jar包：
json-lib-2.2.3-jdk15.jar
struts2-json-plugin-2.3.4.1.jar（此包版本必须和Struts2版本对应，否则会抛错）
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js文件：
query-1.7.js和json2.js。
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页面导入js文件路径：
<script language="javascript" src="scripts/javascript/jquery-1.7.js"></script>
<script language="javascript" src="scripts/javascript/json2.js"></script>
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js函数：
function checkUserName(){
var url='checkUserName';
var params={
    studentName:'1'
};
$.post(
    url,
    params,
    function callback(json){
        var responseData = eval(json);
        var result = responseData.ckUserName;
        if(result=="pass"){
            $("h1").text("pass");
        }else{
            $("h1").text("npass");
        }
    },
    'json'
    ); 
}
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配置Struts2.xml：
        <action name="checkUserName" class="com.shenzhen.management.action.HelloWorldAjaxAction" method = "checkUserName">
<result type="json">
<param name="#root">ckUserName</param>
</result>
</action>
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Action方法（通过Spring注入Service接口抛异常，所以另外写了一个HelloWorldAjaxAction.java文件）：
public String checkUserName()
{
    String retStr = "fail";
   //通过Spring注入Service接口抛异常，未找到原因，现通过ApplicationContext获取bean，知道如何解决的大神请留言，谢谢！
    ServletContext servletContext = ServletActionContext.getServletContext();
    ApplicationContext applicationContext = WebApplicationContextUtils.getRequiredWebApplicationContext(servletContext);
    StudentService studentService = (StudentService)applicationContext.getBean("studentService");
    try{
        ckUserName = "pass";
        retStr = "success";
    }catch(Exception e){
            ckUserName = "npass";
    }
    return retStr;
}