weak Global Reference (*try_take_global …)
• Delete the JNI Weak Global Reference (-w- …)
• Finalize the Hello.MyJLO instance (Finalizing PeerReference=0x0/G …)

The Instance= value can be used to correlate instances; it's the value of RuntimeHelpers.GetHashCode().