Fix #4450: deserialize QName "" as empty, not null (#4468)

Author: cowtowncoder

release-notes/VERSION-2.x

@@ -20,6 +20,8 @@ Project: jackson-databind
   String ".05": not a valid representation
  (reported by @EAlf91)
  (fix by @pjfanning)
+#4450: Empty QName deserialized as `null`
+ (reported by @winfriedgerlach)
 
 2.17.0 (12-Mar-2024)
 

src/main/java/com/fasterxml/jackson/databind/deser/std/FromStringDeserializer.java

@@ -256,7 +256,7 @@ protected Object _deserializeFromEmptyString(DeserializationContext ctxt) throws
         if (act == CoercionAction.AsEmpty) {
             return getEmptyValue(ctxt);
         }
-        // 09-Jun-2020, tatu: semantics for `TryConvert` are bit interesting due to
+        // 09-Jun-2020, tatu: semantics for `TryConvert` are a bit interesting due to
         //    historical reasons
         return _deserializeFromEmptyStringDefault(ctxt);
     }

src/main/java/com/fasterxml/jackson/databind/ext/CoreXMLDeserializers.java

@@ -19,9 +19,11 @@
  */
 public class CoreXMLDeserializers extends Deserializers.Base
 {
+    protected final static QName EMPTY_QNAME = QName.valueOf("");
+
     /**
      * Data type factories are thread-safe after instantiation (and
-     * configuration, if any); and since instantion (esp. implementation
+     * configuration, if any); and since instantiation (esp. implementation
      * introspection) can be expensive we better reuse the instance.
      */
     final static DatatypeFactory _dataTypeFactory;
@@ -125,6 +127,14 @@ protected Object _deserialize(String value, DeserializationContext ctxt)
             throw new IllegalStateException();
         }
 
+        @Override
+        protected Object _deserializeFromEmptyString(DeserializationContext ctxt) throws IOException {
+            if (_kind == TYPE_QNAME) {
+                return EMPTY_QNAME;
+            }
+            return super._deserializeFromEmptyString(ctxt);
+        }
+
         protected XMLGregorianCalendar _gregorianFromDate(DeserializationContext ctxt,
                 Date d)
         {

src/test/java/com/fasterxml/jackson/databind/ext/MiscJavaXMLTypesReadWriteTest.java

@@ -115,6 +115,11 @@ public void testQNameDeser() throws Exception
         String qstr = qn.toString();
         assertEquals(qn, MAPPER.readValue(q(qstr), QName.class),
             "Should deserialize to equal QName (exp serialization: '"+qstr+"')");
+
+        // [databind#4450]
+        qn = MAPPER.readValue(q(""), QName.class);
+        assertNotNull(qn);
+        assertEquals("", qn.getLocalPart());
     }
 
     @Test