Description

The function deserializeJson() parses a JSON input and puts the result in a JsonDocument.

This function behaves differently depending on the type of the input:

  • For a read-only input, it duplicates the strings in the input document. This duplication consumes additional space in the JsonDocument.
  • For a writeable input, it stores pointers to the strings in the input buffer. This is the zero-copy mode. In this mode, the JsonDocument can be smaller, but ArduinoJson needs to modify the input buffer to insert null-terminators and replace escaped characters. Because the JsonDocument stores pointers to the input buffer, you must ensure that this buffer remains in memory.

Prefer the first mode when the input comes from a Stream; prefer the second when the input is in a buffer.

If the input is a char*, ensure the input buffer stays in memory until the JsonDocument is destructed.

Signatures

// writable input => zero-copy
DeserializationError deserializeJson(JsonDocument& doc, char* input);
DeserializationError deserializeJson(JsonDocument& doc, char* input, size_t inputSize);

// read-only input => duplication
DeserializationError deserializeJson(JsonDocument& doc, const char* input);
DeserializationError deserializeJson(JsonDocument& doc, const char* input, size_t inputSize);
DeserializationError deserializeJson(JsonDocument& doc, const __FlashStringHelper* input);
DeserializationError deserializeJson(JsonDocument& doc, const __FlashStringHelper* input, size_t inputSize);
DeserializationError deserializeJson(JsonDocument& doc, const String& input);
DeserializationError deserializeJson(JsonDocument& doc, const std::string& input);
DeserializationError deserializeJson(JsonDocument& doc, Stream& input);
DeserializationError deserializeJson(JsonDocument& doc, std::istream& input);

template<Reader> // custom reader class (see below)
DeserializationError deserializeJson(JsonDocument& doc, Reader& input);

// all overloads also accept an optional parameter of type DeserializationOption::NestingLimit (see below)

Arguments

doc: the JsonDocument that will store the memory representation of the JSON document.

input: the JSON document to parse:

  • const char* is a string in RAM, it should be zero-terminated
  • const __FlashStringHelper* is a Flash string, usually created with F()
  • Stream is Arduino’s I/O stream interface, implemented by:

inputSize: the maximum number of bytes to read from input

This function supports an optional argument of type DeserializationOption::NestingLimit to change the maximum number of nesting level that the parser will accept. See below for the details.

Return value

Returns a DeserializationError.

Nesting limit

The ArduinoJson’s parser contains a recursive function that is called each time an opening brace ({) or opening bracket ([) appears in the input. In other words, each object/array nesting level causes a recursive call.

This recursive call is a security risk because an attacker could craft a JSON input with many opening brackets to cause a stack overflow.

To protect against this security risk, ArduinoJson limits the number of nesting levels. The macro ARDUINOJSON_DEFAULT_NESTING_LIMIT sets the default value.

If your JSON input contains more nesting levels that allowed, you can pass an extra parameter of type DeserializationOption::NestingLimit to deserializeJson(). See the example below.

Configuration

deserializeJson() can be configured with the following settings:

How to view the JSON input?

When you pass a Stream to deserializeJson(), it consumes the input but doesn’t print anything to the serial, which makes troubleshooting difficult.

If you want to see what deserializeJson() consumed, use ReadLoggingStream from the StreamUtils library. See the example below.

Performance

When you pass a Stream to deserializeJson(), it consumes bytes one by one, which can be slow depending on the input you use. For example, if you read from a SPIFFS file, you can read twenty times faster by reading chunks of 64 bytes.

To read the stream in chunks, you can use ReadBufferingStream from the StreamUtils library. See the example below.

Custom reader

If none of the supported input types is suitable for you, you can implement a custom reader class. This class must implement two member functions, as shown below:

struct CustomReader {
  // Reads one byte, or returns -1
  int read();
  // Reads several bytes, returns the number of bytes read.
  size_t readBytes(char* buffer, size_t length);
};

Then, pass a reference to an instance of this class as the second argument of deserializeJson().

Example

Read-only input

const char* json = "{\"hello\":\"world\"}";
StaticJsonDocument<200> doc;
deserializeJson(doc, json);
const char* world = doc["hello"];

Zero-copy mode

char json[] = "{\"hello\":\"world\"}";
StaticJsonDocument<200> doc;
deserializeJson(doc, json);
const char* world = doc["hello"];

Raise the nesting limit

char json[] = "[[[[[[[[[[[[[[[42]]]]]]]]]]]]]]]";
StaticJsonDocument<200> doc;
deserializeJson(doc, json, DeserializationOption::NestingLimit(15));;
int answer = doc[0][0][0][0][0][0][0][0][0][0][0][0][0][0][0];

Show the JSON input stream

This example requires the StreamUtils library.

Suppose your program is:

deserializeJson(doc, wifiClient);

If you want to see what deserializeJson() consumed, replace this line with:

ReadLoggingStream loggingStream(wifiClient, Serial);
deserialzeJson(doc, loggingStream);

The first line creates a new Stream on top of wifiClient that writes everything it reads to Serial.

Improve read performance

This example requires the StreamUtils library.

Suppose your program is:

deserializeJson(doc, file);

If you want to make deserializeJson() read chunks instead of reading bytes one by one, replace this line with:

ReadBufferingStream bufferingStream(file, 64);
deserialzeJson(doc, bufferingStream);

The first line creates a new Stream that reads blocks of 64 bytes from file.

See also