File parsing (JSON parsing)

One . What is? JSON

JSON Is a lightweight data exchange format , It's based on ECMAScript A subset of , Use text format completely independent of programming language to store and represent data . A simple and clear hierarchy makes JSDN Become the ideal data exchange language . Easy to read and understand , At the same time, it is also easy for machine analysis and generation , And effectively improve the network transmission efficiency .

Two .JSON grammar

Use braces { } Save the object , Each object consists of several pieces of data
Each data is generated by key : value Key value pairs make up
Use commas between data , Separate
Use \ Escape special characters

{"key1" : value1,"key1" : value1,......"keyN" : valueN}

Use brackets **[ ]** Save array ( aggregate ), Array ( aggregate )… Can contain multiple objects

[{"key1" : value1,"key1" : value1,......"keyN" : valueN},
{"key1" : value1,"key1" : value1,......"keyN" : valueN},
......]

data:[{"key1" : value1,"key1" : value1,......"keyN" : valueN},
{"key1" : value1,"key1" : value1,......"keyN" : valueN},
......]

3、 ... and .JSON Use of
JSON As a lightweight data exchange format , Its main purpose is to transfer data between computer systems , It has the following advantages ：

·JSON Only use **UTF-8** code , There is no coding problem ;
·JSON The content only includes **key-value** Key value pair , The format is simple , There is no redundant structure , It is a lightweight structure ;
· Browser built in JSON Support , If you use the data JSON Send it to the browser , It can be used **JavaScript** Deal directly with ;

therefore , Development Web When applied , Use JSON As data transmission , It is very convenient on the browser side .JSON Natural fit JavaScript Handle , therefore , most RESTAPI All choices JSON As a data transfer format .

Four .Java analysis JSON
Use a third-party library to JSON Parsing of file format data
Commonly used for parsing JSON The third-party library of ：

·Jackson
·Gson
·Fastjson

Fastjson

Fastjson It's Alibaba's open source JSON Parsing library , It can parse JSON Format string , Support will java The object is serialized into json Formatted data , Can also be json The string of the format is inversely sequenced into Java object .

Fastjson advantage ：
 Fast 
 Widely used 
 The test is complete ( There are many. testcase, Perform regression tests every time you release commas , Ensure the quality is stable )
 Easy to use (fastjson Of API It's very simple )
 Fully functional ( Supports generics , Support streaming large text , Support enumeration , Support serialization and deserialization extension )

.Fastjson The main object of ： The main use of JSON,JSONObject,JSONArray Three categories , among JSONObject and
JSONArray Inherit JSON.

JSON class :
JSON Class is used for original transformation , Common methods are ：

1. take java object **** serialize by json Formatted data :

   JSON.toJSONString(Object object);

        //  Entity data 
		Weather weather = new Weather();
		weather.setCity(" Xi'an ");
		weather.setComfort_index(" Very comfortable ");
		weather.setDate_y("2022 year 07 month 10 Japan ");

		//  Convert to json Format string (  serialize )
		String json = JSON.toJSONString(weather);
		System.out.println(json);

2. take json Format String Inverse sequence become Java object

①. JSONObject class ：
It is mainly used for packaging key-value Key value pair data , It inherited LinkedHashMap Interface .

**** JSON.parseObject()****

// json Format data 
String jsonStr = "{\"temperature\":\"29℃~41℃\",\"weather\":\" It's cloudy \",\"weather_id\":{\"fa\":\"01\",\"fb\":\"02\"},\"wind\":\" The Northeast breeze \",\"week\":\" Sunday \",\"city\":\" Xi'an \",\"date_y\":\"2022 year 07 month 10 Japan \",\"dressing_index\":\" scorching hot \",\"dressing_advice\":\" It's hot , Suggest a short shirt 、 Short skirt 、 shorts 、 Thin type T Cool summer clothes such as T-shirts .\",\"uv_index\":\" secondary \",\"comfort_index\":\"\",\"wash_index\":\" More appropriate \",\"travel_index\":\" Less suitable \",\"exercise_index\":\" Less suitable \",\"drying_index\":\"\"}";

//  convert to JSONObject
JSONObject jsonObj = JSON.parseObject(jsonStr);
System.out.println(" date ：" + jsonObj.getString("date_y"));
System.out.println(" City ：" + jsonObj.getString("city"));
System.out.println(" The weather ：" + jsonObj.getString("weather"));
System.out.println(" temperature ：" + jsonObj.getString("temperature"));

②. JSONArray class ：
JSONArray Class is mainly used to encapsulate the data of array collection class , It is inherited from ArrayList class

JSON.parseArray()

// json Format data 
String jsonStr =  "{\r\n" + 
				"\r\n" + 
				"	\"title\": \" Shanghai municipal police station information \",\r\n" + 
				"	\"list\": [{\r\n" + 
				"		\"name\": \" Hudong University police station of cultural relics protection branch \",\r\n" + 
				"		\"addr\": \" Zhongshan North 1st Road 801 Number \",\r\n" + 
				"		\"tel\": \"22027732\"\r\n" + 
				"	}, {\r\n" + 
				"		\"name\": \" Huxi University police station of cultural relics protection branch \",\r\n" + 
				"		\"addr\": \" Furongjiang road 55 Number \",\r\n" + 
				"		\"tel\": \"62751704\"\r\n" + 
				"	}, {\r\n" + 
				"		\"name\": \" Wusong water police station of water Public Security Bureau \",\r\n" + 
				"		\"addr\": \" Songpu Road 187 Number \",\r\n" + 
				"		\"tel\": \"56671442\"\r\n" + 
				"	}, {\r\n" + 
				"		\"name\": \" Yangpu water police station of water Public Security Bureau \",\r\n" + 
				"		\"addr\": \" Yangshupu Road 1291 Number \",\r\n" + 
				"		\"tel\": \"65898004\"\r\n" + 
				"	}, {\r\n" + 
				"		\"name\": \" Waitan water police station of water Public Security Bureau \",\r\n" + 
				"		\"addr\": \" Zhongshan 2nd Road 8 get 3 Number \",\r\n" + 
				"		\"tel\": \"63305388\"\r\n" + 
				"	}]\r\n" + 
				"}";

//  convert to JSONArray
JSONArray jsonArray = JSON.parseArray(jsonStr);

//  Traverse JSONArray
for(int i =0 ; i <jsonArray.size(); i++) {
    JSONObject item = jsonArray.getJSONObject(i);
    System.out.println(item);
}

Four . common problem

Question 1 ：
FastJson Default filter null value , No display null Value fields .

Map<String, Object> map = new HashMap<String, Object>(){
    {
        put("age", 18);
        put("name", " Zhang San ");
        put("sex", null);
    }
};

System.out.println(JSONObject.toJSONString(map));

【 Running results 】：

【 solve 】：
convert to JSON When the string , Use Feature Enumeration values .

Map<String, Object> map = new HashMap<String, Object>(){
    {
        put("age", 18);
        put("name", " Zhang San ");
        put("sex", null);
    }
};
//  Use Feature Set the enumeration value of type 
System.out.println(JSONObject.toJSONString(map,Feature.WriteMapNullValue));

Question two ： control JSON Field order of

//  Entity class 
public class PoliceStation {
	private String name;
	private String addr;
	private String tel;
	
	
	public String getName() {
		return name;
	}
	public void setName(String name) {
		this.name = name;
	}
	public String getAddr() {
		return addr;
	}
	public void setAddr(String addr) {
		this.addr = addr;
	}
	public String getTel() {
		return tel;
	}
	public void setTel(String tel) {
		this.tel = tel;
	}
}

//  Test class 
PoliceStation ps = new PoliceStation();
ps.setName(" Electronic City police station ");
ps.setAddr(" Wangwang planet ");
ps.setTel("14587091409");

System.out.println(JSON.toJSONString(ps));

【 Running results 】：

【 solve 】： The output result is inconsistent with the field definition order . When defining entity class fields , Use @JSONFiled Annotated ordinal Property for sequential configuration .

import com.alibaba.fastjson2.annotation.JSONField;
public class PoliceStation {
	@JSONField(ordinal = 1)
	private String name;
	
	@JSONField(ordinal = 2)
	private String addr;
	
	@JSONField(ordinal = 3)
	private String tel;
}

Question 3 ：
control JSON Of Date Field format

//  Entity order class 
public class Order{
    //  The order no. 
	private String orderId;
	
    //  Date of creation 
	private LocalDateTime creationTime;
	
	public Order() {
		this.orderId = UUID.randomUUID().toString();
		this.creationTime = LocalDateTime.now();
	}

	public String getOrderId() {
		return orderId;
	}

	public void setOrderId(String orderId) {
		this.orderId = orderId;
	}

	public LocalDateTime getCreationTime() {
		return creationTime;
	}

	public void setCreationTime(LocalDateTime creationTime) {
		this.creationTime = creationTime;
	}
}

//  Test class 
public class Test {
	public static void main(String[] args) {
		Order order1 = new Order();
		String json = JSON.toJSONString(order1);
		System.out.println(json);
	}
}

【 Running results 】
【 solve 】
When outputting the date field , When the default format does not meet the requirements , You can define entity classes in Date Field , Use @JSONFiled Annotated format Property to configure the format .

//  Order class 
public class Order{
	//  The order no. 
	private String orderId;
	
	//  Date of creation 
	@JSONField(format = "yyyy-MM-dd HH:mm:ss")
	private LocalDateTime creationTime;
}