Expand test wrt [databind#5860], improve XmlMapper support for it

src/main/java/tools/jackson/dataformat/xml/XmlMapper.java

@@ -475,7 +475,6 @@ public boolean isEnabled(XmlWriteFeature f) {
         return _serializationConfig.hasFormatFeature(f);
     }
 
-
     /*
     /**********************************************************************
     /* XML-specific access
@@ -497,7 +496,8 @@ public FromXmlParser createParser(XMLStreamReader r) throws IOException {
      */
     public ToXmlGenerator createGenerator(XMLStreamWriter w) throws IOException {
         SerializationContextExt prov = _serializationContext(serializationConfig());
-        return tokenStreamFactory().createGenerator(prov, w);
+        return (ToXmlGenerator) _initializeGenerator(
+                tokenStreamFactory().createGenerator(prov, w));
     }
 
     /**

src/test/java/tools/jackson/dataformat/xml/ser/GeneratorInitializerTest.java

@@ -0,0 +1,76 @@
+package tools.jackson.dataformat.xml.ser;
+
+import java.io.StringWriter;
+
+import javax.xml.stream.XMLOutputFactory;
+import javax.xml.stream.XMLStreamException;
+import javax.xml.stream.XMLStreamWriter;
+
+import tools.jackson.databind.cfg.GeneratorInitializer;
+import tools.jackson.dataformat.xml.XmlMapper;
+import tools.jackson.dataformat.xml.XmlTestUtil;
+
+import org.junit.jupiter.api.Test;
+
+import static org.junit.jupiter.api.Assertions.*;
+
+// Test to ensure that XML module supports
+// [https://github.com/FasterXML/jackson-databind/issues/5860] (GeneratorInitializer
+// abstraction)
+public class GeneratorInitializerTest extends XmlTestUtil
+{
+    private final GeneratorInitializer ISO_8859_INITIALIZER = (config, gen) -> {
+        ToXmlGenerator xmlGen = (ToXmlGenerator) gen;
+        XMLStreamWriter sw = xmlGen.getStaxWriter();
+        try {
+            sw.writeStartDocument("ISO-8859-1", "1.0");
+        } catch (XMLStreamException e) {
+            throw new RuntimeException("Failed to write StartDocument event", e);
+        }
+    };
+
+    // Test via writeValueAsString (goes through ObjectWriter)
+    @Test
+    public void testCustomEncodingViaGeneratorInitializer() throws Exception {
+        XmlMapper mapper = XmlMapper.builder()
+                .generatorInitializer(ISO_8859_INITIALIZER)
+                .build();
+        String xml = mapper.writeValueAsString(new StringBean("test"));
+        assertTrue(xml.contains("encoding='ISO-8859-1'") || xml.contains("encoding=\"ISO-8859-1\""),
+                "Expected ISO-8859-1 encoding in XML declaration, got: " + xml);
+    }
+
+    // Test via writeValue(XMLStreamWriter, Object) — XML-specific path
+    @Test
+    public void testCustomEncodingViaWriteValueXMLStreamWriter() throws Exception {
+        XmlMapper mapper = XmlMapper.builder()
+                .generatorInitializer(ISO_8859_INITIALIZER)
+                .build();
+        StringWriter sw = new StringWriter();
+        XMLStreamWriter xmlSW = XMLOutputFactory.newFactory().createXMLStreamWriter(sw);
+        mapper.writeValue(xmlSW, new StringBean("test"));
+        xmlSW.close();
+        String xml = sw.toString();
+        assertTrue(xml.contains("encoding='ISO-8859-1'") || xml.contains("encoding=\"ISO-8859-1\""),
+                "Expected ISO-8859-1 encoding in XML declaration, got: " + xml);
+    }
+
+    // Test via createGenerator(XMLStreamWriter) — direct generator creation
+    @Test
+    public void testCustomEncodingViaCreateGeneratorXMLStreamWriter() throws Exception {
+        XmlMapper mapper = XmlMapper.builder()
+                .generatorInitializer(ISO_8859_INITIALIZER)
+                .build();
+        StringWriter sw = new StringWriter();
+        XMLStreamWriter xmlSW = XMLOutputFactory.newFactory().createXMLStreamWriter(sw);
+        ToXmlGenerator gen = mapper.createGenerator(xmlSW);
+        gen.setNextName(new javax.xml.namespace.QName("root"));
+        gen.writeStartObject();
+        gen.writeEndObject();
+        gen.close();
+        xmlSW.close();
+        String xml = sw.toString();
+        assertTrue(xml.contains("encoding='ISO-8859-1'") || xml.contains("encoding=\"ISO-8859-1\""),
+                "Expected ISO-8859-1 encoding in XML declaration, got: " + xml);
+    }
+}