mirror of
https://github.com/eledio-devices/thirdparty-ArduinoJson.git
synced 2025-10-31 16:14:11 +01:00
Updated README.md
This commit is contained in:
Benoit Blanchon
@@ -25,7 +25,7 @@ Example
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JsonParser<32> parser;
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char json[] = "{\"sensor\":\"gps\",\"time\":1351824120,\"data\":[48.756080,2.302038]}";
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JsonHashTable root = parser.parse(json);
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JsonObject root = parser.parse(json);
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char* sensor = root["sensor"];
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long time = root["time"];
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@@ -58,7 +58,7 @@ To extract data from the JSON string, you need to create a `JsonParser`, and spe
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> #### How to choose the number of tokens ?
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> A token is an element of the JSON object: either a key, a value, an hash-table or an array.
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> A token is an element of the JSON object: either a key, a value, an object or an array.
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> As an example the `char json[]` on the top of this page contains 12 tokens (don't forget to count 1 for the whole object and 1 more for the array itself).
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> The more tokens you allocate, the more complex the JSON can be, but also the more memory is occupied.
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@@ -71,28 +71,24 @@ To extract data from the JSON string, you need to create a `JsonParser`, and spe
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To use this library, you need to know beforehand what is the type of data contained in the JSON string, which is very likely.
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The root object has to be either a hash-table (like `{"key":"value"}`) or an array (like `[1,2]`).
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The root object has to be either an object (like `{"key":"value"}`) or an array (like `[1,2]`).
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The nested objects can be either arrays, booleans, hash-tables, numbers or strings.
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The nested objects can be either arrays, booleans, objects, numbers or strings.
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If you need other type, you can get the string value and parse it yourself.
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#### Hash-table
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#### Object
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Consider we have a `char json[]` containing to the following JSON string:
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{
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"Name":"Blanchon",
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"Skills":[
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"C",
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"C++",
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"C#"],
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"Age":32,
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"Online":true
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"sensor":"gps",
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"time":1351824120,
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"data":[48.756080,2.302038]
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}
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In this case the root object of the JSON string is a hash-table, so you need to extract a `JsonHashTable`:
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In this case the string contains a JSON object, so you need to extract a `JsonObject`:
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JsonHashTable root = parser.parseHashTable(json);
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JsonObject root = parser.parse(json);
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To check if the parsing was successful, you must check:
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@@ -103,13 +99,10 @@ To check if the parsing was successful, you must check:
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And then extract the member you need:
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char* name = hashTable.getString("Name");
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JsonArray skills = hashTable.getArray("Skills");
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int age = hashTable.getLong("Age");
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bool online = hashTable.getBool("Online");
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char* sensor = root["sensor"];
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long time = root["time"];
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double latitude = root["data"][0];
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double longitude = root["data"][1];
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#### Array
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@@ -122,7 +115,7 @@ Consider we have a `char json[]` containing to the following JSON string:
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In this case the root object of the JSON string is an array, so you need to extract a `JsonArray`:
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JsonArray root = parser.parseArray(json);
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JsonArray root = parser.parse(json);
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To check if the parsing was successful, you must check:
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@@ -133,20 +126,17 @@ To check if the parsing was successful, you must check:
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And then extract the content by its index in the array:
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JsonArray row0 = root.getArray(0);
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double a = row0.getDouble(0);
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or simply:
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double a = root.getArray(0).getDouble(0);
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double a = root[0][0];
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double b = root[0][1];
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double c = root[1][0];
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double d = root[1][1];
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Common pitfalls
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---------------
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### 1. Not enough tokens
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By design, the library has no way to tell you why `JsonParser::parseArray()` or `JsonParser::parseHashTable()` failed.
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By design, the library has no way to tell you why `JsonParser::parse()` failed.
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There are basically two reasons why they may fail:
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@@ -173,7 +163,7 @@ That is why an 8-bit processor is not able to parse long and complex JSON string
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### 3. JsonParser not in memory
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To reduce the memory consumption, `JsonArray` and `JsonHashTable` contains pointer to the token that are inside the `JsonParser`. This can only work if the `JsonParser` is still in memory.
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To reduce the memory consumption, `JsonValue`, `JsonArray` and `JsonObject` contains pointer to the token that are inside the `JsonParser`. This can only work if the `JsonParser` is still in memory.
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For example, don't do this:
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@@ -189,7 +179,7 @@ because the local variable `parser` will be *removed* from memory when the funct
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This will probably never be an issue, but you need to be aware of this feature.
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When you pass a `char[]` to `JsonParser::parseArray()` or `JsonParser::parseHashTable()`, the content of the string will be altered to add `\0` at the end of the tokens.
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When you pass a `char[]` to `JsonParser::parse()`, the content of the string will be altered to add `\0` at the end of the tokens.
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This is because we want functions like `JsonArray::getString()` to return a null-terminating string without any memory allocation.
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@@ -215,188 +205,7 @@ This table is for an 8-bit Arduino, types would be bigger on a 32-bit processor.
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<td>4</td>
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</tr>
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<tr>
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<td>JsonHashTable</td>
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<td>JsonObject</td>
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<td>4</td>
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</tr>
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</table>
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Code size
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---------
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Theses tables has been created by analyzing the map file generated by AVR-GCC after adding `-Wl,-Map,foo.map` to the command line.
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As you'll see the code size is between 1680 and 3528 bytes, depending on the features you use.
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### Minimum setup
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<table>
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<tr>
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<th>Function</th>
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<th>Size in bytes</th>
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</tr>
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<tr>
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<td>strcmp(char*,char*)</td>
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<td>18</td>
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</tr>
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<tr>
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<td>jsmn_init(jsmn_parser*)</td>
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<td>20</td>
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</tr>
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<tr>
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<td>jsmn_parse(jsmn_parser*, char const*, jsmntok_t*, unsigned int)</td>
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<td>960</td>
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</tr>
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<tr>
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<td>JsonParser::parse(char*)</td>
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<td>106</td>
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</tr>
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<tr>
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<td>JsonObjectBase::getNestedTokenCount(jsmntok_t*)</td>
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<td>84</td>
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</tr>
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<tr>
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<td>JsonObjectBase::getStringFromToken(jsmntok_t*)</td>
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<td>68</td>
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</tr>
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<tr>
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<td>JsonArray::JsonArray(char*, jsmntok_t*)</td>
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<td>42</td>
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</tr>
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<tr>
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<td>JsonArray::getToken(int)</td>
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<td>112</td>
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</tr>
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<tr>
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<td>JsonArray::getString(int)</td>
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<td>18</td>
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</tr>
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<tr>
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<td>JsonHashTable::JsonHashTable(char*, jsmntok_t*)</td>
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<td>42</td>
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</tr>
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<tr>
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<td>JsonHashTable::getToken(char*)</td>
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<td>180</td>
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</tr>
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<tr>
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<td>JsonHashTable::getString(char*)</td>
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<td>18</td>
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</tr>
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<tr>
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<td>TOTAL</td>
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<td>1680</td>
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</tr>
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</table>
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### Additional space to parse nested objects
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<table>
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<tr>
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<th>Function</th>
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<th>Size in bytes</th>
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</tr>
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<tr>
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<td>JsonArray::getArray(int)</td>
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<td>42</td>
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</tr>
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<tr>
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<td>JsonArray::getHashTable(int)</td>
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<td>64</td>
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</tr>
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<tr>
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<td>JsonHashTable::getArray(char*)</td>
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<td>64</td>
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</tr>
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<tr>
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<td>JsonHashTable::getHashTable(char*)</td>
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<td>42</td>
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</tr>
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<tr>
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<td>TOTAL</td>
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<td>212</td>
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</tr>
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</table>
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### Additional space to parse `bool` values
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<table>
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<tr>
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<th>Function</th>
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<th>Size in bytes</th>
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</tr>
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<tr>
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<td>JsonObjectBase::getBoolFromToken(jsmntok_t*)</td>
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<td>82</td>
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</tr>
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<tr>
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<td>JsonArray::getBool(int)</td>
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<td>18</td>
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</tr>
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<tr>
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<td>JsonHashTable::getBool(char*)</td>
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<td>18</td>
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</tr>
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<tr>
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<td>TOTAL</td>
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<td>130</td>
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</tr>
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</table>
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### Additional space to parse `double` values
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<table>
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<tr>
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<th>Function</th>
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<th>Size in bytes</th>
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</tr>
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<tr>
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<td>strtod(char*,int)</td>
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<td>704</td>
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</tr>
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<tr>
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<td>JsonObjectBase::getDoubleFromToken(jsmntok_t*)</td>
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<td>44</td>
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</tr>
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<tr>
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<td>JsonArray::getDouble(int)</td>
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<td>18</td>
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</tr>
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<tr>
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<td>JsonHashTable::getDouble(char*)</td>
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<td>18</td>
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</tr>
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<tr>
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<td>TOTAL</td>
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<td>796</td>
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</tr>
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</table>
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### Additional space to parse `long` values
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<table>
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<tr>
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<th>Function</th>
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<th>Size in bytes</th>
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</tr>
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<tr>
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<td>strtol(char*,char**,int)</td>
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<td>606</td>
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</tr>
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<tr>
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<td>JsonObjectBase::getLongFromToken(jsmntok_t*)</td>
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<td>56</td>
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</tr>
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<tr>
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<td>JsonArray::getLong(int)</td>
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<td>18</td>
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</tr>
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<tr>
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<td>JsonHashTable::getLong(char*)</td>
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<td>18</td>
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</tr>
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<tr>
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<td>TOTAL</td>
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<td>710</td>
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</tr>
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</table>
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</table>
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