PlantUMLLanguage specificationPreprocessing Json

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Preprocessing JSON

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.

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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:

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

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Access to data

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

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

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Complex structures

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

Let $foo, the structure:

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@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

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@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]

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@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

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Loading data

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

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Loop [foreach]

If you define array, you can loop over.

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@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

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Full Example

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

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@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

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Self-descriptive example

Here is a self-descriptive example:

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@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]

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%get_json_keys builtin function

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

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@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]

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%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).

🎉 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]

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%json_key_exists builtin function

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]

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%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:
TypeReturn 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]

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