commonmark-java

Java library for parsing and rendering Markdown text according to the CommonMark specification (and some extensions).

Provides classes for parsing input to an abstract syntax tree of nodes (AST), visiting and manipulating nodes, and rendering to HTML. It started out as a port of commonmark.js, but has since evolved into a full library with a nice API and the following features:

Small (minimal dependencies)
Fast (10-20 times faster than pegdown, see benchmarks in repo)
Flexible (manipulate the AST after parsing, customize HTML rendering)
Extensible (tables, strikethrough, autolinking and more, see below)

Requirements:

Java 7 or above
Works on Android, minimum API level 15 (see commonmark-android-test directory)
The core has no dependencies; for extensions, see below

Coordinates for core library (see all on Maven Central):

<dependency>
    <groupId>com.atlassian.commonmark</groupId>
    <artifactId>commonmark</artifactId>
    <version>0.9.0</version>
</dependency>

Note that for 0.x releases of this library, the API is not considered stable yet and may break between minor releases. After 1.0, Semantic Versioning will be followed.

See the spec.txt file if you're wondering which version of the spec is currently implemented. Also check out the CommonMark dingus for getting familiar with the syntax or trying out edge cases.

Usage

Parse and render to HTML

import org.commonmark.node.*;
import org.commonmark.parser.Parser;
import org.commonmark.renderer.html.HtmlRenderer;

Parser parser = Parser.builder().build();
Node document = parser.parse("This is *Sparta*");
HtmlRenderer renderer = HtmlRenderer.builder().build();
renderer.render(document);  // "<p>This is <em>Sparta</em></p>\n"

This uses the parser and renderer with default options. Both builders have methods for configuring their behavior, e.g. calling escapeHtml(true) on HtmlRenderer will escape raw HTML tags and blocks. For all available options, see methods on the builders.

Note that this library doesn't try to sanitize the resulting HTML; that is the responsibility of the caller.

For rendering to plain text, there's also a TextContentRenderer with a very similar API.

Use a visitor to process parsed nodes

After the source text has been parsed, the result is a tree of nodes. That tree can be modified before rendering, or just inspected without rendering:

Node node = parser.parse("Example\n=======\n\nSome more text");
WordCountVisitor visitor = new WordCountVisitor();
node.accept(visitor);
visitor.wordCount;  // 4

class WordCountVisitor extends AbstractVisitor {
    int wordCount = 0;

    @Override
    public void visit(Text text) {
        // This is called for all Text nodes. Override other visit methods for other node types.

        // Count words (this is just an example, don't actually do it this way for various reasons).
        wordCount += text.getLiteral().split("\\W+").length;

        // Descend into children (could be omitted in this case because Text nodes don't have children).
        visitChildren(text);
    }
}

Add or change attributes of HTML elements

Sometimes you might want to customize how HTML is rendered. If all you want to do is add or change attributes on some elements, there's a simple way to do that.

In this example, we register a factory for an AttributeProvider on the renderer to set a class="border" attribute on img elements.

Parser parser = Parser.builder().build();
HtmlRenderer renderer = HtmlRenderer.builder()
        .attributeProviderFactory(new AttributeProviderFactory() {
            public AttributeProvider create(AttributeProviderContext context) {
                return new ImageAttributeProvider();
            }
        })
        .build();

Node document = parser.parse("![text](/url.png)");
renderer.render(document);
// "<p><img src=\"/url.png\" alt=\"text\" class=\"border\" /></p>\n"

class ImageAttributeProvider implements AttributeProvider {
    @Override
    public void setAttributes(Node node, String tagName, Map<String, String> attributes) {
        if (node instanceof Image) {
            attributes.put("class", "border");
        }
    }
}

Customize HTML rendering

If you want to do more than just change attributes, there's also a way to take complete control over how HTML is rendered.

In this example, we're changing the rendering of indented code blocks to only wrap them in pre instead of pre and code:

Parser parser = Parser.builder().build();
HtmlRenderer renderer = HtmlRenderer.builder()
        .nodeRendererFactory(new HtmlNodeRendererFactory() {
            public NodeRenderer create(HtmlNodeRendererContext context) {
                return new IndentedCodeBlockNodeRenderer(context);
            }
        })
        .build();

Node document = parser.parse("Example:\n\n    code");
renderer.render(document);
// "<p>Example:</p>\n<pre>code\n</pre>\n"

class IndentedCodeBlockNodeRenderer implements NodeRenderer {

    private final HtmlWriter html;

    IndentedCodeBlockNodeRenderer(HtmlNodeRendererContext context) {
        this.html = context.getWriter();
    }

    @Override
    public Set<Class<? extends Node>> getNodeTypes() {
        // Return the node types we want to use this renderer for.
        return Collections.<Class<? extends Node>>singleton(IndentedCodeBlock.class);
    }

    @Override
    public void render(Node node) {
        // We only handle one type as per getNodeTypes, so we can just cast it here.
        IndentedCodeBlock codeBlock = (IndentedCodeBlock) node;
        html.line();
        html.tag("pre");
        html.text(codeBlock.getLiteral());
        html.tag("/pre");
        html.line();
    }
}

Add your own node types

In case you want to store additional data in the document or have custom elements in the resulting HTML, you can create your own subclass of CustomNode and add instances as child nodes to existing nodes.

To define the HTML rendering for them, you can use a NodeRenderer as explained above.

API documentation

Javadocs are available online on javadoc.io.

Extensions

Extensions need to extend the parser, or the HTML renderer, or both. To use an extension, the builder objects can be configured with a list of extensions. Because extensions are optional, they live in separate artifacts, so additional dependencies need to be added as well.

Let's look at how to enable tables from GitHub Flavored Markdown. First, add an additional dependency (see Maven Central for others):

<dependency>
    <groupId>com.atlassian.commonmark</groupId>
    <artifactId>commonmark-ext-gfm-tables</artifactId>
    <version>0.9.0</version>
</dependency>

Then, configure the extension on the builders:

import org.commonmark.ext.gfm.tables.TablesExtension;

List<Extension> extensions = Arrays.asList(TablesExtension.create());
Parser parser = Parser.builder()
        .extensions(extensions)
        .build();
HtmlRenderer renderer = HtmlRenderer.builder()
        .extensions(extensions)
        .build();

To configure another extension in the above example, just add it to the list.

The following extensions are developed with this library, each in their own artifact.

Autolink

Turns plain links such as URLs and email addresses into links (based on autolink-java).

Use class AutolinkExtension from artifact commonmark-ext-autolink.

Strikethrough

Enables strikethrough of text by enclosing it in ~~. For example, in hey ~~you~~, you will be rendered as strikethrough text.

Use class StrikethroughExtension in artifact commonmark-ext-gfm-strikethrough.

Tables

Enables tables using pipes as in GitHub Flavored Markdown.

Use class TablesExtension in artifact commonmark-ext-gfm-tables.

Heading anchor

Enables adding auto generated "id" attributes to heading tags. The "id" is based on the text of the heading.

# Heading will be rendered as:

<h1 id="heading">Heading</h1>

Use class HeadingAnchorExtension in artifact commonmark-ext-heading-anchor.

In case you want custom rendering of the heading instead, you can use the IdGenerator class directly together with a HtmlNodeRendererFactory (see example above).

Ins

Enables underlining of text by enclosing it in ++. For example, in hey ++you++, you will be rendered as underline text. Uses the <ins> tag.

Use class InsExtension in artifact commonmark-ext-ins.

YAML front matter

Adds support for metadata through a YAML front matter block. This extension only supports a subset of YAML syntax. Here's an example of what's supported:

---
key: value
list:
  - value 1
  - value 2
literal: |
  this is literal value.

  literal values 2
---

document start here

Use class YamlFrontMatterExtension in artifact commonmark-ext-yaml-front-matter. To fetch metadata, use YamlFrontMatterVisitor.

Contributing

Pull requests, issues and comments welcome ☺. For pull requests:

Add tests for new features and bug fixes
Follow the existing style (always use braces, 4 space indent)
Separate unrelated changes into multiple pull requests

See the existing "help wanted" issues for things to start contributing.

For bigger changes, make sure you start a discussion first by creating an issue and explaining the intended change.

License

BSD (2-clause) licensed, see LICENSE.txt file.

vdesmet93/commonmark-java