Question

I am currently writting a JNI wrapper for a C++ class and I’m not sure if what I have done so far is the most correct way.

As I understand it, it is not possible to declare a Java constructor native, so I ended up writing something like:

package log;

public class Logger
{
    private long internalPtr = 0;

    private native long createNativeInstance(String application, String module, String function);

    public Logger(String application, String module, String function)
    {
        this.internalPtr = createNativeInstance(application, module, function);
    }

    public native String getApplication();

    static { System.loadLibrary("log_java"); }
}

Basically, my internalPtr field holds the pointer to my underlying C++ instance and I create it in a pure Java constructor, using the static native method createNativeInstance().

Is this the correct way to do things ?

Another question which I could get an answer for is: “How should I handle the deallocation ?”

My Java skills are extremely limited, so do not hesitate to suggest even the more “obvious” solutions.

Thank you very much.

Answer 1

That is the way I have always implemented native classes which wrap arround a C/C++ object. I think writing a native constructor could be a bit of a pain, so have never tried.

For de-allocation, you do a simolar thing. Write a C/C++ function which de-allocates the memory for your C/C++ object, and then call this from the java destructor (finalise method). So something like this:

private native void destroyNativeInstance( long p_internalPtr );

public void finalize()
{
    destroyNativeInstance( this.internalPtr );
}