Struts2 and Ajax are used to transfer json objects, and then the secondary linkage of menus is realized

The following is the source code of my js file:

var mail={
//Initialize the
init:{
//Initialization data
initdata:{
did:'',
ttitle:'',
sendpassword:'',
description:''
},
//Initialization event
initevent:{
DataEvent:function(){
$("#did").unbind("change");//Gets a level one menu to bind events
$("#did").bind("change",function(){
//alert($(this).val());
mail.init.initdata.did=$(this).val();
//alert(mail.init.initdata.did);
mail.init.initevent.getuser();
});
},
<span style="color:#ff0000;">getuser:function(){
$.post("mailAction_showUsers?did="+mail.init.initdata.did,null,function(data){
var uidoption=$("#uid") ;//Get secondary menu
uidoption.empty();//Empty the list
for(var i=0;i<data.users.length;i++){
//Fill it with options, through the loop
uidoption.append("<option value="+data.users[i].uid+ " >"+data.users[i].username+"</option>" );
}
});</span>
},
submitCheck:function(){
$("#send").unbind("click");
$("#send").bind("click",function(){
mail.init.initdata.description=$("input[type='textarea']").text();
mail.init.initdata.sendpassword=$("input[name='sendpassword']").val();
mail.init.initdata.ttitle=$("input[name='ttitle']").val();

if(mail.init.initdata.sendpassword==""){
alert(" Please enter your password !");
return false;
}
else if(mail.init.initdata.ttitle==""){
alert(" Please enter the theme! ");
return false ;
}
else if(mail.init.initdata.description==""){
alert(" Please enter the content! ");
return false;
}
else
return true;
});
}
}
}
}

$().ready(function(){
mail.init.initevent.DataEvent();
mail.init.initevent.submitCheck();

});

This is the action in the background:

private int did;
public String getDid(){
...
}
public void setDid(Strign did){
...
}
public String showUsers(){

users=(ArrayList<User>)this.userService.getUsersByDid(did);
System.out.println(users.size()+"...");
return SUCCESS;
}

Struts.xml configuration:

<package name="users" namespace="/" extends="json-default">
<action name="mailAction_showUsers" method="showUsers" class="mailAction">
<result type="json"></result>
</action>

</package>

Foreground JSP page:

<td width="65px"> recipient </td>
<td> <s:select list="departmentlist" listKey="did" listValue="dname" id="did" name="did"></s:select>
<s:select list="users" listKey="uid" listValue="username" id="uid" name="uid"></s:select>
</td>

This is a screenshot of the run after implementation:   Write to someone in need… Refer to