Failing test for #787

src/main/java/com/fasterxml/jackson/dataformat/xml/deser/FromXmlParser.java

@@ -786,8 +786,21 @@ public JsonToken nextToken() throws IOException
                 return _updateToken(JsonToken.START_OBJECT);
             }
             if (_parsingContext.inArray()) {
-                // Yup: in array, so this element could be verified; but it won't be
-                // reported anyway, and we need to process following event.
+                // Validate that all array elements have the same element name
+                String currentElementName = _xmlTokens.getLocalName();
+                String expectedElementName = _parsingContext.getExpectedArrayElementName();
+
+                if (expectedElementName != null) {
+                    // We have an expected name, validate it matches
+                    if (!expectedElementName.equals(currentElementName)) {
+                        _reportError("Unexpected element name '%s' in array; expected '%s'",
+                                currentElementName, expectedElementName);
+                    }
+                } else {
+                    // First element in array, set as expected name
+                    _parsingContext.setExpectedArrayElementName(currentElementName);
+                }
+
                 token = _nextToken();
                 _mayBeLeaf = true;
                 continue;

src/main/java/com/fasterxml/jackson/dataformat/xml/deser/XmlReadContext.java

@@ -51,6 +51,14 @@ public final class XmlReadContext
      */
     protected String _wrappedName;
 
+    /**
+     * Expected element name for array elements. When set, all elements
+     * in this array context must have this local name.
+     *
+     * @since 2.21
+     */
+    protected String _expectedArrayElementName;
+
     /*
     /**********************************************************************
     /* Simple instance reuse slots; speeds up things a bit (10-15%)
@@ -99,6 +107,7 @@ protected final void reset(int type, int lineNr, int colNr)
         _currentName = null;
         _currentValue = null;
         _namesToWrap = null;
+        _expectedArrayElementName = null;
         if (_dups != null) {
             _dups.reset();
         }
@@ -231,6 +240,20 @@ public boolean shouldWrap(String localName) {
         return (_namesToWrap != null) && _namesToWrap.contains(localName);
     }
 
+    /**
+     * @since 2.21
+     */
+    public String getExpectedArrayElementName() {
+        return _expectedArrayElementName;
+    }
+
+    /**
+     * @since 2.21
+     */
+    public void setExpectedArrayElementName(String name) {
+        _expectedArrayElementName = name;
+    }
+
     protected void convertToArray() {
         _type = TYPE_ARRAY;
     }

src/test/java/com/fasterxml/jackson/dataformat/xml/lists/ListDeser787Test.java

@@ -0,0 +1,95 @@
+package com.fasterxml.jackson.dataformat.xml.lists;
+
+import java.util.ArrayList;
+import java.util.List;
+import java.util.Objects;
+
+import org.junit.jupiter.api.Test;
+
+import com.fasterxml.jackson.databind.JsonMappingException;
+import com.fasterxml.jackson.databind.ObjectMapper;
+import com.fasterxml.jackson.dataformat.xml.XmlTestUtil;
+import com.fasterxml.jackson.dataformat.xml.annotation.JacksonXmlProperty;
+import com.fasterxml.jackson.dataformat.xml.annotation.JacksonXmlText;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+import static org.junit.jupiter.api.Assertions.assertNotNull;
+import static org.junit.jupiter.api.Assertions.assertThrows;
+import static org.junit.jupiter.api.Assertions.assertTrue;
+
+/**
+ * Test for [dataformat-xml#787]: List deserialization should throw an exception
+ * when encountering mismatched element types (e.g., <bar> elements when
+ * expecting <foo> in a list)
+ */
+public class ListDeser787Test extends XmlTestUtil
+{
+    static class Root787 {
+        @JacksonXmlProperty
+        List<Foo787> foos = new ArrayList<>();
+    }
+
+    static class Foo787 {
+        @JacksonXmlText
+        String text;
+
+        @JacksonXmlProperty(isAttribute = true)
+        Integer sequenceNr;
+
+        public Foo787() { }
+
+        public Foo787(String text, Integer sequenceNr) {
+            this.text = text;
+            this.sequenceNr = sequenceNr;
+        }
+
+        @Override
+        public boolean equals(Object o) {
+            if (this == o) return true;
+            if (o == null || getClass() != o.getClass()) return false;
+            Foo787 foo = (Foo787) o;
+            return Objects.equals(text, foo.text) && Objects.equals(sequenceNr, foo.sequenceNr);
+        }
+
+        @Override
+        public int hashCode() {
+            return Objects.hash(text, sequenceNr);
+        }
+
+        @Override
+        public String toString() {
+            return "Foo787{" +
+                    "text='" + text + '\'' +
+                    ", sequenceNr=" + sequenceNr +
+                    '}';
+        }
+    }
+
+    private final ObjectMapper MAPPER = newMapper();
+
+    // [dataformat-xml#787]: Should throw exception on non-matching elements in list
+    @Test
+    public void testDeser787MixedElements() throws Exception
+    {
+        String xml =
+                "<root>\n" +
+                "    <foos>\n" +
+                "        <foo sequenceNr=\"1\">somefoo</foo>\n" +
+                "        <foo sequenceNr=\"2\">otherfoo</foo>\n" +
+                "        <bar>something very different</bar>\n" +
+                "    </foos>\n" +
+                "</root>";
+
+        // Should throw exception when encountering <bar> element in a list expecting <foo> elements
+        JsonMappingException exception = assertThrows(JsonMappingException.class, () -> {
+            MAPPER.readValue(xml, Root787.class);
+        });
+
+        // Verify the error message mentions the mismatched element names
+        String message = exception.getMessage();
+        assertTrue(message.contains("Unexpected element name 'bar'"),
+                "Error message should mention 'bar': " + message);
+        assertTrue(message.contains("expected 'foo'"),
+                "Error message should mention expected 'foo': " + message);
+    }
+}