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PlantUML Language specification Preprocessing 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.

JSON Object

Example: 

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

Corresponding of this structure:

JSON Array

Example: 

!$foo = ["abc", "def", "ghi"]

Corresponding of this structure:

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Access to data

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

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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:

You can acess to the values:

Or using intermediate variables: to acess to the values:

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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.

[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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Self-descriptive example

Here is a self-descriptive example:

[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.

[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).

[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).

[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:
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

[Ref. QA-14901]

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%json_add builtin function

You can use %json_add to add element on an existing JSON structure (returns the final JSON structure).

[Ref. PR-1742 and PR-1757]

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%json_remove builtin function

You can use %json_remove to remove element on an existing JSON structure (returns the final JSON structure).

[Ref. PR-1742]

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%json_set builtin function

You can use %json_set to set element on an existing JSON structure (returns the final JSON structure).

[Ref. PR-1761]

[Ref. PR-1782]

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%json_merge builtin function

You can use %json_merge to merge JSON structures (returns the final JSON structure).

[Ref. PR-1763]

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%str2json builtin function

You can use %str2json to transform a string value to a JSON structure.

Here are some example:

🚩 Known issue: Unfortunately, be careful with quote management.

Some issues occur when a single quote is contained in a string when the entire string is surrounded by single quote.

[Ref. PR-1742]