1、JSON Introduce

        JSON：JavaScript Object Notation, namely JavaScript Object notation , Is a lightweight data exchange format , Independent of language , It can be read and used as data by any programming language .

        JSON The values of can be of the following types ：

                - Numbers （ Integer or floating point ）

                - character string （ Must be enclosed in double quotation marks ）

                - Boolean value （true or false）

                - Numbers （ Use square brackets to indicate ）

                - object （ In curly brackets ）

                - null

        JSON There are two structural forms ： Objects and arrays .

                The object structure is {} Express , The middle contains 0 Or more key:value Yes , Data is passed through obj["key"] or obj.key How to get ;

                Arrays are [] Express , In the middle by 0 Or a list of values , The data is orderly , therefore , It can be done by obj[index] To get data .

// 1. Object structure 
var obj = {
    "id":44021,
    "name":" You know ",
    "appPackage":"com.zhihu.android",
    "size":23720,
    "versionCode":442
};
console.log(obj.name);// You know 
console.log(obj["name"]);// You know 

// 2. Array structure 
var obj = [
    200,
    "success",
    {
        "subCode":1,
        "errorMsg":null
    }
];
console.log(obj[0]);//200
console.log(obj[2].subCode);//1

2、 Application scenarios

        （1） Interface returns data

                - In the front and rear end separation system , The data transmitted from the back end to the front end

                - Ajax Asynchronous access to data

                - to open up API Interface , Such as Baidu translation 、 Weather information, etc

                - The interface of cooperation between enterprises

                ...... wait

        （2） serialize

                serialize ： Convert the object into a sequence of bytes that can be transferred ;

                Deserialization ： The process of restoring a byte sequence to an object

                The ultimate purpose of serialization is to realize cross platform storage and network transmission , And our way of cross platform storage and network transmission is IO,IO The supported data is byte array .

                JSON Is one of the common serialization methods . When the object is serialized , Will save class Information down , But the data types of different languages are different , It is possible that the serialized data cannot be restored to its original appearance . but JSON When serializing an object, only the attribute data of the object is saved , Will not save class Information , It can read and write data across languages .

                another ： We use some json Parsed Library , Such as java Of fastjson and python Of json library , Has helped us achieve serialization and deserialization , We don't have to write code explicitly .

        （3） Generate Token

        （4） The configuration file

                The classic example is npm Package management profile for package.json

3、JSON Data operation

        3.1 JavaScript

                （1）eval()

                        This function takes a string parameter , And execute it as script code . such as ：eval("{name:' Zhang San '}"),{} Is parsed into code blocks , Represents declaring an object . thus , Using this function will json Convert string to object , There are two ways ：

                        The first one is ： stay json Concatenate in front of the string "var obj = ";

                        The second kind ： stay json Use before and after the string () Cover up , such eval Will not {} Parse into code blocks , It's an expression .

//  The first way 
var str = '{"id":"44021","name"," You know "}';
eval('var obj = ' + str);
console.log(obj["name"]);// You know 

//  The second way 
var str = '{"id":"44021","name"," You know "}';
var obj = eval("(" + str + ")");
console.log(obj["name"]);// You know 

                （2）JSON.parse()

                        This function receives json Format string , Convert it to JavaScript object .

                        Special Usage ： The function also has an argument reviver, It's a function , You can define some operations in it , Yes json The value of is processed and then returned .

var str = '{"id":"44021","name"," You know "}';
var obj = JSON.parse(str);
console.log(obj["name"]);// You know 


var str = '[{"id":"44021","name"," You know "},{"id":"40019","name"," TaoBao "},{"id":"45001","name"," Public comment "}]';
var obj = JSON.parse(str);
console.log(obj[1].name);// TaoBao 


//  Special Usage 
var text = '{"name":" Xiao Zhan ","birth":"1991-10-05","city":" Chongqing "}';
var obj = JSON.parse(text,function(key,value){
    //  Convert date string to date object 
	if (key == "birth"){
		return new Date(value);
	}else{
		return value;
	}
});

                （3）JSON.stringify()

                        This function takes JavaScript Object to json character string ： to key Parameters are enclosed in double quotes ,value unchanged .

var obj = {
	name:" Wang Yibo ",
	age:24,
	city:" luoyang "
};
var jsonStr = JSON.stringify(obj);
console.log(jsonStr[1]);//"

        3.2 Java

                fastjson There are three main categories ：JSON、JSONObject、JSONArray.

                among ,JSON Is the parent of the other two classes , Mainly used for json String and object conversion , Including string and JavaBean、JSONObject、JSONArray The mutual transformation between them ; and JSONObject and JSONArray Is used for parsing json character string 、 get data .

                （1）JSON

// json String to javabean
String str = "{\"format\":\"png\",\"leval\":1,\"title\":\"json String to javabean\"}";
Material m = JSON.parseObject(str,Material.class);



// json String to list
String str = "[{\"format\":\"png\",\"leval\":1,\"title\":\" First object \"},{\"format\":\"jpg\",\"leval\":2,\"title\":\" The second object \"},{\"format\":\"gif\",\"leval\":3,\"title\":\" The third object \"}]";
List<Material> materials = JSON.parseArray(str,Material.class);
for (Material m : materials)
{
    System.out.println(m.toString);
}



// jsonobject To javabean
// jaonarray To list
JSONArray mUrls = item.getJSONArray("mUrls");
for (int i = 0; i < mUrls.size(); i++)
{
    MUrl url = JSON.toJavaObject(mUrls.getJSONObject(i),MUrl.class);
    list.add(url);
}



// map To json character string 
ArrayList<Material> list = new ArrayList<>();
list.add(material);
list.add(material2);
list.add(material3);
HashMap<String,Object> map = new HashMap<>();
map.put("leval",1);
map.put("title","map To json character string ");
map.put("list",list);
map.put("format","png");
System.out.println(JSON.toJSONString(map));

                （2）JSONObject

                        getxxx(key)： obtain key Corresponding value , And turn it into a certain type , Such as getInteger()、getString、getJSONArray() etc. .

String msg = (String)jsonObject.get("msg");
Integer code = jsonObject.getInteger("code");
JsonArray objects = jsonObject.getJSONArray("objects");

                （3）JSONArray

                        And JSONObject be similar , Data is also obtained through getxxx(index) The way , It's just that the parameter is the index index.

                        in addition , because JSONArray It stands for json An array of objects , Class also provides direct operations json Object method , Such as add()、remove() etc. .

        3.3 Python