struts2 ajax jquery返回json类型

三个页面，

<?xml version="1.0" encoding="UTF-8" ?>

<!DOCTYPE struts PUBLIC "-//Apache Software Foundation//DTD Struts Configuration 2.1//EN" "http://struts.apache.org/dtds/struts-2.1.dtd">

<struts>

    <package name="default" namespace="/" extends="json-default">

        <action name="checkuser" class="checkuserAction">

            <result name="success" type="json">

                <param name="root">result</param>

            </result>

        </action>

    </package>

</struts>

import java.util.HashMap;

import java.util.Map;

import net.sf.json.JSONObject;

import com.opensymphony.xwork2.ActionSupport;

public class checkuserAction extends ActionSupport {

    private String username;

    private JSONObject obj;

    private String result;

    /**

     * @return

     */

    public String execute() {

        String msg = "";

        if (username.equals("admin")) {

            msg="已经注册啦，不行";

        }

        else {

            msg="可以注册，放心";

        }

        Map<String,String>  map = new HashMap<String,String>();

        map.put("msg", msg);

        obj=JSONObject.fromObject(map);

        result=obj.toString();

        return SUCCESS;

    }

    public String getUsername() {

        return username;

    }

    public void setUsername(String username) {

        this.username = username;

    }

    public JSONObject getObj() {

        return obj;

    }

    public void setObj(JSONObject obj) {

        this.obj = obj;

    }

    public String getResult() {

        return result;

    }

    public void setResult(String result) {

        this.result = result;

    }

}

<%@ page language="java" import="java.util.*" pageEncoding="utf-8"%>

<%

    String path = request.getContextPath();

    String basePath = request.getScheme() + "://"

            + request.getServerName() + ":" + request.getServerPort()

            + path + "/";

%>

<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">

<html>

<head>

<base href="<%=basePath%>">

<title>My JSP 'index.jsp' starting page</title>

<meta http-equiv="pragma" content="no-cache">

<meta http-equiv="cache-control" content="no-cache">

<meta http-equiv="expires" content="0">

<meta http-equiv="keywords" content="keyword1,keyword2,keyword3">

<meta http-equiv="description" content="This is my page">

<!--

    <link rel="stylesheet" type="text/css" href="styles.css">

    -->

<script type="text/javascript" src="jquery-2.1.4.js"></script>

<script type="text/javascript">

    $(document).ready(function() {

        $("#username").bind("blur", function() {

            var username = $("#username").val();

            $.ajax({

                type : "post",

                url : "checkuser",

                dataType : "json",

                data : {

                    'username' : username

                },

                success : function(data) {

                    var d = eval("(" + data + ")");//将数据转换成json类型，可以把data用alert()输出出来看看到底是什么样的结构

                    //得到的d是一个形如{"key":"value","key1":"value1"}的数据类型，然后取值出来

                    $("#msg").text("" + d.msg + "");

                }

            });

        });

    });

</script>

</head>

<body>

    <input type="text" name="username" id="username">

    <div id="msg"></div>

</body>

</html>

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struts2 ajax jquery返回json类型