feedc0de/ArduinoJson
1
0
Fork 0
forked from bblanchon/ArduinoJson
Code Pull Requests Activity

Added "features" section

Browse Source
This commit is contained in:
Benoit Blanchon
2014-01-22 21:50:24 +01:00
parent 1c55c19680 d3cb228922
commit f5a1e1d5e0
1 changed files with 268 additions and 165 deletions
Download Patch File Download Diff File Expand all files Collapse all files

135
README.md
View File

@ -1,12 +1,16 @@
# A malloc-free JSON parser for Arduino
A malloc-free JSON parser for Arduino
=====================================
The library is an thin C++ wrapper around the *jsmn* tokenizer: http://zserge.com/jsmn.html
This library is an thin C++ wrapper around the *jsmn* tokenizer: http://zserge.com/jsmn.html
It's design to be very lightweight, works without any allocation on the heap (no malloc) and supports nested objects.
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.
## Features
Features
-------
* Based on the well-proven [jsmn](http://zserge.com/jsmn.html) tokenizer
* Supports nested objects
@ -14,7 +18,9 @@ It has been written with Arduino in mind, but it isn't linked to Arduino librari
* Low footprint
* MIT License
## Example
Example
-------
char* json = "{\"Name\":\"Blanchon\",\"Skills\":[\"C\",\"C++\",\"C#\"],\"Age\":32,\"Online\":true}";
@ -35,19 +41,23 @@ It has been written with Arduino in mind, but it isn't linked to Arduino librari
bool online = hashTable.getBool("Online");
## How to use ?
### 1. Install the the library
How to use ?
-------------
### 1. Install the library
Download the library and extract it to:
<your Arduino Sketch folder>/libraries/ArduinoJonsParser
<your Arduino Sketch folder>/libraries/ArduinoJsonParser
### 2. Import in your sketch
Just add the following line on the to of your `.ino` file:
Just add the following line on the top of your `.ino` file:
#include <JonsParser.h>
#include <JsonParser.h>
### 3. Create a parser
@ -57,20 +67,25 @@ To extract data from the JSON string, you need to create a `JsonParser`, and spe
> #### How to choose the number of tokens ?
> First you need to know exactly what a token is. A token is an element af the JSON object: either a key, a value, an hash-table or an array.
> A token is an element of the JSON object: either a key, a value, an hash-table or an array.
> 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).
> The more tokens you allocate, the more complex the JSON can be, but also the more memory will be occupied.
> The more tokens you allocate, the more complex the JSON can be, but also the more memory is occupied.
> Each token takes 8 bytes, so `sizeof(JsonParser<32>)` is 256 bytes which is quite big in an Arduino with only 2KB of RAM.
> Don't forget that you also have to store the JSON string in memory and it's probably big.
> Don't forget that you also have to store the JSON string in RAM and it's probably big.
> 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.
### 4. Extract data
To use this library, you need to know beforehand what is the type of data contained in the JSON string, which is extremely likely.
To use this library, you need to know beforehand what is the type of data contained in the JSON string, which is very likely.
#### Hash table
The root object has to be either a hash-table (like `{"key":"value"}`) or an array (like `[1,2]`).
The nested objects can be either arrays, booleans, hash-tables, numbers or strings.
If you need other type, you can get the string value and parse it yourself.
#### Hash-table
Consider we have a `char* json` pointing to the following JSON string:
@ -84,7 +99,7 @@ Consider we have a `char* json` pointing to the following JSON string:
"Online":true
}
In this case the root object of the JSON string is a hash table, so you need to extract a `JsonHashTable`:
In this case the root object of the JSON string is a hash-table, so you need to extract a `JsonHashTable`:
JsonHashTable root = parser.parseHashTable(json);
@ -134,7 +149,95 @@ or simply:
double a = root.getArray(0).getDouble(0);
## Code size
Common pitfalls
---------------
### 1. Not enough tokens
By design, the library has no way to tell you why `JsonParser::parseArray()` or `JsonParser::parseHashTable()` failed.
There are basically two reasons why they may fail:
1. the JSON string is invalid
2. the JSON string contains more tokens that the parser can store
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.
### 2. Not enough memory
You may go into unpredictable trouble if you allocate more memory than your processor really has.
It's a very common issue in embedded development.
To diagnose this, look at every big objects in you code and sum their size to check that they fit in RAM.
For example, don't do this:
char json[1024]; // 1 KB
JsonParser<64> parser; // 512 B
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.
That is why an 8-bit processor is not able to parse long and complex JSON strings.
### 3. JsonParser not in memory
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.
For example, don't do this:
JsonArray getArray(char* json)
{
JsonParser<16> parser;
return parser.parseArray(parser);
}
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.
### 4. JSON string is altered
This will probably never be an issue, but you need to be aware of this feature.
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.
This is because we want functions like `JsonArray::getString()` to return a null-terminating string without any memory allocation.
Memory usage
------------
Here are the size of the main classes of the library.
This table is for an 8-bit Arduino, types would be bigger on a 32-bit processor.
<table>
<tr>
<th>Type</th>
<th>Size in bytes</th>
</tr>
<tr>
<td>Parser&lt;N&gt;</td>
<td>8 x N</td>
</tr>
<tr>
<td>JsonArray</td>
<td>4</td>
</tr>
<tr>
<td>JsonHashTable</td>
<td>4</td>
</tr>
</table>
Code size
---------
Theses tables has been created by analyzing the map file generated by AVR-GCC after adding `-Wl,-Map,foo.map` to the command line.