同步请求和异步请求的区别（个人理解）

同步和异步的区别？

• 同步就是等这个方法请求完成后，再执行下一个请求方法
• 异步就是在你请求这个方法的时候，可以在当前的页面写一个ajax请求后台查询对应数据，然后在后台返回数据，这就在请求页面同时也在请求数据返回其数据，这就是异步请求

前台onclick事件绑定两个方法，分别到后台请求两个方法，然后后台两个方法分别指向model数据给同一个前端页面，这是同步请求，会打开两个页面导致数据不一致

同步请求：

• 这是前台onclick绑定的两个方法

  <input class="btn btn-primary" type="button" onclick="putOnRrecordAffix(),putOnRrecord()" value="备案函打印"/>

• 这是onclik对应绑定的方法

 //备案函通知书打印(windows.print)
        function putOnRrecord(){
            var arr = new Array();
            if ($(".unclaimedId:checked").length === 0) {
                $.jBox.alert('请至少选择一条数据打印');
                return false;
            } else {
                //获取需要打印的数据
                $(".unclaimedId:checked").each(function () {
                    if ($(this).attr("checked") === "checked") {
                        arr.push($(this).val());
                    }
                });
            }
            url = "${ctx}/bz/unclaimed/putOnRrecord?projectArr=" + arr;
            window.open(url);
        }

   //备案函附件列表打印(windows.print)
        function putOnRrecordAffix(){
            var arr = new Array();
            if ($(".unclaimedId:checked").length === 0) {
                $.jBox.alert('请至少选择一条数据打印');
                return false;
            } else {
                //获取需要打印的数据
                $(".unclaimedId:checked").each(function () {
                    if ($(this).attr("checked") === "checked") {
                        arr.push($(this).val());
                    }
                });
            }
            url = "${ctx}/bz/unclaimed/putOnRrecordAffix?ids=" + arr;
            window.open(url);
        }

• 这是后台对应两个方法，返回同样数据给前端的页面，只能等请求一个页面完成后再打开一个页面，所以会打开两个页面，这就是同步请求

  /**
     * 打印公告函附件
     *
     * @param bzUnclaimed
     * @param request
     * @param model
     * @return
     */
    @RequestMapping(value = "putOnRrecordAffix")
    public String putOnRrecordAffix(BzUnclaimed bzUnclaimed, String[] ids, HttpServletRequest request, Model model) {
        if (ids != null) bzUnclaimed.setProjectArr(ids);
        List<BzUnclaimed> list = bzUnclaimedService.appendixForm(bzUnclaimed);
        model.addAttribute("list", list);
        if (list.size() > 0) {
            model.addAttribute("announcementDate", list.get(0).getAnnouncementDate());
            model.addAttribute("announcementEndDate", list.get(0).getAnnouncementEndDate());
            BzUnclaimed bzUnclaimeddata = new BzUnclaimed();
                BzUnclaimed bzUnclaimed1 = list.get(0);
                String serialNumber = bzUnclaimed1.getSerialNumber();
                Date announcementDate = bzUnclaimed1.getAnnouncementDate();
                Date announcementEndDate = bzUnclaimed1.getAnnouncementEndDate();
                int count = list.size();
                bzUnclaimeddata.setAnnouncementDate(announcementDate);
                bzUnclaimeddata.setAnnouncementEndDate(announcementEndDate);
                bzUnclaimeddata.setCounts(count);
                bzUnclaimeddata.setSerialNumber(serialNumber);
                model.addAttribute("counts", bzUnclaimeddata.getCounts());
                model.addAttribute("serialNumber", bzUnclaimeddata.getSerialNumber());
                SimpleDateFormat dateFormat = new SimpleDateFormat("yyyy 年 MM 月 dd 日");
                String announcementTime = dateFormat.format(bzUnclaimeddata.getAnnouncementDate());
                String announcementEndTime = dateFormat.format(bzUnclaimeddata.getAnnouncementEndDate());
                String nowTime = dateFormat.format(new Date());
                model.addAttribute("announcementTime", announcementTime);
                model.addAttribute("announcementEndTime", announcementEndTime);
                model.addAttribute("nowTime", nowTime);

        }
        return "modules/bz/unclaimedRemains/printPutOnRrecord";
    }



  /**
     * 打印公告函附件
     *
     * @param bzUnclaimed
     * @param request
     * @param model
     * @return
     */
    @RequestMapping(value = "putOnRrecordAffix")
    public String putOnRrecordAffix(BzUnclaimed bzUnclaimed, String[] ids, HttpServletRequest request, Model model) {
        if (ids != null) bzUnclaimed.setProjectArr(ids);
        List<BzUnclaimed> list = bzUnclaimedService.appendixForm(bzUnclaimed);
        model.addAttribute("list", list);
        if (list.size() > 0) {
            model.addAttribute("announcementDate", list.get(0).getAnnouncementDate());
            model.addAttribute("announcementEndDate", list.get(0).getAnnouncementEndDate());
            BzUnclaimed bzUnclaimeddata = new BzUnclaimed();
                BzUnclaimed bzUnclaimed1 = list.get(0);
                String serialNumber = bzUnclaimed1.getSerialNumber();
                Date announcementDate = bzUnclaimed1.getAnnouncementDate();
                Date announcementEndDate = bzUnclaimed1.getAnnouncementEndDate();
                int count = list.size();
                bzUnclaimeddata.setAnnouncementDate(announcementDate);
                bzUnclaimeddata.setAnnouncementEndDate(announcementEndDate);
                bzUnclaimeddata.setCounts(count);
                bzUnclaimeddata.setSerialNumber(serialNumber);
                model.addAttribute("counts", bzUnclaimeddata.getCounts());
                model.addAttribute("serialNumber", bzUnclaimeddata.getSerialNumber());
                SimpleDateFormat dateFormat = new SimpleDateFormat("yyyy 年 MM 月 dd 日");
                String announcementTime = dateFormat.format(bzUnclaimeddata.getAnnouncementDate());
                String announcementEndTime = dateFormat.format(bzUnclaimeddata.getAnnouncementEndDate());
                String nowTime = dateFormat.format(new Date());
                model.addAttribute("announcementTime", announcementTime);
                model.addAttribute("announcementEndTime", announcementEndTime);
                model.addAttribute("nowTime", nowTime);

        }
        return "modules/bz/unclaimedRemains/printPutOnRrecord";
    }

异步请求：

function putOnRrecords(arr) { //在页面请求的ajax异步方法
    $.ajax({
        type: "get",
        url: "${ctx}/bz/unclaimed/putOnRrecords?projectArr=" + arr,
        dataType: 'JSON',
        async: true,
        success(data) {
            objDece = data;
        }
    })
}

/**
 * 打印备案函数据(lodop.print)
 *
 * @param bzUnclaimed
 * @param request
 * @param response
 * @param model
 * @return
 */
@RequestMapping(value = "putOnRrecords")//后台ajax请求异步方法
public String putOnRrecords(BzUnclaimed bzUnclaimed, HttpServletRequest request, HttpServletResponse response, Model model) {
    List<BzUnclaimed> bzUnclaimedList = bzUnclaimedService.appendixForm(bzUnclaimed);
    BzUnclaimed bzUnclaimeddata = new BzUnclaimed();
    if (bzUnclaimedList.size() > 0) {
        BzUnclaimed bzUnclaimed1 = bzUnclaimedList.get(0);
        String serialNumber = bzUnclaimed1.getSerialNumber();
        Date announcementDate = bzUnclaimed1.getAnnouncementDate();
        Date announcementEndDate = bzUnclaimed1.getAnnouncementEndDate();
        int count = bzUnclaimedList.size();
        bzUnclaimeddata.setAnnouncementDate(announcementDate);
        bzUnclaimeddata.setAnnouncementEndDate(announcementEndDate);
        bzUnclaimeddata.setCounts(count);
        bzUnclaimeddata.setSerialNumber(serialNumber);
    }
    //获取当前日期
    bzUnclaimeddata.setDateTime(new Date());
    return renderString(response, bzUnclaimeddata);
}

总结：所谓的同步就是指阻塞式操作，而异步就是非阻塞式操作

ajax异步和同步

ajax异步（默认async：true）
异步：并行处理，程序向服务器发送一个请求，在结果返回之前，程序还是可以执行其它操作（以前台界面为例，用户依然可以输入其它信息，并且和服务器进行其它交付），大大节省了用户的时间，也提高用户的体验

二、ajax同步（设置async：false）
同步：顺序处理，程序向服务器发送一个请求，在结果返回之前，程序要一直等待结果返回才可以执行下一步操作

同步是指：发送方发出数据后，等接收方发回响应以后才发下一个数据包的通讯方式。
异步是指：发送方发出数据后，不等接收方发回响应，接着发送下个数据包的通讯方式。
（注：除非有特殊要求或需求，尽量不要用同步）