How to use Ajax and JavaScript in JSP to achieve cascading effects

With the rapid development of Web applications, front-end technology is becoming more and more important. In this article, we'll take a deep dive into how to use Ajax and JavaScript in JSP to achieve cascading effects.

1. What is cascade?

Cascading means that after selecting an option in one drop-down list, the options in another drop-down list will also change accordingly. For example, if you select Beijing in the "Province" drop-down list, the options in the "City" drop-down list will automatically change to the city to which Beijing belongs.

2. Ajax and JavaScript in JSP

In JSP, we can use Ajax and JavaScript to interact with the interface. Among them, Ajax is the abbreviation of Asynchronous JavaScript and XML (asynchronous JavaScript and XML). It uses JavaScript and XML technologies to update a portion of a page without reloading the entire page. JavaScript is a scripting language that can be run in the browser. It can operate HTML pages and achieve dynamic effects on web pages.

3. How to realize cascading?

1. Front-end part

In the front-end, we need to define the code for the two drop-down lists. For example, we define a "Province" and a "City" drop-down list:

<label for="province">省份</label>
<select id="province">
  <option value="0">请选择</option>
  <option value="1">北京市</option>
  <option value="2">上海市</option>
  <option value="3">广东省</option>
</select>

<label for="city">城市</label>
<select id="city">
  <option value="0">请选择</option>
</select>

Here we use the id attribute to name the drop-down lists so that they can be operated in JavaScript.

2. Backend part

In the background, we need to define an interface to obtain a city list. For example, we can use the Spring MVC framework to implement this function:

@RestController
@RequestMapping("/city")
public class CityController {

    @GetMapping("/{provinceId}")
    public List<City> getCityList(@PathVariable int provinceId) {
        List<City> cityList = null;
        // 查询城市列表的代码
        return cityList;
    }

    @Data
    public static class City {
        private int id;
        private String name;
    }

}

Here we define a CityController class and obtain and return the city list in its getCityList method. Note that we also define an inner class City to represent city information.

3. JavaScript part

In JavaScript, we need to perform a series of operations, including:

• Listen to the selection of the "Province" drop-down list Event
• Send Ajax request to the background to get the city list
• Dynamicly update the contents of the "City" drop-down list based on the city list

For example, we can use jQuery Library to implement this function:

$(document).ready(function () {
  $("#province").change(function () {
    var provinceId = $(this).val();
    if (provinceId > 0) {
      $.get("/city/" + provinceId, function (data) {
        var citySelect = $("#city");
        citySelect.empty().append('<option value="0">请选择</option>');
        $.each(data, function (index, city) {
          citySelect.append('<option value="&#39; + city.id + &#39;">' + city.name + '</option>');
        });
      });
    } else {
      $("#city").empty();
    }
  });
});

In this JavaScript code, we define a ready method to be executed after the page is loaded. Then, we listened to the selection event of the "Province" drop-down list and obtained its value. If the value is greater than 0, it means that the user has selected a valid province, and we use the $.get method to send an Ajax request to the background to obtain the corresponding city list. After successfully obtaining the city list, we dynamically updated the contents of the "City" drop-down list. Otherwise, if the user selects "Please select", we clear the "City" drop-down list.

4. Summary

This article deeply explores the method of using Ajax and JavaScript in JSP to achieve cascading effects. By defining drop-down lists on the front end, defining interfaces on the backend, and invoking JavaScript events and Ajax methods, we can easily achieve cascading effects, improve user experience, and make web applications more convenient and efficient.

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