使用cJSON库解析和构建JSON字符串

前言

其实之前的两篇博文已经介绍了json格式和如何使用cJSON库来解析JSON：

• 使用cJSON库解析JSON
• JSON简介

当时在MCU平台上使用时，会出现时间长了死机的情况，在调用cJSON_Print输出格式化后的JSON数据之后，

    LOG("JSON数据:\n%s\n", cJSON_Print(root));    

要使用cJSON_Delete释放内存，否则会导致内存泄漏。

cJSON_Delete(root);     //调用cJSON_Print时才需要

这一点在嵌入式开发平台要格外注意。

解析和构建JSON的示例程序，我都已经上传到代码托管平台上，示例工程基于CodeBlocks开发环境。

Github仓库地址：

Gitee仓库地址：

或者通过下面的命令clone到本地：

Github：

git clone https://gitee.com/whik/cJSON_Demo.git

Gitee：

git clone https://github.com/whik/cJSON_Demo.git

JSON解析示例包括：

• 和风天气实时数据
• 心知天气实时数据
• 心知天气3天预报数据
• 城市空气质量AQI信息
• 全国油价信息
• 北京时间等。

JSON的构建：

• 简单的键值对
• JSON对象作为键的值
• JSON数组
• JSON数组的嵌套

JSON的构建

cJSON是一个基于C语言的JSON解析库，这个库非常简单，只有cJSON.c和cJSON.h两个文件，支持JSON的解析和构建，需要调用时，只需要#include "cJSON.h"就可以使用了。

由于JSON的解析之前已经介绍过了：使用cJSON库解析JSON，所以本篇博文主要介绍使用cJSON来构建JSON，强大的cJSON库在构建JSON上也是非常的简单。

1.一个简单的JSON键值对构建

构建函数：

void Create_Simple_JSON(void)
{cJSON *root;root = cJSON_CreateObject();//创建一个json对象cJSON_AddItemToObject(root, "CSDN", cJSON_CreateString("https://blog.csdn.net/whik1194"));cJSON_AddItemToObject(root, "cnblogs", cJSON_CreateString("https://home.cnblogs.com/u/whik/"));cJSON_AddItemToObject(root, "Github", cJSON_CreateString("https://github.com/whik/"));cJSON_AddStringToObject(root, "Blog", "http://www.wangchaochao.top/");printf("构建的JSON:\n%s\n", cJSON_Print(root));cJSON_Delete(root);
}

输出结果：

{"CSDN": "https://blog.csdn.net/whik1194","cnblogs": "https://home.cnblogs.com/u/whik/","Github": "https://github.com/whik/","Blog": "http://www.wangchaochao.top/"
}

2.键的值是一个JSON对象

构建函数：

void Create_BJTime_JSON(void)
{cJSON *root;cJSON *result;root = cJSON_CreateObject();//创建一个json对象result = cJSON_CreateObject();//result构建cJSON_AddItemToObject(result, "timestamp", cJSON_CreateString("ok"));
//等效于下面
//    cJSON_AddStringToObject(result, "timestamp", "ok");cJSON_AddItemToObject(result, "datetime_1", cJSON_CreateString("2019-07-21 10:46:57"));cJSON_AddItemToObject(result, "datetime_2", cJSON_CreateString("2019年07月21日 10时46分57秒"));cJSON_AddItemToObject(result, "week_1", cJSON_CreateString("0"));cJSON_AddItemToObject(result, "week_2", cJSON_CreateString("星期日"));cJSON_AddItemToObject(result, "week_3", cJSON_CreateString("周日"));cJSON_AddItemToObject(result, "week_4", cJSON_CreateString("Sunday"));//等效于cJSON_AddNumberToObject(root, "ok", 1);cJSON_AddItemToObject(root, "status", cJSON_CreateString("success"));cJSON_AddItemToObject(root, "result", result);cJSON_AddStringToObject(root, "Blog", "www.wangchaochao.top");printf("构建的JSON:\n%s\n", cJSON_Print(root));cJSON_Delete(root);
}

输出结果：

{"status": "success","result": {"timestamp": "ok","datetime_1": "2019-07-21 10:46:57","datetime_2": "2019年07月21日 10时46分57秒","week_1": "0","week_2": "星期日","week_3": "周日","week_4": "Sunday"},"Blog": "www.wangchaochao.top"
}

3.JSON数组，元素是字符串

构建函数：

void Create_Array_Str_JSON(void)
{cJSON *root;const char *strings[7]={"Sunday","Monday","Tuesday","Wednesday","Thursday","Friday","Saturday"};root=cJSON_CreateStringArray(strings,7);printf("%s\n",cJSON_Print(root));cJSON_Delete(root);
}

输出结果：

["Sunday", "Monday", "Tuesday", "Wednesday", "Thursday", "Friday", "Saturday"]

4.键的值是一个数组，数组包含多个对象元素

构建函数：

void Create_Array_JSON(void)
{cJSON *root;cJSON *forceast;cJSON *day1, *day2, *day3;  //数组day1 = cJSON_CreateObject();day2 = cJSON_CreateObject();day3 = cJSON_CreateObject();cJSON_AddStringToObject(day1, "date", "2019-07-21");    //日期cJSON_AddStringToObject(day1, "cond_txt", "多云");      //天气状况cJSON_AddStringToObject(day1, "cond_code", "101");      //天气代码cJSON_AddStringToObject(day1, "hum", "23");             //湿度cJSON_AddStringToObject(day1, "tmp_H", "31");           //最高温度cJSON_AddStringToObject(day1, "tmp_L", "25");           //最低温度cJSON_AddStringToObject(day2, "date", "2019-07-22");cJSON_AddStringToObject(day2, "cond_txt", "晴");cJSON_AddStringToObject(day2, "cond_code", "100");cJSON_AddStringToObject(day2, "hum", "20");cJSON_AddStringToObject(day2, "tmp_H", "33");cJSON_AddStringToObject(day2, "tmp_L", "26");cJSON_AddStringToObject(day3, "date", "2019-07-23");cJSON_AddStringToObject(day3, "cond_txt", "阵雨");cJSON_AddStringToObject(day3, "cond_code", "107");cJSON_AddStringToObject(day3, "hum", "45");cJSON_AddStringToObject(day3, "tmp_H", "32");cJSON_AddStringToObject(day3, "tmp_L", "25");forceast = cJSON_CreateArray();//注意顺序，索引依次递增cJSON_AddItemToArray(forceast, day1);   //元素0cJSON_AddItemToArray(forceast, day2);   //元素1cJSON_AddItemToArray(forceast, day3);   //元素2root = cJSON_CreateObject();    //创建一个json对象cJSON_AddStringToObject(root, "status", "ok");cJSON_AddItemToObject(root, "weather", forceast);cJSON_AddStringToObject(root, "update", "2019-07-21 11:00");cJSON_AddStringToObject(root, "Blog", "www.wangchaochao.top");//等效于:cJSON_AddItemToObject(root, "update", cJSON_CreateString("2019-07-21 11:00");printf("构建的JSON:\n%s\n", cJSON_Print(root));cJSON_Delete(root);
}

输出结果：

{"status": "ok","weather": [{"date": "2019-07-21","cond_txt": "多云","cond_code": "101","hum": "23","tmp_H": "31","tmp_L": "25"}, {"date": "2019-07-22","cond_txt": "晴","cond_code": "100","hum": "20","tmp_H": "33","tmp_L": "26"}, {"date": "2019-07-23","cond_txt": "阵雨","cond_code": "107","hum": "45","tmp_H": "32","tmp_L": "25"}],"update": "2019-07-21 11:00","Blog": "www.wangchaochao.top"
}

5.数组内嵌套了5个数组，每个数组内有5个字符串元素

构建函数：

void Create_Array_Nest_JSON(void)
{struct oil_stu{char *city;          //城市名称char *oil_92_price;  //92号汽油价格char *oil_95_price;char *oil_98_price;char *oil_0_price;};cJSON *root;cJSON *data;  //包含多个数组cJSON *table, *data_bj, *data_sh, *data_js, *data_tj;const char *bj_str[5] = {"北京", "6.78", "7.21", "8.19", "6.45"};const char *sh_str[5] = {"上海", "6.74", "7.17", "7.87", "6.39"};const char *js_str[5] = {"江苏", "6.75", "7.18", "8.06", "6.37"};const char *tj_str[5] = {"天津", "6.77", "7.15", "8.07", "6.41"};const char *talbe_str[5] = {"地区", "92号汽油", "95号汽油", "98号汽油", "0号柴油"};data_bj = cJSON_CreateStringArray(bj_str, 5);   //只包含5个字符串的数组data_sh = cJSON_CreateStringArray(sh_str, 5);data_js = cJSON_CreateStringArray(js_str, 5);data_tj = cJSON_CreateStringArray(tj_str, 5);table = cJSON_CreateStringArray(talbe_str, 5);data = cJSON_CreateArray();cJSON_AddItemToArray(data, table);cJSON_AddItemToArray(data, data_bj);cJSON_AddItemToArray(data, data_sh);cJSON_AddItemToArray(data, data_js);cJSON_AddItemToArray(data, data_tj);root = cJSON_CreateObject();cJSON_AddStringToObject(root, "status", "ok");cJSON_AddStringToObject(root, "msg", "2019-07-21 11:00");cJSON_AddStringToObject(root, "update", "2019-07-21 11:00");cJSON_AddItemToObject(root, "data", data);cJSON_AddStringToObject(root, "About", "wcc");cJSON_AddStringToObject(root, "Blog", "www.wangchaochao.top");printf("构建的JSON:\n%s\n", cJSON_Print(root));cJSON_Delete(root);
}

输出结果：

{"status": "ok","msg": "全国各省份汽柴油价格信息","update": "2019-07-21","data": [["地区", "92号汽油", "95号汽油", "98号汽油", "0号柴油"],["北京", "6.78", "7.21", "8.19", "6.45"],["上海", "6.74", "7.17", "7.87", "6.39"],["江苏", "6.75", "7.18", "8.06", "6.37"],["天津", "6.77", "7.15", "8.07", "6.41"]],"About": "wcc","Blog": "www.wangchaochao.top"
}

参考资料

• cJSON库源码：cJSON download
• JSON官方网站：json

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