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C-3PO is a static website generator for the JVM.

First and foremost C-3PO is using the Thymeleaf 2.1.3 templating system. Thymeleaf is a template engine (like JSP) but also involves good layout support (like Tiles).


C-3PO requires a Java 11 JRE installed on you computer. Upon installation you also need Gradle installed. C-3PO has been tested with Gradle 3.0 and 2.12.

You'd also need to install purifycss if you'd like to purge unused CSS.


At the moment, only installing from source is supported. Follow these steps

  • ensure a Java SE JDK 11 is installed on your computer
  • ensure Gradle (version 3.0, version 2.12 should be fine too) is installed on your computer
  • clone C-3PO's repository to your machine
  • in the repository's root directory call gradle installDist
  • add the directory build/install/C-3PO/bin to your PATH


C-3PO is a command line tool, both for Windows and Unix. C-3PO accepts these command line paramters:

  • -src <dir-name> ... the root source directory of your website.
  • -dest <dir-name> ... the root destination directory in which the website should be generated into.
  • -a ... if the flag is set, C-3PO builds the website as soon as files have changed in the source directory tree. This is a useful option when fiddling around with CSS for example.
  • --fingerprint ... if set, C-3PO fingerprints static asset files like stylesheets, JavaScript files and images (supported image file extensions are .png, .jpg, .jpeg, .svg, .gif, .webp) and replaces references to them in generated HTML documents accordingly.
  • --purge-unused-css ... if set, attempts to purge unused CSS rules in all CSS files beneath ./css. For this to work, purifycss needs to be installed and configured properly in .c3posettings.
  • -p ... stands for production and automatically sets --fingerprint and --purge-unused-css.

Heads up! C-3PO is preventing you from accidentally using the same src and dest directories because this would mean that the source files would be overwritten by their generated counterparts.

Here's an example

c-3po -src . -dest site -a

For each file within the project directory structure C-3PO decides what to do with it. At the moment C-3PO can

  • process Thymeleaf based HTML5 files
    • Thymeleaf's layout dialect is enabled
    • Thymeleaf's LEGACYHTML5 template mode is enabled
  • copy static resources like CSS and JS files into the destination directory

C-3PO does not require a certain project structure. However, it is recommended to follow well-established standards. Here's an example:

  • css/ --> your own CSS or SASS stylesheets
  • css/vendor --> third-party CSS stylesheets
  • js/ --> your own JavaScript files
  • js/vendor/ --> third-party JavaScript files
  • img/ --> image files


C-3PO comes with a sample website that would server as a good starting point for creating a new website. Take a look at samples/base-website. The sample website illustrates how to:

  • configure .c3poignore to ignore certain files
  • configure .c3posettings to trigger convenience functions like creating a sitemap.xml file
  • show how to use Thymeleaf's decorator-based layout system (Thymeleaf layout dialect)
  • show how to use markdown to created web pages faster and in a more convenient way
  • include useful stuff for web development like CSS resets and a HTML5 shiv takes makes older browsers recognize new HTML5 elements Building the sample website is done like this: c-3po -src samples/base-website -dest <your-dir-of-choice>.


C-3PO looks for a .c3posettings file in the top-level source directory. It's a Java standard properties file holding configuration preferences.

Here is a list of available settings:

  • baseUrl ... the base URL of the deployed website. If not set, C-3PO does not generate a sitemap.xml file.
  • nodejsHome ... the home directory of a nodejs binary which is required for running purifycss to purge unused CSS. If you're using nvm to manage nodejs installations, this would look something like this: nodejsHome=/home/robert/.nvm/versions/node/v10.15.3/bin.
  • purifycssHome ... the home directory of the purifycss installation which is required by C-3PO to purge unused CSS. If you're using nvm to manage nodejs installations, this would look something like this: purifycssHome=/home/robert/.nvm/versions/node/v10.15.3/bin/.
  • purifycssWhitelist ... value supplied to purifycss' whitelist parameter which allows to whitelist CSS selectors from being purged.

Generating sitemap.xml and robots.txt

C-3PO is able to generate a sitemap.xml (as specified at file and a robots.txt file. In order to generate a sitemap.xml, C-3PO requires two prerequisites to be fulfilled:

  • there must not exist a sitemap.xml file in the source directory (the same is true for robots.txt)
  • the baseUrl (e.g. setting must be set in .c3posettings

Sometimes it's also desirable to exclude URLs from being crawled. This can be done in the .c3poignore file as described in the corresponding section.

When there is no robots.txt in the source folder, the C-3PO generates a minimal one putting the URL of sitemap.xml into it. This gives search crawlers a hint where to look for a sitemap file.

Heads up! Generation of sitemap.xml and robots.txt is not supported in autoBuild mode.

Ignoring certain files

You'll want to ignore certain files, e.g. the .git folder. Place a text file named .c3poignore into the root directory (defined by -src). Therein list the files and directories (one line for each) that should not be processed by C-3PO.

Can I use wildcards?

Yes. The glob syntax is supported.

.c3poignore example


Ignoring files for sitemap generation only

As already mentioned in the sitemap generation section, C-3PO allows to ignore files and directories only in the context of sitemap generation. Simply place es (exlude in sitemap) within square brackets [...] after an entry of .c3poignore.

In the following example the directory private is only excluded in terms of sitemap generation:

private [es]

Ignoring output of files for destination folder

Most likely you'll have a Thymeleaf layout file somewhere in the project that is an important part of the end result but that should not end up in the build directory itself. Given a _layouts/main-layout.html file you can exclude it from the build directory with the [er] modifier like this:

private [es]
_layouts [er]

Heads Up! Those files and directories still trigger a build when being modified in autoBuild mode.

Using Markdown

C-3PO allows you to write in markdown. To be precise commonmark is used. Why? Because it's an effort to standardize markdown syntax.

Anyways, how do you use markdown with C-3PO?

Create a markdown file. Then create a Thymeleaf template called md-template.html in the same directory. Within md-template.html you are able to access two context objects called markdownContent (the HTML elements that result from processing the markdown file as a String) and markdownHead (an object representing meta tags and title to be included in the page's head; further description below). md-template.html is simply a wrapper for the markdown content that allows us to integrate with the site's layout and so on.

Heads up! If C-3PO does not find a file called md-template.html in the directory, it will not process the markdown file.

Heads up! C-3PO does not do HTML escaping of markdown content itself since markdown allows to have inline HTML markup like <cite> in markdown text.

Example of a md-template.html file:

<!DOCTYPE html>
<html layout:decorator="_layouts/main-layout">
		<title th:text="${markdownHead.title}"></title>
		<meta th:each="metaTagEntry : ${markdownHead.metaTags}"
		<div layout:fragment="content">
			<div th:utext="${markdownContent}">
				This is replaced by markdown based blog content.

Note the use of th:utext to spit out the HTML string in markdownContent. Beware that th:utext renders unescaped text.

Define HTML title and meta tags in markdown

C-3PO introduced an extension to commonmark allowing editors to define the title and meta tags for the resulting HTML page.

Why is this useful?

  1. Reader Experience: people like when browser tabs show meaningful titles
  2. SERPs: the contents of the meta description tag is shown on search engine result pages (SERP). Ideally a description is 150 to 160 characters long. A good meta description will raise the chances that search engine users click through to your site.

So, how do I define these meta tags?

$meta-title: A catchy page title
$meta-description: A summary that describes the contents (ideally 150 to 160 characters)

# Some heading

They must start with $meta-. Everything between $meta- and the colon : will become the name of the meta tag. The rest after the colon : will be the content of the meta tag.

When processing a markdown file, C-3PO will put this data as an object called markdownHead into the template's context. You'll be able to use the markdownHead object in the Thymeleaf template file md-template.html like this:

<title th:text="${markdownHead.title}"></title>
<meta th:each="metaTagEntry : ${markdownHead.metaTags}"

Access the name of the markdown file

In some cases you'll want to access the name of the markdown file, that is being processed, in your templates (i.e. layout templates). You might know that Thymeleaf passes the name of the current template being processed to ${execInfo.templateName}. But if a markdown file is processed, the template in use is by convention md-template.html, meaning that ${execInfo.templateName} basically resolves to md-template.html. But for some situations you'll want to know the name of the markdown file that's being wrapped by md-template.html. C-3PO provides that in markdownFileName that you can access with ${markdownFileName}.

Heads up! Before using it in your templates, you probably want to check if it is even set (e.g. when mixing markdown and html content). Here's an expression that does that: ${markdownFileName} != null.


C-3PO is able to process SASS / SCSS stylesheets. SASS / SCSS is a CSS preprocessor and enables you to use useful things like selector nesting or variables in your stylesheets. Read more about it at

Beware that there's a difference between .scss and .sass files. E.g. in .sass files you can omit curly brackets and the indentation level of your code is important. However, seemingly the SASS owners introduced SCSS later on because SASS was causing some confusion.

Note: C-3PO is minifying CSS output by default.

See the sample project samples/base-website for a basic SASS example.

Fingerprinting assets to support cache busting

C-3PO has asset fingerprinting built-in. Right now, it supports fingerprinting CSS, JavaScript and image files. For image files, it supports the following file extensions: .png, .jpg, .jpeg, .svg, .gif, .webp.

To activate fingerprinting, either pass --fingerprint or -p on the command line. You don't have to modify your HTML (or Markdown) files for fingerprinting to work. This is in contrast to the majority of web frameworks, build tools and static site generators that require you to use special syntax to load assets. For example see the fingerprinting section in the Rails Guide or the article How do web frameworks implement asset fingerprinting?

C-3PO fingerprinting supports all kinds of URL forms as described in Absolute and relative URLs in HTML and it is able to recognize whether an asset is served by the site under construction or by an external site. One caveat here is when referencing an asset through its absolute URL, it is only considered to be controlled by the website if the asset's base URL matches baseUrl in .c3posettings. This means, given baseURL=, is considered to be an asset of the site while the www-variant is not. This would be a cool feature, but read the solution log for more details.

Fingerprinting by the way means that a hash of the file in question is calculated and appended to the file name. In case of C-3PO, ./css/main.css turns into something like ./css/main.6180d1743d1be0d975ed1afbdc3b4c0bfb134124.css. The original file is kept in place. This is a safety measure for the case something goes wrong when replacing asset references in HTML by their fingerprinted counterparts. Fingerprinting does not change the contents of the file. The hash algorithm in use is SHA-1. C-3PO shall produce the same hashes as the Unix commandsha1sum.

Fingerprinting limitations

  • There's only one way to load external CSS and JavaScript assets in HTML. For images, there are multiple ways, and so far only <img src="..."> and <img srcset="..."> is supported. For srcset, asset URLs containing a comma will not be replaced correctly. This is due to the fact, that parsing the srcset syntax is not trivial.
  • Audio and video assets are not supported simply because this requirement didn't come up so far.
  • C-3PO only fingerprints stylesheets located beneath ./css, JavaScript files beneath ./js and image files beneath ./img.

Purging unused CSS

Heads up! This has only been tested on Linux so far.

Purging unused CSS is the process of removing CSS rules not used on the website. Unlike with fingerprinting, C-3PO does not have this feature built-in. Instead, it relies on purifycss. purifycss can be installed through npm like npm i -g purify-css.

purifycss has been chosen because it has the most practical CLI interface. Running such tool within the JVM's JavaScript engine (is there still one in Java 11?) was not an option.

For it to work, either supply the --purge-unused-css or -p command line arguments. In addition you need to set the nodejsHome and purifycssHome properties in .c3posettings. See the settings section for more information.

How does purging unused CSS relate to fingerprinting? Not much. It runs before fingerprinting and it simply replaces the original CSS file by the purified one. This means, that fingerprinting is not aware that unused CSS is purged before.

Purging unused CSS only applies to CSS files beneath the ./css folder (including sub-directories).

FAQ for Website Editing

When using Thymeleaf's Layout dialect how to pass parameters from a template to its layout?

When you use a decorator-based layout, you may want to pass parameters from the content template to the layout template. To accomplish this simply use th:with in the content template like this:

<html layout:decorator="_layouts/main-layout" th:with="activeMainNavEntry='home'">

This is useful for example when the main navigation is defined in the layout page and you want to control which navigation entry is rendered as active.

How to control which navigation entry is active when using Thymeleaf's Layout dialect?

Assuming you're using Thymeleaf's Layout dialect in conjunction with decorator-based layouts there are two possibilities:

  • Evaluate the standard ${execInfo.templateName} in the layout template to determine which template is currently being decorated. This is useful for smaller sites, probably without a sub navigation.
  • Pass a parameter, for example activeMainNavEntry, to the layout template and and evaluate it when rendering the navigation. This is more useful for larger sites where you want to control a main and a sub-navigation and the name of the template currently being decorated doesn't reflect the main navigation category that is supposed to be shown as active.

How to avoid obsolete <div> elements resulting from a decorator-based layout (layout dialect)?

Usually you do something like this in your layout.html

<div layout:fragment="content">
  <!-- This will be replaced by content from content template -->

... and in your content template content1.html something like this

<div layout:fragment="content">
  <h1>Foo Bar</h1>

But what if you want to include page-specific scripts that way? The div elements would surround the script declarations. In this case just replace div elements with th:block elements in both the decorator (the layout) and the content template (the content page).


How to build C-3PO?

C-3PO builds with Gradle 3.0 and uses its application plugin.

  • gradle build ... builds (compile, test etc.) the project
  • gradle distZip ... creates a ZIP-packaged distribution of C-3PO
  • gradle installDist ... installs C-3PO into build/install

Hint: you can put the /bin directory within the install directory to your operating system's search PATH. This way C-3PO will always be available on the command line. This is very useful when developing C-3PO and building a website with C-3PO at the same time.

Solution log

Decide which absolute asset URLs to consider when fingerprinting assets, 2020-04-22

When fingerprinting an asset, which hostnames are considered to be the same when it comes to replacing an absolute URL to this asset with its fingerprinted counterpart? Suppose the base URL is Should be considered the same? For example, should be replaced by given the base URL (defined in .c3posettings) is

This would be a cool feature for sure. But the problem is the implementation. Unless you would also consider something like and the same, finding out the lower-level domain (the "example" in, the "foo" in, sometimes also called second-level domain) isn't easy at all. First and foremost, because top-level domains can have two levels as well, such as Hence, the common advice is to use Guava's InternetDomainName class. It uses generated regular expressions to recognize valid top-level domains. Whenever a new top-level domain gets introduced, these regular expressions need to be adapted and regenerated.

I decided not to support that feature because I didn't want to introduce a big library like Guava for such a small win. Also, it is questionable to reference one's assets through absolute URLs anyways and doing so by using either the non-www or www variant even more so. Just think about changing the domain name one time. You've got to change all those absolute URLs. Use relative URLs instead.

More Resources

If you want to dive deeper into C-3PO, have a look into the Wiki.


Java-based static web site generator






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