JQuery实现跨域访问
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??? PrintWriterout = response.getWriter();
??? out.println(output);
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补充说明：
??? 我调试是用的http://localhost:8080/ 域，虽然都是本地服务器，但是，在http://localhost:8080/下访问
http://127.0.0.1:8080/，当然是不同域了。如果调试时，地址栏上用的域与你jQuery访问时用的域不一致，同样会涉及跨域访问的知识。
??? 这也是我发现这个问题的原因。
使用getJSON函数是，为了实现跨域访问，必须加上callback=?,因为request.getParameter("callback")。
服务器端必须得到jQuery动态生成的回调函数名，再传给客户端。
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我的这个试例，客户端响应为：
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jQuery152028319909285452693_1302780724731({'symbol' : 'IBM', 'price' : '91.42'});?
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