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JSON notes

2022-07-28 06:58:00 【wr＿01】

One 、 summary

Concept ：JavaScript Object Notation.JavaScript Object notation .

The following is JavaScript Object definition format ：

{
    
	name:"zhangsan",
	age:23,
	city:" Beijing "
}

Let's take a look at JSON The format of ：

{
    
	"name":"zhangsan",
	"age":23,
	"city":" Beijing "
}

Pass above js Object format and json Compare the format , Two formats are found to be particularly similar to .

It's just js Attribute names in objects can be quoted （ It can be a single quotation mark , Or double quotes ）; and json The key in the format must be enclosed in double quotation marks , This is a json The rules of the form .

json What is the use of formatted data ？

effect ： Because of its simple syntax format , Distinct hierarchy , Now it is mostly used as Data carriers , Data transmission in the network . As shown in the figure below, it is the data that the server responds to the browser , This data is relatively simple , If you need it now JAVA If the data encapsulated in the object responds back to the browser , What kind of data transmission should be used ？

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You remember ajax The concept of ？ yes Asynchronous JavaScript and xml. there xml This is the way of data transmission in the past , as follows ：

<student>
    <name> Zhang San </name>
    <age>23</age>
    <city> Beijing </city>
</student>

Look again json Describe how the above data is written ：

{
    	
	"name":" Zhang San ",
    "age":23,
    "city":" Beijing "
}

After comparing the above two formats, you will find json Simple format of data , And less bytes .

Two 、JSON Basic grammar

2.1 Define format

JSON The essence is a string , However, the string content has certain format requirements . The definition format is as follows ：

var  Variable name  = '{"key":value,"key":value,...}';

JSON The key of the string must be enclosed in double quotation marks , The value depends on the type to represent .value The data types of are as follows

Numbers （ Integer or floating point number ）
character string （ Use double quotes ）
Boolean value （true perhaps false）
Array （ In square brackets ）
object （ In curly brackets ）
null

Example ：

var jsonStr = '{"name":"zhangsan","age":23,"addr":[" Beijing "," Shanghai "," Xi'an "]}'

2.2 Code demonstration

Create a page , On this page <script> Defined in label json character string

<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>
</head>
<body>
<script> //1.  Definition JSON character string  var jsonStr = '{"name":"zhangsan","age":23,"addr":[" Beijing "," Shanghai "," Xi'an "]}' alert(jsonStr); </script>
</body>
</html>

Open via browser , The page effect is shown in the figure below

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Now we need to get the JSON In a string name Property value , What should I do ？

If it is a js object , We can go through js object . Property name To get the data .JS Provides an object JSON , This object has the following two methods ：

parse(str) ： take JSON String to js object . The way to use it is ： var jsObject = JSON.parse(jsonStr);
stringify(obj) ： take js Object to JSON strand . The way to use it is ：var jsonStr = JSON.stringify(jsObject)

Code demonstration ：

<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>
</head>
<body>
<script> //1.  Definition JSON character string  var jsonStr = '{"name":"zhangsan","age":23,"addr":[" Beijing "," Shanghai "," Xi'an "]}' alert(jsonStr); //2.  take  JSON  String to  JS  object  let jsObject = JSON.parse(jsonStr); alert(jsObject) alert(jsObject.name) //3.  take  JS  Object to  JSON  character string  let jsonStr2 = JSON.stringify(jsObject); alert(jsonStr2) </script>
</body>
</html>

2.3 Send asynchronous request with parameters

We use axios When sending a request , If you want to carry complex data, you will use JSON Format , as follows

axios({
    
    method:"post",
    url:"http://localhost:8080/ajax-demo/axiosServlet",
    data:"username=zhangsan"
}).then(function (resp) {
    
    alert(resp.data);
})

It is impossible for us to splice strings by ourselves for request parameters ？ Definitely not , You can define a in advance js object , Used to encapsulate the parameters to be submitted , And then use JSON.stringify(js object ) Convert to JSON strand , And then JSON String as axios Of data Attribute value to submit the request parameters . as follows ：

var jsObject = {
    name:" Zhang San "};

axios({
    
    method:"post",
    url:"http://localhost:8080/ajax-demo/axiosServlet",
    data: JSON.stringify(jsObject)
}).then(function (resp) {
    
    alert(resp.data);
})

and axios It's a very powerful tool . We just need to encapsulate the parameters to be submitted into js object , And put the js Object as axios Of data Property value , It will automatically js Object to JSON Submit in series . as follows ：

var jsObject = {
    name:" Zhang San "};

axios({
    
    method:"post",
    url:"http://localhost:8080/ajax-demo/axiosServlet",
    data:jsObject  // here  axios  Will be the js Object to  json  List of 
}).then(function (resp) {
    
    alert(resp.data);
})

Be careful ：
js Provided JSON We just need to know the object . because axios Will automatically js Objects and JSON String for want to change conversion .
When sending an asynchronous request , If the request parameter is JSON Format , The way of request must be POST. because JSON The string needs to be placed in the request body .

3、 ... and 、JSON String sum Java The transformation of objects

After learning json after , Let's talk about json The role of . In the future, we will json Format data for front-end and back-end interaction . When the front end sends a request , If it is complex data, it will be in the form of json Submit to the back end ; If the back-end needs to respond to some complex data , It also needs to be json Format the data response back to the browser .

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On the back end, we need to focus on the following two parts of operation ：

Request data ：JSON String to Java object
The response data ：Java Object to JSON character string

Four 、Fastjson

4.1 Fastjson summary

Fastjson It's one provided by Alibaba Java High performance and perfect function of language JSON library , Is currently the Java The fastest in the language JSON library , Can achieve Java Objects and JSON The mutual conversion of strings .

4.2 Fastjson Use

Fastjson The use is also relatively simple , It is divided into the following three steps to complete

Import coordinates

<dependency>
    <groupId>com.alibaba</groupId>
    <artifactId>fastjson</artifactId>
    <version>1.2.62</version>
</dependency>

Java Object turn JSON
```
String jsonStr = JSON.toJSONString(obj);
```
take Java Object to JSON strand , Just use Fastjson Provided JSON Class toJSONString() Static method .
JSON String rotation Java object
```
User user = JSON.parseObject(jsonStr, User.class);
```
take json Convert to Java object , Just use Fastjson Provided JSON Class parseObject() Static method .

4.3 Code demonstration

Introduce coordinates

Create a class , Designed to test Java Objects and JSON Mutual conversion of strings , The code is as follows ：

public class FastJsonDemo {
      

    public static void main(String[] args) {
      
        //1.  take Java Object to JSON character string 
        User user = new User();
        user.setId(1);
        user.setUsername("zhangsan");
        user.setPassword("123");

        String jsonString = JSON.toJSONString(user);
        System.out.println(jsonString);//{"id":1,"password":"123","username":"zhangsan"}


        //2.  take JSON String to Java object 
        User u = JSON.parseObject("{\"id\":1,\"password\":\"123\",\"username\":\"zhangsan\"}", User.class);
        System.out.println(u);
    }
}