Add JavaDoc and test wrt custom JsonDeserializers null handling (#4594)

Author: JooHyukKim

src/main/java/com/fasterxml/jackson/databind/JsonDeserializer.java

@@ -95,10 +95,17 @@ public abstract class JsonDeserializer<T>
      * fails, event that was not recognized or usable, which may be
      * the same event as the one it pointed to upon call).
      *<p>
-     * Note that this method is never called for JSON null literal,
-     * and thus deserializers need (and should) not check for it.
+     * <strong>Handling null values (JsonToken.VALUE_NULL)</strong>
+     * <br>
+     * : Note that this method is never called for the JSON {@code null} literal to avoid
+     * every deserializer from having to handle null values. Instead, the
+     * {@link JsonDeserializer#getNullValue(DeserializationContext)} method
+     * is called to produce a null value. To influence null handling,
+     * custom deserializers should override
+     * {@link JsonDeserializer#getNullValue(DeserializationContext)}
+     * and usually also {@link JsonDeserializer#getNullAccessPattern()}.
      *
-     * @param p Parsed used for reading JSON content
+     * @param p Parser used for reading JSON content
      * @param ctxt Context that can be used to access information about
      *   this deserialization activity.
      *

src/test/java/com/fasterxml/jackson/databind/deser/CustomDeserializers4225NullCacheTest.java

@@ -0,0 +1,70 @@
+package com.fasterxml.jackson.databind.deser;
+
+import java.io.IOException;
+import java.util.ArrayList;
+import java.util.List;
+
+import org.junit.jupiter.api.Test;
+
+import com.fasterxml.jackson.core.JacksonException;
+import com.fasterxml.jackson.core.JsonParser;
+import com.fasterxml.jackson.databind.*;
+import com.fasterxml.jackson.databind.annotation.JsonDeserialize;
+import com.fasterxml.jackson.databind.testutil.DatabindTestUtil;
+
+import static org.assertj.core.api.Assertions.assertThat;
+
+/**
+ * Test to check that getNullValue for deserializer is not cached, by default.
+ */
+public class CustomDeserializers4225NullCacheTest extends DatabindTestUtil
+{
+    static class CustomListDeserializer extends JsonDeserializer<List<String>> {
+
+        private static int getNullValueInvocationCount = 0;
+
+        @Override
+        public List<String> deserialize(JsonParser p, DeserializationContext ctxt) throws IOException, JacksonException {
+            return makeList("regular");
+        }
+
+        @Override
+        public List<String> getNullValue(DeserializationContext ctxt) throws JsonMappingException {
+            // Increment invocation count
+            getNullValueInvocationCount++;
+            return makeList("nullVal_" + getNullValueInvocationCount);
+        }
+
+        public List<String> makeList(String content) {
+            List<String> randomList = new ArrayList<>();
+            randomList.add(content);
+            return randomList;
+        }
+    }
+
+    static class Bean4225 {
+        @JsonDeserialize(using = CustomListDeserializer.class)
+        public List<String> myList;
+    }
+
+    @Test
+    public void testGetNullValueIsCached() throws Exception
+    {
+        ObjectMapper mapper = newJsonMapper();
+
+        // First time deserializing null
+        verifyGetNullValueInvokedTimes(mapper, 1);
+        // Second time deserializing null, should be invoked twice
+        verifyGetNullValueInvokedTimes(mapper, 2);
+    }
+
+    private void verifyGetNullValueInvokedTimes(ObjectMapper mapper, int times)
+            throws Exception
+    {
+        Bean4225 someBean = mapper.readValue(a2q("{'myList': null}"), Bean4225.class);
+
+        assertThat(someBean.myList).hasSize(1);
+        assertThat(someBean.myList.get(0)).isEqualTo("nullVal_" + times);
+        assertThat(CustomListDeserializer.getNullValueInvocationCount).isEqualTo(times);
+    }
+}