JSON Library
The package json
contains a library for serialization, deserialization, and manipulation of JSON
data. The central part of the library is the class json.JsonValue
that represents a JSON
value.
JSON Values
As mentioned above, the class json.JsonValue
is used to represent arbitrary JSON values.
An instance of JsonValue
can be thought of as a variant that is specialized for JSON usage. In
particular, it implements both the interface for an array and for a map, so that it is convenient
to access the data without explicit typechecking. Furthermore, a JsonValue
that is created with
its default constructor is initially empty (i.e. null
), but can become any type if you try to add
data to it. This only works once, however. Once the JsonValue
contains a value, it will throw an
exception if used improperly.
Creating Values
Creating a JsonValue
that contains a scalar type (i.e. booleans, numbers, and strings) is done
using one of its constructors. These constructors are marked cast constructors to allow the
conversion to happen implicitly. Therefore, it is possible to do the following in Basic Storm:
JsonValue b = false; JsonValue n = 32; JsonValue d = 3.5; JsonValue s = "test";
To create arrays or maps, the most convenient way is to create an empty JsonValue
and start
populating it. Note that JsonValue
replicates the interface of Map<T>
and Array<T>
. There are
also constructors that accept JsonValue[]
and Str->JsonValue
respectively for programs that
produce the contents separately.
JsonValue array; array << 20 << 30; JsonValue object; object["key"] = "value"; object["number"] = 12; // or object.put("key", "value"); object.put("number", 12);
Note that array
and object
above will both have the value null
before elements are added to
them. If you wish to create empty arrays and objects, you can use the static functions
json.JsonValue emptyArray()
and json.JsonValue emptyObject()
. This is mainly a
concern if you need to serialize the representation into empty objects or arrays down the line.
Inspecting Values
Inspecting the contents of a JsonValue
is similarly aimed at being similar to existing
conventions, but without requiring explicit checks all the time. Instead, the API is designed to
throw exceptions whenever the expectations of the program do not match the structure.
The first part of inspecting the contents of a JsonValue
are the typecast members. As with many
other types in the standard library, these are named after the type casted to. All of these throw an
exception if the JsonValue
does not contain the expected type. They are as follows:
-
byte
,int
,nat
,long
,word
Extract an integer type. Note that this only works for types that were integer types from the start (e.g. in the serialization source). Creating a
JsonValue
from a floating point value and trying to extract an integer type will result in an error. Also note that values are stored internally ascore.Long
.You can check if the
JsonValue
contains an integer number usingisInteger
. -
float
,double
Extract a floating point type. Note that integer types are automatically converted to floating point types if required. Values are stored internally as
core.Double
.You can check if the
JsonValue
contains a floating point number usingisNumber
. Note that if the value contains an integer, bothisNumber
andisInteger
will return true. -
str
Extract a
core.Str
. You can check if the value contains a string usingisStr
. -
array
Extract an array of contained elements. In general, this is only necessary when you wish to iterate through the contents of the container. The number of elements can be retrieved using
count
. You can check if the value contains a string usingisArray
. -
object
Extract a map of contained elements. In general, this is only necessary when you wish to iterate through the contents of the container. The number of elements can be retrieved using
count
. You can check if the value contains a string usingisObject
.
For arrays and objects, JsonValue
additionally implements the interface for arrays and maps. As
such, it is possible to access elements using the appropriate operators on the value directly. There
is, however, one minor difference regarding the behavior of []
for objects. Namely, that []
behaves like get
in that it throws an exception when trying to read a key that does not exist.
Using this API, a JSON object can be inspected as below:
JsonValue array; array << 20 << 20.5; JsonValue object; object["a"] = "string"; object["b"] = array; for (k, v in object.object) { print("${k} -> ${v}"); } Str aValue = object["a"].str; JsonValue bValue = object["b"]; for (id, v in bValue.array) { print("${id}: ${v}"); } Double first = bValue[0].double; Double second = bValue[1].double;
The json.JsonValue
also contains a ==
operator to allow comparing arbitrary JSON
values. It implements a deep comparison.
JSON Literals
The library also provides a syntax extension that allows embedding JSON literals into Basic Storm
code. Literals start with the keyword json
and continues with either an object or an array using
the standard JSON syntax. All parts of the JSON hierarchy can be replaced by arbitrary Basic Storm
expressions except for the keys in object literals.
Keys in JSON objects do not have to be enclosed in quotes if it only contains alphanumeric
characters and underscores (which is otherwise required by JSON). As such, to use Basic Storm
expressions in keys, either use the interpolated string syntax (i.e. "${expr}"
) or enclose the
expression in ${expr}
.
Below is an example of a json literal. Note that it captures values from the surrounding code.
Str s = "string"; Int i = 15; Str name = "keyname"; JsonValue value = json{ "normal key": "value", unquoted-key: s, array: [s, i, name], "${name}": "key is named 'keyname'", ${name + "!"}: "key is named 'keyname!'", };
Serialization
The json.JsonValue
class serializes JSON to a proper string representation using its
toS
method as usual. By default, the toS
generates formatted JSON documents, using line breaks
and indentation to make it easier to read the structure.
The class provides overloads to change the formatting options. If one parameter is passed to toS
,
it indicates the indentation depth (in number of spaces). If zero is passed, it produces a single
line, compact representation. A second parameter to toS
indicates if keys in objects should be
sorted alphabetically. Since Map<T>
does not preserve the insertion order of elements in Storm,
element ordering in objects is otherwise unpredictable.
Similar options are available for the toS
overload that accepts a StrBuf
as its first parameter.
However, the second parameter is a boolean that instructs if the compact representation should be
used rather than a number since this overload uses the StrBuf
's standard indentation mechanism.
The output from the serialization is always ASCII (i.e. non-ascii characters are escaped). As such,
it can be converted to binary data using core.io.Buffer toUtf8(core.Str str)
without issues.
Deserialization is provided via the function parseJson
. There are two overloads, one that accepts
a string and another that accepts a core.io.Buffer
. The second one allows parsing
UTF-8-encoded binary data before decoding it first.
Exceptions
All exceptions thrown by the JSON library inherit from json.JsonError
. There are two
subtypes, json.JsonParseError
that is thrown by the JSON parser, and
json.JsonAccessError
that is thrown on incorrect accesses to the json.JsonValue
class. The latter of the two also captures a stacktrace to ease debugging.
Object Serialization
The library additionally contains the decorator jsonSerializable
that automatically generates code
that converts between json.JsonValue
and regular Storm classes to make it easier to
consume and produce JSON data in a structured manner. This is not too dissimilar from the normal
serialization mechanism.
To illustrate how the decorator works, consider the following class:
class Employee : jsonSerializable { Str name; Nat salary = 500; Str? speciality; }
In this case, the jsonSerializable
decorator adds the following members to the class:
class Employee : jsonSerializable { Str name; Nat salary = 500; Str? speciality; init(JsonValue json) { init { name = json["name"].str; salary = if (value = json.at("salary")) { value.nat; } else { 500; }; speciality = { element = json["speciality"]; if (element.isNull) { Str?(); } else { Str?(element.str); } }; } } JsonValue toJson() { var out = JsonValue:emptyObject(); out.put("name", JsonValue(name)); out.put("salary", JsonValue(salary)); if (speciality) { out.put("speciality", JsonValue(speciality)); } else { out.put("speciality", JsonValue()); } return out; } }
As we can see, jsonSerializable
adds a constructor that converts a JsonValue
into the type, as
well as a toJson
function that converts the type into JSON. As such, we can use the functions as
follows:
var src = json{ "name": "Test", "salary": 1000, "speciality": null }; var converted = src.Employee; // or var converted = Employee(src);
To convert back, we can of course just call converted.toJson
.
It is worth noting that the deserialization will only allow members that have explicit default
values set to be missing from the JSON. For example, salary
is allowed to be missing, while
speciality
is not allowed to, even though it is a maybe type.
Finally, even though it is not illustrated above, the serialization library supports serializing and
deserializing other types that have an appropriate constructor and a toJson
function. It also
supports arrays, maps with string keys, and maybe types natively. Inheritance is also supported, but
since the actual type of an object is not stored in the JSON representation, the support is not as
robust as the serialization library in Storm. That is, if we would serialize a subclass to
Employee
using its toJson
, we will always get Employee
if we deserialize it using
json.Employee()
since the system does not know which subclass was originally serialized.