Brief introduction and use of JSON

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Json

One 、 What is? Json

• JSON(JavaScript Object Notation, JS Object tag ) Is a lightweight data exchange format , At present, it is widely used .
• Using programming language independent Text format To store and represent data .
• A simple and clear hierarchy makes JSON Become the ideal data exchange language .
• Easy to read and write , At the same time, it is also easy for machine analysis and generation , And effectively improve the network transmission efficiency .

stay JavaScript In language , Everything is an object . therefore , whatever JavaScript All types of support are available through JSON To express , Like strings 、 Numbers 、 object 、 Array etc. . Look at his requirements and grammar ：

• Objects are represented as key value pairs , Data is separated by commas
• Curly braces hold objects
• Square brackets hold arrays

JSON Key value pair It's for keeping JavaScript A way of targeting , and JavaScript The writing method of the object is also similar , key / The key name in the value pair combination is written before and in double quotation marks “” The parcel , Use a colon : Separate , And then it's worth ：

json Data instance ：

{"name": "QinJiang"}
{"age": "3"}
{"sex": " male "}

Json and JS The object looks like , Easy to confuse , In fact, you can put Json It means JS String representation of object , It uses text to represent a JS Object information , The essence is a string .

contrast ：

// This is an object , Note that key names can also be enclosed in quotation marks 
var obj = {a: 'Hello', b: 'World'}; 
// This is a  JSON  character string , The essence is a string 
var json = "{"a": "Hello", "b": "World"}"; 

In the front end JS It also encapsulates methods that can make json and js Objects can be transformed into each other , If you are interested, you can search by yourself , The following is mainly about Java Yes Json The operation of .

Two 、Java How to convert objects and Json

1、JackSon

1.1、 Preparation for use

SSM project ：

You need to manually import dependencies ：

        <dependency>
            <groupId>com.fasterxml.jackson.core</groupId>
            <artifactId>jackson-core</artifactId>
            <version>2.9.6</version>
        </dependency>

        <dependency>
            <groupId>com.fasterxml.jackson.core</groupId>
            <artifactId>jackson-annotations</artifactId>
            <version>2.9.6</version>
        </dependency>

        <dependency>
            <groupId>com.fasterxml.jackson.core</groupId>
            <artifactId>jackson-databind</artifactId>
            <version>2.9.6</version>
        </dependency>

SpringBoot project ：

SpringBoot Bring their own JackSon, There is no need to manually introduce dependencies

1.2、 Easy to use

pojo Entity class ：

@Data
@NoArgsConstructor
@AllArgsConstructor
public class User {
    public Integer id;
    public Integer age;
    public String name;
}

controller：

    @RequestMapping(path = "/json")
    public String json() throws JsonProcessingException {
        // Create a jackson Object mapper for , Used to parse data 
        ObjectMapper mapper = new ObjectMapper();
        // Create an object 
        User user = new User(1, 3, " Summer sail ");
        // Resolve our object to json Format 
        String json = mapper.writeValueAsString(user);

        return json;
    }

Running results ：

1.3、 Processing date type

Get into ObjectMapper Object mapping class discovery , It returns a sequence for date processing ：

    public DateFormat getDateFormat() {
        return this._serializationConfig.getDateFormat();
    }

meanwhile , It also provides a set Method to modify the date format ：

    public ObjectMapper setDateFormat(DateFormat dateFormat) {
        this._deserializationConfig = (DeserializationConfig)this._deserializationConfig.with(dateFormat);
        this._serializationConfig = this._serializationConfig.with(dateFormat);
        return this;
    }

So for operations with dates , We can customize it ：

DateFormat Abstract class , We generally use its subclasses SimpleDateFormat To operate , The string operation rules of this subclass are as follows ：

Examples are as follows ：

pojo Entity class ：

@Data
@NoArgsConstructor
@AllArgsConstructor
public class User {
    public Integer id;
    public Integer age;
    public String name;
    public Date date;
}

controller：

    @RequestMapping(path = "/json")
    public String json() throws JsonProcessingException {
        // Create a jackson Object mapper for , Used to parse data 
        ObjectMapper mapper = new ObjectMapper();
        // Create an object 
        User user = new User(1, 3, " Summer sail ",new Date());
        // to ObjectMapper Object to customize the date format 
        SimpleDateFormat simpleDateFormat = new SimpleDateFormat("yyyy year mm month dd Japan ");
        mapper.setDateFormat(simpleDateFormat);
        // Resolve our object to json Format 
        String json = mapper.writeValueAsString(user);

        return json;
    }

Running results ：

2、FastJson

fastjson.jar It's a product developed by Ali, which is specially used for Java Developed packages , Easy to implement json Object and the JavaBean Object conversion , Realization JavaBean Object and the json String conversion , Realization json Object and the json String conversion . Realization json There are many ways to convert , The final result is the same . The biggest advantage is high performance .

2.1、 Preparation for use

Whether it's SSM still SpringBoot All need to introduce dependencies ：

<dependency>
   <groupId>com.alibaba</groupId>
   <artifactId>fastjson</artifactId>
   <version>1.2.60</version>
</dependency>

2.2、 Three main operation classes

JSONObject representative json object

• JSONObject Realized Map Interface , guess JSONObject The underlying operation is made up of Map Realized .
• JSONObject Corresponding json object , Through various forms of get() Method can obtain json Data in object , You can also use things like size(),isEmpty() And so on " key ： value " The number of pairs and whether the judgment is empty . Its essence is to achieve Map Interface and call methods in the interface .

JSONArray representative json An array of objects

• Inside there is List To complete the operation in the interface .

JSON representative JSONObject and JSONArray The transformation of

• JSON Class source code analysis and use
• Look closely at these methods , Mainly to achieve json object ,json An array of objects ,javabean object ,json Mutual conversion between strings .

2.3、 Easy to use

pojo Entity class ：

@Data
@NoArgsConstructor
@AllArgsConstructor
public class User {
    public Integer id;
    public Integer age;
    public String name;
}

controller：

    @RequestMapping(path = "/fjson")
    public String testFjson() throws JsonProcessingException, JSONException {

        User user1 = new User(1, 1, " Summer sail 1");
        User user2 = new User(2, 2, " Summer sail 2");
        User user3 = new User(3, 3, " Summer sail 3");
        List<User> list = new ArrayList<User>();
        list.add(user1);
        list.add(user2);
        list.add(user3);

        String l1 = JSON.toJSONString(list);
        String u1 = JSON.toJSONString(user1);
        String u2 = JSON.toJSONString(user2);
        String u3 = JSON.toJSONString(user3);

        return l1;
    }

Running results ：