Synchronization: submit the request - > wait for the server to process - > return after processing. During this period, the client browser cannot do anything
Asynchronous: the request is triggered by an event - > processed by the server (at this time, the browser can still do other things) - > processing is completed
Synchronization refers to the communication mode in which the sender sends data and the receiver sends a response before sending the next data packet.
Asynchrony refers to the communication mode in which the sender sends data, does not wait for the receiver to send back the response, and then sends the next data packet
Easy to understand:
Asynchronous transmission: you can transmit. I'll do my job. Let me know when the transmission is over
Synchronous transmission: you transmit now. I want to see your transmission complete before I do anything else
What is Ajax?
This object was first introduced in Internet Explorer 5. It is a technology that supports asynchronous requests. In short, XmlHttpRequest enables you to make requests to the server and process responses using JavaScript without blocking users.
AJAX:(Asynchronous JavaScript and XML) is not a new technology, but a combination of many technologies, including Javascript, XHTML and CSS, DOM, XML and XMLHttpRequest
What is the difference between get and post in Ajax?
If any of the following conditions are met, use the POST method:
*The result of the request has persistent side effects, such as adding new data rows to the database.
*If you use GET method, data collected on form may make URL too long.
*The data to be transmitted is not encoded in 7-Bit ASCII.
If any of the following conditions are met, use the GET method:
*The request is to find resources, and the HTML form data is only used to help search.
*The request results have no persistent side effects.
*The total length of the collected data and the name of the input field in the HTML form shall not exceed 1024 characters.
How to realize the three-level linkage between provinces and cities?
<%@ page contentType="text/html;charset=UTF-8" language="java" %>
<html>
<head>
<title>Title</title>
<script src="js/jquery-1.8.3.js"type="text/javascript"></script>
</head>
<script type="text/javascript">
$(function (){
$.ajax({
type:"get",
url:"provinceAll",
dataType:"json",
success:function (data){
for (var i = 0; i <data.length ; i++) {
$("#province").append("<option value='"+data[i].pid+"'>"+data[i].pname+"</option>");
}
}
});
$("#province").change(function (){
$("#city option:gt(0)").remove();
$("#district option:gt(0)").remove();
$.ajax({
type:"get",
url:"cityAll?pid="+$("#province").val(),
dataType:"json",
success:function (data){
for (var i = 0; i <data.length ; i++) {
$("#city").append("<option value='"+data[i].cid+"'>"+data[i].cname+"</option>");
}
}
});
});
$("#city").change(function (){
$("#district option:gt(0)").remove();
$.ajax({
type:"get",
url:"districtAll?cid="+$("#city").val(),
dataType:"json",
success:function (data){
for (var i = 0; i <data.length ; i++) {
$("#district").append("<option value='"+data[i].did+"'>"+data[i].dname+"</option>");
}
}
});
});
});
</script>
<body>
<select id="province">
<option>Please select Province</option>
</select>province
<select id="city">
<option>Please select a city</option>
</select>city
<select id="district">
<option>Please select an area</option>
</select>area
</body>
</html>
package Level3LinkageServlet;
import JDBC.DBHelper;
import Provinces.City;
import Provinces.Province;
import net.sf.json.JSONArray;
import javax.servlet.*;
import javax.servlet.http.*;
import javax.servlet.annotation.*;
import java.io.IOException;
import java.sql.Connection;
import java.sql.PreparedStatement;
import java.sql.ResultSet;
import java.sql.SQLException;
import java.util.ArrayList;
import java.util.List;
@WebServlet(name = "cityAllServlet", value = "/cityAll")
public class cityAllServlet extends HttpServlet {
@Override
protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
String pid=request.getParameter("pid");
int pid1=Integer.parseInt(pid);
Connection con= DBHelper.getConn();
String sql="select*from city where pid=?";
List<City> clist=new ArrayList<City>();
City ci=null;
try {
PreparedStatement ps=con.prepareStatement(sql);
ps.setInt(1,pid1);
ResultSet rs=ps.executeQuery();
while (rs.next()){
ci=new City(rs.getInt(1),rs.getString(2),rs.getInt(3));
clist.add(ci);
}
} catch (SQLException e) {
e.printStackTrace();
}
JSONArray json=JSONArray.fromObject(clist);
response.getWriter().write(json.toString());
}
@Override
protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
}
}
package Level3LinkageServlet;
import JDBC.DBHelper;
import Provinces.Province;
import net.sf.json.JSONArray;
import net.sf.json.JSONObject;
import javax.servlet.*;
import javax.servlet.http.*;
import javax.servlet.annotation.*;
import java.io.IOException;
import java.sql.Connection;
import java.sql.PreparedStatement;
import java.sql.ResultSet;
import java.sql.SQLException;
import java.util.ArrayList;
import java.util.List;
@WebServlet(name = "provinceAllServlet", value = "/provinceAll")
public class provinceAllServlet extends HttpServlet {
@Override
protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
Connection con= DBHelper.getConn();
String sql="select*from province";
List<Province> plist=new ArrayList<Province>();
Province pv=null;
try {
PreparedStatement ps=con.prepareStatement(sql);
ResultSet rs=ps.executeQuery();
while (rs.next()){
pv=new Province(rs.getInt(1),rs.getString(2));
plist.add(pv);
}
} catch (SQLException e) {
e.printStackTrace();
}
JSONArray json=JSONArray.fromObject(plist);
response.getWriter().write(json.toString());
}
@Override
protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
}
}
package Level3LinkageServlet;
import JDBC.DBHelper;
import Provinces.City;
import Provinces.District;
import net.sf.json.JSONArray;
import javax.servlet.*;
import javax.servlet.http.*;
import javax.servlet.annotation.*;
import java.io.IOException;
import java.sql.Connection;
import java.sql.PreparedStatement;
import java.sql.ResultSet;
import java.sql.SQLException;
import java.util.ArrayList;
import java.util.List;
@WebServlet(name = "districtAllServlet", value = "/districtAll")
public class districtAllServlet extends HttpServlet {
@Override
protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
String cid=request.getParameter("cid");
int cid1=Integer.parseInt(cid);
Connection con= DBHelper.getConn();
String sql="select*from district where cid=?";
List<District> dlist=new ArrayList<District>();
District di=null;
try {
PreparedStatement ps=con.prepareStatement(sql);
ps.setInt(1,cid1);
ResultSet rs=ps.executeQuery();
while (rs.next()){
di=new District(rs.getInt(1),rs.getString(2),rs.getInt(3));
dlist.add(di);
}
} catch (SQLException e) {
e.printStackTrace();
}
JSONArray json=JSONArray.fromObject(dlist);
System.out.println(json);
response.getWriter().write(json.toString());
}
@Override
protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
}
}