From b0b6834157e17f0ff916b2c39dcfd38fba65b376 Mon Sep 17 00:00:00 2001
From: Marcono1234 <Marcono1234@users.noreply.github.com>
Date: Mon, 22 Aug 2022 15:55:31 +0200
Subject: [PATCH] Mention that GsonBuilder.registerTypeAdapter makes
 (de-)serializers null-safe (#1704)

---
 gson/src/main/java/com/google/gson/GsonBuilder.java         | 6 ++++++
 gson/src/test/java/com/google/gson/GsonTypeAdapterTest.java | 6 ++++++
 2 files changed, 12 insertions(+)

diff --git a/gson/src/main/java/com/google/gson/GsonBuilder.java b/gson/src/main/java/com/google/gson/GsonBuilder.java
index 3c9f0f52..81fa022d 100644
--- a/gson/src/main/java/com/google/gson/GsonBuilder.java
+++ b/gson/src/main/java/com/google/gson/GsonBuilder.java
@@ -534,6 +534,12 @@ public final class GsonBuilder {
    * types! For example, applications registering {@code boolean.class} should also register {@code
    * Boolean.class}.
    *
+   * <p>{@link JsonSerializer} and {@link JsonDeserializer} are made "{@code null}-safe". This
+   * means when trying to serialize {@code null}, Gson will write a JSON {@code null} and the
+   * serializer is not called. Similarly when deserializing a JSON {@code null}, Gson will emit
+   * {@code null} without calling the deserializer. If it is desired to handle {@code null} values,
+   * a {@link TypeAdapter} should be used instead.
+   *
    * @param type the type definition for the type adapter being registered
    * @param typeAdapter This object must implement at least one of the {@link TypeAdapter},
    * {@link InstanceCreator}, {@link JsonSerializer}, and a {@link JsonDeserializer} interfaces.
diff --git a/gson/src/test/java/com/google/gson/GsonTypeAdapterTest.java b/gson/src/test/java/com/google/gson/GsonTypeAdapterTest.java
index 2e00dc9b..d92994fa 100644
--- a/gson/src/test/java/com/google/gson/GsonTypeAdapterTest.java
+++ b/gson/src/test/java/com/google/gson/GsonTypeAdapterTest.java
@@ -53,10 +53,16 @@ public class GsonTypeAdapterTest extends TestCase {
       fail("Type Adapter should have thrown an exception");
     } catch (IllegalStateException expected) { }
 
+    // Verify that serializer is made null-safe, i.e. it is not called for null
+    assertEquals("null", gson.toJson(null, AtomicLong.class));
+
     try {
       gson.fromJson("123", AtomicLong.class);
       fail("Type Adapter should have thrown an exception");
     } catch (JsonParseException expected) { }
+
+    // Verify that deserializer is made null-safe, i.e. it is not called for null
+    assertNull(gson.fromJson(JsonNull.INSTANCE, AtomicLong.class));
   }
 
   public void testTypeAdapterProperlyConvertsTypes() throws Exception {