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）

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