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406 lines
10 KiB
Markdown
406 lines
10 KiB
Markdown
A malloc-free JSON parser for Arduino
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=====================================
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This library is an thin C++ wrapper around the *jsmn* tokenizer: http://zserge.com/jsmn.html
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It's design to be very lightweight, works without any allocation on the heap (no malloc) and supports nested objects.
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It has been written with Arduino in mind, but it isn't linked to Arduino libraries so you can use this library on any other C++ project.
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Example
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-------
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char* json = "{\"Name\":\"Blanchon\",\"Skills\":[\"C\",\"C++\",\"C#\"],\"Age\":32,\"Online\":true}";
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JsonParser<32> parser;
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JsonHashTable hashTable = parser.parseHashTable(json);
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if (!hashTable.success())
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{
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return;
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}
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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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How to use ?
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-------------
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### 1. Install the library
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Download the library and extract it to:
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<your Arduino Sketch folder>/libraries/ArduinoJsonParser
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### 2. Import in your sketch
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Just add the following line on the top of your `.ino` file:
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#include <JsonParser.h>
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### 3. Create a parser
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To extract data from the JSON string, you need to create a `JsonParser`, and specify the number of token you allocate for the parser itself:
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JsonParser<32> parser;
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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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> 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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> Each token takes 8 bytes, so `sizeof(JsonParser<32>)` is 256 bytes which is quite big in an Arduino with only 2KB of RAM.
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> Don't forget that you also have to store the JSON string in RAM and it's probably big.
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> 32 tokens may seem small but it's very descent for an 8-bit processor, you wouldn't get better results with other JSON libraries.
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### 4. Extract data
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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 nested objects can be either arrays, booleans, hash-tables, 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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Consider we have a `char* json` pointing 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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}
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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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JsonHashTable root = parser.parseHashTable(json);
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To check if the parsing was successful, you must check:
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if (!root.success())
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{
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// Parsing fail: could be an invalid JSON, or too many tokens
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}
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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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#### Array
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Consider we have a `char* json` pointing to the following JSON string:
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[
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[ 1.2, 3.4 ],
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[ 5.6, 7.8 ]
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]
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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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To check if the parsing was successful, you must check:
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if (!root.success())
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{
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// Parsing fail: could be an invalid JSON, or too many tokens
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}
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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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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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There are basically two reasons why they may fail:
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1. the JSON string is invalid
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2. the JSON string contains more tokens that the parser can store
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So, if you are sure the JSON string is correct and you still can't parse it, you should slightly increase the number of token of the parser.
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### 2. Not enough memory
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You may go into unpredictable trouble if you allocate more memory than your processor really has.
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It's a very common issue in embedded development.
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To diagnose this, look at every big objects in you code and sum their size to check that they fit in RAM.
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For example, don't do this:
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char json[1024]; // 1 KB
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JsonParser<64> parser; // 512 B
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because it may be too big for a processor with only 2 KB: you need free memory to store other variables and the call stack.
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That is why an 8-bit processor is not able to parse long and complex JSON strings.
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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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For example, don't do this:
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JsonArray getArray(char* json)
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{
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JsonParser<16> parser;
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return parser.parseArray(parser);
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}
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because the local variable `parser` will be *removed* from memory when the function `getArray()` returns, and the pointer inside `JsonArray` will point to an invalid location.
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### 4. JSON string is altered
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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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This is because we want functions like `JsonArray::getString()` to return a null-terminating string without any memory allocation.
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Memory usage
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------------
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Here are the size of the main classes of the library.
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This table is for an 8-bit Arduino, types would be bigger on a 32-bit processor.
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<table>
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<tr>
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<th>Type</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>Parser<N></td>
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<td>8 x N</td>
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</tr>
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<tr>
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<td>JsonArray</td>
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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>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 if 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>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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