Fix #214

datatypes/src/main/java/com/fasterxml/jackson/datatype/jdk8/OptionalDeserializer.java

@@ -60,7 +60,9 @@ public Optional<?> referenceValue(Object contents) {
 
     @Override
     public Object getReferenced(Optional<?> reference) {
-        return reference.get();
+        // 23-Apr-2021, tatu: [modules-java8#214] Need to support empty
+        //    for merging too
+        return reference.orElse(null);
     }
 
     @Override // since 2.9

datatypes/src/test/java/com/fasterxml/jackson/datatype/jdk8/OptionalMergeTest.java

@@ -0,0 +1,33 @@
+package com.fasterxml.jackson.datatype.jdk8;
+
+import java.util.Arrays;
+import java.util.List;
+import java.util.Optional;
+
+import com.fasterxml.jackson.annotation.JsonMerge;
+import com.fasterxml.jackson.databind.ObjectMapper;
+
+public class OptionalMergeTest extends ModuleTestBase
+{
+    // [modules-java8#214]
+    static class OptionalListWrapper {
+        @JsonMerge
+        public Optional<List<String>> list = Optional.empty();
+    }
+
+    private final ObjectMapper MAPPER = mapperWithModule();
+
+    // [modules-java8#214]: ReferenceType of List, merge
+    public void testMergeToListViaRef() throws Exception
+    {
+        OptionalListWrapper base = MAPPER.readValue(aposToQuotes("{'list':['a']}"),
+                OptionalListWrapper.class);
+        assertNotNull(base.list);
+        assertEquals(Arrays.asList("a"), base.list.get());
+
+        OptionalListWrapper merged = MAPPER.readerForUpdating(base)
+                .readValue(aposToQuotes("{'list':['b']}"));
+        assertSame(base, merged);
+        assertEquals(Arrays.asList("a", "b"), base.list.get());
+    }
+}

release-notes/VERSION-2.x

@@ -8,6 +8,12 @@ Modules:
 === Releases ===
 ------------------------------------------------------------------------
 
+2.12.4 (not yet released)
+
+#214: readerForUpdating(objectToUpdate).readValue(json) behaves unexpectedly
+   on Optional<List>
+ (reported by jc84-dev@github)
+
 2.12.3 (12-Apr-2021)
 
 #207: Fail to serialize `TemporalAdjuster` type with 2.12