AJAX 异步传输数据的问题

要异步传输的数据：
xml代码
....
<action xsi:type="basic:javascript" script="index += 1;"/>
....
ajax异步传输代码：
js代码

var postdata = "input="+ escape(inputjson) +"&script="+escape(xml)+
"&feedgeneral=" + escape(feedgeneral);
xmlhttprequest.open("post",url,true);
xmlhttprequest.setrequestheader("content-type","application/x-www-form-urlencoded");
xmlhttprequest.send(postdata);

postdata在encode和unencode，最终导致在后台servlet中得到得到数据+被空格代替，使得script中的index += 1;变成了index = 1;从而导致后台java代码在跑script出现死循环。
在网上搜索，发现content-type使用application/x-www-form-urlencoded后：
[来自http://www.w3.org/tr/html401/interact/forms.html#h-17.13.4.1]写道

control names and values are escaped. space characters are replaced by `+', and then reserved characters are escaped as
described in [rfc1738], section 2.2: non-alphanumeric characters are replaced by `%hh', a percent sign and two hexadecimal
digits representing the ascii code of the character. line breaks are represented as "cr lf" pairs (i.e., `%0d%0a').

然而使用form来提交方式来发起request却不会出现类似的问题，而form默认的content-type也是application/x-www-form-urlencoded:
js代码

$('test').innerhtml = "<form target='_blank' id='test_form' action='./gen_feed' method='post'>"
+ "<input type='text' name='input' /><input type='text' name='script' />"
+ "<input type='text' name='feedgeneral' /><input type='hidden' name='format' value='" + this.feed_type + "'
/>"
+ "<input type='submit' value='gen' /></form>";
var test_form = $('test_form');
test_form.elements[0].value = inputjson;
test_form.elements[1].value = script;
test_form.elements[2].value = feedgeneral;
test_form.submit();

仍未发现问题到底出在何处，暂做备忘。暂时把script中的‘+'都用‘-'代替，index += 1;改成index -= -1;呵呵，以后有人看到这段自动生成的诡异脚本，不知道会作何感想，但现在也只能如此。