Drawing diagrams with PlantUML
Custom Images
In addition to the integrated icons, you can also create custom images. Listing 9 shows the source for embedding a custom image, and Figure 8 shows its output.
Listing 9
Embedding Images
01 @startuml 02 sprite tux linuxtux.png 03 Linux->World : I am here <$tux>, where are you? 04 Linux<-World : I am SO glad to see you! 05 @enduml
To do this, define a sprite
(a graphical element) with a label (tux
in my example) and associate it with an actual image on your hard drive. Next, you can place the image where you want by calling its label with the syntax shown in line 3 of Listing 8. In addition to regular images, you may define and embed custom sprites [7], which are monochrome images that are defined similarly to traditional ASCII art right inside your UML code:
startuml sprite $foo1 { FFFFFFFFFFFFFFF F0123456789ABCF F0123456789ABCF FFFFFFFFFFFFFFF }
This format is less esoteric than it may seem. Each hexadecimal digit inside the curly braces corresponds to one pixel of the image, and its value corresponds to the gray level of that pixel, with
being white, and F
being black. For further details on embedded sprites, see the PlantUML guide [5].
UML Programming
UML's creators have given it several capabilities of a real programming language.
You can include comments in your UML files to make them more readable. Single line comments start with one simple quote ('
), and multiline comments are enclosed by /'
and '/
tags.
Lines that start with exclamation marks (!
) are preprocessing directives, with which you can give PlantUML general instructions or make it interact with your system. The following
!log Now generating a Gantt Diagram
tells PlantUML to log a message, by writing it to its standard output.
PlantUML also uses variables and functions. PlantUML variables can only contain strings or integer numbers; it is good practice to give them names starting with a dollar sign ($
), like Perl. Besides storing values for later use, variables can also be used for simple flow control, to decide what to draw where:
Alice -> Bob : Are you free tonight? !if ($day == "Saturday") Alice <- Bob : yes !else Alice -> Bob : no, I'm sorry !endif
PlantUML supports three different types of functions: built-in, void, and return functions. Built-in functions, which have names starting with %
, include operators like %strlen
(string length) or %substr
(substring extraction). Other built-in functions give access to system information, like file location or the current date.
Void and return functions are recognizable, because, like variables, their names must start with a dollar sign ($
). The difference is simple between void and return functions and summarized in Listing 10.
Listing 10
Void and Return Functions
01 !function $warning($source, $recipient) 02 $source --> $recipient : Warning! You have been hacked! 03 !endfunction 04 !function $halfvalue($a) 05 !return $a/2 06 !endfunction
Void functions directly insert something (normally one statement) into the UML diagram description. In Listing 10, the $warning
function (line 1) will print a warning from user $source
to user $recipient
whenever it is called, obviously replacing $source
and $recipient
with the current values of those variables. Return functions perform a calculation or string processing and return the result (lines 4 and 5); they are often called from other functions.
In both void and return functions, you can define local variables, which are not visible from outside the function, and set default values for the arguments.
Code Management
In UML, as in software in general, reuse is almost always good, and duplication is almost always bad. You can put all the configuration and formatting functions that you regularly use in one file and make PlantUML load it as follows:
java -jar /path/to/plantuml.jar -config "./config.cfg" dir1
To do the same thing with the actual drawing code (e.g., the initial part of a sequence exchange or flow diagram) that you want to duplicate in other diagrams, you can write the corresponding code once (maybe packaging it as a function) in one file. Then !include
that file in other files every time you need it as follows:
@startuml !include common-diagrams-section.iuml .. your other UML code here @enduml
By doing this, any change in that single file will be seen and reloaded by all the files that include it the next time you run PlantUML, just as if you had copied and pasted common-diagrams-section
in each file. The !include
directive supports URLs, so you may even load code directly from the Internet or your company network. By default, a file can only be included once. You may include the same file several times in your code with the !include_many
directive, but think twice before doing it: It may make your code unmanageable.
Syntax-wise, you may also put several independent blocks of common code in the same file, each within its own @startuml
/@enduml
statements, and identify each of those blocks by their number. A directive like !include myuml.txt!1
will then load only the second (numbering starts from
) block within myuml.txt
. To improve code readability, you can assign names to a block
@startuml(id=SOME_IDENTIFIER)
and then include only that block with this statement
!include foo.txt!SOME_IDENTIFIER
Buy this article as PDF
(incl. VAT)
Buy Linux Magazine
Subscribe to our Linux Newsletters
Find Linux and Open Source Jobs
Subscribe to our ADMIN Newsletters
Support Our Work
Linux Magazine content is made possible with support from readers like you. Please consider contributing when you’ve found an article to be beneficial.
News
-
Red Hat Adds New Deployment Option for Enterprise Linux Platforms
Red Hat has re-imagined enterprise Linux for an AI future with Image Mode.
-
OSJH and LPI Release 2024 Open Source Pros Job Survey Results
See what open source professionals look for in a new role.
-
Proton 9.0-1 Released to Improve Gaming with Steam
The latest release of Proton 9 adds several improvements and fixes an issue that has been problematic for Linux users.
-
So Long Neofetch and Thanks for the Info
Today is a day that every Linux user who enjoys bragging about their system(s) will mourn, as Neofetch has come to an end.
-
Ubuntu 24.04 Comes with a “Flaw"
If you're thinking you might want to upgrade from your current Ubuntu release to the latest, there's something you might want to consider before doing so.
-
Canonical Releases Ubuntu 24.04
After a brief pause because of the XZ vulnerability, Ubuntu 24.04 is now available for install.
-
Linux Servers Targeted by Akira Ransomware
A group of bad actors who have already extorted $42 million have their sights set on the Linux platform.
-
TUXEDO Computers Unveils Linux Laptop Featuring AMD Ryzen CPU
This latest release is the first laptop to include the new CPU from Ryzen and Linux preinstalled.
-
XZ Gets the All-Clear
The back door xz vulnerability has been officially reverted for Fedora 40 and versions 38 and 39 were never affected.
-
Canonical Collaborates with Qualcomm on New Venture
This new joint effort is geared toward bringing Ubuntu and Ubuntu Core to Qualcomm-powered devices.