Instantiating an abstract class

[Igor R]

Hello,

In jni code I attempt the subj. This works well on the simulator and
on some devices, but fails with InstantiationException on some other
devices.
What's the reason of such a behavior? Is it configurable?

Thanks.

[Tim Mensch]

That sounds like a Java question, since if you tried to instantiate an
abstract class in C++, it simply wouldn't compile.

This being the NDK list, you might have better luck asking on another
list, where the topic is Java.

Tim

[Doug Schaefer]

Even in Java, you shouldn't have been able to instantiate an abstract
class. Sounds like you found a bug in your first attempt and should
not count on that happening again.

[alan]

I imagine that unless check jni is tunred on you are probably able to instantiate an abstract class. I imagine as soon as you try to use it though bad/random things will happen

[Igor R]

> That sounds like a Java question, since if you tried to instantiate an
> abstract class in C++, it simply wouldn't compile.
>
> This being the NDK list, you might have better luck asking on another
> list, where the topic is Java.

I probably didn't explain myself well... I know that both Java c++
forbid to instantiate abstract classes :).
I mean the following: knowing that jni allows to ignore some java
rules (like member protection), I attempted to instantiate abstract
java classes this way:
jclass cls = env->FindClass("path/to/myclass");
jmethodID method = env->GetMethodID(cls, "<init>", "()V");
jobject result = env->NewObject(cls, method);

In most cases this *does* work.
But on some device it thows InstantiationException.

The question is: why? What it depends on?

[Igor R]

> I imagine that unless check jni is tunred on you are probably able to instantiate an abstract class.

Oh I see.
But I didn't change my project settings, when switched from one device
to another, and I also didn't tweak device settings.

> I imagine as soon as you try to use it though bad/random things will happen

Why? In case the class is fully defined, "abstract" is just syntactic
restriction (just like "private", which jni ignores).

[alan]

probably whether checkjni is turned on, I beleive that it is normally on by default on emulators and off by default on devices but some manufacturers may have it turned on in a device. If your class is fully defined then instantiating it will probably do no harm but if checkjni is on then it will probably crash. The only solution is to not try to create abstract classes, jni is for making faster or more portable code it is not for bypassing jvm checks

[Linux Box Solutions]

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On Jul 4, 2011, at 6:13 AM, alan <al...@birtles.org.uk> wrote:

probably whether checkjni is turned on, I beleive that it is normally on by default on emulators and off by default on devices but some manufacturers may have it turned on in a device. If your class is fully defined then instantiating it will probably do no harm but if checkjni is on then it will probably crash. The only solution is to not try to create abstract classes, jni is for making faster or more portable code it is not for bypassing jvm checks

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[David Turner]

On Tue, Jul 5, 2011 at 12:30 AM, Linux Box Solutions <linuxbox...@gmail.com> wrote:

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[Dianne Hackborn]

From what I can tell, you are saying "I am trying to do this thing that I know I am not supposed to do and isn't supported, but when I do it I am getting different kinds of errors in different situations and why is that?"

If so...  well, I think the problem mostly lies in the first part of that not the second. :}  If it is not something that should work, the failure behavior may vary.  But since this is not supposed to work, isn't the solution to just not do it?

As far as the differences in behavior...  you haven't really given much information for people to speculate.  Maybe this undefined different devices you are trying it on are running different versions of the platform?  Maybe some manufacturers have some different behavior?

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[fadden]

Back in Cupcake-era Android it was possible to instantiate abstract
classes through reflection (Constructor.newInstance()). This was
fixed some time ago.

AllocObject and NewObject still allowed this up until about a year
ago. Here's the change that fixed it:

http://android.git.kernel.org/?p=platform/dalvik.git;a=commit;h=b76594d71daec00e5947a5f6f14282f8f9edebe4

I'm not sure what release that first appeared in.

Bottom line: it's not allowed, it should never have been allowed, you
will get nothing but sadness if you try to use it.

[Christopher Van Kirk]

I love your responses sometimes, Dianne.

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