Add NullSafeTypeAdapter to prevent TypeAdapter.nullSafe()

from returning nested null-safe type adapters (#2729)

gson/src/main/java/com/google/gson/TypeAdapter.java

@@ -289,24 +289,29 @@ 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);
+ }
  }
 }

gson/src/test/java/com/google/gson/TypeAdapterTest.java

@@ -45,6 +45,27 @@ public String read(JsonReader in) {
  assertThat(adapter.fromJson("null")).isNull();
  }
 
+ @Test
+ public void testNullSafe_ReturningNotNestedNullSafeTypeAdapter() 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<?> nullSafeAdapter = adapter.nullSafe();
+
+ assertThat(adapter.toJson(null)).isEqualTo("null");
+ assertThat(adapter.fromJson("null")).isNull();
+ assertThat(nullSafeAdapter.nullSafe()).isSameInstanceAs(nullSafeAdapter);
+ }
+
  /**
  * Tests behavior when {@link TypeAdapter#write(JsonWriter, Object)} manually throws {@link
  * IOException} which is not caused by writer usage.