Testing JsonAdapter Annotation Implementation in google/gson

google/gson

gson/src/test/java/com/google/gson/functional/JsonAdapterAnnotationOnClassesTest.java

            
/*
 * Copyright (C) 2014 Google Inc.
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *      http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

package com.google.gson.functional;

import static com.google.common.truth.Truth.assertThat;
import static org.junit.Assert.assertThrows;

import com.google.common.base.Splitter;
import com.google.gson.Gson;
import com.google.gson.GsonBuilder;
import com.google.gson.InstanceCreator;
import com.google.gson.JsonDeserializationContext;
import com.google.gson.JsonDeserializer;
import com.google.gson.JsonElement;
import com.google.gson.JsonPrimitive;
import com.google.gson.JsonSerializationContext;
import com.google.gson.JsonSerializer;
import com.google.gson.TypeAdapter;
import com.google.gson.TypeAdapterFactory;
import com.google.gson.annotations.JsonAdapter;
import com.google.gson.reflect.TypeToken;
import com.google.gson.stream.JsonReader;
import com.google.gson.stream.JsonWriter;
import java.io.IOException;
import java.lang.reflect.ParameterizedType;
import java.lang.reflect.Type;
import java.util.List;
import java.util.Locale;
import org.junit.Test;

/** Functional tests for the {@link JsonAdapter} annotation on classes. */
@SuppressWarnings("ClassNamedLikeTypeParameter") // for dummy classes A, B, ...
public final class JsonAdapterAnnotationOnClassesTest {

  @Test
  public void testJsonAdapterInvoked() {
    Gson gson = new Gson();
    String json = gson.toJson(new A("bar"));
    assertThat(json).isEqualTo("\"jsonAdapter\"");

    // Also invoke the JsonAdapter javadoc sample
    json = gson.toJson(new User("Inderjeet", "Singh"));
    assertThat(json).isEqualTo("{\"name\":\"Inderjeet Singh\"}");
    User user = gson.fromJson("{'name':'Joel Leitch'}", User.class);
    assertThat(user.firstName).isEqualTo("Joel");
    assertThat(user.lastName).isEqualTo("Leitch");

    json = gson.toJson(Foo.BAR);
    assertThat(json).isEqualTo("\"bar\"");
    Foo baz = gson.fromJson("\"baz\"", Foo.class);
    assertThat(baz).isEqualTo(Foo.BAZ);
  }

  @Test
  public void testJsonAdapterFactoryInvoked() {
    Gson gson = new Gson();
    String json = gson.toJson(new C("bar"));
    assertThat(json).isEqualTo("\"jsonAdapterFactory\"");
    C c = gson.fromJson("\"bar\"", C.class);
    assertThat(c.value).isEqualTo("jsonAdapterFactory");
  }

  @Test
  public void testRegisteredAdapterOverridesJsonAdapter() {
    TypeAdapter<A> typeAdapter =
        new TypeAdapter<>() {
          @Override
          public void write(JsonWriter out, A value) throws IOException {
            out.value("registeredAdapter");
          }

          @Override
          public A read(JsonReader in) throws IOException {
            return new A(in.nextString());
          }
        };
    Gson gson = new GsonBuilder().registerTypeAdapter(A.class, typeAdapter).create();
    String json = gson.toJson(new A("abcd"));
    assertThat(json).isEqualTo("\"registeredAdapter\"");
  }

  /** The serializer overrides field adapter, but for deserializer the fieldAdapter is used. */
  @Test
  public void testRegisteredSerializerOverridesJsonAdapter() {
    JsonSerializer<A> serializer =
        (src, typeOfSrc, context) -> new JsonPrimitive("registeredSerializer");
    Gson gson = new GsonBuilder().registerTypeAdapter(A.class, serializer).create();
    String json = gson.toJson(new A("abcd"));
    assertThat(json).isEqualTo("\"registeredSerializer\"");
    A target = gson.fromJson("abcd", A.class);
    assertThat(target.value).isEqualTo("jsonAdapter");
  }

  /** The deserializer overrides Json adapter, but for serializer the jsonAdapter is used. */
  @Test
  public void testRegisteredDeserializerOverridesJsonAdapter() {
    JsonDeserializer<A> deserializer = (json, typeOfT, context) -> new A("registeredDeserializer");
    Gson gson = new GsonBuilder().registerTypeAdapter(A.class, deserializer).create();
    String json = gson.toJson(new A("abcd"));
    assertThat(json).isEqualTo("\"jsonAdapter\"");
    A target = gson.fromJson("abcd", A.class);
    assertThat(target.value).isEqualTo("registeredDeserializer");
  }

  @Test
  public void testIncorrectTypeAdapterFails() {
    Gson gson = new Gson();
    ClassWithIncorrectJsonAdapter obj = new ClassWithIncorrectJsonAdapter("bar");
    assertThrows(ClassCastException.class, () -> gson.toJson(obj));
  }

  @Test
  public void testSuperclassTypeAdapterNotInvoked() {
    String json = new Gson().toJson(new B("bar"));
    assertThat(json).doesNotContain("jsonAdapter");
  }

  @Test
  public void testNullSafeObject() {
    Gson gson = new Gson();
    NullableClass fromJson = gson.fromJson("null", NullableClass.class);
    assertThat(fromJson).isNull();

    fromJson = gson.fromJson("\"ignored\"", NullableClass.class);
    assertThat(fromJson).isNotNull();

    String json = gson.toJson(null, NullableClass.class);
    assertThat(json).isEqualTo("null");

    json = gson.toJson(new NullableClass());
    assertThat(json).isEqualTo("\"nullable\"");
  }

  /**
   * Tests behavior when a {@link TypeAdapterFactory} registered with {@code @JsonAdapter} returns
   * {@code null}, indicating that it cannot handle the type and Gson should try a different factory
   * instead.
   */
  @Test
  public void testFactoryReturningNull() {
    Gson gson = new Gson();

    assertThat(gson.fromJson("null", WithNullReturningFactory.class)).isNull();
    assertThat(gson.toJson(null, WithNullReturningFactory.class)).isEqualTo("null");

    TypeToken<WithNullReturningFactory<String>> stringTypeArg = new TypeToken<>() {};
    WithNullReturningFactory<?> deserialized = gson.fromJson("\"a\"", stringTypeArg);
    assertThat(deserialized.t).isEqualTo("custom-read:a");
    assertThat(gson.fromJson("null", stringTypeArg)).isNull();
    assertThat(gson.toJson(new WithNullReturningFactory<>("b"), stringTypeArg.getType()))
        .isEqualTo("\"custom-write:b\"");
    assertThat(gson.toJson(null, stringTypeArg.getType())).isEqualTo("null");

    // Factory should return `null` for this type and Gson should fall back to reflection-based
    // adapter
    TypeToken<WithNullReturningFactory<Integer>> numberTypeArg = new TypeToken<>() {};
    deserialized = gson.fromJson("{\"t\":1}", numberTypeArg);
    assertThat(deserialized.t).isEqualTo(1);
    assertThat(gson.toJson(new WithNullReturningFactory<>(2), numberTypeArg.getType()))
        .isEqualTo("{\"t\":2}");
  }

  // Also set `nullSafe = true` to verify that this does not cause a NullPointerException if the
  // factory would accidentally call `nullSafe()` on null adapter
  @JsonAdapter(value = WithNullReturningFactory.NullReturningFactory.class, nullSafe = true)
  private static class WithNullReturningFactory<T> {
    T t;

    public WithNullReturningFactory(T t) {
      this.t = t;
    }

    static class NullReturningFactory implements TypeAdapterFactory {
      @Override
      public <T> TypeAdapter<T> create(Gson gson, TypeToken<T> type) {
        // Don't handle raw (non-parameterized) type
        if (type.getType() instanceof Class) {
          return null;
        }
        ParameterizedType parameterizedType = (ParameterizedType) type.getType();
        // Makes this test a bit more realistic by only conditionally returning null (instead of
        // always)
        if (parameterizedType.getActualTypeArguments()[0] != String.class) {
          return null;
        }

        @SuppressWarnings("unchecked")
        TypeAdapter<T> adapter =
            (TypeAdapter<T>)
                new TypeAdapter<WithNullReturningFactory<String>>() {
                  @Override
                  public void write(JsonWriter out, WithNullReturningFactory<String> value)
                      throws IOException {
                    out.value("custom-write:" + value.t);
                  }

                  @Override
                  public WithNullReturningFactory<String> read(JsonReader in) throws IOException {
                    return new WithNullReturningFactory<>("custom-read:" + in.nextString());
                  }
                };
        return adapter;
      }
    }
  }

  @JsonAdapter(A.JsonAdapter.class)
  private static class A {
    final String value;

    A(String value) {
      this.value = value;
    }

    static final class JsonAdapter extends TypeAdapter<A> {
      @Override
      public void write(JsonWriter out, A value) throws IOException {
        out.value("jsonAdapter");
      }

      @Override
      public A read(JsonReader in) throws IOException {
        String unused = in.nextString();
        return new A("jsonAdapter");
      }
    }
  }

  @JsonAdapter(C.JsonAdapterFactory.class)
  private static class C {
    final String value;

    C(String value) {
      this.value = value;
    }

    static final class JsonAdapterFactory implements TypeAdapterFactory {
      @Override
      public <T> TypeAdapter<T> create(Gson gson, TypeToken<T> type) {
        return new TypeAdapter<>() {
          @Override
          public void write(JsonWriter out, T value) throws IOException {
            out.value("jsonAdapterFactory");
          }

          @SuppressWarnings("unchecked")
          @Override
          public T read(JsonReader in) throws IOException {
            String unused = in.nextString();
            return (T) new C("jsonAdapterFactory");
          }
        };
      }
    }
  }

  private static final class B extends A {
    B(String value) {
      super(value);
    }
  }

  // Note that the type is NOT TypeAdapter<ClassWithIncorrectJsonAdapter> so this
  // should cause error
  @JsonAdapter(A.JsonAdapter.class)
  private static final class ClassWithIncorrectJsonAdapter {
    @SuppressWarnings("unused")
    final String value;

    ClassWithIncorrectJsonAdapter(String value) {
      this.value = value;
    }
  }

  // This class is used in JsonAdapter Javadoc as an example
  @JsonAdapter(UserJsonAdapter.class)
  private static class User {
    final String firstName;
    final String lastName;

    User(String firstName, String lastName) {
      this.firstName = firstName;
      this.lastName = lastName;
    }
  }

  private static class UserJsonAdapter extends TypeAdapter<User> {
    @Override
    public void write(JsonWriter out, User user) throws IOException {
      // implement write: combine firstName and lastName into name
      out.beginObject();
      out.name("name");
      out.value(user.firstName + " " + user.lastName);
      out.endObject();
    }

    @Override
    public User read(JsonReader in) throws IOException {
      // implement read: split name into firstName and lastName
      in.beginObject();
      String unused = in.nextName();
      List<String> nameParts = Splitter.on(" ").splitToList(in.nextString());
      in.endObject();
      return new User(nameParts.get(0), nameParts.get(1));
    }
  }

  // Implicit `nullSafe=true`
  @JsonAdapter(value = NullableClassJsonAdapter.class)
  private static class NullableClass {}

  private static class NullableClassJsonAdapter extends TypeAdapter<NullableClass> {
    @Override
    public void write(JsonWriter out, NullableClass value) throws IOException {
      out.value("nullable");
    }

    @Override
    public NullableClass read(JsonReader in) throws IOException {
      String unused = in.nextString();
      return new NullableClass();
    }
  }

  @JsonAdapter(FooJsonAdapter.class)
  private static enum Foo {
    BAR,
    BAZ
  }

  private static class FooJsonAdapter extends TypeAdapter<Foo> {
    @Override
    public void write(JsonWriter out, Foo value) throws IOException {
      out.value(value.name().toLowerCase(Locale.US));
    }

    @Override
    public Foo read(JsonReader in) throws IOException {
      return Foo.valueOf(in.nextString().toUpperCase(Locale.US));
    }
  }

  @Test
  public void testIncorrectJsonAdapterType() {
    Gson gson = new Gson();
    WithInvalidAdapterClass obj = new WithInvalidAdapterClass();
    var e = assertThrows(IllegalArgumentException.class, () -> gson.toJson(obj));
    assertThat(e)
        .hasMessageThat()
        .isEqualTo(
            "Invalid attempt to bind an instance of java.lang.Integer as a @JsonAdapter for "
                + WithInvalidAdapterClass.class.getName()
                + ". @JsonAdapter value must be a TypeAdapter, TypeAdapterFactory, JsonSerializer"
                + " or JsonDeserializer.");
  }

  @JsonAdapter(Integer.class)
  private static final class WithInvalidAdapterClass {
    @SuppressWarnings("unused")
    final String value = "a";
  }

  /**
   * Verifies that {@link TypeAdapterFactory} specified by {@code @JsonAdapter} can call {@link
   * Gson#getDelegateAdapter} without any issues, despite the factory not being directly registered
   * on Gson.
   */
  @Test
  public void testDelegatingAdapterFactory() {
    @SuppressWarnings("unchecked")
    WithDelegatingFactory<String> deserialized =
        new Gson().fromJson("{\"custom\":{\"f\":\"de\"}}", WithDelegatingFactory.class);
    assertThat(deserialized.f).isEqualTo("de");

    deserialized =
        new Gson()
            .fromJson(
                "{\"custom\":{\"f\":\"de\"}}", new TypeToken<WithDelegatingFactory<String>>() {});
    assertThat(deserialized.f).isEqualTo("de");

    WithDelegatingFactory<String> serialized = new WithDelegatingFactory<>("se");
    assertThat(new Gson().toJson(serialized)).isEqualTo("{\"custom\":{\"f\":\"se\"}}");
  }

  @JsonAdapter(WithDelegatingFactory.Factory.class)
  private static class WithDelegatingFactory<T> {
    T f;

    WithDelegatingFactory(T f) {
      this.f = f;
    }

    static class Factory implements TypeAdapterFactory {
      @Override
      public <T> TypeAdapter<T> create(Gson gson, TypeToken<T> type) {
        if (type.getRawType() != WithDelegatingFactory.class) {
          return null;
        }

        TypeAdapter<T> delegate = gson.getDelegateAdapter(this, type);

        return new TypeAdapter<>() {
          @Override
          public T read(JsonReader in) throws IOException {
            // Perform custom deserialization
            in.beginObject();
            assertThat(in.nextName()).isEqualTo("custom");
            T t = delegate.read(in);
            in.endObject();

            return t;
          }

          @Override
          public void write(JsonWriter out, T value) throws IOException {
            // Perform custom serialization
            out.beginObject();
            out.name("custom");
            delegate.write(out, value);
            out.endObject();
          }
        };
      }
    }
  }

  /**
   * Similar to {@link #testDelegatingAdapterFactory}, except that the delegate is not looked up in
   * {@code create} but instead in the adapter methods.
   */
  @Test
  public void testDelegatingAdapterFactory_Delayed() {
    WithDelayedDelegatingFactory deserialized =
        new Gson().fromJson("{\"custom\":{\"f\":\"de\"}}", WithDelayedDelegatingFactory.class);
    assertThat(deserialized.f).isEqualTo("de");

    WithDelayedDelegatingFactory serialized = new WithDelayedDelegatingFactory("se");
    assertThat(new Gson().toJson(serialized)).isEqualTo("{\"custom\":{\"f\":\"se\"}}");
  }

  @JsonAdapter(WithDelayedDelegatingFactory.Factory.class)
  private static class WithDelayedDelegatingFactory {
    String f;

    WithDelayedDelegatingFactory(String f) {
      this.f = f;
    }

    static class Factory implements TypeAdapterFactory {
      @Override
      public <T> TypeAdapter<T> create(Gson gson, TypeToken<T> type) {
        return new TypeAdapter<>() {
          // suppress Error Prone warning; should be clear that `Factory` refers to enclosing class
          @SuppressWarnings("SameNameButDifferent")
          private TypeAdapter<T> delegate() {
            return gson.getDelegateAdapter(Factory.this, type);
          }

          @Override
          public T read(JsonReader in) throws IOException {
            // Perform custom deserialization
            in.beginObject();
            assertThat(in.nextName()).isEqualTo("custom");
            T t = delegate().read(in);
            in.endObject();

            return t;
          }

          @Override
          public void write(JsonWriter out, T value) throws IOException {
            // Perform custom serialization
            out.beginObject();
            out.name("custom");
            delegate().write(out, value);
            out.endObject();
          }
        };
      }
    }
  }

  /**
   * Tests behavior of {@link Gson#getDelegateAdapter} when <i>different</i> instances of the same
   * factory class are used; one registered on the {@code GsonBuilder} and the other implicitly
   * through {@code @JsonAdapter}.
   */
  @Test
  public void testDelegating_SameFactoryClass() {
    Gson gson =
        new GsonBuilder().registerTypeAdapterFactory(new WithDelegatingFactory.Factory()).create();

    // Should use both factories, and therefore have `{"custom": ... }` twice
    WithDelegatingFactory<?> deserialized =
        gson.fromJson("{\"custom\":{\"custom\":{\"f\":\"de\"}}}", WithDelegatingFactory.class);
    assertThat(deserialized.f).isEqualTo("de");

    WithDelegatingFactory<String> serialized = new WithDelegatingFactory<>("se");
    assertThat(gson.toJson(serialized)).isEqualTo("{\"custom\":{\"custom\":{\"f\":\"se\"}}}");
  }

  /**
   * Tests behavior of {@link Gson#getDelegateAdapter} when the <i>same</i> instance of a factory is
   * used (through {@link InstanceCreator}).
   *
   * <p><b>Important:</b> This situation is likely a rare corner case; the purpose of this test is
   * to verify that Gson behaves reasonable, mainly that it does not cause a {@link
   * StackOverflowError} due to infinite recursion. This test is not intended to dictate an expected
   * behavior.
   */
  @Test
  public void testDelegating_SameFactoryInstance() {
    WithDelegatingFactory.Factory factory = new WithDelegatingFactory.Factory();

    Gson gson =
        new GsonBuilder()
            .registerTypeAdapterFactory(factory)
            // Always provides same instance for factory
            .registerTypeAdapter(
                WithDelegatingFactory.Factory.class, (InstanceCreator<?>) type -> factory)
            .create();

    // Current Gson.getDelegateAdapter implementation cannot tell when call is related to
    // @JsonAdapter or not, it can only work based on the `skipPast` factory, so if the same factory
    // instance is used the one registered with `GsonBuilder.registerTypeAdapterFactory` actually
    // skips past the @JsonAdapter one, so the JSON string is `{"custom": ...}` instead of
    // `{"custom":{"custom":...}}`
    WithDelegatingFactory<?> deserialized =
        gson.fromJson("{\"custom\":{\"f\":\"de\"}}", WithDelegatingFactory.class);
    assertThat(deserialized.f).isEqualTo("de");

    WithDelegatingFactory<String> serialized = new WithDelegatingFactory<>("se");
    assertThat(gson.toJson(serialized)).isEqualTo("{\"custom\":{\"f\":\"se\"}}");
  }

  /**
   * Tests behavior of {@link Gson#getDelegateAdapter} when <i>different</i> instances of the same
   * factory class are used; one specified with {@code @JsonAdapter} on a class, and the other
   * specified with {@code @JsonAdapter} on a field of that class.
   *
   * <p><b>Important:</b> This situation is likely a rare corner case; the purpose of this test is
   * to verify that Gson behaves reasonable, mainly that it does not cause a {@link
   * StackOverflowError} due to infinite recursion. This test is not intended to dictate an expected
   * behavior.
   */
  @Test
  public void testDelegating_SameFactoryClass_OnClassAndField() {
    Gson gson =
        new GsonBuilder()
            .registerTypeAdapter(
                String.class,
                new TypeAdapter<String>() {
                  @Override
                  public String read(JsonReader in) throws IOException {
                    return in.nextString() + "-str";
                  }

                  @Override
                  public void write(JsonWriter out, String value) throws IOException {
                    out.value(value + "-str");
                  }
                })
            .create();

    // Should use both factories, and therefore have `{"custom": ... }` once for class and once for
    // the field, and for field also properly delegate to custom String adapter defined above
    WithDelegatingFactoryOnClassAndField deserialized =
        gson.fromJson(
            "{\"custom\":{\"f\":{\"custom\":\"de\"}}}", WithDelegatingFactoryOnClassAndField.class);
    assertThat(deserialized.f).isEqualTo("de-str");

    WithDelegatingFactoryOnClassAndField serialized =
        new WithDelegatingFactoryOnClassAndField("se");
    assertThat(gson.toJson(serialized)).isEqualTo("{\"custom\":{\"f\":{\"custom\":\"se-str\"}}}");
  }

  /**
   * Tests behavior of {@link Gson#getDelegateAdapter} when the <i>same</i> instance of a factory is
   * used (through {@link InstanceCreator}); specified with {@code @JsonAdapter} on a class, and
   * also specified with {@code @JsonAdapter} on a field of that class.
   *
   * <p><b>Important:</b> This situation is likely a rare corner case; the purpose of this test is
   * to verify that Gson behaves reasonable, mainly that it does not cause a {@link
   * StackOverflowError} due to infinite recursion. This test is not intended to dictate an expected
   * behavior.
   */
  @Test
  public void testDelegating_SameFactoryInstance_OnClassAndField() {
    WithDelegatingFactoryOnClassAndField.Factory factory =
        new WithDelegatingFactoryOnClassAndField.Factory();

    Gson gson =
        new GsonBuilder()
            .registerTypeAdapter(
                String.class,
                new TypeAdapter<String>() {
                  @Override
                  public String read(JsonReader in) throws IOException {
                    return in.nextString() + "-str";
                  }

                  @Override
                  public void write(JsonWriter out, String value) throws IOException {
                    out.value(value + "-str");
                  }
                })
            // Always provides same instance for factory
            .registerTypeAdapter(
                WithDelegatingFactoryOnClassAndField.Factory.class,
                (InstanceCreator<?>) type -> factory)
            .create();

    // Because field type (`String`) differs from declaring class,
    // JsonAdapterAnnotationTypeAdapterFactory does not confuse factories and this behaves as
    // expected: Both the declaring class and the field each have `{"custom": ...}` and delegation
    // for the field to the custom String adapter defined above works properly
    WithDelegatingFactoryOnClassAndField deserialized =
        gson.fromJson(
            "{\"custom\":{\"f\":{\"custom\":\"de\"}}}", WithDelegatingFactoryOnClassAndField.class);
    assertThat(deserialized.f).isEqualTo("de-str");

    WithDelegatingFactoryOnClassAndField serialized =
        new WithDelegatingFactoryOnClassAndField("se");
    assertThat(gson.toJson(serialized)).isEqualTo("{\"custom\":{\"f\":{\"custom\":\"se-str\"}}}");
  }

  // Same factory class specified on class and one of its fields
  @JsonAdapter(WithDelegatingFactoryOnClassAndField.Factory.class)
  private static class WithDelegatingFactoryOnClassAndField {
    // suppress Error Prone warning; should be clear that `Factory` refers to nested class
    @SuppressWarnings("SameNameButDifferent")
    @JsonAdapter(Factory.class)
    String f;

    WithDelegatingFactoryOnClassAndField(String f) {
      this.f = f;
    }

    static class Factory implements TypeAdapterFactory {
      @Override
      public <T> TypeAdapter<T> create(Gson gson, TypeToken<T> type) {
        TypeAdapter<T> delegate = gson.getDelegateAdapter(this, type);

        return new TypeAdapter<>() {
          @Override
          public T read(JsonReader in) throws IOException {
            // Perform custom deserialization
            in.beginObject();
            assertThat(in.nextName()).isEqualTo("custom");
            T t = delegate.read(in);
            in.endObject();

            return t;
          }

          @Override
          public void write(JsonWriter out, T value) throws IOException {
            // Perform custom serialization
            out.beginObject();
            out.name("custom");
            delegate.write(out, value);
            out.endObject();
          }
        };
      }
    }
  }

  /** Tests usage of {@link JsonSerializer} as {@link JsonAdapter} value */
  @Test
  public void testJsonSerializer() {
    Gson gson = new Gson();
    // Verify that delegate deserializer (reflection deserializer) is used
    WithJsonSerializer deserialized = gson.fromJson("{\"f\":\"test\"}", WithJsonSerializer.class);
    assertThat(deserialized.f).isEqualTo("test");

    String json = gson.toJson(new WithJsonSerializer());
    // Uses custom serializer which always returns `true`
    assertThat(json).isEqualTo("true");
  }

  @JsonAdapter(WithJsonSerializer.Serializer.class)
  private static class WithJsonSerializer {
    String f = "";

    static class Serializer implements JsonSerializer<WithJsonSerializer> {
      @Override
      public JsonElement serialize(
          WithJsonSerializer src, Type typeOfSrc, JsonSerializationContext context) {
        return new JsonPrimitive(true);
      }
    }
  }

  /** Tests usage of {@link JsonDeserializer} as {@link JsonAdapter} value */
  @Test
  public void testJsonDeserializer() {
    Gson gson = new Gson();
    WithJsonDeserializer deserialized =
        gson.fromJson("{\"f\":\"test\"}", WithJsonDeserializer.class);
    // Uses custom deserializer which always uses "123" as field value
    assertThat(deserialized.f).isEqualTo("123");

    // Verify that delegate serializer (reflection serializer) is used
    String json = gson.toJson(new WithJsonDeserializer("abc"));
    assertThat(json).isEqualTo("{\"f\":\"abc\"}");
  }

  @JsonAdapter(WithJsonDeserializer.Deserializer.class)
  private static class WithJsonDeserializer {
    String f;

    WithJsonDeserializer(String f) {
      this.f = f;
    }

    static class Deserializer implements JsonDeserializer<WithJsonDeserializer> {
      @Override
      public WithJsonDeserializer deserialize(
          JsonElement json, Type typeOfT, JsonDeserializationContext context) {
        return new WithJsonDeserializer("123");
      }
    }
  }

  /**
   * Tests creation of the adapter referenced by {@code @JsonAdapter} using an {@link
   * InstanceCreator}.
   */
  @Test
  public void testAdapterCreatedByInstanceCreator() {
    CreatedByInstanceCreator.Serializer serializer =
        new CreatedByInstanceCreator.Serializer("custom");
    Gson gson =
        new GsonBuilder()
            .registerTypeAdapter(
                CreatedByInstanceCreator.Serializer.class, (InstanceCreator<?>) t -> serializer)
            .create();

    String json = gson.toJson(new CreatedByInstanceCreator());
    assertThat(json).isEqualTo("\"custom\"");
  }

  @JsonAdapter(CreatedByInstanceCreator.Serializer.class)
  private static class CreatedByInstanceCreator {
    static class Serializer implements JsonSerializer<CreatedByInstanceCreator> {
      private final String value;

      @SuppressWarnings("unused")
      public Serializer() {
        throw new AssertionError("should not be called");
      }

      public Serializer(String value) {
        this.value = value;
      }

      @Override
      public JsonElement serialize(
          CreatedByInstanceCreator src, Type typeOfSrc, JsonSerializationContext context) {
        return new JsonPrimitive(value);
      }
    }
  }

  /** Tests creation of the adapter referenced by {@code @JsonAdapter} using JDK Unsafe. */
  @Test
  public void testAdapterCreatedByJdkUnsafe() {
    String json = new Gson().toJson(new CreatedByJdkUnsafe());
    assertThat(json).isEqualTo("false");
  }

  @JsonAdapter(CreatedByJdkUnsafe.Serializer.class)
  private static class CreatedByJdkUnsafe {
    static class Serializer implements JsonSerializer<CreatedByJdkUnsafe> {
      // JDK Unsafe leaves this at default value `false`
      private boolean wasInitialized = true;

      // Explicit constructor with args to remove implicit no-args constructor
      @SuppressWarnings("unused")
      public Serializer(int i) {
        throw new AssertionError("should not be called");
      }

      @Override
      public JsonElement serialize(
          CreatedByJdkUnsafe src, Type typeOfSrc, JsonSerializationContext context) {
        return new JsonPrimitive(wasInitialized);
      }
    }
  }
}