seek好像不支持,给你一段拷贝文件的代码
    /**
     *  文件拷贝。
     *
     *@param  from             源路径。
     *@param  to               目标路径。
     *@exception  IOException  Description of the Exception
     */
    public static void copy( String from, String to )
                     throws IOException
    {
        int BUFF_SIZE = 100000;
        byte[] buffer = new byte[ BUFF_SIZE ];
        InputStream in = null;
        OutputStream out = null;        try
        {
            in = new FileInputStream( from );
            out = new FileOutputStream( to );            while ( true )
            {
                synchronized ( buffer )
                {
                    int amountRead = in.read( buffer );                    if ( amountRead == -1 )
                    {
                        break;
                    }                    out.write( buffer, 0, amountRead );
                }
            }
        }
        finally
        {
            if ( in != null )
            {
                in.close();
            }            if ( out != null )
            {
                out.close();
            }
        }
    }