Some JSON preprocessing capabilities are included in PlantUML, and available for all diagrams. That extends the current preprocessing. 🛈 If you are looking for how to display JSON data: see rather Display JSON Data. In addition to existing type (String and Integer), you can define JSON variable. Example:
!$foo = { "name": "John", "age" : 30 }
Corresponding of this structure: 🎉 Copied! 
 | @startjson
{
"$foo": { "name": "John", "age" : 30 }
}
@endjson
|
Once a variable is defined, you can access to its members: 🎉 Copied! 
 | @startuml
!$foo = { "name": "John", "age" : 30 }
Alice -> Bob : Do you know **$foo.name** ?
@enduml
|
It is possible to use complex JSON objects and arrays, with definition on several lines. Let $foo , the structure: 🎉 Copied! 
 | @startjson
{
"$foo":{ "company": "Skynet", "employees" : [
{"name" : "alice", "salary": 100 },
{"name" : "bob", "salary": 50} ]
}
}
@endjson
| You can acess to the values: $foo.employees[0].name $foo.employees[0].salary
🎉 Copied! 
 | @startuml
!$foo = { "company": "Skynet", "employees" : [
{"name" : "alice", "salary": 100 },
{"name" : "bob", "salary": 50} ]
}
start
:The salary of <u>$foo.employees[0].name</u> is <u>$foo.employees[0].salary</u>;
@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]
🎉 Copied! 
 | @startuml
!$foo = { "company": "Skynet", "employees" : [
{"name" : "alice", "salary": 100 },
{"name" : "bob", "salary": 50} ]
}
!$attribute_1="name"
!$attribute_2="salary"
start
:The salary of <u>$foo.employees[0][$attribute_1]</u> is <u>$foo.employees[0][$attribute_2]</u>;
@enduml
|
Some standard function provides a way to load JSON object from URL or local files:
!$foo = %load_json("http://foo.net/users/list.json")
!$foo2 = %load_json("myDir/localFile.json")
Available since 1.2021.15 If you define array, you can loop over. 🎉 Copied! 
 | @startmindmap
!$foo = { "company": "Skynet", "employees" : [
{"name" : "alice", "salary": 100 },
{"name" : "bob", "salary": 50} ]
}
* The salary of
!foreach $emp in $foo.employees
** **$emp.name**
*** is
**** **$emp.salary**
!endfor
@endmindmap
| [SW] Some remarks - for or better foreach ? ->
foreach - It would be nice to also have "break" and "continue"
- It would be nice to also have the for or while loop with a standard variable
From a example worked in a forum question, with this JSON structure: 🎉 Copied! 
 | @startjson
{
"data":
{
"participants": [
{"name": "XYZ", "as": "xyz"},
{"name": "RST", "as": "rst"},
{"name": "UVW", "as": "uvw"}]
}
}
@endjson
|
🎉 Copied! 
 | @startuml
!unquoted function DRAW($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", $data)
@enduml
|
Here is a self-descriptive example: 🎉 Copied! 
 | @startuml
left to right direction
!$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 --> $part.name
!endfor
}
}
@enduml
| [Adapted from QA-12917] You can use %get_json_keys to get all the keys of one level on a JSON structure. 🎉 Copied! 
 | @startuml
!$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 $key
!endfor
@enduml
|
🎉 Copied! 
 | @startwbs
!$json_object = {
"name": "Mark McGwire", "hr": 65, "avg": 0.278
}
* json_object
* keys of json_object
!foreach $key in %get_json_keys($json_object)
* $key
!endfor
@endwbs
| [Ref. QA-15360, QA-15423] You can use %get_json_type to get the type of an element of a JSON structure (returns a string). 🎉 Copied! 
 | @startuml
!$json_object = {
"name": "Mark McGwire", "hr": 65, "avg": 0.278,
"letters": ["a", "b", "c"]
}
label l [
=json_object:
{{json
$json_object
}}
|= $variable |= <U+0025>get_json_type($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] You can use %json_key_exists to know if a key exists on a JSON structure (returns a boolean). 🎉 Copied! 
 | @startuml
!$json_object= {
"name": "Mark McGwire", "hr": 65, "avg": 0.278
}
label l [
|= key |= <U+0025>json_key_exists(json_object, key) |
| "hr" | %json_key_exists($json_object, "hr") |
| "foo" | %json_key_exists($json_object, "foo")|
| null | %json_key_exists($json_object, null) |
]
@enduml
| [Ref. QA-15423] 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 |
🎉 Copied! 
 | @startuml
!$json_object= {
"name" : "Mark McGwire",
"hr" : 65,
"avg" : 0.278,
"letters": ["a", "b", "c"]
}
label l [
|= $variable |= <U+0025>get_json_type($var) |= <U+0025>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] | |