Disallow null as skipPast

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

@@ -642,33 +642,28 @@ public <T> TypeAdapter<T> getAdapter(TypeToken<T> type) {
    * types, our stats factory will not count the number of String or primitives that will be
    * read or written.
    *
-   * <p>If {@code skipPast} is {@code null} or a factory which has neither been registered
-   * on the {@link GsonBuilder} nor specified with the {@link JsonAdapter @JsonAdapter} annotation
-   * on a class, then this method behaves as if {@link #getAdapter(TypeToken)} had been called.
-   * This also means that for fields with {@code @JsonAdapter} annotation this method behaves
-   * normally like {@code getAdapter} (except for corner cases where a custom {@link InstanceCreator}
-   * is used to create an instance of the factory).
+   * <p>If {@code skipPast} is a factory which has neither been registered on the {@link GsonBuilder}
+   * nor specified with the {@link JsonAdapter @JsonAdapter} annotation on a class, then this
+   * method behaves as if {@link #getAdapter(TypeToken)} had been called. This also means that
+   * for fields with {@code @JsonAdapter} annotation this method behaves normally like {@code getAdapter}
+   * (except for corner cases where a custom {@link InstanceCreator} is used to create an
+   * instance of the factory).
    *
    * @param skipPast The type adapter factory that needs to be skipped while searching for
    *   a matching type adapter. In most cases, you should just pass <i>this</i> (the type adapter
-   *   factory from where {@code getDelegateAdapter} method is being invoked). May be {@code null}.
+   *   factory from where {@code getDelegateAdapter} method is being invoked).
    * @param type Type for which the delegate adapter is being searched for.
    *
    * @since 2.2
    */
   public <T> TypeAdapter<T> getDelegateAdapter(TypeAdapterFactory skipPast, TypeToken<T> type) {
+    Objects.requireNonNull(skipPast, "skipPast must not be null");
     Objects.requireNonNull(type, "type must not be null");
 
-    if (skipPast != null) {
-      if (jsonAdapterFactory.isClassJsonAdapterFactory(type.getRawType(), skipPast)) {
-        skipPast = jsonAdapterFactory;
-      } else if (!factories.contains(skipPast)) {
-        // Probably a factory from @JsonAdapter on a field
-        skipPast = null;
-      }
-    }
-
-    if (skipPast == null) {
+    if (jsonAdapterFactory.isClassJsonAdapterFactory(type.getRawType(), skipPast)) {
+      skipPast = jsonAdapterFactory;
+    } else if (!factories.contains(skipPast)) {
+      // Probably a factory from @JsonAdapter on a field
       return getAdapter(type);
     }
 

gson/src/main/java/com/google/gson/internal/bind/JsonAdapterAnnotationTypeAdapterFactory.java

@@ -45,7 +45,13 @@ private static class DummyTypeAdapterFactory implements TypeAdapterFactory {
    * Factory used for {@link TreeTypeAdapter}s created for {@code @JsonAdapter}
    * on a class.
    */
-  private static final TypeAdapterFactory TREE_TYPE_DUMMY_FACTORY = new DummyTypeAdapterFactory();
+  private static final TypeAdapterFactory TREE_TYPE_CLASS_DUMMY_FACTORY = new DummyTypeAdapterFactory();
+
+  /**
+   * Factory used for {@link TreeTypeAdapter}s created for {@code @JsonAdapter}
+   * on a field.
+   */
+  private static final TypeAdapterFactory TREE_TYPE_FIELD_DUMMY_FACTORY = new DummyTypeAdapterFactory();
 
   private final ConstructorConstructor constructorConstructor;
   /**
@@ -115,12 +121,17 @@ TypeAdapter<?> getTypeAdapter(ConstructorConstructor constructorConstructor, Gso
           ? (JsonDeserializer<?>) instance
           : null;
 
-      TypeAdapterFactory skipPast = null;
+      TypeAdapterFactory skipPast;
       if (isClassAnnotation) {
-        // Use dummy `skipPast` value; otherwise TreeTypeAdapter's call to `Gson.getDelegateAdapter` would
-        // cause infinite recursion because it would keep returning adapter specified by @JsonAdapter
-        skipPast = TREE_TYPE_DUMMY_FACTORY;
+        // Use dummy `skipPast` value and put it into factory map; otherwise TreeTypeAdapter's call to
+        // `Gson.getDelegateAdapter` would cause infinite recursion because it would keep returning the
+        // adapter specified by @JsonAdapter
+        skipPast = TREE_TYPE_CLASS_DUMMY_FACTORY;
         adapterFactoryMap.put(type.getRawType(), skipPast);
+      } else {
+        // Use dummy `skipPast` value, but don't put it into factory map; this way `Gson.getDelegateAdapter`
+        // will return regular adapter for field type without actually skipping past any factory
+        skipPast = TREE_TYPE_FIELD_DUMMY_FACTORY;
       }
       @SuppressWarnings({ "unchecked", "rawtypes" })
       TypeAdapter<?> tempAdapter = new TreeTypeAdapter(serializer, deserializer, gson, type, skipPast, nullSafe);

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

@@ -325,12 +325,12 @@ public <T> TypeAdapter<T> create(Gson gson, TypeToken<T> type) {
 
     TypeToken<?> type = TypeToken.get(Number.class);
 
+    assertThrows(NullPointerException.class, () -> gson.getDelegateAdapter(null, type));
+    assertThrows(NullPointerException.class, () -> gson.getDelegateAdapter(factory1, null));
+
     // For unknown factory the first adapter for that type should be returned
     assertThat(gson.getDelegateAdapter(new DummyFactory(new DummyAdapter(0)), type)).isEqualTo(adapter2);
 
-    // For null as 'skipPast' the first adapter for that type should be returned
-    assertThat(gson.getDelegateAdapter(null, type)).isEqualTo(adapter2);
-
     assertThat(gson.getDelegateAdapter(factory2, type)).isEqualTo(adapter1);
     // Default Gson adapter should be returned
     assertThat(gson.getDelegateAdapter(factory1, type)).isNotInstanceOf(DummyAdapter.class);