Fix #4355: don't fail getting serializer for Enum with toString() returning null (#4360)

Author: cowtowncoder

release-notes/VERSION-2.x

@@ -17,6 +17,9 @@ Project: jackson-databind
 #4316: NPE when deserializing `JsonAnySetter` in `Throwable`
  (reported by @jpraet)
  (fix contributed by Joo-Hyuk K)
+#4355: Jackson 2.16 fails attempting to obtain `ObjectWriter` for an `Enum` of which
+  some value returns null from `toString()`
+ (reported by @YutaHiguchi-bsn)
 
 2.16.1 (24-Dec-2023)
 

src/main/java/com/fasterxml/jackson/databind/util/EnumValues.java

@@ -129,6 +129,11 @@ public static EnumValues constructFromToString(MapperConfig<?> config, Annotated
             if (name == null) {
                 Enum<?> en = enumConstants[i];
                 name = en.toString();
+                // 01-Feb-2024, tatu: [databind#4355] Nulls not great but... let's
+                //   coerce into "" for backwards compatibility
+                if (name == null) {
+                    name = "";
+                }
             }
             if (useLowerCase) {
                 name = name.toLowerCase();

src/test/java/com/fasterxml/jackson/databind/deser/enums/EnumWithNullToString4355Test.java

@@ -0,0 +1,32 @@
+package com.fasterxml.jackson.databind.deser.enums;
+
+import com.fasterxml.jackson.databind.*;
+
+public class EnumWithNullToString4355Test extends BaseMapTest
+{
+    // [databind#4355]
+    enum Enum4355 {
+        ALPHA("A"),
+        BETA("B"),
+        UNDEFINED(null);
+
+        private final String s;
+
+        Enum4355(String s) {
+            this.s = s;
+        }
+
+        @Override
+        public String toString() {
+            return s;
+        }
+    }
+
+    private final ObjectMapper MAPPER = newJsonMapper();
+
+    // [databind#4355]
+    public void testWithNullToString() throws Exception
+    {
+        assertEquals("\"ALPHA\"", MAPPER.writeValueAsString(Enum4355.ALPHA));        
+    }
+}