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

Those functionalities are very similar to the C language preprocessor, except that the special character # has been changed to the exclamation mark !.

Migration notes

The actual preprocessor is an update from some legacy preprocessor.

Even if some legacy features are still supported with the actual preprocessor, you should not use them any more (they might be removed in some long term future).

  • You should not use !define and !definelong anymore. Use !function, !procedure or variable definition instead.
    • !define should be replaced by return !function
    • !definelong should be replaced by !procedure.
  • !include now allows multiple inclusions : you don't have to use !include_many anymore
  • !include now accepts a URL, so you don't need !includeurl
  • Some features (like %date%) have been replaced by builtin functions (for example %date())
  • When calling a legacy !definelong macro with no arguments, you do have to use parenthesis. You have to use my_own_definelong() because my_own_definelong without parenthesis is not recognized by the new preprocessor.

Please contact us if you have any issues.

Variable definition

Although this is not mandatory, we highly suggest that variable names start with a $.

There are two types of data:
  • Integer number (int);
  • String (str) - these must be surrounded by single quote or double quote.

Variables created outside function are global, that is you can access them from everywhere (including from functions). You can emphasize this by using the optional global keyword when defining a variable.

!$ab = "foo1"
!$cd = "foo2"
!$ef = $ab + $cd

Alice -> Bob : $ab
Alice -> Bob : $cd
Alice -> Bob : $ef

Boolean expression

Boolean represention [0 is false]

There is not real boolean type, but PlantUML use this integer convention:

  • Integer 0 means false
  • and any non-null number (as 1) or any string (as "1", or even "0") means true.

[Ref. QA-9702]

Boolean operation and operator [&&, ||, ()]

You can use boolean expression, in the test, with :
  • parenthesis ();
  • and operator &&;
  • or operator ||.

(See next example, within if test.)

Boolean builtin functions [%false(), %true(), %not(<exp>)]

For convenience, you can use those boolean builtin functions:

  • %false()
  • %true()
  • %not(<exp>)

[See also Builtin functions]

Conditions [!if, !else, !elseif, !endif]

  • You can use expression in condition.
  • else and elseif are also implemented

!$a = 10
!$ijk = "foo"
Alice -> Bob : A
!if ($ijk == "foo") && ($a+10>=4)
Alice -> Bob : yes
Alice -> Bob : This should not appear
Alice -> Bob : B

While loop [!while, !endwhile]

You can use !while and !endwhile keywords to have repeat loops.

!procedure $foo($arg)
  :procedure start;
  !while $arg!=0
    !while $i!=0
      :arg=$arg and i=$i;
      !$i = $i - 1
    !$arg = $arg - 1
  :procedure end;


[Adapted from QA-10838]

Procedure [!procedure, !endprocedure]

  • Procedure names should start with a $
  • Argument names should start with a $
  • Procedures can call other procedures


!procedure $msg($source, $destination)
  $source --> $destination

!procedure $init_class($name)
  class $name {

!procedure $addCommonMethod()

$msg("foo1", "foo2")

Variables defined in procedures are local. It means that the variable is destroyed when the procedure ends.

Return function [!function, !endfunction]

A return function does not output any text. It just define a function that you can call:
  • directly in variable definition or in diagram text
  • from other return functions
  • from procedures

  • Function name should start with a $
  • Argument names should start with a $

!function $double($a)
!return $a + $a

Alice -> Bob : The double of 3 is $double(3)

It is possible to shorten simple function definition in one line:

!function $double($a) !return $a + $a

Alice -> Bob : The double of 3 is $double(3)
Alice -> Bob : $double("This work also for strings.")

As in void function, variable are local by default (they are destroyed when the function is exited). However, you can access to global variables from function. However, you can use the local keyword to create a local variable if ever a global variable exists with the same name.

!function $dummy()
!local $ijk = "local"
!return "Alice -> Bob : " + $ijk

!global $ijk = "foo"

Alice -> Bob : $ijk
Alice -> Bob : $ijk

Default argument value

In both procedure and return functions, you can define default values for arguments.

!function $inc($value, $step=1)
!return $value + $step

Alice -> Bob : Just one more $inc(3)
Alice -> Bob : Add two to three : $inc(3, 2)

Only arguments at the end of the parameter list can have default values.

!procedure defaulttest($x, $y="DefaultY", $z="DefaultZ")
note over Alice
  x = $x
  y = $y
  z = $z
end note

defaulttest(1, 2, 3)
defaulttest(1, 2)

Unquoted procedure or function [!unquoted]

By default, you have to put quotes when you call a function or a procedure. It is possible to use the unquoted keyword to indicate that a function or a procedure does not require quotes for its arguments.

!unquoted function id($text1, $text2="FOO") !return $text1 + $text2

alice -> bob : id(aa)
alice -> bob : id(ab,cd)

Keywords arguments

Like in Python, you can use keywords arguments :


!unquoted procedure $element($alias, $description="", $label="", $technology="", $size=12, $colour="green")
rectangle $alias as "


$element(myalias, "This description is\non several lines", $size=10, $technology="Java")

Including files or URL [!include, !include_many, !include_once]

Use the !include directive to include file in your diagram. Using URL, you can also include file from Internet/Intranet.

Imagine you have the very same class that appears in many diagrams. Instead of duplicating the description of this class, you can define a file that contains the description.


interface List
List : int size()
List : void clear()
List <|.. ArrayList

File List.iuml

interface List
List : int size()
List : void clear()

The file List.iuml can be included in many diagrams, and any modification in this file will change all diagrams that include it.

You can also put several @startuml/@enduml text block in an included file and then specify which block you want to include adding !0 where 0 is the block number. The !0 notation denotes the first diagram.

For example, if you use !include foo.txt!1, the second @startuml/@enduml block within foo.txt will be included.

You can also put an id to some @startuml/@enduml text block in an included file using @startuml(id=MY_OWN_ID) syntax and then include the block adding !MY_OWN_ID when including the file, so using something like !include foo.txt!MY_OWN_ID.

By default, a file can only be included once. You can use !include_many instead of !include if you want to include some file several times. Note that there is also a !include_once directive that raises an error if a file is included several times.

Including Subpart [!startsub, !endsub, !includesub]

You can also use !startsub NAME and !endsub to indicate sections of text to include from other files using !includesub. For example:



A -> A : stuff1
!startsub BASIC
B -> B : stuff2
C -> C : stuff3
!startsub BASIC
D -> D : stuff4

file1.puml would be rendered exactly as if it were:


A -> A : stuff1
B -> B : stuff2
C -> C : stuff3
D -> D : stuff4

However, this would also allow you to have another file2.puml like this:



title this contains only B and D
!includesub file1.puml!BASIC

This file would be rendered exactly as if:


title this contains only B and D
B -> B : stuff2
D -> D : stuff4

Builtin functions [%]

Some functions are defined by default. Their name starts by %

Name Description Example Return
%date Retrieve current date. You can provide an optional format for the date %date("yyyy.MM.dd' at 'HH:mm") Current date
%dirpath Retrieve current dirpath %dirpath() Current path
%false Return always false %false() false
%file_exists Check if a file exists on the local filesystem %file_exists("c:/foo/dummy.txt") true if the file exists
%filename Retrieve current filename %filename() Current filename
%function_exists Check if a function exists %function_exists("$some_function") true if the function has been defined
%get_variable_value Retrieve some variable value %get_variable_value("$my_variable") the value of the variable
%getenv Retrieve environment variable value %getenv("OS") The value of OS variable
%intval Convert a String to Int %intval("42") 42
%lower Return a lowercase string %lower("Hello") hello in that example
%not Return the logical negation of an expression %not(2+2==4) false in that example
%set_variable_value Set a global variable %set_variable_value("$my_variable", "some_value") An empty string
%string Convert an expression to String %string(1 + 2) 3 in the example
%strlen Calculate the length of a String %strlen("foo") 3 in the example
%strpos Search a substring in a string %strpos("abcdef", "ef") 4 (position of ef)
%substr Extract a substring. Takes 2 or 3 arguments %substr("abcdef", 3, 2) "de" in the example
%true Return always true %true() true
%upper Return an uppercase string %upper("Hello") HELLO in that example
%variable_exists Check if a variable exists %variable_exists("$my_variable") true if the variable has been defined exists
%version Return PlantUML current version %version() 1.2020.08 for example

Logging [!log]

You can use !log to add some log output when generating the diagram. This has no impact at all on the diagram itself. However, those logs are printed in the command line's output stream. This could be useful for debug purpose.

!function bold($text)
!$result = "<b>"+ $text +"</b>"
!log Calling bold function with $text. The result is $result
!return $result

Alice -> Bob : This is bold("bold")
Alice -> Bob : This is bold("a second call")

Memory dump [!memory_dump]

You can use !memory_dump to dump the full content of the memory when generating the diagram. An optional string can be put after !memory_dump. This has no impact at all on the diagram itself. This could be useful for debug purpose.

!function $inc($string)
!$val = %intval($string)
!log value is $val
!return $val+1

Alice -> Bob : 4 $inc("3")
!unused = "foo"
!dump_memory EOF

Assertion [!assert]

You can put assertions in your diagram.

Alice -> Bob : Hello
!assert %strpos("abcdef", "cd")==3 : "This always fails"

Building custom library [!import, !include]

It's possible to package a set of included files into a single .zip or .jar archive. This single zip/jar can then be imported into your diagram using !import directive.

Once the library has been imported, you can !include file from this single zip/jar.



!import /path/to/
' This just adds "" in the search path

!include myFolder/myFile.iuml
' Assuming that myFolder/myFile.iuml is located somewhere
' either inside "" or on the local filesystem


Search path

You can specify the java property plantuml.include.path in the command line.

For example:

java -Dplantuml.include.path="c:/mydir" -jar plantuml.jar atest1.txt

Note the this -D option has to put before the -jar option. -D options after the -jar option will be used to define constants within plantuml preprocessor.

Argument concatenation [##]

It is possible to append text to a macro argument using the ## syntax.

!unquoted procedure COMP_TEXTGENCOMP(name)
[name] << Comp >>
interface Ifc << IfcType >> AS name##Ifc
name##Ifc - [name]

Dynamic invocation [%invoke_procedure(), %call_user_func()]

You can dynamically invoke a procedure using the special %invoke_procedure() procedure. This procedure takes as first argument the name of the actual procedure to be called. The optional following arguments are copied to the called procedure.

For example, you can have:

!procedure $go()
  Bob -> Alice : hello

!$wrapper = "$go"


!procedure $go($txt)
  Bob -> Alice : $txt

%invoke_procedure("$go", "hello from Bob...")

For return functions, you can use the corresponding special function %call_user_func() :

!function bold($text)
!return "<b>"+ $text +"</b>"

Alice -> Bob : %call_user_func("bold", "Hello") there

Evaluation of addition depending of data types [+]

Evaluation of $a + $b depending of type of $a or $b

<#LightBlue>|= |=  $a |=  $b |=  <U+0025>string($a + $b)|
<#LightGray>| type | str | str | str (concatenation) |
| example |= "a" |= "b" |= %string("a" + "b") |
<#LightGray>| type | str | int | str (concatenation) |
| ex.|= "a" |=  2  |= %string("a" + 2)   |
<#LightGray>| type | str | int | str (concatenation) |
| ex.|=  1  |= "b" |= %string(1 + "b")   |
<#LightGray>| type | bool | str | str (concatenation) |
| ex.|= <U+0025>true() |= "b" |= %string(%true() + "b") |
<#LightGray>| type | str | bool | str (concatenation) |
| ex.|= "a" |= <U+0025>false() |= %string("a" + %false()) |
<#LightGray>| type |  int  |  int | int (addition of int) |
| ex.|=  1  |=  2  |= %string(1 + 2)     |
<#LightGray>| type |  bool  |  int | int (addition) |
| ex.|= <U+0025>true() |= 2 |= %string(%true() + 2) |
<#LightGray>| type |  int  |  bool | int (addition) |
| ex.|=  1  |= <U+0025>false() |= %string(1 + %false()) |
<#LightGray>| type |  int  |  int | int (addition) |
| ex.|=  1  |=  <U+0025>intval("2")  |= %string(1 + %intval("2")) |
end title

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