&＃xfeff;前言
之前我曾经写过几个JSON解析库的使用方法&＃xff1a;
Qt平台下使用QJson解析和构建JSON字符串
使用cJSON库解析JSON
使用cJSON库构建JSON
对于嵌入式开发&＃xff0c;比较常用的就是cJSON解析库了&＃xff0c;但是使用这个库对内存开销比较大&＃xff0c;需要大量使用malloc和free进行动态内存分配和释放。对于单片机这种资源短缺的芯片来说&＃xff0c;简直是太吃力了。使用STM32等单片机&＃xff0c;不能解析多层JSON时&＃xff0c;还需要调大堆栈。
今天来了解一下一个和cJSON非常类似的解析库——Jansson的使用&＃xff0c;支持解析和构建JSON字符串&＃xff0c;不需要动态分配内存&＃xff0c;使用方法和cJSON几乎一样。
本文以Keil环境下的Jansson为例&＃xff0c;介绍多种不同格式的JSON字符串解析。
示例工程下载&＃xff1a;whik1194-JanssonDemo.rar
Jansson的安装
Jansson的安装非常简单&＃xff0c;到Keil官网下载一个Jansson的Pack包即可&＃xff1a; Keil.Jansson.1.0.0.pack&＃xff0c;点击直接下载。
安装完成之后&＃xff0c;如果要在当前项目中添加Jansson解析库&＃xff0c;在Keil中勾选上Jansson即可&＃xff0c;选择完成之后&＃xff0c;在当前工程下会出现两个文件&＃xff1a;jansson_CM3LE.lib和jansson_config.c。
在要使用的文件中包含头文件&＃xff1a;#include
为了方便调试&＃xff0c;自定义实现了一个串口格式化输出函数&＃xff1a;LOG1。
void LOG1(char *fmt,...)
{unsigned char UsartPrintfBuf[296];va_list ap;unsigned char *pStr &＃61; UsartPrintfBuf;va_start(ap, fmt);vsnprintf((char *)UsartPrintfBuf, sizeof(UsartPrintfBuf), fmt, ap); //格式化va_end(ap);while(*pStr !&＃61; 0){while(USART_GetFlagStatus(USART1, USART_FLAG_TC) &＃61;&＃61; RESET);USART_SendData(USART1, *pStr&＃43;&＃43;);}
}
示例1:纯键值对
JSON字符串&＃xff0c;包括字符串、整形和布尔类型三种类型的键值对&＃xff1a;
{"status": true,"age": 20,"name": "whik1194","blog": "https://blog.csdn.net/whik1194","wechat": "mcu149"
}
解析函数&＃xff1a;
uint8_t Jansson1_Demo(void)
{const char *text &＃61; "{"status":true,"age":20,"score":78.4,"name":"whik1194","blog":"https://blog.csdn.net/whik1194","wechat":"mcu149"}";json_error_t error;json_t *root;json_t *status;char *name, *blog, *wechat;int age;double score;root &＃61; json_loads((const char*)text, 0, &error); if(json_is_object(root)){LOG1("root format correctrn");/* string 类型的解析 */name &＃61; (char *)json_string_value(json_object_get(root, "name"));blog &＃61; (char *)json_string_value(json_object_get(root, "blog"));wechat &＃61; (char *)json_string_value(json_object_get(root, "wechat"));LOG1("name: %srn", name);LOG1("blog: %srn", blog);LOG1("wechat: %srn", wechat);/* int 类型的解析 */age &＃61; json_integer_value(json_object_get(root, "age"));LOG1("age: %drn", age);/* double 类型的解析 */score &＃61; json_real_value(json_object_get(root, "score"));LOG1("score: %.1frn", score);/* bool 类型的解析 */status &＃61; json_object_get(root, "status");if(json_is_true(status))LOG1("status: truern");else if(json_is_false(status))LOG1("status: falsern");}else{printf("root format error:%d-%srn", error.line, error.text);return 1;}return 0;
}
运行结果&＃xff1a;
示例2:键的值是一个对象
JSON字符串&＃xff0c;其中result键的值是一个JSON对象&＃xff0c;包括几组键值对&＃xff1a;
{"success": "1","result": {"timestamp": "1592640249","datetime_1": "2020-06-20 16:04:09","week_1": "6","week_2": "Saturday"}
}
解析函数&＃xff1a;
uint8_t Jansson2_Demo(void)
{const char *text &＃61; "{"success":"1","result":{"timestamp":"1592640249","datetime_1":"2020-06-20 16:04:09","week_1":"6","week_2":"Saturday"}}";json_error_t error;json_t *root;json_t *result;char *success;char *timestamp;char *datetime_1;char *week_1;root &＃61; json_loads((const char*)text, 0, &error); if(json_is_object(root)){LOG1("root format correctrn");success &＃61; (char *)json_string_value(json_object_get(root, "success"));LOG1("success:%srn", success);result &＃61; json_object_get(root, "result");if(json_is_object(result)){timestamp &＃61; (char *)json_string_value(json_object_get(result, "timestamp"));datetime_1 &＃61; (char *)json_string_value(json_object_get(result, "datetime_1"));week_1 &＃61; (char *)json_string_value(json_object_get(result, "week_1"));LOG1("timestamp:%srn", timestamp);LOG1("datetime_1:%srn", datetime_1);LOG1("week_1:%srn", week_1);}else {LOG1("reuslt format errorrn");return 1;}}else{LOG1("root format error:%d-%srn", error.line, error.text);return 1;}return 0;
}
运行结果&＃xff1a;
示例3:键的值是一个数组
JSON字符串&＃xff0c;键的值是一个JSON数组&＃xff0c;包含两个JSON对象&＃xff1a;
{"location": [{"name": "Faye","address": "Beijing"}, {"name": "Andy","address": "ShangHai"}, {"name": "Lisa","address": "ShenZhen"}],"time": "2018-11-17"
}
解析函数&＃xff1a;
uint8_t Jansson3_Demo(void)
{const char *text &＃61; "{"location":[{"name":"Faye","address":"Beijing"},{"name":"Andy","address":"ShangHai"},{"name":"Lisa","address":"ShenZhen"}],"time":"2018-11-17"}";json_error_t error;json_t *root;json_t *location_arr;json_t *location;char *name, *address;char *time;int idx;root &＃61; json_loads((const char*)text, 0, &error); if(json_is_object(root)){LOG1("root format correctrn");time &＃61; (char *)json_string_value(json_object_get(root, "time"));location_arr &＃61; json_object_get(root, "location");if(json_is_array(location_arr)){for(idx &＃61; 0; idx }
运行结果&＃xff1a;
注意
如果出现解析失败&＃xff0c;或者解析错误&＃xff0c;可以尝试增加Heap_Size大小&＃xff1a;
不知道是不是编码的问题&＃xff0c;Jansson并不支持含有中文的JSON字符串解析&＃xff0c;会报格式错误。
示例工程下载&＃xff1a;whik1194-JanssonDemo.rar
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