JSON字符串解析函数cJSON_Parse()
cJSON * cJSON_Parse(const char *value)
{
//第一个参数：要解析的字符串
//第二个参数：最后一个被解析的字符（传出）
//第三个参数：被解析的字符串是否以null结尾
return cJSON_ParseWithOpts(value, 0, 0);
}
typedef struct
{
const unsigned char *content; //字符串
size_t length; //字符串长度
size_t offset; //偏移量（解析到哪里了）
size_t depth; /* How deeply nested (in arrays/objects) is the input at the current offset. */
internal_hooks hooks; //钩子函数
} parse_buffer;
cJSON * cJSON_ParseWithOpts(const char *value, const char **return_parse_end, cJSON_bool require_null_terminated)
{
parse_buffer buffer = { 0, 0, 0, 0, { 0, 0, 0 } };
cJSON *item = NULL;
/* reset error position */
global_error.json = NULL;
global_error.position = 0;
if (value == NULL)
{
goto fail;
}
//初始化parse_buffer结构体
buffer.content = (const unsigned char*)value;
buffer.length = strlen((const char*)value) + sizeof("");
buffer.offset = 0;
buffer.hooks = global_hooks;
item = cJSON_New_Item(&global_hooks);
if (item == NULL) /* memory fail */
{
goto fail;
}
//parse_value是真正的解析函数
if (!parse_value(item, buffer_skip_whitespace(skip_utf8_bom(&buffer))))
{
/* parse failure. ep is set. */
goto fail;
}
/* if we require null-terminated JSON without appended garbage, skip and then check for a null terminator */
//检查被解析的字符是否以NULL结尾
if (require_null_terminated)
{
buffer_skip_whitespace(&buffer);
if ((buffer.offset >= buffer.length) || buffer_at_offset(&buffer)[0] != '\0')
{
goto fail;
}
}
//返回最后一个被解析的字符
if (return_parse_end)
{
*return_parse_end = (const char*)buffer_at_offset(&buffer);
}
return item;
//异常处理
fail:
if (item != NULL)
{
cJSON_Delete(item);
}
if (value != NULL)
{
//存放错误的json字符串和错误位置
error local_error;
local_error.json = (const unsigned char*)value;
local_error.position = 0;
if (buffer.offset < buffer.length)
{
local_error.position = buffer.offset;
}
else if (buffer.length > 0)
{
local_error.position = buffer.length - 1;
}
if (return_parse_end != NULL)
{
*return_parse_end = (const char*)local_error.json + local_error.position;
}
global_error = local_error;
}
return NULL;
}
真正的解析函数parse_value
/* check if the given size is left to read in a given parse buffer (starting with 1) */
#define can_read(buffer, size) ((buffer != NULL) && (((buffer)->offset + size) <= (buffer)->length))
/* check if the buffer can be accessed at the given index (starting with 0) */
#define can_access_at_index(buffer, index) ((buffer != NULL) && (((buffer)->offset + index) < (buffer)->length))
#define cannot_access_at_index(buffer, index) (!can_access_at_index(buffer, index))
/* get a pointer to the buffer at the position */
#define buffer_at_offset(buffer) ((buffer)->content + (buffer)->offset)
/* Parser core - when encountering text, process appropriately. */
static cJSON_bool parse_value(cJSON * const item, parse_buffer * const input_buffer)
{
if ((input_buffer == NULL) || (input_buffer->content == NULL))
{
return false; /* no input */
}
/* parse the different types of values */
/* null */
//解析null字符串，can_read函数判断offset+4有没有超过length，buffer_at_offset函数返回一个指针指向下一个未解析的位置。
if (can_read(input_buffer, 4) && (strncmp((const char*)buffer_at_offset(input_buffer), "null", 4) == 0))
{
item->type = cJSON_NULL;
//offset+4，offset是已经解析过的字符数
input_buffer->offset += 4;
return true;
}
/* false */
if (can_read(input_buffer, 5) && (strncmp((const char*)buffer_at_offset(input_buffer), "false", 5) == 0))
{
item->type = cJSON_False;
input_buffer->offset += 5;
return true;
}
/* true */
if (can_read(input_buffer, 4) && (strncmp((const char*)buffer_at_offset(input_buffer), "true", 4) == 0))
{
item->type = cJSON_True;
item->valueint = 1;
input_buffer->offset += 4;
return true;
}
/* string */
//解析字符串，必需以 " 字符开头
if (can_access_at_index(input_buffer, 0) && (buffer_at_offset(input_buffer)[0] == '\"'))
{
return parse_string(item, input_buffer);
}
/* number */
//数字，以-号或者数字开头
if (can_access_at_index(input_buffer, 0) && ((buffer_at_offset(input_buffer)[0] == '-') || ((buffer_at_offset(input_buffer)[0] >= '0') && (buffer_at_offset(input_buffer)[0] <= '9'))))
{
return parse_number(item, input_buffer);
}
/* array */
//数组，以 [ 开头
if (can_access_at_index(input_buffer, 0) && (buffer_at_offset(input_buffer)[0] == '['))
{
return parse_array(item, input_buffer);
}
/* object */
//object对象类型，以 { 开头
if (can_access_at_index(input_buffer, 0) && (buffer_at_offset(input_buffer)[0] == '{'))
{
return parse_object(item, input_buffer);
}
return false;
}
字符串类型解析函数
/* Parse the input text into an unescaped cinput, and populate item. */
static cJSON_bool parse_string(cJSON * const item, parse_buffer * const input_buffer)
{
const unsigned char *input_pointer = buffer_at_offset(input_buffer) + 1;
const unsigned char *input_end = buffer_at_offset(input_buffer) + 1;
unsigned char *output_pointer = NULL;
unsigned char *output = NULL;
/* not a string */
if (buffer_at_offset(input_buffer)[0] != '\"')
{
goto fail;
}
//这一段为什么要用{}包起来，没有看懂。
{
/* calculate approximate size of the output (overestimate) */
size_t allocation_length = 0;
size_t skipped_bytes = 0;
//如果没有超过了字符串的长度以及没有遇到结尾的 " 字符，循环继续
while (((size_t)(input_end - input_buffer->content) < input_buffer->length) && (*input_end != '\"'))
{
/* is escape sequence */
//如果是转义字符，如果字符串中有'\' 'n'这样的字符(这个是两个字符)，将它们变成'\n'（一个字符）
if (input_end[0] == '\\')
{
if ((size_t)(input_end + 1 - input_buffer->content) >= input_buffer->length)
{
/* prevent buffer overflow when last input character is a backslash */
goto fail;
}
skipped_bytes++;
input_end++;
}
input_end++;
}
//如果不是以 " 字符结尾，报错
if (((size_t)(input_end - input_buffer->content) >= input_buffer->length) || (*input_end != '\"'))
{
goto fail; /* string ended unexpectedly */
}
/* This is at most how much we need for the output */
//给output指针分配空间
allocation_length = (size_t) (input_end - buffer_at_offset(input_buffer)) - skipped_bytes;
output = (unsigned char*)input_buffer->hooks.allocate(allocation_length + sizeof(""));
if (output == NULL)
{
goto fail; /* allocation failure */
}
}
output_pointer = output;
/* loop through the string literal */
while (input_pointer < input_end)
{
if (*input_pointer != '\\')
{
*output_pointer++ = *input_pointer++;
}
/* escape sequence */
else
{
unsigned char sequence_length = 2;
if ((input_end - input_pointer) < 1)
{
goto fail;
}
// 将'\'和后面的字符变成一个字符。
switch (input_pointer[1])
{
case 'b':
*output_pointer++ = '\b';
break;
case 'f':
*output_pointer++ = '\f';
break;
case 'n':
*output_pointer++ = '\n';
break;
case 'r':
*output_pointer++ = '\r';
break;
case 't':
*output_pointer++ = '\t';
break;
case '\"':
case '\\':
case '/':
*output_pointer++ = input_pointer[1];
break;
/* UTF-16 literal */
case 'u':
sequence_length = utf16_literal_to_utf8(input_pointer, input_end, &output_pointer);
if (sequence_length == 0)
{
/* failed to convert UTF16-literal to UTF-8 */
goto fail;
}
break;
default:
goto fail;
}
input_pointer += sequence_length;
}
}
/* zero terminate the output */
*output_pointer = '\0';
item->type = cJSON_String;
item->valuestring = (char*)output;
input_buffer->offset = (size_t) (input_end - input_buffer->content);
//为什么还要++？
input_buffer->offset++;
return true;
fail:
if (output != NULL)
{
input_buffer->hooks.deallocate(output);
}
if (input_pointer != NULL)
{
input_buffer->offset = (size_t)(input_pointer - input_buffer->content);
}
return false;
}
解析数字
/* Parse the input text to generate a number, and populate the result into item. */
static cJSON_bool parse_number(cJSON * const item, parse_buffer * const input_buffer)
{
double number = 0;
unsigned char *after_end = NULL;
unsigned char number_c_string[64];
unsigned char decimal_point = get_decimal_point();
size_t i = 0;
if ((input_buffer == NULL) || (input_buffer->content == NULL))
{
return false;
}
/* copy the number into a temporary buffer and replace '.' with the decimal point
* of the current locale (for strtod)
* This also takes care of '\0' not necessarily being available for marking the end of the input */
for (i = 0; (i < (sizeof(number_c_string) - 1)) && can_access_at_index(input_buffer, i); i++)
{
switch (buffer_at_offset(input_buffer)[i])
{
case '0':
case '1':
case '2':
case '3':
case '4':
case '5':
case '6':
case '7':
case '8':
case '9':
case '+':
case '-':
case 'e':
case 'E':
number_c_string[i] = buffer_at_offset(input_buffer)[i];
break;
case '.':
number_c_string[i] = decimal_point;
break;
default:
goto loop_end;
}
}
loop_end:
number_c_string[i] = '\0';
number = strtod((const char*)number_c_string, (char**)&after_end);
if (number_c_string == after_end)
{
return false; /* parse_error */
}
item->valuedouble = number;
/* use saturation in case of overflow */
if (number >= INT_MAX)
{
item->valueint = INT_MAX;
}
else if (number <= (double)INT_MIN)
{
item->valueint = INT_MIN;
}
else
{
item->valueint = (int)number;
}
item->type = cJSON_Number;
input_buffer->offset += (size_t)(after_end - number_c_string);
return true;
}
数组解析
#define CJSON_NESTING_LIMIT 1000
/* Build an array from input text. */
static cJSON_bool parse_array(cJSON * const item, parse_buffer * const input_buffer)
{
cJSON *head = NULL; /* head of the linked list */
cJSON *current_item = NULL;
//防止嵌套超过1000层，防止堆栈溢出
if (input_buffer->depth >= CJSON_NESTING_LIMIT)
{
return false; /* to deeply nested */
}
input_buffer->depth++;
if (buffer_at_offset(input_buffer)[0] != '[')
{
/* not an array */
goto fail;
}
input_buffer->offset++;
//跳过空白字符
buffer_skip_whitespace(input_buffer);
if (can_access_at_index(input_buffer, 0) && (buffer_at_offset(input_buffer)[0] == ']'))
{
/* empty array */
goto success;
}
/* check if we skipped to the end of the buffer */
if (cannot_access_at_index(input_buffer, 0))
{
input_buffer->offset--;
goto fail;
}
/* step back to character in front of the first element */
//为什么要回退一个字符？
input_buffer->offset--;
/* loop through the comma separated array elements */
do
{
/* allocate next item */
cJSON *new_item = cJSON_New_Item(&(input_buffer->hooks));
if (new_item == NULL)
{
goto fail; /* allocation failure */
}
/* attach next item to list */
if (head == NULL)
{
/* start the linked list */
current_item = head = new_item;
}
else
{
/* add to the end and advance */
current_item->next = new_item;
new_item->prev = current_item;
current_item = new_item;
}
/* parse next value */
input_buffer->offset++;
buffer_skip_whitespace(input_buffer);
//递归调用
if (!parse_value(current_item, input_buffer))
{
goto fail; /* failed to parse value */
}
buffer_skip_whitespace(input_buffer);
}
while (can_access_at_index(input_buffer, 0) && (buffer_at_offset(input_buffer)[0] == ','));
if (cannot_access_at_index(input_buffer, 0) || buffer_at_offset(input_buffer)[0] != ']')
{
goto fail; /* expected end of array */
}
success:
input_buffer->depth--;
item->type = cJSON_Array;
item->child = head;
input_buffer->offset++;
return true;
fail:
if (head != NULL)
{
cJSON_Delete(head);
}
return false;
}
object类型解析
/* Build an object from the text. */
static cJSON_bool parse_object(cJSON * const item, parse_buffer * const input_buffer)
{
cJSON *head = NULL; /* linked list head */
cJSON *current_item = NULL;
//和数组一样要判断深度
if (input_buffer->depth >= CJSON_NESTING_LIMIT)
{
return false; /* to deeply nested */
}
input_buffer->depth++;
//如果不是以 '{' 开头，不是object类型
if (cannot_access_at_index(input_buffer, 0) || (buffer_at_offset(input_buffer)[0] != '{'))
{
goto fail; /* not an object */
}
input_buffer->offset++;
buffer_skip_whitespace(input_buffer);
if (can_access_at_index(input_buffer, 0) && (buffer_at_offset(input_buffer)[0] == '}'))
{
goto success; /* empty object */
}
/* check if we skipped to the end of the buffer */
if (cannot_access_at_index(input_buffer, 0))
{
input_buffer->offset--;
goto fail;
}
/* step back to character in front of the first element */
input_buffer->offset--;
/* loop through the comma separated array elements */
do
{
/* allocate next item */
cJSON *new_item = cJSON_New_Item(&(input_buffer->hooks));
if (new_item == NULL)
{
goto fail; /* allocation failure */
}
/* attach next item to list */
if (head == NULL)
{
/* start the linked list */
current_item = head = new_item;
}
else
{
/* add to the end and advance */
current_item->next = new_item;
new_item->prev = current_item;
current_item = new_item;
}
/* parse the name of the child */
//解析key的名称
input_buffer->offset++;
buffer_skip_whitespace(input_buffer);
if (!parse_string(current_item, input_buffer))
{
goto fail; /* failed to parse name */
}
buffer_skip_whitespace(input_buffer);
/* swap valuestring and string, because we parsed the name */
current_item->string = current_item->valuestring;
current_item->valuestring = NULL;
if (cannot_access_at_index(input_buffer, 0) || (buffer_at_offset(input_buffer)[0] != ':'))
{
goto fail; /* invalid object */
}
/* parse the value */
//解析value
input_buffer->offset++;
buffer_skip_whitespace(input_buffer);
if (!parse_value(current_item, input_buffer))
{
goto fail; /* failed to parse value */
}
buffer_skip_whitespace(input_buffer);
}
while (can_access_at_index(input_buffer, 0) && (buffer_at_offset(input_buffer)[0] == ','));
if (cannot_access_at_index(input_buffer, 0) || (buffer_at_offset(input_buffer)[0] != '}'))
{
goto fail; /* expected end of object */
}
success:
input_buffer->depth--;
item->type = cJSON_Object;
item->child = head;
input_buffer->offset++;
return true;
fail:
if (head != NULL)
{
cJSON_Delete(head);
}
return false;
}
关于将json对象转换成字符串的部分放到下一篇。
