如何得到某一个文件夹下带某个特定后缀名的文件总数，注意这个文件夹下可以再套文件夹，再套的文件夹还可以套文件夹

package com.cloudwalk.ibis.model.system;

import org.springframework.util.Assert;

import java.io.File;
import java.util.ArrayList;

/**
 * @author qinlinsen
 */
public class FileTest {
    private static final String FILE_NAME = "D:\cloudwalk\IBIS3.0\src\main\resources\cn\cloudwalk\mapper";

    public static void countFileNumber(String directoryPath, String fileExtensiveName) {
        ArrayList<File> filesWithSpecialExtensiveName = new ArrayList<File>();
        ArrayList<File> filesWithoutSpecialExtensiveName = new ArrayList<File>();
        Assert.notNull(directoryPath, "file is required");
        Assert.notNull(fileExtensiveName, "file extensive name is required");
        File file = new File(directoryPath);
        Assert.state(file.isDirectory(), "This require is directory rather than file,please check you input");
        //get all files of this directory
        File[] files = file.listFiles();
        iteratorDirectory(fileExtensiveName, filesWithSpecialExtensiveName, filesWithoutSpecialExtensiveName, files);
        System.out.println("the total number of file with " + fileExtensiveName
                + " is " + filesWithSpecialExtensiveName.size() +
                " without the " + fileExtensiveName + " the count number is : "
                + filesWithoutSpecialExtensiveName.size());
    }

    private static void iteratorDirectory(String fileExtensiveName, ArrayList<File> filesWithSpecialExtensiveName, ArrayList<File> filesWithoutSpecialExtensiveName, File[] files) {
        for (File f : files) {
            if(f.isDirectory()){
                iteratorDirectory(fileExtensiveName,filesWithSpecialExtensiveName,filesWithoutSpecialExtensiveName,f.listFiles());
            }
            if (f.isFile()) {
                //only ends with special extensive name can add to filesWithSpecialExtensiveName
                String fileName = f.getName();
                if (fileName.endsWith(fileExtensiveName)) {
                    filesWithSpecialExtensiveName.add(f);
                } else {
                    filesWithoutSpecialExtensiveName.add(f);
                }
            }
        }
    }


    public static void main(String[] args) {

        countFileNumber("D:\cloudwalk\IBIS3.0\src\main\resources\com\timo",".xml");

    }
}