从MAC OSX上的URL下载文件？

AM尝试使用此代码在java中使用url＆urlconnection类从网上下载文件 .

BufferedInputStream bis = null;
            BufferedOutputStream bos = null;
            FileOutputStream fos = null;
            try {
                fos = new FileOutputStream(destination);
            } catch (FileNotFoundException ex) {
            }
            try {
                URL url = null;
                try {
                    url = new URL("here is the url");
//                    
                } catch (MalformedURLException ex) {
                    ex.printStackTrace();
                }
                URLConnection urlc = null;

                try {
                    urlc = url.openConnection();
                    System.out.println("Conneceted");
                } catch (IOException ex) {
                    ex.printStackTrace();
                }
                try {
                    bis = new BufferedInputStream(urlc.getInputStream());
                } catch (IOException ex) {
                    ex.printStackTrace();
                }

                bos = new BufferedOutputStream(fos);
                //          System.out.println("absolute path="+destination.getAbsolutePath());

                int i;
                try {
                    while ((i = bis.read()) != -1) {
                        System.out.print((char) i);
                        bos.write(i);
                    }

                    JOptionPane.showMessageDialog(this, "downloadeded sucessfully");
                    this.dispose();
                } catch (IOException ex) {
                    JOptionPane.showMessageDialog(this, "could not be diownloaded \n please try again");
                    ex.printStackTrace();
                }
            } finally {
                if (bis != null) {
                    try {
                        bis.close();
                    } catch (IOException ioe) {
                        ioe.printStackTrace();
                    }
                }
                if (bos != null) {
                    try {

                        bos.close();
                    } catch (IOException ioe) {
                        ioe.printStackTrace();
                    }
                }
            }

当我在我的Windows系统上运行此代码时，它运行得很完美但是当我把我的代码带到mac os时，它给了我一个例外

Exception in thread "Thread-2" java.lang.NullPointerException
 at java.io.BufferedOutputStream.flushBuffer(BufferedOutputStream.java:65)
 at java.io.BufferedOutputStream.flush(BufferedOutputStream.java:123)
 at java.io.FilterOutputStream.close(FilterOutputStream.java:140)
 at FileDownloader.Download(FileDownloader.java:201)
 at FileDownloader$2.run(FileDownloader.java:114)
 at java.lang.Thread.run(Thread.java:637)

谁能告诉我原因是什么？以及我如何解决这个问题？

提前致谢