Using CNI on inner classes

Using CNI on inner classes

[Vaijayanthi Mala Suresh]

Hi,

I have an Test.java as shown below


public class Test {

    private native void natRegister(Inner obj);
    Inner myInnerClassObj;

    class Inner
    {
      private void printline()
      {
        System.out.printline("Prints from printline");
      }
      Inner()
      {
      }
    }

    public Test()
    {
      myInnerClassObj = new Inner();
    }

    public void register()
    {
	natRegister(myInnerClassObj);
    }


    public static void main(String [] args) {
      System.out.println("Hello from main");
      Test ht = new Test();
      ht.register();
    }
  }

I have the Test.cc defined as shown below
// This file is intended to give you a head start on implementing native
// methods using CNI.
// Be aware: running 'gcjh -stubs' once more for this class may
// overwrite any edits you have made to this file.

#include <gcj/cni.h>
#include <java/lang/UnsupportedOperationException.h>

#include "Test.h"
#include "Test$Inner.h"
void
Test::natRegister(::Test$Inner *innerObj)
{
  innerObj->printline();
}

I need to access the private method from the native code. This was
possible in JNI but it is not possible in CNI.

Can you please suggest me a solution for this using CNI.

Thanks
Mala

Re: Using CNI on inner classes

[Andrew Haley]

Vaijayanthi Mala Suresh wrote:
> Hi,
> 
> I have an Test.java as shown below
> 
> 
> public class Test {
> 
>     private native void natRegister(Inner obj);
>     Inner myInnerClassObj;
> 
>     class Inner
>     {
>       private void printline()
>       {
>         System.out.printline("Prints from printline");
>       }
>       Inner()
>       {
>       }
>     }
> 
>     public Test()
>     {
>       myInnerClassObj = new Inner();
>     }
> 
>     public void register()
>     {
> 	natRegister(myInnerClassObj);
>     }
> 
> 
>     public static void main(String [] args) {
>       System.out.println("Hello from main");
>       Test ht = new Test();
>       ht.register();
>     }
>   }
> 
> I have the Test.cc defined as shown below
> // This file is intended to give you a head start on implementing native
> // methods using CNI.
> // Be aware: running 'gcjh -stubs' once more for this class may
> // overwrite any edits you have made to this file.
> 
> #include <gcj/cni.h>
> #include <java/lang/UnsupportedOperationException.h>
> 
> #include "Test.h"
> #include "Test$Inner.h"
> void
> Test::natRegister(::Test$Inner *innerObj)
> {
>   innerObj->printline();
> }
> 
> I need to access the private method from the native code. This was
> possible in JNI but it is not possible in CNI.
> 
> Can you please suggest me a solution for this using CNI.

If the method is declared private, it really will be private, even
to cni.

If you really need to breach privacy, you can do this in Test$Inner.h:

class Test$Inner : public ::java::lang::Object
{
  friend class ::Test;

but I can't really see the point of private methods in an inner class
that you actually intend to be called from the enclosing class.

Andrew.

Re: Using CNI on inner classes

[Bryce McKinlay]

On Thu, Aug 13, 2009 at 10:10 AM, Andrew Haley<aph@redhat.com> wrote:

> If the method is declared private, it really will be private, even
> to cni.
>
> If you really need to breach privacy, you can do this in Test$Inner.h:
>
> class Test$Inner : public ::java::lang::Object
> {
>  friend class ::Test;
>
> but I can't really see the point of private methods in an inner class
> that you actually intend to be called from the enclosing class.

In Java the private methods are accessible to the enclosing class. So,
ideally gcjh would add friend declarations, like the above,
automatically to inner classes to approximate the Java semantics.

But, the easiest fix here is just to make the private method
package-private instead.

Re: Using CNI on inner classes

[Andrew Haley]

Bryce McKinlay wrote:
> On Thu, Aug 13, 2009 at 10:10 AM, Andrew Haley<aph@redhat.com> wrote:
>
>> If the method is declared private, it really will be private, even
>> to cni.
>>
>> If you really need to breach privacy, you can do this in Test$Inner.h:
>>
>> class Test$Inner : public ::java::lang::Object
>> {
>>  friend class ::Test;
>>
>> but I can't really see the point of private methods in an inner class
>> that you actually intend to be called from the enclosing class.
>
> In Java the private methods are accessible to the enclosing class.

Ah yes, that rule, the one that requires all the "synthetic access$0"
business.  I just did a web search to find out the reason for this
rather odd rule, and I found "... inner classes can access all members
of the declaring class, even private members. In fact, the inner class
itself is said to be a member of the class; therefore, following the
rules of object-oriented engineering, it should have access to all
members of the class."
http://onjava.com/pub/a/onjava/excerpt/HardcoreJava_chap06/index.html

> So, ideally gcjh would add friend declarations, like the above,
> automatically to inner classes to approximate the Java semantics.

That's a nice idea.  There's no reason why not; I just think no-one
ever thought of it.

> But, the easiest fix here is just to make the private method
> package-private instead.

I think so.

Andrew.

Re: Using CNI on inner classes

[Andrew Haley]

Andrew Haley wrote:
> Bryce McKinlay wrote:
>> On Thu, Aug 13, 2009 at 10:10 AM, Andrew Haley<aph@redhat.com> wrote:
>>
>>> If the method is declared private, it really will be private, even
>>> to cni.
>>>
>>> If you really need to breach privacy, you can do this in Test$Inner.h:
>>>
>>> class Test$Inner : public ::java::lang::Object
>>> {
>>>  friend class ::Test;
>>>
>>> but I can't really see the point of private methods in an inner class
>>> that you actually intend to be called from the enclosing class.
>> In Java the private methods are accessible to the enclosing class.
> 
> Ah yes, that rule, the one that requires all the "synthetic access$0"
> business.  I just did a web search to find out the reason for this
> rather odd rule, and I found "... inner classes can access all members
> of the declaring class, even private members. In fact, the inner class
> itself is said to be a member of the class; therefore, following the
> rules of object-oriented engineering, it should have access to all
> members of the class."
> http://onjava.com/pub/a/onjava/excerpt/HardcoreJava_chap06/index.html

Actually, that doesn't explain it: it only explains why inner classes
can access the private methods of enclosing classes, not vice versa.

Andrew.