Enable adding own markdown renderers in builder

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

@@ -136,9 +136,9 @@ private RendererContext(MarkdownWriter writer) {
             }
             additionalTextEscapes = Collections.unmodifiableSet(escapes);
 
-            // The first node renderer for a node type "wins".
-            for (int i = nodeRendererFactories.size() - 1; i >= 0; i--) {
-                MarkdownNodeRendererFactory nodeRendererFactory = nodeRendererFactories.get(i);
+            // The first node renderer for a node type "wins". The NodeRendererMap
+            // disallows overwriting.
+            for (MarkdownNodeRendererFactory nodeRendererFactory : nodeRendererFactories) {
                 // Pass in this as context here, which uses the fields set above
                 NodeRenderer nodeRenderer = nodeRendererFactory.create(this);
                 nodeRendererMap.add(nodeRenderer);

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

@@ -0,0 +1,73 @@
+package org.commonmark.renderer.markdown;
+
+import static org.junit.Assert.assertEquals;
+import static org.junit.Assert.assertTrue;
+
+import java.util.Set;
+
+import org.commonmark.node.Heading;
+import org.commonmark.node.Node;
+import org.commonmark.parser.Parser;
+import org.commonmark.renderer.NodeRenderer;
+import org.junit.Test;
+
+public class NodeRendererFactoriesTest {
+
+    @Test
+    public void testAddedNodeRendererFactoryOverrides() {
+        String input = "# Header";
+        TestCoreMarkdownNodeRenderer.called = false;
+        parseAndRender(input, new TestNodeRendererFactory());
+        assertTrue(TestCoreMarkdownNodeRenderer.called);
+    }
+
+    private void assertRoundTrip(String input, MarkdownNodeRendererFactory factory) {
+        String rendered = parseAndRender(input, factory);
+        assertEquals(input, rendered);
+    }
+
+    private String parseAndRender(String source, MarkdownNodeRendererFactory factory) {
+        Node parsed = parse(source);
+        return render(parsed, factory);
+    }
+
+    private Node parse(String source) {
+        return Parser.builder().build().parse(source);
+    }
+
+    private String render(Node node, MarkdownNodeRendererFactory factory) {
+        return MarkdownRenderer.builder().nodeRendererFactory(factory).build().render(node);
+    }
+
+    private static class TestNodeRendererFactory implements MarkdownNodeRendererFactory {
+        @Override
+        public NodeRenderer create(MarkdownNodeRendererContext context) {
+            return new TestCoreMarkdownNodeRenderer(context);
+        }
+
+        @Override
+        public Set<Character> getSpecialCharacters() {
+            return Set.of();
+        }
+    }
+
+    private static class TestCoreMarkdownNodeRenderer extends CoreMarkdownNodeRenderer {
+        static boolean called = false;
+
+        public TestCoreMarkdownNodeRenderer(MarkdownNodeRendererContext context) {
+            super(context);
+        }
+
+        @Override
+        public Set<Class<? extends Node>> getNodeTypes() {
+            return super.getNodeTypes();
+        }
+
+        @Override
+        public void visit(Heading heading) {
+            called = true;
+            super.visit(heading);
+        }
+    }
+
+}