在采用jquery ajax调用http请求时，发现了一系列问题：
如采用firebug调试API请求（这个API是自己服务器的应用），看到服务器明明返回200状态，response返回数据也是json格式，但ajax返回的error。
在排除json数据格式不正确的原因之后，发现了ajax error函数返回“networkerror failed to execute ‘send' on ‘xmlhttprequest' failed to load ‘http //“ XMLHttpRequest.status=0，就是没有初始化。
后来才知道是跨域问题（CORS），因为程序调用的是远程服务器的API，服务器不允许跨域调用。如果只是简单的方法，只需要在程序的response添加支持跨域的header添加属性”Access-Control-Allow-Origin: *“即可。如java 服务器代码：
yourownvariable.setHeader("Access-Control-Allow-Origin:", "origin url of your site");
yourownvariable.setHeader("Access-Control-Allow-Methods", "GET, POST,PUT");
如果是配置nginx服务器（如果是其他服务器，可以参考：I want to add CORS support to my server），需要在nginx.conf配置文件添加一下内容：
#
# Wide-open CORS config for nginx
#
location / {
if ($request_method = 'OPTIONS') {
add_header 'Access-Control-Allow-Origin' '*';
add_header 'Access-Control-Allow-Methods' 'GET, POST, OPTIONS';
#
# Custom headers and headers various browsers *should* be OK with but aren't
#
add_header 'Access-Control-Allow-Headers' 'DNT,X-CustomHeader,Keep-Alive,User-Agent,X-Requested-With,If-Modified-Since,Cache-Control,Content-Type,Content-Range,Range';
#
# Tell client that this pre-flight info is valid for 20 days
#
add_header 'Access-Control-Max-Age' 1728000;
add_header 'Content-Type' 'text/plain charset=UTF-8';
add_header 'Content-Length' 0;
return 204;
}
if ($request_method = 'POST') {
add_header 'Access-Control-Allow-Origin' '*';
add_header 'Access-Control-Allow-Methods' 'GET, POST, OPTIONS';
add_header 'Access-Control-Allow-Headers' 'DNT,X-CustomHeader,Keep-Alive,User-Agent,X-Requested-With,If-Modified-Since,Cache-Control,Content-Type,Content-Range,Range';
add_header 'Access-Control-Expose-Headers' 'DNT,X-CustomHeader,Keep-Alive,User-Agent,X-Requested-With,If-Modified-Since,Cache-Control,Content-Type,Content-Range,Range';
}
if ($request_method = 'GET') {
add_header 'Access-Control-Allow-Origin' '*';
add_header 'Access-Control-Allow-Methods' 'GET, POST, OPTIONS';
add_header 'Access-Control-Allow-Headers' 'DNT,X-CustomHeader,Keep-Alive,User-Agent,X-Requested-With,If-Modified-Since,Cache-Control,Content-Type,Content-Range,Range';
add_header 'Access-Control-Expose-Headers' 'DNT,X-CustomHeader,Keep-Alive,User-Agent,X-Requested-With,If-Modified-Since,Cache-Control,Content-Type,Content-Range,Range';
}
}