JNI: Access a class declared in C++

Dear all,

I have a class declared in C++ and I want to access it in Java code. The problem is I need several instances of the class, so I can't make a global reference to it.

Is there any method that I can pass a pointer of C++ to the Java code? And I need to do something with the class in Java code.

Thank you.

pigpig
pigpigAsked:
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Igor BazarnyConnect With a Mentor Commented:

> int apt = (int) &a;
you take address of a itself. It should be apt = (int)a;
> A *b = (A *)&apt;
Here again, you take address of apt and convert it into pointer. Write: A* b = (A*)apt;
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Igor BazarnyCommented:
Hi,

You need to write JNI wrapper for your C++ class. Usually C++ pointer fits into java int, so your java class would look like:

class JavaWrapper{
    private int cppClass;
    public JavaWrapper(){
        init();
    }
    public native void doSomething();
    private native void init();
}

On JNI side, create instance of C++ class in init() implementation and store it in the cppClass field. In other methods you can get cppClass fiels and cast it back to C++ pointer type.

Possibly you will need to take care of memory issues--and it's non-trivial. In java, you know when you need new instance, but don't know when you don't need it anymore. You can use finalize() or weak references to clean up your C++ memory.

Regards,
Igor Bazarny,
Brainbench MVP for Java 1
 
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pigpigAuthor Commented:
Hello bazarny,

Do you have some sample codes in doing the pointer conversion in C++? I have some trouble in it.

thx
pigpig
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Igor BazarnyCommented:

What kind of problem do you have? Is it access to java field or pure C++ problem? I think you just need to cast pointer to long and set value to java field. When you need it back, cast long to same pointer type (unless you are very strict about inheritance/virtual functions use, numeric values of different pointer types pointing to tha same instance may differ)

Regards,
Igor Bazarny


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pigpigAuthor Commented:
Can u see my problem?
I have a piece of code like that in C++:

#include <iostream.h>

class A
{
public:
     A(int n) : no(n) {}
     int no;
};

void main()
{
     A *a = new A(9);
     
     int apt = (int) &a;

        A *b = (A *)&apt;
        cout << b->no << endl;

     delete a;
}
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pigpigAuthor Commented:
done !!!!!!!!
thx very much~~~~~~
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