Preprocessing JSON

Some JSON preprocessing capabilities are included in PlantUML, and available for all diagrams.

That extends the current link::preprocessing[preprocessing].

🛈 If you are looking for how to display JSON data: see rather link::json[Display JSON Data].

Variable definition

In addition to existing type (String and Integer), you can define JSON variable.

Example:

!$foo = { "name": "John", "age" : 30 }

Corresponding of this structure:

@startjson
{
"$foo": { "name": "John", "age" : 30 }
}
@endjson

Access to data

Once a variable is defined, you can access to its members:

@startuml
!$foo = { "name": "John", "age" : 30 }
Alice -> Bob : Do you know **$foo.name** ?
@enduml

Complex structures

It is possible to use complex JSON objects and arrays, with definition on several lines.

Let $foo, the structure:

@startjson
{
"$foo":{ "company": "Skynet", "employees" : [
  {"name" : "alice", "salary": 100 },
  {"name" : "bob", "salary": 50} ]
}
}
@endjson

You can acess to the values: * $foo.employees[0].name * $foo.employees[0].salary

@startuml
!$foo = { "company": "Skynet", "employees" : [
  {"name" : "alice", "salary": 100 },
  {"name" : "bob", "salary": 50} ]
}
start
:The salary of $foo.employees[0].name is $foo.employees[0].salary;
@enduml

Or using intermediate variables: * !$attribute_1="name" * !$attribute_2="salary" to acess to the values: * $foo.employees[0][$attribute_1] * $foo.employees[0][$attribute_2]

@startuml
!$foo = { "company": "Skynet", "employees" : [
  {"name" : "alice", "salary": 100 },
  {"name" : "bob", "salary": 50} ]
}

!$attribute_1="name"
!$attribute_2="salary"

start
:The salary of $foo.employees[0][$attribute_1] is $foo.employees[0][$attribute_2];
@enduml

Loading data

Some standard function provides a way to load JSON object from URL or local files:

!$foo = %loadJSON("http://foo.net/users/list.json")
!$foo2 = %loadJSON("myDir/localFile.json")

Available since 1.2021.15

Loop [foreach]

If you define array, you can loop over.

@startmindmap
!$foo = { "company": "Skynet", "employees" : [
  {"name" : "alice", "salary": 100 },
  {"name" : "bob", "salary": 50} ]
}

* The salary of
!foreach $emp in $foo.employees
  ** **$emp.name**
  *** is
  **** **$emp.salary**
!endfor
@endmindmap

[SW] Some remarks * for or better foreach ? → foreach * It would be nice to also have "break" and "continue" * It would be nice to also have the for or while loop with a standard variable

Full Example

From a example worked in a forum question, with this JSON structure:

@startjson
{
"data":
  {
  "participants": [
    {"name": "XYZ", "as": "xyz"},
    {"name": "RST", "as": "rst"},
    {"name": "UVW", "as": "uvw"}]
  }
}
@endjson
@startuml
!unquoted function DRAW($x) return %set_variable_value($x, 1)

!procedure addComponent($part, $component, $as)
    !if %variable_exists($part)
        participant "$component" as $as
    !endif
!end procedure

!procedure addBox2($part, $box, $colour, $data)
    !if %variable_exists($part)
        box "$box" #$colour
            !foreach $item in $data.participants
                addComponent($part, $item.name, $item.as)
            !endfor
        end box
    !endif
!end procedure

DRAW(PART25)

!ifdef PART25
title  TESTING  (Boxes & Participants)  //Part25//
!endif

!$data={
  "participants": [
    {"name": "XYZ", "as": "xyz"},
    {"name": "RST", "as": "rst"},
    {"name": "UVW", "as": "uvw"}]
}

addBox2("PART25", "New Box", "white", $data)
@enduml

Self-descriptive example

Here is a self-descriptive example:

@startuml
left to right direction

!$data={"parts":[
{"shape": "cloud",    "name": "id1", "colour": "#palegreen", "desc": "some text"},
{"shape": "folder",   "name": "id2", "colour": "#lightblue", "desc": "more text"},
{"shape": "database", "name": "id3", "colour": "#pink",      "desc": "even more text"}
]}

rectangle Outer {
rectangle Inner #tan as "
{{json
$data
}}
"
together {
!foreach $part in $data.parts
  $part.shape $part.colour $part.name as "$part.desc"
  Inner --> $part.name
!endfor
}
}
@enduml

[Adapted from QA-12917]

%get_json_keys builtin function

You can use %get_json_keys to get all the keys of one level on a JSON structure.

@startuml
!$myjson = {
"root" : [{
    "fruits": [
        {"name": "apple", "colorId": "1"},
        {"name": "pear", "colorId": "2"},
        {"name": "pineapple", "colorId": "3"}
    ]
},
{
    "colors": [
        {"id": "1", "name": "red"},
        {"id": "2", "name": "green"},
        {"id": "3", "name": "yellow"}
    ]
}]
}

!foreach $key in %get_json_keys($myjson.root)
     rectangle $key
!endfor
@enduml
@startwbs
!$json_object = {
  "name": "Mark McGwire", "hr": 65, "avg":  0.278
}

* json_object
 * keys of json_object
!foreach $key in %get_json_keys($json_object)
  * $key
!endfor
@endwbs

[Ref. QA-15360, QA-15423]

%get_json_type builtin function

You can use %get_json_type to get the type of an element of a JSON structure (returns a string).

@startuml
!$json_object = {
  "name": "Mark McGwire", "hr": 65, "avg":  0.278,
  "letters": ["a", "b", "c"]
}

label l [
=json_object:
{{json
$json_object
}}

|= $variable          |= get_json_type($var)         |
| json_object         | %get_json_type($json_object)         |
| json_object.name    | %get_json_type($json_object.name)    |
| json_object.hr      | %get_json_type($json_object.hr)      |
| json_object.letters | %get_json_type($json_object.letters) |

Test on type:
!if %get_json_type($json_object.letters)=="array"
  json_object.letters is an **%get_json_type($json_object.letters)**
!endif
]
@enduml

[Ref. QA-15360]

+ %json_key_exists+ builtin function

You can use %json_key_exists to know if a key exists on a JSON structure (returns a boolean).

@startuml
!$json_object= {
  "name": "Mark McGwire", "hr": 65, "avg":  0.278
}

label l [
|= key  |= json_key_exists(json_object, key)   |
| "hr"  | %json_key_exists($json_object, "hr") |
| "foo" | %json_key_exists($json_object, "foo")|
| null  | %json_key_exists($json_object, null) |
]
@enduml

[Ref. QA-15423]

+ %size+ builtin function

You can use %size to know the size of different elements on a JSON structure.

For each type here are the return value:

Type Return value

JSON Object

the number of pairs it contains

JSON Array

the number of values it contains

string value

the number of characters it contains

numeric value

zero

true/false/null

zero

 
@startuml
!$json_object= {
  "name"   : "Mark McGwire",
  "hr"     : 65,
  "avg"    : 0.278,
  "letters": ["a", "b", "c"]
}

label l [
|= $variable          |= get_json_type($var)         |= size($var)         |
| json_object         | %get_json_type($json_object)         | %size($json_object)         |
| json_object.name    | %get_json_type($json_object.name)    | %size($json_object.name)    |
| json_object.hr      | %get_json_type($json_object.hr)      | %size($json_object.hr)      |
| json_object.letters | %get_json_type($json_object.letters) | %size($json_object.letters) |
]
@enduml

[Ref. QA-14901]