Fix #4688 2.18.0-rc1 Regression deserializing with no-arg delegating JsonCreator (#4689)

Author: carterkozak

release-notes/CREDITS-2.x

@@ -849,6 +849,8 @@ Carter Kozak (carterkozak@github)
   * Contributed #3876: `TypeFactory` cache performance degradation with
     `constructSpecializedType()`
    (2.15.0)
+  * Contributed #4688: Should allow deserializing with no-arg `@JsonCreator(mode = DELEGATING)`
+   (2.18.0)
 
 Reinhard Prechtl (dnno@github)
   * Reported #2034: Serialization problem with type specialization of nested generic types

release-notes/VERSION-2.x

@@ -70,6 +70,8 @@ Project: jackson-databind
  (contributed by Sim Y-T)
 #4678: Java records don't serialize with `MapperFeature.REQUIRE_SETTERS_FOR_GETTERS`
  (reported by Mathijs V)
+#4688: Should allow deserializing with no-arg `@JsonCreator(mode = DELEGATING)`
+ (contributed by Carter K)
 
 2.17.2 (05-Jul-2024)
 

src/main/java/com/fasterxml/jackson/databind/deser/BasicDeserializerFactory.java

@@ -468,6 +468,13 @@ private boolean _addExplicitDelegatingCreator(DeserializationContext ctxt,
         int ix = -1;
         final int argCount = candidate.paramCount();
         SettableBeanProperty[] properties = new SettableBeanProperty[argCount];
+        // [databind#4688]: Should still accept 0-arg (explicitly delegated) creator
+        //   for backwards-compatibility (worked in 2.17 and before)
+        if (argCount == 0) {
+            // "Convert" to property-based since that works well
+            creators.addPropertyCreator(candidate.creator(), true, properties);
+            return true;
+        }
         for (int i = 0; i < argCount; ++i) {
             AnnotatedParameter param = candidate.parameter(i);
             JacksonInject.Value injectId = candidate.injection(i);

src/test/java/com/fasterxml/jackson/databind/deser/creators/SingletonDelegatingCreatorTest.java

@@ -0,0 +1,78 @@
+package com.fasterxml.jackson.databind.deser.creators;
+
+import org.junit.jupiter.api.Test;
+
+import com.fasterxml.jackson.annotation.JsonCreator;
+import com.fasterxml.jackson.databind.ObjectMapper;
+import com.fasterxml.jackson.databind.testutil.DatabindTestUtil;
+
+import static org.junit.jupiter.api.Assertions.assertSame;
+
+// [databind#4688]
+public class SingletonDelegatingCreatorTest extends DatabindTestUtil
+{
+    static final class NoFieldSingletonWithDelegatingCreator {
+        static final NoFieldSingletonWithDelegatingCreator INSTANCE = new NoFieldSingletonWithDelegatingCreator();
+
+        private NoFieldSingletonWithDelegatingCreator() {}
+
+        @JsonCreator(mode = JsonCreator.Mode.DELEGATING)
+        static NoFieldSingletonWithDelegatingCreator of() {
+            return INSTANCE;
+        }
+    }
+
+    static final class NoFieldSingletonWithPropertiesCreator {
+        static final NoFieldSingletonWithPropertiesCreator INSTANCE = new NoFieldSingletonWithPropertiesCreator();
+
+        private NoFieldSingletonWithPropertiesCreator() {}
+
+        @JsonCreator(mode = JsonCreator.Mode.PROPERTIES)
+        static NoFieldSingletonWithPropertiesCreator of() {
+            return INSTANCE;
+        }
+    }
+
+    static final class NoFieldSingletonWithDefaultCreator {
+        static final NoFieldSingletonWithDefaultCreator INSTANCE = new NoFieldSingletonWithDefaultCreator();
+
+        private NoFieldSingletonWithDefaultCreator() {}
+
+        @JsonCreator
+        static NoFieldSingletonWithDefaultCreator of() {
+            return INSTANCE;
+        }
+    }
+
+    /*
+    /**********************************************************************
+    /* Test methods
+    /**********************************************************************
+     */
+
+    private final ObjectMapper MAPPER = newJsonMapper();
+
+    @Test
+    public void testNoFieldSingletonWithDelegatingCreator() throws Exception
+    {
+        NoFieldSingletonWithDelegatingCreator deserialized = MAPPER.readValue("{}",
+                NoFieldSingletonWithDelegatingCreator.class);
+        assertSame(NoFieldSingletonWithDelegatingCreator.INSTANCE, deserialized);
+    }
+
+    @Test
+    public void testNoFieldSingletonWithPropertiesCreator() throws Exception
+    {
+        NoFieldSingletonWithPropertiesCreator deserialized = MAPPER.readValue("{}",
+                NoFieldSingletonWithPropertiesCreator.class);
+        assertSame(NoFieldSingletonWithPropertiesCreator.INSTANCE, deserialized);
+    }
+
+    @Test
+    public void testNoFieldSingletonWithDefaultCreator() throws Exception
+    {
+        NoFieldSingletonWithDefaultCreator deserialized = MAPPER.readValue("{}",
+                NoFieldSingletonWithDefaultCreator.class);
+        assertSame(NoFieldSingletonWithDefaultCreator.INSTANCE, deserialized);
+    }
+}