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javascript - ajax 异步和同步的问题

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Jun 06, 2016 pm 08:12 PM

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我们问问大神们什么是ajax异步，什么是ajax同步，两者有什么区别，或则两者适用场合有什么不同。

回复内容：

我们问问大神们什么是ajax异步，什么是ajax同步，两者有什么区别，或则两者适用场合有什么不同。

同步异步和 ajax有什么关系吗
Ajax是Asynchronous Javascript And XML的缩写
本身就是异步的何谈同步

异步就是当前线程请求操作后天开新线程处理完毕后回调结果给当前线程

同步意味着，在执行这段程序的时候，不能接着执行其他代码，放在Ajax上，就相当于，当你Ajax获取消息期间什么都干不了，一看就不科学，所以异步是为了让他在获取时可以正常运行其他部分的代码，然后callback回调处理结果就行了。

在一系列需要执行的事件队列里面：
同步操作：需要编译器对语句一件件的执行下去，如果某个事件还没有执行完毕，那么系统就需要一直等待，直到该事件执行完毕！
异步操作：就好比吧这个事件拉出来放到另外一队，做一个单独的事情，程序执行到这个异步的时候，会分成两部分，一部分继续执行下面的程序，另一部分会单独去执行这个异步，互不影响程序加载！
在用户角度来说，这样不会让用户感觉他在等待响应！

ajax.open方法中，第3个参数是设同步或者异步。prototype等js类库一般都默认为异步，即设为true。
先说下同步的情况下，js会等待请求返回，获取status。不需要onreadystatechange事件处理函数。而异步则需要onreadystatechange事件处理，且值为4再正确处理下面的内容。

我自己的理解是这样的，ajax同步，即向服务器请求并在此等待服务器的回应，直到服务器回应了才继续往下走；ajax异步，即向服务器发送请求后，无需等待服务器的回应，直接往后走。

举个例子说明吧。
我们假设有三句语句

method1() 需要0.5秒
$.ajax() 需要2 秒
method2() 需要0.5秒

同步：就是1执行完后，执行2的时候，阻塞，直到ajax全部结束，执行方法3 一共3秒
异步：执行完1后，2开始执行的同时，3也马上执行非阻塞，往下走。理论全部结束一共 2.5秒
同步=串行，异步=并行，这样子。

本来没有ajax异步和同步的说法，AJAX本身便是异步请求数据的意思，但是请求数据还有同步的方式。

即XMLHttpRequest对象包含了同步请求和异步请求两种使用方式。当年因为异步技术给开发带来了深刻影响，所以AJAX技术火了，人们也习惯把向服务器请求数据的技术称为AJAX，但实际上还有同步，只是称呼上就被叫做了AJAX同步了，约定俗成而已

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