com.github.messenger4j.internal.gson.LowercaseEnumTypeAdapterFactory Maven / Gradle / Ivy

package com.github.messenger4j.internal.gson;

import com.google.gson.Gson;
import com.google.gson.TypeAdapter;
import com.google.gson.TypeAdapterFactory;
import com.google.gson.annotations.SerializedName;
import com.google.gson.reflect.TypeToken;
import com.google.gson.stream.JsonReader;
import com.google.gson.stream.JsonToken;
import com.google.gson.stream.JsonWriter;
import java.io.IOException;
import java.util.HashMap;
import java.util.Map;

/**
 * @author Andriy Koretskyy
 * @author Max Grabenhorst
 * @since 1.0.0
 */
final class LowercaseEnumTypeAdapterFactory implements TypeAdapterFactory {

  @Override
  public  TypeAdapter create(Gson gson, TypeToken type) {

    @SuppressWarnings("unchecked")
    final Class rawType = (Class) type.getRawType();
    if (!rawType.isEnum()) {
      return null;
    }

    final TypeAdapter delegateAdapter = gson.getDelegateAdapter(this, type);
    final Map transformedToConstant = new HashMap<>();
    final Map constantToTransformed = new HashMap<>();
    for (T constant : rawType.getEnumConstants()) {
      final String transformedConstant = transform(constant, delegateAdapter);
      transformedToConstant.put(transformedConstant, constant);
      constantToTransformed.put(constant, transformedConstant);
    }

    return new TypeAdapter() {
      @Override
      public void write(JsonWriter out, T value) throws IOException {
        if (value == null) {
          out.nullValue();
        } else {
          out.value(constantToTransformed.get(value));
        }
      }

      @Override
      public T read(JsonReader reader) throws IOException {
        if (reader.peek() == JsonToken.NULL) {
          reader.nextNull();
          return null;
        } else {
          return transformedToConstant.get(reader.nextString());
        }
      }
    };
  }

  /**
   * Transforms the given enum constant to lower case. If a {@code SerializedName} annotation is
   * present, the default adapter result is returned.
   *
   * @param enumConstant the enumeration constant
   * @param delegateAdapter the default adapter of the given type
   * @return the transformed string representation of the constant
   */
  private  String transform(T enumConstant, TypeAdapter delegateAdapter) {
    try {
      final String enumValue = ((Enum) enumConstant).name();
      final boolean hasSerializedNameAnnotation =
          enumConstant.getClass().getField(enumValue).isAnnotationPresent(SerializedName.class);
      return hasSerializedNameAnnotation
          ? delegateAdapter.toJsonTree(enumConstant).getAsString()
          : enumValue.toLowerCase();
    } catch (NoSuchFieldException e) {
      // should never happen
      throw new RuntimeException(e);
    }
  }
}