add feature to include standalone=yes in xml declaration

Author: duoduobingbing

src/main/java/com/fasterxml/jackson/dataformat/xml/ser/ToXmlGenerator.java

@@ -51,6 +51,15 @@ public enum Feature implements FormatFeature
          */
         WRITE_XML_DECLARATION(false),
 
+        /**
+         * Feature that controls whether XML declaration should include the standalone attribute
+         * when generator is initialized (true) or not (false). Only honored when
+         * {@link Feature#WRITE_XML_DECLARATION WRITE_XML_DECLARATION} is enabled
+         *
+         * @since 2.19
+         */
+        WRITE_STANDALONE_YES_TO_XML_DECLARATION(false),
+
         /**
          * Feature that controls whether output should be done as XML 1.1; if so,
          * certain aspects may differ from default (1.0) processing: for example,
@@ -297,15 +306,24 @@ public void initGenerator()  throws IOException
         _initialized = true;
         try {
             boolean xmlDeclWritten;
-            if (Feature.WRITE_XML_1_1.enabledIn(_formatFeatures)) {
-                _xmlWriter.writeStartDocument("UTF-8", "1.1");
-                xmlDeclWritten = true;
-            } else if (Feature.WRITE_XML_DECLARATION.enabledIn(_formatFeatures)) {
-                _xmlWriter.writeStartDocument("UTF-8", "1.0");
+
+            if(Feature.WRITE_XML_1_1.enabledIn(_formatFeatures) ||
+                    Feature.WRITE_XML_DECLARATION.enabledIn(_formatFeatures)) {
+
+                String xmlVersion = Feature.WRITE_XML_1_1.enabledIn(_formatFeatures) ? "1.1" : "1.0";
+                String encoding = "UTF-8";
+
+                if(Feature.WRITE_STANDALONE_YES_TO_XML_DECLARATION.enabledIn(_formatFeatures)) {
+                    _xmlWriter.writeStartDocument(xmlVersion, encoding, true);
+                }else{
+                    _xmlWriter.writeStartDocument(encoding, xmlVersion);
+                }
+
                 xmlDeclWritten = true;
             } else {
                 xmlDeclWritten = false;
             }
+
             // as per [dataformat-xml#172], try adding indentation
             if (xmlDeclWritten && (_xmlPrettyPrinter != null)) {
                 // ... but only if it is likely to succeed:

src/test/java/com/fasterxml/jackson/dataformat/xml/ser/TestXmlDeclaration.java

@@ -32,5 +32,26 @@ public void testXml11Declaration() throws Exception
         String xml = mapper.writeValueAsString(new StringBean("abcd"));
         assertEquals(xml, "<?xml version='1.1' encoding='UTF-8'?><StringBean><text>abcd</text></StringBean>");
     }
+
+    @Test
+    public void testXml11DeclarationWithStandalone() throws Exception
+    {
+        XmlMapper mapper = new XmlMapper();
+        mapper.configure(ToXmlGenerator.Feature.WRITE_XML_DECLARATION, true);
+        mapper.configure(ToXmlGenerator.Feature.WRITE_XML_1_1, true);
+        mapper.configure(ToXmlGenerator.Feature.WRITE_STANDALONE_YES_TO_XML_DECLARATION, true);
+        String xml = mapper.writeValueAsString(new StringBean("abcd"));
+        assertEquals("<?xml version='1.1' encoding='UTF-8' standalone='yes'?><StringBean><text>abcd</text></StringBean>", xml);
+    }
+
+    @Test
+    public void testXml10DeclarationWithStandalone() throws Exception
+    {
+        XmlMapper mapper = new XmlMapper();
+        mapper.configure(ToXmlGenerator.Feature.WRITE_XML_DECLARATION, true);
+        mapper.configure(ToXmlGenerator.Feature.WRITE_STANDALONE_YES_TO_XML_DECLARATION, true);
+        String xml = mapper.writeValueAsString(new StringBean("abcd"));
+        assertEquals("<?xml version='1.0' encoding='UTF-8' standalone='yes'?><StringBean><text>abcd</text></StringBean>", xml);
+    }
 
 }