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Moved documentation to the wiki

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Benoit Blanchon
2014-11-29 16:59:22 +01:00
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8
README.md
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double latitude = root["data"][0]; double latitude = root["data"][0];
double longitude = root["data"][1]; double longitude = root["data"][1];
[See complete guide](/doc/Decoding JSON.md)
#### Encoding / Generating #### Encoding / Generating
StaticJsonBuffer<200> jsonBuffer; StaticJsonBuffer<200> jsonBuffer;
@@ -53,7 +51,11 @@ Quick start
// This prints: // This prints:
// {"sensor":"gps","time":1351824120,"data":[48.756080,2.302038]} // {"sensor":"gps","time":1351824120,"data":[48.756080,2.302038]}
[See complete guide](/doc/Encoding JSON.md)
Documentation
-------------
The documentation is available online in the [Arduino JSON wiki](https://github.com/bblanchon/ArduinoJson/wiki)
Testimonials Testimonials
------------ ------------

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doc/Avoiding pitfalls.md
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Avoiding common pitfalls in Arduino JSON
========================================
As `StaticJsonBuffer` is the corner stone of this library, you'll see that every pitfall listed here are related to a wrong understanding of the memory model.
Make sure you read [Arduino JSON memory model](Memory model.md) before going further.
## 1. Make `StaticJsonBuffer` big enough
By design, the library has no way to tell you why `parseArray()` or `parseObject()` failed.
There are basically two reasons why they may fail:
1. the JSON string is invalid,
2. the JSON string contains more values that the buffer can store.
So, if you are sure the JSON string is correct and you still can't parse it, you should try to increase the size of the `StaticJsonBuffer`.
## 2. Make sure everything fits in 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
StaticJsonBuffer<512> buffer; // 514 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. Keep the `StaticJsonBuffer` in memory long enough
Remember that `StaticJsonBuffer`'s function return references.
References don't contain data, they are just pointer to the actual.
So they can only work if the actual data is in memory.
For example, don't do this:
JsonArray& getArray(char* json)
{
StaticJsonBuffer<200> buffer;
return buffer.parseArray(json);
}
because the local variable `buffer` will be *removed* from memory when the function `parseArray()` returns, and the `JsonArray&` will point to an invalid location.
## 4. Don't reuse the same `StaticJsonBuffer`
During is lifetime a `StaticJsonBuffer` growth until it's discarded. If you try to reuse the same instance several time, it will rapidly get full.
For this reason, you should not use a global variable for your `StaticJsonBuffer`. I don't think there is any scenario in which a global `StaticJsonBuffer` would be a valid option.
The best practice is to declare it in a local scope, so that it's discarded as soon as possible. My advice it to declare it in a function which unique role is to handle the JSON serialization.
## 5. Keep the JSON string in memory long enough
The library never make memory duplication.
This has an important implication on string values, it means that the library will return pointer to chunks of the string.
For instance, let's imagine that you parse `["hello","world"]`, like this:
char[] json = "[\"hello\",\"world\"]";
StaticJsonBuffer<32> buffer;
JsonArray& array = buffer.parseArray(json);
const char* first = array[0];
const char* second = array[1];
In that case, both `first` and `second` are pointers to the content of the original string `json`.
So this will only work if `json` is still in memory.
## 6. JSON string is altered
If you read carefully the previous section, you may have come to the conclusion that the JSON parser modifies the JSON string.
Indeed, the parser modifies the string for two reasons:
1. it inserts `\0` to terminate substrings,
2. it translate escaped characters like `\n` or `\t`.
Most of the time this wont be an issue, but there are some corner cases that can be problematic.
Let take the example bellow:
char[] json = "[\"hello\",\"world\"]";
StaticJsonBuffer<32> buffer;
JsonArray& array = buffer.parseArray(json);
If you replace it by:
char* json = "[\"hello\",\"world\"]";
StaticJsonBuffer<32> buffer;
JsonArray& array = buffer.parseArray(json);
Depending on your platform, you may have an exception because the parser tries to write at a location that is read-only.
In the first case `char json[]` declares an array of `char` initialized to the specified string.
In the second case `char* json` declares a pointer to a read-only string, in fact it should be a `const char*` instead of a `char*`.

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Contributing to Arduino JSON
============================
If you want to contribute to the project, please:
1. Use GitHub pull request feature
2. Follow the coding conventions
3. Write tests
About the coding conventions: I try to follow the [Google C++ Style Guide](http://google-styleguide.googlecode.com/svn/trunk/cppguide.html) with few variations to match the Arduino conventions.
I use [ClangFormat](http://clang.llvm.org/docs/ClangFormat.html) to format the code for me.
I use [CppLint](http://google-styleguide.googlecode.com/svn/trunk/cpplint/cpplint.py) to detect non-compliant stuff.
You should have a look at the `scripts/` folder as it contains a few helpers scripts.

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doc/Decoding JSON.md
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Decoding JSON with Arduino JSON
===============================
Before writing any code, don't forget to include the header:
#include <ArduinoJson.h>
For instructions on how to install the library, please read [Using the library with Arduino](Using the library with Arduino.md) or [Using the library without Arduino](Using the library without Arduino.md).
## Example
Here an example that parse the string `{"sensor":"gps","time":1351824120,"data":[48.756080,2.302038]}`:
char json[] = "{\"sensor\":\"gps\",\"time\":1351824120,\"data\":[48.756080,2.302038]}";
//
// Step 1: Reserve memory space
//
StaticJsonBuffer<200> jsonBuffer;
//
// Step 2: Deserialize the JSON string
//
JsonObject& root = jsonBuffer.parseObject(json);
if (!root.success())
{
Serial.println("parseObject() failed");
return;
}
//
// Step 3: Retrieve the values
//
const char* sensor = root["sensor"];
long time = root["time"];
double latitude = root["data"][0];
double longitude = root["data"][1];
## Step 1: Reserve memory space
Arduino JSON uses a preallocated memory pool to store the object tree, this is done by the `StaticJsonBuffer`.
Before continuing please read the page [Arduino JSON memory model](Memory model.md) that explains everything you need to know about `StaticJsonBuffer`.
## Step 2: Parse the JSON string
You invoke the JSON parser through the instance of `StaticJsonBuffer`.
It exposes two functions for parsing JSON:
1. `parseArray()` that returns a reference to a `JsonArray`
2. `parseObject()` that returns a reference to a `JsonObject`
Let's see an example.
Say we want to parse `{"sensor":"gps","time":1351824120,"data":[48.756080,2.302038]}`, it's an object so we call `parseObject()` as follows:
char json[] = "{\"sensor\":\"gps\",\"time\":1351824120,\"data\":[48.756080,2.302038]}";
JsonObject& root = jsonBuffer.parseObject(json);
As you can see `parseObject()` takes a `char*` as a parameter.
Be careful, it's not a `const char*`, the memory must be writable.
Indeed, the parser will modify the string in two cases:
1. to insert string endings (character `\0`),
2. to translate escaped characters (like `\n` or `\t`).
Another thing that you must keep in mind is that the string (`char json[]` in the example above) must stay in memory during the whole parsing process.
That is because the in memory object tree will store pointer to chunks of the string, so as to avoid any memory duplication.
Now, to check if the parsing was successful, you can call `JsonObject::success()`:
if (!root.success())
{
// Parsing fail
}
The result can be `false` for tree reasons:
1. the JSON string is invalid,
2. the JSON string doesn't represent an object,
3. the `StaticJsonBuffer` is too small.
We just saw how to parse an object, there is nothing more to say for arrays, the procedure is exactly the same.
## Step 3: Retrieve the values
Now that the object or array is in memory, you can extract the data very easily.
In this section, we'll see how to do it with a `JsonObject`.
Once again, there is nothing more to say about arrays, `JsonArray` works exactly the same as `JsonObject`.
#### Subscript operator
The simplest way is to use the subscript operator of `JsonObject`:
const char* sensor = root["sensor"];
long time = root["time"];
You can chain the subscript operator if you have nested arrays or objects:
double latitude = root["data"][0];
double longitude = root["data"][1];
But alternatively, you can get a reference to the nested array:
JsonArray& nestedArray = root["data"];
#### Casting values
In the previous examples, the values were implicitly casted to the target type.
You can also do this explicitly
const char* sensor = root["sensor"].asString();
long time = root["time"].as<long>();
JsonArray& nestedArray = root["data"].asArray();
If the actual value doesn't match the target type, a default value will be return:
1. `false` for boolean values
2. `0` for integer values
3. `NULL` for string values
4. `JsonArray::invalid()` for nested arrays
5. `JsonObject::invalid()` for nested object
#### Check values
If you want to know if some value is present, call `containsKey()`:
if (root.contains("extra"))
{
// root["extra"] is valid
}
If you want to check the type value has a certain type, call `is<T>()`:
if (root["extra"].is<JsonArray&>())
{
// root["extra"] is an array
}
You can also iterate through the key-value pairs of the object:
for (JsonObject::itertor it=root.begin(); it!=root.end(); ++it)
{
Serial.println(it->key);
Serial.println(i->value.asString());
}

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Encoding JSON with Arduino JSON
===============================
Before writing any code, don't forget to include the header:
#include <ArduinoJson.h>
For instructions on how to install the library, please read [Using the library with Arduino](Using the library with Arduino.md) or [Using the library without Arduino](Using the library without Arduino.md).
## Example
Here is an example to generate `{"sensor":"gps","time":1351824120,"data":[48.756080,2.302038]}`
//
// Step 1: Reserve memory space
//
StaticJsonBuffer<200> jsonBuffer;
//
// Step 2: Build object tree in memory
//
JsonObject& root = jsonBuffer.createObject();
root["sensor"] = "gps";
root["time"] = 1351824120;
JsonArray& data = root.createNestedArray("data");
data.add(48.756080, 6); // 6 is the number of decimals to print
data.add(2.302038, 6); // if not specified, 2 digits are printed
//
// Step 3: Generate the JSON string
//
root.printTo(Serial);
## Step 1: Reserve memory space
Arduino JSON uses a preallocated memory pool to store the object tree, this is done by the `StaticJsonBuffer`.
Before continuing please read the page [Arduino JSON memory model](Memory model.md) that explains everything you need to know about `StaticJsonBuffer`.
## Step 2: Build object tree in memory
Now that you have enough memory hold by the `StaticJsonBuffer`, you can use it to build your in-memory representation of the JSON string.
#### Arrays
You create an array like this:
JsonArray& array = jsonBuffer.createArray();
Don't forget the `&` after `JsonArray`, it needs to be a reference to the array.
Then you can add strings, integer, booleans, etc:
array.add("bazinga!");
array.add(42);
array.add(true);
There are two syntaxes for floating point values:
array.add(3.1415, 4); // 4 digits: "3.1415"
array.add(3.1415); // 2 digits: "3.14"
> ##### About floating point precision
> The overload of `add()` with 2 parameters allows you to specify the number of decimals to save in the JSON string.
> When you use the overload with one parameter, you use the default number of decimals which is 2.
> Note that this behavior is the exact same as Arduino's `Print::print(double,int);` which is implemented by `Serial`, so you may already be familiar with this behavior.
You can add a nested array or object if you have a reference to it.
Or simpler, you can create nested array or nested objects from the array:
JsonArray& nestedArray = array.createNestedArray();
JsonObject& nestedObject = array.createNestedObject();
#### Objects
You create an object like this:
JsonObject& object = jsonBuffer.createObject();
Again, don't forget the `&` after `JsonObject`, it needs to be a reference to the object.
Then you can add strings, integer, booleans, etc:
object["key1"] = "bazinga!";
object["key2"] = 42;
object["key3"] = true;
As for the arrays, there are two syntaxes for the floating point values:
object["key4"].set(3.1415, 4); // 4 digits "3.1415"
object["key5"] = 3.1415; // default: 2 digits "3.14"
You can add a nested array or object if you have a reference to it.
Or simpler, you can create nested array or nested objects from the object:
JsonArray& nestedArray = object.createNestedArray("key6");
JsonObject& nestedObject = object.createNestedObject("key7");
> ##### Other JsonObject functions
> * `object.add(key, value)` is a synonym for `object[key] = value`
> * `object.containsKey(key)` returns `true` is the `key` is present in `object`
> * `object.remove(key)` removes the `value` associated with `key`
## Step 3: Generate the JSON string
There are two ways tho get the resulting JSON string.
Depending on your project, you may need to dump the string in a classic `char[]` or send it to a `Print` implementation like `Serial` or `EthernetClient `.
Both ways are the easy way :-)
#### Use a classic `char[]`
Whether you have a `JsonArray&` or a `JsonObject&`, simply call `printTo()` with the destination buffer, like so:
char buffer[256];
array.printTo(buffer, sizeof(buffer));
> ##### Want an indented output?
> By default the generated JSON is as small as possible. It contains no extra space, nor line break.
> But if you want an indented, more readable output, you can.
> Simply call `prettyPrintTo` instead of `printTo()`:
>
> array.prettyPrintTo(buffer, sizeof(buffer));
#### Send to a `Print` implementation
It's very likely that the generated JSON will end up in a stream like `Serial` or `EthernetClient `, so you can save some time and memory by doing this:
array.printTo(Serial);
And, of course if you need an indented JSON string:
array.prettyPrintTo(Serial);
> ##### About the Print interface
> The library is designed to send the JSON string to an implementation of the `Print` interface that is part of Arduino.
> In the example above we used `Serial`, but they are many other implementations that would work as well, including: `HardwareSerial`, `SoftwareSerial`, `LiquidCrystal`, `EthernetClient`, `WiFiClient`, `Wire`...
> When you use this library out of the Arduino environment, it will use it's own implementation of `Print` and everything will be the same.

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Arduino JSON memory model
=========================
## Fixed memory allocation
### Introducing `StaticJsonBuffer`
Arduino JSON uses a preallocated memory pool to store the object tree, this is done by the `StaticJsonBuffer` class.
Before using any function of the library you need to create a `StaticJsonBuffer`. Then you can use this instance to create arrays and objects, or parse a JSON string.
`StaticJsonBuffer` has a template parameter that determines its capacity. For example, the following line create a `StaticJsonBuffer` with a capacity of 200 bytes:
StaticJsonBuffer<200> jsonBuffer;
The bigger the buffer is, the more complex the object tree can be, but also the more memory you need.
### How to determine the buffer size?
So the big question you should have in mind right now is *How can I determine the size?*.
There are basically two approaches here:
1. either you can predict the content of the object tree,
2. or, you know how much memory is available.
In the first case, you know some constraints on the object tree. For instance, let's say that your know in advance (and by that I mean "at compilation time") that you want to generate an object with 3 values, one of them being an array with 2 values, like the following:
{"sensor":"gps","time":1351824120,"data":[48.756080,2.302038]}
To determine the memory usage of this object tree, you use the two macros `JSON_ARRAY_SIZE(n)` and `JSON_OBJECT_SIZE(n)`, both take the number of elements as an argument.
For the example above, it would be:
const int BUFFER_SIZE = JSON_OBJECT_SIZE(3) + JSON_ARRAY_SIZE(2);
StaticJsonBuffer<BUFFER_SIZE> jsonBuffer;
In the second case, let's say you dynamically generate a JSON object tree of a random complexity so you can't put a limit based on that. But on the other hand, you don't want your program to crash because the object tree doesn't fit in memory.
The solution here is to determine how much memory is available, or in other words how much memory you can afford for the JSON object tree.
### Why choosing fixed allocation?
This fixed allocation approach may seem a bit strange, especially if your a desktop application developer used to dynamic allocation, but it make a lot of sense in an embedded context:
1. the code is smaller
2. it uses less memory
3. it doesn't create memory fragmentation
4. it predictable
Don't forget that the memory is "freed" as soon as the `StaticJsonBuffer` is out of scope, like any other variable. It only hold the memory for a short amount of time.
## Memory usage
#### Object size for 8-bit AVR
| Type | Size |
|-------------------------|------------|
| JsonArray of N element | 4 + 8 * N |
| JsonObject of N element | 4 + 10 * N |

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Migrating code written for Arduino JSON v3 to v4
================================================
Arduino JSON v4 was a major rewrite of the library, and the API changed significantly.
## Includes
Arduino JSON v3 had two include files:
#include <JsonParser.h>
#include <JsonGenerator.h>
Arduino JSON v4 only has one:
#include <ArduinoJson.h>
## Namespaces
Arduino JSON v3 had two namespaces:
using namespace ArduinoJson::Parser;
using namespace ArduinoJson::Generator;
Arduino JSON v4 doesn't require the `using namespace` statement.
It has a namespace but the `using namespace` is done in the header file.
## StaticJsonBuffer
Arduino JSON v3 had different memory allocation models for the parser:
JsonParser<16> parser; // 16 being the capacity in "tokens"
and for the generator:
JsonArray<4> array; // 4 being the number of element
JsonObject<4> object;
Arduino JSON v4 only has one memory allocation model:
StaticJsonBuffer<128> buffer; // 128 being the capacity in bytes
## Return values for the parser
Arduino JSON v3 returned value types:
JsonArray array = parser.parseArray(json);
JsonObject object = parser.parseObject(json);
Arduino JSON v4 returns references types:
JsonArray& array = buffer.parseArray(json);
JsonObject& object = buffer.parseObject(json);
Everything else is compatible
## Creating arrays and objects
Arduino JSON v3 allowed to create `JsonArray` and `JsonObject` directly:
JsonArray<4> array;
JsonObject<4> object;
Arduino JSON v4 requires that you use a `StaticJsonBuffer` for that:
JsonArray& array = buffer.createArray();
JsonObject& object = buffer.createObject();
Note: you don't have to specify the capacity anymore.
## Printable interface
Arduino JSON v3 used to implement the Printable interface, which allowed statements like:
Serial.print(array);
But Arduino JSON v4 doesn't, instead you need to write this:
array.printTo(Serial);
Note: there was a good reason for removing that feature, and it's reducing the size of `JsonArray` and `JsonObject`.

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Using the library with Arduino
==============================
This library is primarily design to be used with the Arduino IDE and therefore has a simplified setup procedure for that environment.
If you don't use the Arduino IDE, please read [Using the library without Arduino](Using the library without Arduino.md).
## Install the library
[Download the zip package](https://github.com/bblanchon/ArduinoJson/releases) and extract it to:
<your Arduino Sketch folder>/libraries/ArduinoJson
Then restart the Arduino IDE.
## Run the examples sketches
Click `File` / `Example` / `ArduinoJson`.
![Screen capture of Arduino IDE](http://i.imgur.com/g5UwkVh.png)
## Use the library in your sketches
Just add the following line at the top of your program:
#include <ArduinoJson.h>
Then follow the instructions:
1. [Parsing JSON](Parsin JSON.md)
2. [Generating JSON](Generating JSON.md)

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Using the library without Arduino
=================================
This library is primarily design to be used with the Arduino IDE and therefore has a simplified setup procedure for that environment.
If you use the Arduino IDE, please read [Using the library with Arduino](Using the library with Arduino.md).
However, it can be used without Arduino IDE with very little effort.
## Compiling the library
Step 1: Download source code:
git clone https://github.com/bblanchon/ArduinoJson.git
Step 2: Generate the `Makefile` for your environment
cd ArduinoJson
cmake .
Step 3: Build
make
## File paths
Assuming you installed the library into `<arduino-json>`, you need to add:
1. `<arduino-json>/include` to your include path
2. `<arduino-json>/lib` to your library path
----------
You are now ready to follow the instructions:
1. [Parsing JSON](Parsin JSON.md)
2. [Generating JSON](Generating JSON.md)