From 011c870f749d6a5f0d0455e967937e2335e0a446 Mon Sep 17 00:00:00 2001
From: Lyubomyr Shaydariv <lyubomyr-shaydariv@users.noreply.github.com>
Date: Fri, 30 Aug 2024 12:11:51 +0300
Subject: [PATCH] Add NullSafeTypeAdapter to prevent TypeAdapter.nullSafe()
 from returning nested null-safe type adapters (#2729)

---
 .../java/com/google/gson/TypeAdapter.java     | 42 +++++++++------
 .../java/com/google/gson/TypeAdapterTest.java | 51 ++++++++++++++-----
 2 files changed, 65 insertions(+), 28 deletions(-)

diff --git a/gson/src/main/java/com/google/gson/TypeAdapter.java b/gson/src/main/java/com/google/gson/TypeAdapter.java
index 90d330e382..3d9b2b9e45 100644
--- a/gson/src/main/java/com/google/gson/TypeAdapter.java
+++ b/gson/src/main/java/com/google/gson/TypeAdapter.java
@@ -289,24 +289,34 @@ public final T fromJsonTree(JsonElement jsonTree) {
    * Note that we didn't need to check for nulls in our type adapter after we used nullSafe.
    */
   public final TypeAdapter<T> nullSafe() {
-    return new TypeAdapter<T>() {
-      @Override
-      public void write(JsonWriter out, T value) throws IOException {
-        if (value == null) {
-          out.nullValue();
-        } else {
-          TypeAdapter.this.write(out, value);
-        }
+    if (!NullSafeTypeAdapter.class.isInstance(this)) {
+      return new NullSafeTypeAdapter();
+    }
+    return this;
+  }
+
+  private final class NullSafeTypeAdapter extends TypeAdapter<T> {
+    @Override
+    public void write(JsonWriter out, T value) throws IOException {
+      if (value == null) {
+        out.nullValue();
+      } else {
+        TypeAdapter.this.write(out, value);
       }
+    }
 
-      @Override
-      public T read(JsonReader reader) throws IOException {
-        if (reader.peek() == JsonToken.NULL) {
-          reader.nextNull();
-          return null;
-        }
-        return TypeAdapter.this.read(reader);
+    @Override
+    public T read(JsonReader reader) throws IOException {
+      if (reader.peek() == JsonToken.NULL) {
+        reader.nextNull();
+        return null;
       }
-    };
+      return TypeAdapter.this.read(reader);
+    }
+
+    @Override
+    public String toString() {
+      return "NullSafeTypeAdapter[" + TypeAdapter.this + "]";
+    }
   }
 }
diff --git a/gson/src/test/java/com/google/gson/TypeAdapterTest.java b/gson/src/test/java/com/google/gson/TypeAdapterTest.java
index e5aa8a422a..e95b029aa5 100644
--- a/gson/src/test/java/com/google/gson/TypeAdapterTest.java
+++ b/gson/src/test/java/com/google/gson/TypeAdapterTest.java
@@ -28,23 +28,50 @@
 public class TypeAdapterTest {
   @Test
   public void testNullSafe() throws IOException {
-    TypeAdapter<String> adapter =
-        new TypeAdapter<String>() {
-          @Override
-          public void write(JsonWriter out, String value) {
-            throw new AssertionError("unexpected call");
-          }
-
-          @Override
-          public String read(JsonReader in) {
-            throw new AssertionError("unexpected call");
-          }
-        }.nullSafe();
+    TypeAdapter<String> adapter = assertionErrorAdapter.nullSafe();
 
     assertThat(adapter.toJson(null)).isEqualTo("null");
     assertThat(adapter.fromJson("null")).isNull();
   }
 
+  @Test
+  public void testNullSafe_ReturningSameInstanceOnceNullSafe() {
+    TypeAdapter<?> nullSafeAdapter = assertionErrorAdapter.nullSafe();
+
+    assertThat(nullSafeAdapter.nullSafe()).isSameInstanceAs(nullSafeAdapter);
+    assertThat(nullSafeAdapter.nullSafe().nullSafe()).isSameInstanceAs(nullSafeAdapter);
+    assertThat(nullSafeAdapter.nullSafe().nullSafe().nullSafe()).isSameInstanceAs(nullSafeAdapter);
+  }
+
+  @Test
+  public void testNullSafe_ToString() {
+    TypeAdapter<?> adapter = assertionErrorAdapter;
+
+    assertThat(adapter.toString()).isEqualTo("assertionErrorAdapter");
+    assertThat(adapter.nullSafe().toString())
+        .isEqualTo("NullSafeTypeAdapter[assertionErrorAdapter]");
+    assertThat(adapter.nullSafe().nullSafe().toString())
+        .isEqualTo("NullSafeTypeAdapter[assertionErrorAdapter]");
+  }
+
+  private static final TypeAdapter<String> assertionErrorAdapter =
+      new TypeAdapter<>() {
+        @Override
+        public void write(JsonWriter out, String value) {
+          throw new AssertionError("unexpected call");
+        }
+
+        @Override
+        public String read(JsonReader in) {
+          throw new AssertionError("unexpected call");
+        }
+
+        @Override
+        public String toString() {
+          return "assertionErrorAdapter";
+        }
+      };
+
   /**
    * Tests behavior when {@link TypeAdapter#write(JsonWriter, Object)} manually throws {@link
    * IOException} which is not caused by writer usage.