Implement #845: add writeComment() in ToXmlGenerator

release-notes/VERSION

@@ -114,6 +114,8 @@ Version: 3.x (for earlier see VERSION-2.x)
 #802: Serialization with Polymorphisme and `EXTERNAL_PROPERTY` = duplicate property
  (reported by @ Adrien-dev25 )
  (fix by @cowtowncoder, w/ Claude code)
+#845: Implement `JsonGenerator` methods `writeComment()` and `canWriteComments()`
+ (implemented by @cowtowncoder, w/ Claude code)
 
 3.1.1 (27-Mar-2026)
 

src/main/java/tools/jackson/dataformat/xml/ser/ToXmlGenerator.java

@@ -253,21 +253,30 @@ public void initGenerator() throws JacksonException
 
     /*
     /**********************************************************************
-    /* Overridden output state handling methods
+    /* Overrides: capability introspection
     /**********************************************************************
      */
-
-    @Override
-    public final TokenStreamContext streamWriteContext() { return _streamWriteContext; }
+
+    @Override // @since 3.2
+    public boolean canWriteComments() { return true; }
+
+    // Base class impl fine:
+    //@Override public boolean canWriteObjectId() { return false; }
+
+    // Base class impl fine:
+    //@Override public boolean canOmitProperties() { return true; }
+
+    // Base class impl fine:
+    //@Override public boolean canWriteTypeId() { return false; }
 
     @Override
-    public final Object currentValue() {
-        return _streamWriteContext.currentValue();
+    public boolean has(StreamWriteCapability capability) {
+        return DEFAULT_TEXTUAL_WRITE_CAPABILITIES.isEnabled(capability);
     }
 
     @Override
-    public final void assignCurrentValue(Object v) {
-        _streamWriteContext.assignCurrentValue(v);
+    public JacksonFeatureSet<StreamWriteCapability> streamWriteCapabilities() {
+        return DEFAULT_TEXTUAL_WRITE_CAPABILITIES;
     }
 
     /*
@@ -297,6 +306,25 @@ public int streamWriteOutputBuffered() {
         return -1;
     }
 
+    /*
+    /**********************************************************************
+    /* Overridden output state handling methods
+    /**********************************************************************
+     */
+
+    @Override
+    public final TokenStreamContext streamWriteContext() { return _streamWriteContext; }
+
+    @Override
+    public final Object currentValue() {
+        return _streamWriteContext.currentValue();
+    }
+
+    @Override
+    public final void assignCurrentValue(Object v) {
+        _streamWriteContext.assignCurrentValue(v);
+    }
+
     /*
     /**********************************************************************
     /* Extended API, configuration
@@ -316,16 +344,6 @@ public ToXmlGenerator configure(XmlWriteFeature f, boolean state) {
         return this;
     }
 
-    @Override
-    public boolean has(StreamWriteCapability capability) {
-        return DEFAULT_TEXTUAL_WRITE_CAPABILITIES.isEnabled(capability);
-    }
-
-    @Override
-    public JacksonFeatureSet<StreamWriteCapability> streamWriteCapabilities() {
-        return DEFAULT_TEXTUAL_WRITE_CAPABILITIES;
-    }
-
     public boolean inRoot() {
         return _streamWriteContext.inRoot();
     }
@@ -906,6 +924,27 @@ public JsonGenerator writeRaw(char c) throws JacksonException
         return writeRaw(String.valueOf(c));
     }
 
+    /*
+    /**********************************************************************
+    /* Output method implementations, comments
+    /**********************************************************************
+     */
+
+    @Override // @since 3.2
+    public JsonGenerator writeComment(String comment) throws JacksonException
+    {
+        try {
+            if (comment != null) {
+                _xmlWriter.writeComment(comment);
+            } else {
+                _xmlWriter.writeSpace("\n");
+            }
+        } catch (XMLStreamException e) {
+            StaxUtil.throwAsWriteException(e, this);
+        }
+        return this;
+    }
+
     /*
     /**********************************************************************
     /* Output method implementations, base64-encoded binary

src/test/java/tools/jackson/dataformat/xml/stream/XmlGeneratorTest.java

@@ -12,6 +12,7 @@
 import tools.jackson.dataformat.xml.ser.ToXmlGenerator;
 
 import static org.junit.jupiter.api.Assertions.assertEquals;
+import static org.junit.jupiter.api.Assertions.assertTrue;
 
 public class XmlGeneratorTest extends XmlTestUtil
 {
@@ -312,4 +313,52 @@ public void testRawCharArratAttribute() throws Exception
         xml = removeSjsxpNamespace(xml);
         assertEquals("<root attr=\"value\"/>", xml);
     }
+
+    // [dataformat-xml#845]
+    @Test
+    public void testCanWriteComments() throws Exception
+    {
+        StringWriter out = new StringWriter();
+        ToXmlGenerator gen = (ToXmlGenerator) MAPPER.createGenerator(out);
+        assertTrue(gen.canWriteComments());
+        // Need to write something to avoid empty document error on close
+        gen.setNextName(new QName("root"));
+        gen.writeStartObject();
+        gen.writeEndObject();
+        gen.close();
+    }
+
+    // [dataformat-xml#845]
+    @Test
+    public void testWriteCommentInObject() throws Exception
+    {
+        StringWriter out = new StringWriter();
+        ToXmlGenerator gen = (ToXmlGenerator) MAPPER.createGenerator(out);
+        gen.setNextName(new QName("root"));
+        gen.writeStartObject();
+        gen.writeComment("a comment");
+        gen.writeName("elem");
+        gen.writeString("value");
+        gen.writeEndObject();
+        gen.close();
+        String xml = removeSjsxpNamespace(out.toString());
+        assertEquals("<root><!--a comment--><elem>value</elem></root>", xml);
+    }
+
+    // [dataformat-xml#845]
+    @Test
+    public void testWriteCommentNullWritesEmptyLine() throws Exception
+    {
+        StringWriter out = new StringWriter();
+        ToXmlGenerator gen = (ToXmlGenerator) MAPPER.createGenerator(out);
+        gen.setNextName(new QName("root"));
+        gen.writeStartObject();
+        gen.writeComment(null);
+        gen.writeName("elem");
+        gen.writeString("value");
+        gen.writeEndObject();
+        gen.close();
+        String xml = removeSjsxpNamespace(out.toString());
+        assertEquals("<root>\n<elem>value</elem></root>", xml);
+    }
 }