How to embed an native application running in debian into Java SWT application?

Question

I am trying to embed a native app running in Debian such as LibreOffice into a Java SWT application. I have written the SWT code in Java. I send the handle of SWT Composite through the JNI call and embed the child window into it. But it is not getting embedded. Can anyone please help me with what I have done wrong?

Thanks.

This is the Java code:

import java.util.logging.Logger;
import org.eclipse.swt.*;
import org.eclipse.swt.widgets.*;
import org.eclipse.swt.layout.*;
import org.eclipse.swt.graphics.*;
import org.eclipse.swt.internal.gtk.*;
import org.eclipse.swt.widgets.Shell;
import org.eclipse.swt.widgets.Display;
import org.eclipse.swt.internal.gtk.OS;

public class DebianWindowEmbedder {
private static final Logger logger = Logger.getLogger(DebianWindowEmbedder.class.getName());

static {
    System.load("path_to_my_library_so_file");
    logger.info("Loaded lib");
}
private static native void embedWindow(String windowTitle, long parentWindowXid)
public static void main(String[] args) {

Display display = new Display();
Shell shell = new Shell(display);
shell.setLayout(new FillLayout());
shell.setText("Main Application");
String title = "Mozilla Firefox";
shell.setSize(1366, 768);
Composite debianComposite = new Composite(shell, SWT.EMBEDDED);
debianComposite.setLayout(new FillLayout());

long parentXid = debianComposite.handle;

embedWindow(title,parentXid);


shell.open();
while (!shell.isDisposed()) {
if (!display.readAndDispatch())
display.sleep();
}
display.dispose();
}
}

This is the C code:

#include <jni.h>
#include <gtk/gtk.h>
#include <gdk/gdkx.h>
#include <gdk/gdk.h>
#include<stdio.h>


JNIEXPORT void JNICALL Java_DebianWindowEmbedder_embedWindow(JNIEnv *env, jclass clazz, jstring      windowTitle, jlong parentWindowXid) {

GtkWidget* pParentWidget = (GtkWidget*)(parentWindowXid);        
GdkWindow* pParentWindow = gtk_widget_get_window(pParentWidget);

//For time being, I have hardcoded the xid.
Window child_xid = 0x02a00009 ;
    
GdkWindow *childWindow = gdk_window_lookup(child_xid);
  
GdkDisplay *gdk_display =  gdk_display_get_default();
        
gdk_window_reparent(childWindow , pParentWindow , 0, 0);
           
}