PMDâs documentation uses Jekyll with the Iâd rather be writing Jekyll Theme.
Here are some quick tips.
FormatThe pages are in general in Github Flavored Markdown.
StructureThe documentation sources can be found in two places based on how they are generated:
All handwritten documentation is stored in the subfolders under docs/pages
. The folder structure resembles the sidebar structure. Since all pages use a simple permalink, in the rendered html pages, all pages are flattened in one directory. This makes it easy to view the documentation also offline.
The categories for a language %lang%
are located in pmd-%lang%/src/main/resources/category/%lang%
. So for Java the categories can be found under pmd-java/src/main/resources/category/java. The XML category files in this directory are transformed during build into markdown pages describing the rules they contain. These pages are placed under docs/
like the handwritten documentation, and are then rendered with Jekyll like the rest of them. The rule documentation generator is the separate submodule pmd-doc
.
Modifying the documentation of a rule should thus not be done on the markdown page, but directly on the XML rule
tag corresponding to the rule, in the relevant category file.
The XML documentation of rules can contain GitHub flavoured markdown. Just wrap the markdown inside CDATA section in the xml. CDATA sections preserve all formatting inside the delimiters, and allow to write code samples without escaping special xml characters. For example:
<rule ...>
<description>
<![CDATA[
Full description, can contain markup
And paragraphs
]]>
</description>
...
</rule>
We have some additional custom liquid tags that help in writing the documentation.
Hereâs a short overview:
For the javadoc tags, the standard PMD maven modules are already defined as namespaces, e.g. core
, java
, apex
, â¦.
For the implementation of these tags, see the _plugins folder.
BuildingThere are two ways, to execute jekyll:
Using bundler. This will install all the needed ruby packages locally and execute jekyll:
# this is required only once, to download and install the dependencies
bundle install
# this builds the documentation under _site
bundle exec jekyll build
# this runs a local webserver as http://localhost:4005
bundle exec jekyll serve
Using docker. This will create a local docker image, into which all needed ruby packages and jekyll is installed.
# this is required only once to create a local docker image named "pmd-doc"
docker build --no-cache -t pmd-doc .
# this builds the documentation under _site
docker run --rm=true -v "$PWD:/src" pmd-doc build -H 0.0.0.0
# this runs a local webserver as http://localhost:4005
docker run --rm=true -v "$PWD:/src" -p 4005:4005 pmd-doc serve -H 0.0.0.0
The built site is stored locally in the (git ignored) directory _site
. You can point your browser to _site/index.html
to see the pmd documentation.
Alternatively, you can start the local webserver, that will serve the documentation. Just go to http://localhost:4005. If a page is modified, the documentation will automatically be rendered again and all you need to do, is refreshing the page in your browser.
See also the script pmd-jekyll.sh. It starts the jekyll server in the background and doesnât block the current shell.
The sidebar is stored as a YAML document under _data/sidebars/pmd_sidebar.yml
.
Make sure to add an entry there, whenever you create a new page.
The frontmatterEach page in jekyll begins with a YAML section at the beginning. This section is separated by 3 dashes (---
). Example:
---
title: Writing Documentation
last_updated: January 2025 (7.10.0)
permalink: pmd_devdocs_writing_documentation.html
---
Some Text
# Some header
There are a couple of possible fields. Most important and always required are title and permalink.
By default, a page toc (table of contents) is automatically generated. You can prevent this with âtoc: falseâ.
You can add keywords, that will be used for the on-site search: âkeywords: documentation, jekyll, markdownâ
Itâs useful to maintain a last_update field. This will be added at the bottom of the page.
A summary can also be provided. It will be added in a box before the content.
For a more exhaustive list, see Pages - Frontmatter.
Alerts and CalloutsSee Alerts.
For example, a info-box can be created like this:
{% include note.html content="This is a note." %}
It renders as:
Note: This is a note.
Other available types are:
A callout is created like this:
{% include callout.html content="This is a callout of type default.<br/><br/>There are the following types available: danger, default, primary, success, info, and warning." type="default" %}
It renders as:
This is a callout of type default.
There are the following types available: danger, default, primary, success, info, and warning.
Code samples with syntax highlightingThis is as easy as:
``` java
public class Foo {
public void bar() { System.out.println("x"); }
}
```
This looks as follows:
public class Foo {
public void bar() { System.out.println("x"); }
}
Checking for dead links
mvn verify -pl pmd-doc
. This only checks links within the site. HTTP links can be checked by specifying -Dpmd.doc.checkExternalLinks=true
on the command line.
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