I found solution  by myself, of course!
here is my code (converting jasper  to jrxml)
enjoy now

import java.io.File;

import net.sf.jasperreports.engine.JRException;

import net.sf.jasperreports.engine.JasperReport;
import net.sf.jasperreports.engine.design.JasperDesign;

import net.sf.jasperreports.engine.util.JRLoader;

import net.sf.jasperreports.engine.xml.JRXmlWriter;

public class JasperToJRXML {

    public static String sourcePath,
                         destinationPath,
                         xml;


    public static JasperDesign jd = new JasperDesign();

    public static void main(String[] args) throws JRException {


        // Paths
        File dir = new File("C:\\jaspers\\");


        if (dir.isDirectory()) {
            File[] jaspers = dir.listFiles();

            for (int i = 0; i < jaspers.length; i++) {
                String outputPath = "c:\\jrxml\\";
                String ext = getFileExtenstion(jaspers[i]);

                if (ext.equalsIgnoreCase(".jasper")) {

                    JasperReport report = 
                        (JasperReport)JRLoader.loadObject(jaspers[i]);
                    outputPath += getFileName(jaspers[i]) + ".jrxml";
                    JRXmlWriter.writeReport(report, outputPath, "UTF-8");

                }
                System.out.println(ext);
            }
        }

    }

    private static String getFileExtenstion(File file) {
        String fileName = file.getName();
        int i = fileName.indexOf(".");
        return fileName.substring(i, fileName.length());
    }

    private static String getFileName(File file) {
        String fileName = file.getName();
        int i = fileName.indexOf(".");
        return fileName.substring(0, i);
    }
}