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Fix possible premature collection #5405
Description
The problem with new JavaObject(obj).Handle is that there is a window wherein the JavaObject is collected after the .Handle property is accessed, but before .Handle is returned to Java code, which would "normally" keep the JavaObject instance alive.
Related:
This statement should instead be:
return JNIEnv.ToLocalJniHandle (new JavaObject (obj));Additionally, JNIEnv.ToLocalJniHandle() needs to add a GC.KeepAlive(): https://github.com/xamarin/xamarin-android/blob/f495778d433e45fee2fb61435293f059d6942620/src/Mono.Android/Android.Runtime/JNIEnv.cs#L746-L754
It should be:
public static IntPtr ToLocalJniHandle (IJavaObject? value)
{
if (value == null)
return IntPtr.Zero;
var ex = value as IJavaObjectEx;
if (ex != null)
return ex.ToLocalJniHandle ();
try {
return NewLocalRef (value.Handle);
}
finally {
GC.KeepAlive (value);
}
}This will ensure that value isn't likewise GC'd immediately after the .Handle access but before NewLocalRef() is invoked.
Activity
jonpryor commentedon Dec 10, 2020
Should likewise audit the rest of the codebase to check for places where
GC.KeepAlive()is needed. :-(jonpryor commentedon Dec 18, 2020
Mono.Android.Exportlikely needs to be updated to insertGC.KeepAlive()calls in the generated marshal methods.[Mono.Android] Prevent premature JNI handle collection (#12315)
[Mono.Android] Address review feedback on threshold removal
[Mono.Android] Address review feedback on threshold removal
[Mono.Android] Address review feedback on threshold removal