Fix #3271

Author: cowtowncoder

release-notes/VERSION-2.x

@@ -73,6 +73,8 @@ Project: jackson-databind
  (reported by saneksanek@github)
 #3259: Support for BCP 47 `java.util.Locale` serialization/deserialization
  (contributed by Abishek R)
+#3271: String property deserializes null as "null" for JsonTypeInfo.As.EXISTING_PROPERTY
+ (reported by jonc2@github)
 - Fix to avoid problem with `BigDecimalNode`, scale of `Integer.MIN_VALUE` (see
   [dataformats-binary#264] for details)
 - Extend handling of `FAIL_ON_NULL_FOR_PRIMITIVES` to cover coercion from (Empty) String

src/main/java/com/fasterxml/jackson/databind/jsontype/impl/AsPropertyTypeDeserializer.java

@@ -104,7 +104,11 @@ public Object deserializeTypedFromObject(JsonParser p, DeserializationContext ct
             p.nextToken(); // to point to the value
             if (name.equals(_typePropertyName)
                     || (ignoreCase && name.equalsIgnoreCase(_typePropertyName))) { // gotcha!
-                return _deserializeTypedForId(p, ctxt, tb, p.getText());
+                // 09-Sep-2021, tatu: [databind#3271]: Avoid converting null to "null"
+                String typeId = p.getValueAsString();
+                if (typeId != null) {
+                    return _deserializeTypedForId(p, ctxt, tb, typeId);
+                }
             }
             if (tb == null) {
                 tb = ctxt.bufferForInputBuffering(p);
@@ -116,7 +120,8 @@ public Object deserializeTypedFromObject(JsonParser p, DeserializationContext ct
     }
 
     protected Object _deserializeTypedForId(JsonParser p, DeserializationContext ctxt,
-            TokenBuffer tb, String typeId) throws IOException {
+            TokenBuffer tb, String typeId) throws IOException
+    {
         JsonDeserializer<Object> deser = _findDeserializer(ctxt, typeId);
         if (_typeIdVisible) { // need to merge id back in JSON input?
             if (tb == null) {

src/test/java/com/fasterxml/jackson/databind/jsontype/ExistingPropertyTest.java

@@ -6,7 +6,6 @@
 import com.fasterxml.jackson.annotation.JsonSubTypes.Type;
 import com.fasterxml.jackson.annotation.JsonTypeInfo.As;
 import com.fasterxml.jackson.annotation.JsonTypeInfo.Id;
-import com.fasterxml.jackson.annotation.JsonPropertyOrder;
 
 import com.fasterxml.jackson.core.type.TypeReference;
 
@@ -192,10 +191,26 @@ static class Bean1635A extends Bean1635 {
 
     static class Bean1635Default extends Bean1635 { }
 
+    // [databind#3271]
+    @JsonTypeInfo(use = JsonTypeInfo.Id.NAME, include = JsonTypeInfo.As.EXISTING_PROPERTY,
+            visible = true, property = "type", defaultImpl = DefaultShape3271.class)
+    @JsonSubTypes({@JsonSubTypes.Type(value = Square3271.class, name = "square")})
+    static abstract class Shape3271 {
+        public String type;
+
+        public String getType() { return this.type; }
+
+        public void setType(String type) { this.type = type; }
+    }
+
+    static class Square3271 extends Shape3271 {}
+
+    static class DefaultShape3271 extends Shape3271 {}
+
     /*
-    /**********************************************************
+    /**********************************************************************
     /* Mock data
-    /**********************************************************
+    /**********************************************************************
      */
 
     private static final Orange mandarin = new Orange("Mandarin Orange", "orange");
@@ -226,12 +241,12 @@ static class Bean1635Default extends Bean1635 { }
     private static final String carListJson = "[" + camryJson + "," + accordJson + "]";
 
     /*
-    /**********************************************************
+    /**********************************************************************
     /* Test methods
-    /**********************************************************
+    /**********************************************************************
      */
 
-    private final ObjectMapper MAPPER = new ObjectMapper();
+    private final ObjectMapper MAPPER = newJsonMapper();
 
     /**
      * Fruits - serialization tests for simple property on sub-classes
@@ -443,4 +458,21 @@ public void testExistingEnumTypeIdViaDefault() throws Exception
         assertEquals(Bean1635Default.class, result.getClass());
         assertEquals(ABC.C, result.type);
     }
+
+    // [databind#3271]: verify that `null` token does not become "null" String
+
+    public void testDeserializationWithValidType() throws Exception {
+        Shape3271 deserShape = MAPPER.readValue("{\"type\":\"square\"}", Shape3271.class);
+        assertEquals("square", deserShape.getType());
+    }
+
+    public void testDeserializationWithInvalidType() throws Exception {
+        Shape3271 deserShape = MAPPER.readValue("{\"type\":\"invalid\"}", Shape3271.class);
+        assertEquals("invalid", deserShape.getType());
+    }
+
+    public void testDeserializationNull() throws Exception {
+        Shape3271 deserShape = MAPPER.readValue("{\"type\":null}", Shape3271.class);
+        assertNull(deserShape.getType()); // error: "expected null, but was:<null>"
+    }
 }