New option lineSeparator for MarkdownRenderer.Builder

CHANGELOG.md

@@ -6,6 +6,11 @@ The format is based on [Keep a Changelog](http://keepachangelog.com/en/1.0.0/).
 This project adheres to [Semantic Versioning](http://semver.org/spec/v2.0.0.html),
 with the exception that 0.x versions can break between minor versions.
 
+## [Unreleased]
+### Added
+- New option `lineSeparator` for `MarkdownRenderer.Builder` to change the default line
+  separator from `\n` (e.g. to `\r\n`) (#442)
+
 ## [0.29.0] - 2026-06-20
 ### Added
 - Support rendering GFM task list items to Markdown (#433)
@@ -560,6 +565,7 @@ API breaking changes (caused by changes in spec):
 Initial release of commonmark-java, a port of commonmark.js with extensions
 for autolinking URLs, GitHub flavored strikethrough and tables.
 
+[Unreleased]: https://github.com/commonmark/commonmark-java/compare/commonmark-parent-0.29.0...HEAD
 [0.29.0]: https://github.com/commonmark/commonmark-java/compare/commonmark-parent-0.28.0...commonmark-parent-0.29.0
 [0.28.0]: https://github.com/commonmark/commonmark-java/compare/commonmark-parent-0.27.1...commonmark-parent-0.28.0
 [0.27.1]: https://github.com/commonmark/commonmark-java/compare/commonmark-parent-0.27.0...commonmark-parent-0.27.1

commonmark/src/main/java/org/commonmark/renderer/markdown/MarkdownRenderer.java

@@ -27,9 +27,11 @@
  */
 public class MarkdownRenderer implements Renderer {
 
+    private final String lineSeparator;
     private final List<MarkdownNodeRendererFactory> nodeRendererFactories;
 
     private MarkdownRenderer(Builder builder) {
+        this.lineSeparator = builder.lineSeparator;
         this.nodeRendererFactories = new ArrayList<>(builder.nodeRendererFactories.size() + 1);
         this.nodeRendererFactories.addAll(builder.nodeRendererFactories);
         // Add as last. This means clients can override the rendering of core nodes if they want.
@@ -58,7 +60,7 @@ public static Builder builder() {
 
     @Override
     public void render(Node node, Appendable output) {
-        RendererContext context = new RendererContext(new MarkdownWriter(output));
+        var context = new RendererContext(new MarkdownWriter(output, lineSeparator));
         context.render(node);
     }
 
@@ -74,6 +76,7 @@ public String render(Node node) {
      */
     public static class Builder {
 
+        private String lineSeparator = "\n";
         private final List<MarkdownNodeRendererFactory> nodeRendererFactories = new ArrayList<>();
 
         /**
@@ -83,6 +86,18 @@ public MarkdownRenderer build() {
             return new MarkdownRenderer(this);
         }
 
+        /**
+         * Set the line separator (line terminator) that the writer should use. The default is a
+         * newline ({@code \n}) on all systems.
+         *
+         * @param lineSeparator
+         * @return {@code this}
+         */
+        public Builder lineSeparator(String lineSeparator) {
+            this.lineSeparator = lineSeparator;
+            return this;
+        }
+
         /**
          * Add a factory for instantiating a node renderer (done when rendering). This allows to
          * override the rendering of node types or define rendering for custom node types.

commonmark/src/main/java/org/commonmark/renderer/markdown/MarkdownWriter.java

@@ -8,6 +8,7 @@
 public class MarkdownWriter {
 
     private final Appendable buffer;
+    private final String lineSeparator;
 
     private int blockSeparator = 0;
     private char lastChar;
@@ -21,7 +22,12 @@ public class MarkdownWriter {
     private final LinkedList<CharMatcher> rawEscapes = new LinkedList<>();
 
     public MarkdownWriter(Appendable out) {
-        buffer = out;
+        this(out, "\n");
+    }
+
+    public MarkdownWriter(Appendable out, String lineSeparator) {
+        this.buffer = out;
+        this.lineSeparator = lineSeparator;
     }
 
     /** Write the supplied string (raw/unescaped except if {@link #pushRawEscape} was used). */
@@ -53,9 +59,9 @@ public void text(String s, CharMatcher escape) {
         atLineStart = false;
     }
 
-    /** Write a newline (line terminator). */
+    /** Write a line separator (line terminator). */
     public void line() {
-        write('\n');
+        write(lineSeparator);
         writePrefixes();
         atLineStart = true;
     }
@@ -167,6 +173,17 @@ private void write(String s, CharMatcher escape) {
         atLineStart = false;
     }
 
+    private void write(String s) {
+        try {
+            buffer.append(s);
+        } catch (IOException e) {
+            throw new RuntimeException(e);
+        }
+
+        lastChar = s.charAt(s.length() - 1);
+        atLineStart = false;
+    }
+
     private void write(char c) {
         try {
             append(c, null);
@@ -192,10 +209,10 @@ private void writePrefixes() {
      */
     private void flushBlockSeparator() {
         if (blockSeparator != 0) {
-            write('\n');
+            write(lineSeparator);
             writePrefixes();
             if (blockSeparator > 1) {
-                write('\n');
+                write(lineSeparator);
                 writePrefixes();
             }
             blockSeparator = 0;

commonmark/src/test/java/org/commonmark/renderer/markdown/MarkdownRendererTest.java

@@ -312,6 +312,13 @@ public void testSoftLineBreaks() {
         assertRoundTrip("foo\nbar\n");
     }
 
+    @Test
+    public void testCustomLineSeparator() {
+        var renderer = MarkdownRenderer.builder().lineSeparator("\r\n").build();
+        assertThat(renderer.render(parse("# Heading\n\nBody\n")))
+                .isEqualTo("# Heading\r\n\r\nBody\r\n");
+    }
+
     @Test
     public void overrideNodeRender() {
         var nodeRendererFactory =