Core Java

Automate Technical Documentation using Jamal


Writing good technical documentation is an art.

An art is the expression or application of human creative skill and imagination, … to be appreciated primarily for their beauty or emotional power.

But every art, like sculpting, has a craft part. You need chisels, hammers to form the sculpture out of the blob of marble. You need to learn the craft to master the art. Technical documentation writing is similar.

Writing sentences that are easy to read, entertaining for the reader is the art part. Fixing typos and grammatical errors is more like a craft. Making the documentation precise, to the point, and well structured is also the craft part. Crafts can be learned and aided with the right tool.

In technical documentation writing, the tools help address those tasks that are often performed manually though they could be automated. In this article, I will write about a tool that helps in that manner and which I used successfully to write documentation, many articles — also this one –and books.

What can be automated

Many things can be automated for technical document writing. I tried to gather a list from my experience, but it may not be complete. The list is the following:

  • Eliminate manual text repetition.
  • Transclude information from the documented system.
  • Checks internal consistency of the documentation.
  • Check the consistency of the documentation with the documented system.

In the following, I will talk shortly about these tasks, and then I will explain the tool that I use to address these.

DRY in Documentation

The DRY (Don’t Repeat Yourself) is a fundamental and old principle in programming. If there are the same lines in the source, they should be singled out, moving the common code into a separate method, class, or other coding structure. Copy/Paste programming is evil and must not be done. It does not mean that there is no repeated code in the compiled binary code. Code generators are free to repeat code if they think that is better than in some way eliminating it. One famous example is when a short loop is extended, and the code is repeated instead of creating a binary looping construct. It may consume more memory, but at the same time, optimization may find it faster.

The same should happen when you write the documentation. Except, you do not have methods or classes in the documents. You can reorganize your document into smaller sections, and then you can refer to the areas. It may have an impact on the readability. If the reader has to turn the pages instead of linear reading, comprehending the document becomes challenging. Using non-printed, non-linear documentation, a.k.a. hypertext eases a bit the page-turning, but the reader still can get mentally lost in the maze of the non-linear documentation. The ideal solution would be to have documentation, which is linear and contains all the interesting text for the particular user, reading it in the order as they want to read it.

Eventually, it is impossible. With today’s technology, you cannot create a document that contains precisely what the reader wants to read at the very moment and changes for each reader and even for each reading. The best approach we have is repeating some of the text in the documentation. Some readers may find it boring, while others will just get what they need. Your document “source” should be DRY, and the repeating of the text, the copy-paste operation has to be automated. The advantage is: any change in the text is consistently propagated to every occurrence of the text.

Information Transclusion

A living document has to follow the change of the system it documents. In the case of software, this can partially be automated. A lot of data that may need to be included in the document is available in the source code. For example, the current version of the application, a numeric value, may be included in the documentation at different locations. Updating it to the latest version manually is almost always some error. Sooner or later, one or more references may skip the update and become stale. The solution is partial when we use the technique that eliminates DRY. We define the version in the document in one place, and it will be referred to in other places. It still needs that one place to be updated. Fetching the version number from the source code automatically is one level more automation.

Usage samples are also an excellent example for transclusion. When the usage samples are automatically fetched from the unit tests, they are guaranteed to run during the test execution.

Internal Consistency

Ensuring internal consistency of the document can also be automated to some level. It is such an essential factor that many documentation systems support it related to cross-references. The examples can be various.

You may create a document with use cases. In the use cases, you use actors in the description. A document management system can ensure that all the actors used in the document are also defined. A similar check can be done for abbreviations and other things. Consistency, if it can be formally defined, can be checked by automated tools.

External Consistency

Just as well as the different parts of the document should be consistent and without contradiction, the documentation should also be consistent with the system it documents. It is similar to transcluding information from the source. The difference is that the information, in this case, is mainly existence only. For example, you reference a file, a directory, a method in a Java class. A tool can check that the directory, file, or method exists; it was not renamed nor deleted. Similarly, other consistency checks can be programmed.

Document Writing is Programming

There may be some other cases where some automation may come into the picture. The general approach, however, should be to manage the document similar to the program source. Technical documents need maintenance. Documents have a source, and they should be structured. One change in the documented system should be followed by a single change in the document. Every other occurrence in the output should be created automatically.

It is very much similar to programming. The programmers write source code in a high-level programming language, and the compiler generates the machine code. Sometimes the compilation process is a long chain involving many tools. Programming in machine code is an art of the past. The advantages of using a high-level language fairly compensate for the extra effort using the compiler chain.

In technical documentation, the advantages, at least in the short run, are not so appealing. Creating a document using some WYSIWYG editor is easy as opposed to programming in assembly. It is easy to lure the technical writer to avoid some extra work at the start and avoid the document source code creation.

A work to be done tomorrow is always cheaper today than the avoidable work of now.

The same will not be true tomorrow. Creating the more complex but less redundant documentation source almost always payback, especially if we consider document quality coming from consistency and up-to-date-ness.

The Tool: Java Macro Language

In the rest of this article, I will describe a tool that can automate document management tasks. The tool is the Java version of the text processor Jamal. Originally the name was standing for Just Another Macro Language, and it was created in the late 1990-ies in Perl. A few years ago, I rewrote the implementation in Java, with the original functionality enhanced. Since the application is based on Java, it is now named Java Macro Language, abbreviated as Jamal.

The basic concept of Jamal is that the input text containing free text and macros mixed is processed. The output is a text with all the macros executed and evaluated. The syntax of the macros is free. The only requirement is that each of them starts and ends with a specific string. The start and end string can be defined when the macro processor is initialized. It can also be changed on the fly in the input text. When I document Java programs, then I usually use {% as start string and %} as end string. That way, a simple macro definition will be

{%@define lastUpdated=2021-02-17 16:00%}

Later you can refer to this macro as


and it will be replaced by the value 2021-02-17 16:00 for each use.

Jamal distinguishes user-defined and built-in macros. The example above, named lastUpdated is a user-defined macro, as it is defined in the input text. The macro defining it, named define is built-in. It is implemented as a Java class implementing the Jamal Macro interface. The built-in, Java implemented macros are provided in JAR files, in libraries. The core package contains the essential macros, like define, import, begin, end, options, comment, and a few others. These macros are not task-specific. They are needed generally.

Other libraries, like the jamal-snippet library, contain macros that support some specific task. The mentioned jamal-snippet library supports document management.

Snippet Handling, Transclude

The original idea of the snippets is not new. The basic approach to use the source code as part of the documentation originates from D. Knuth with Web and Tangle as early as 1984. Creating a program that contains the documentation and the execution code did not become popular as it needed a lot of extra work from the developers and an additional compilation step. The current trend includes the documentation into the source code as a comment. In the case of Java programs, this is JavaDoc. It is also a trend to use unit tests as a form of documentation.

The two are separate, and both lack the aspect that the other provides. JavaDoc does not show sample use unless someone copies some sample code into it manually. The unit test does not contain a proper explanation unless someone copies fragments or the whole from the JavaDoc to the unit test comments. JavaDoc is converted to navigable HTML pages. Unit tests are source code. Although the best documentation is the source code, it would be nice to have a better, more document-like format.

When we talk about snippets, then we copy code fragments automatically into the documentation. In practice, the documentation format is Asciidoc or MarkDown these days. Both formats allow code samples in the document.

using Jamal, the snippets can be marked in the Java source code or any other source code with

snippet snippetName
    end snippet

lines. The snippetName should be replaced by a unique name that identifies the snippet, and all the lines between the snippet and end snippet lines will be the snippet itself. The snippets are gathered using the {%@snip:collect directory%} macro. Here directory is either a directory or a single file. The collection process reads each file and collects the snippets. After this the snippets can be referenced using the {%@snip snippetName%} macro. When Jamal runs, the macro is replaced with the actual value of the snippet. It ensures that the code sample in the documentation is up-to-date.

Other macros can trim the content, replace some strings in the samples, number the lines, skip some lines, and so on. With these, you can include any code sample.

Snippets are suitable for code samples, but not only for code samples. As JavaDoc is included in the source code, some parts of the documentation can also be included in the code as comments.

For example, the implementation of the macro directory contains the following lines:

// snippet dirMacroFormatPlaceholders
    "$name", name, // gives the name of the directory as was specified on the macro
    "$absolutePath", dir.getAbsolutePath(), // gives the name of the directory as was specified on the macro
    "$parent", dir.getParent() // the parent directory
    "$canonicalPath", dir::getCanonicalPath // the canonical path
    //end snippet

These lines list the different placeholders and their values that the built-in template handler knows. The documentation includes this snippet with the following lines:

{%@define replace=|^.*?"(.*?)"|* `$1`!|!.*?//||%}
{%@define pattern=\)\.and\(%}
{%#replaceLines{%#killLines{%@snip dirMacroFormatPlaceholders %}%}%}

(Note: the actual version is a bit more complicated, as you will see later.) It inserts the content of the snippet evaluating the snip macro. The content of the sippet is then passed to the macro killLines. This macro will delete all the lines that match the regular expression defined in the macro pattern. The result is still further modified by the replaceLines macro. It executes the Java String replaceAll() method on each line with the arguments defined in the macro replace. The final result, inserted into the output is:

* `$name` gives the name of the file as was specified on the macro
* `$absolutePath` the absolute path to the file
* `$parent` the parent directory where the file is
* `$canonicalPath` the canonical path

This way, the document is much easier to maintain. The documentation of the parameters is along with the code, and that way, it is harder to forget to update the documentation. Also, the name of the placeholder is taken directly from the source code. Even if the developer makes a typo naming the placeholder in the example above, the documentation will contain the name as it is in the code and the characters it has to be used.

Snippets can come from other sources, not only from file snippet fragments. The built-in macro snip:xml reads a while XML file and assigns it to a macro name. This macro is similar to the built-in core macro define. It also defines a user-defined macro. In this case, however, the macro is not a constant string with argument placeholders as those defined, calling the macro define. In this case, the content is a whole parsed XML file, and the one argument the macro can and should have when invoked must be an XPath. As you can guess, the result of the macro call is the value in the XML found by the XPath.

As an example, the module documentation README.adoc.jam for jamal-plantuml contains the following lines close to the start of the file:

{%@snip:xml pom=pom.xml%}\
{%#define PLANTUML_VERSION={%pom /project/dependencies/dependency/artifactId[text()="plantuml"]/following-sibling::version/text()%}%}\
{%#define VERSION={%pom /project/version/text()%}%}\

It reads the pom.xml file of the macro and defines the PLANTUML_VERSION and VERSION macros to hold the current version of the used PlantUml library and the version of the project, respectively. Later in the documentation, both {%PLANTUML_VERSION%} and {%VERSION%} can be used and will be replaced in the output with the up-to-date version.

We have seen that snippet texts can be fetched from arbitrary source files and XML files. In addition to that, snippets can also be defined in .properties files (even XML format properties file) and can also be defined as a macro. The snippet definition as a macro using the snip:define built-in has a particular use that we will discuss later with the snip:update macro.

File, Directory, Class, Method => Consistency

The macros file, directory, java:class, and java:method are macros that can keep the code consistent with the system. These macros add barely any formatting to the output; therefore, their use needs discipline. They check that the argument file, directory, class, or method exists. If the entity does not exist, then the macro throws an exception. If the entity was renamed, moved, or deleted, the documentation has to be updated, or else it does not compile.

The use of the macros file and directory is straightforward. They check the existence of the file and directory specified as the argument. The name can either be absolute or relative to the input document.

Checking the existence of a class or method is not that straightforward. It needs a Java environment that has the class on the classpath. It is recommended to invoke Jamal from a unit test to convert the document from the input to output. This article is also written using Jamal as a preprocessor, and it is converted from a unit test of the module jamal-snippet using the following code:

private static void generateDoc(final String directory, final String fileName, final String ext) throws Exception {
    final var in = FileTools.getInput(directory + "/" + fileName + "." + ext + ".jam");
    final var processor = new Processor("{%", "%}");
    final var result = processor.process(in);
    FileTools.writeFileContent(directory + "/" + fileName + "." + ext, result);

void convertSnippetArticle() throws Exception {
    generateDoc(".", "ARTICLE", "wp");

During the unit test’s execution, the classes of the documented system are on the classpath or on the module path, and that way, these macros, java:class and java:method work.

Updating the Input

The jamal-snippet library has a particular macro, snip:update, which does something exceptional.

Built-in macro implementations get the part of the input, which is between the opening and closing string. It is the part of the input that they are supposed to work on. What they get is the input object containing not only the character but also a position coordinate. This coordinate contains the file name and the line/column position of the input in the file. Some macros use this coordinate to report the position of some error. Other macros, like include or import, use the file name to calculate the imported or included file path relative to the one that contains the macro.

The macro snip:update uses the file name to access the file and modify it physically. The macro scans the file and looks for lines that look like

{%@snip id

When the lines with that pattern are found, then the lines between the first and last line, practically the lines denoted with ... above, are replaced with the snippet’s actual content. It will help the maintenance of the input document. When you write the document, it is easier to see the actual snippet and not only the reference to the snippet. It is also easier to debug the line killing, character replacement, and other snippet formatting transformations.

The macro snip is not disturbed by these lines. The syntax of the snip macro is like snip id ... anything treated as a comment... to allow this particular use case.

The invocation of the macro updating of the input should occur at the end of the document when all snippets are already defined. It is also essential to save the input to the version control before converting. The use of this possibility makes it possible to include into the document the formatted snippets. It is done, for example, in the documentation of the macro directory. The sample presented before was a simplified one. Here you can see the real one making use of updates.

{%#snip:define dirMacroFormatPlaceholdersFormatted=
{%#replaceLines{%#killLines{%@snip dirMacroFormatPlaceholders %}%}%}%}

{%@snip dirMacroFormatPlaceholdersFormatted
* `$name` gives the name of the directory as was specified on the macro
* `$absolutePath` gives the name of the directory as was specified on the macro
* `$parent` the parent directory
* `$canonicalPath` the canonical path

This structure includes the snippet dirMacroFormatPlaceholders and converts enclosing it into macros killLines and replaceLines. The final formatted result, however, does not get directly into the output. It is assigned to a new snippet using the macro snip:define. The name of the new snippet is dirMacroFormatPlaceholdersFormatted.

After this, when this new, already formatted snippet is defined, it is referenced using the snip macro to be included in the output. When the macro snip:update is used at the end of the file, this second use of the snip macro is updated, and the formatted lines are inserted there, as you can see.

The first use of the macro snip is not updated because there are extra characters before using the macro. Also, there are extra characters after the snippet identifier.

Creating Diagrams

Using diagrams are very important in the documentation. As the saying goes, a picture is worth a thousand words, especially if your readers are non-native and do not even know a thousand words. An excellent tool to create diagrams is PlantUml. The source for the diagrams in this tool is a text that describes the UML diagram structurally. A simple sequence diagram can look like the following:

Aladdin -> Jasmine : I love you
Jasmine -> Rajah : Aladdin loves me
Rajah --> Aladdin : wtf buddy?


Putting this text into the macro

{%@plantuml sample.svg
Aladdin -> Jasmine : I love you
Jasmine -> Rajah : Aladdin loves me
Rajah --> Aladdin : wtf buddy?

will create the image, and it can then be referenced in the document to get

PlantUml is a widely used tool, and it has integration with many document processors. That way, it is integrated with Markdown and Asciidoc as well. Using Jamal as a preprocessor instead of the PlantUml direct integration has a few advantages, however.

You do not need to have the integration for PlantUml installed on the environment where the document rendering executes. You do not have it, for example, on GitHub or GitLab. Using Jamal, the PlantUml processing is done in your local environment, and after that, you just have a standard Markdown, Asciidoc, or whatever format you use. For example, this document uses WordPress markup, which does not have PlantUml integration, but it does not matter. The source named ARTICLE.wp.jam is processed by Jamal generating ARTICLE.wp, and it has everything it needs. Pictures are generated.

The Jamal preprocessing has other advantages. In this article, as an example, the text of the UML diagram appears three times. Once when I display for the example of how a UML digram is defined in PlantUml. The second time when I show how it is integrated using a Jamal macro. The third time it appears as an image.

The source input contains it only once before the first use. The user-defined macro, named alilove, contains the actual UML, and the latter only references this macro to get the same text. If there is a need to update the structure, it must be done only in one place.

Another advantage is that the macros can access the running Java environment. It is already used when we check the existence and the naming of specific classes and methods. I also plan to extend the PlantUml integration with macros that can leverage the Java environment when we document our code. Running the conversion of the Jamal input during the unit tests reflection can get access to the classes. Using those, I plan to develop macros that need only the listing of the classes you want to be shown on a class diagram. The macro will discover all the relations between the classes and create a UML source to be converted to a diagram using PlantUml. Should your class structure change, the diagrams will also change automatically.

Summary and Takeaway

You should approach technical documentation like programming. Document writers should maintain document source code and compile the document from the source code.

You should automate document content management as much as possible. Use automated tools to transclude information from the documented system. Use automated tools to check the consistency of the documentation. Document everything close to the system documented. Use automated tools to integrate your document source.

Give it a try and use Jamal.

Published on Java Code Geeks with permission by Peter Verhas, partner at our JCG program. See the original article here: Automate Technical Documentation using Jamal

Opinions expressed by Java Code Geeks contributors are their own.

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