ApiMessageHttpResponseParser.java

/*
 * Copyright 2018 Google LLC
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 * modification, are permitted provided that the following conditions are
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 *
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 * notice, this list of conditions and the following disclaimer.
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 * in the documentation and/or other materials provided with the
 * distribution.
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 * contributors may be used to endorse or promote products derived from
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 *
 * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
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 * LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
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 * OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
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package com.google.api.gax.httpjson;

import com.google.auto.value.AutoValue;
import com.google.gson.Gson;
import com.google.gson.GsonBuilder;
import com.google.gson.JsonIOException;
import com.google.gson.JsonSyntaxException;
import com.google.gson.TypeAdapter;
import com.google.gson.stream.JsonReader;
import com.google.gson.stream.JsonWriter;
import com.google.protobuf.TypeRegistry;
import java.io.InputStream;
import java.io.InputStreamReader;
import java.io.Reader;
import java.lang.reflect.Type;
import java.nio.charset.StandardCharsets;

/** Utility class to parse {@link ApiMessage}s from HTTP responses. */
@AutoValue
public abstract class ApiMessageHttpResponseParser<ResponseT extends ApiMessage>
    implements HttpResponseParser<ResponseT> {

  public abstract ResponseT getResponseInstance();

  protected abstract Gson getResponseMarshaller();

  /**
   * Constructs an ApiMessageHttpResponseParser from an instance of the message type to be
   * formatted.
   */
  private static <ResponseT extends ApiMessage> ApiMessageHttpResponseParser<ResponseT> create(
      final ResponseT responseInstance) {
    final Gson baseGson = new GsonBuilder().create();
    TypeAdapter responseTypeAdapter;
    Gson responseMarshaller = null;

    if (responseInstance != null) {
      responseTypeAdapter =
          new TypeAdapter<ResponseT>() {
            @Override
            public void write(JsonWriter out, ResponseT value) {
              baseGson.toJson(value, responseInstance.getClass(), out);
            }

            @Override
            public ResponseT read(JsonReader in) {
              return baseGson.fromJson(in, responseInstance.getClass());
            }
          };
      responseMarshaller =
          new GsonBuilder()
              .registerTypeAdapter(responseInstance.getClass(), responseTypeAdapter)
              .create();
    }

    return new AutoValue_ApiMessageHttpResponseParser<>(responseInstance, responseMarshaller);
  }

  public static <ResponseT extends ApiMessage>
      ApiMessageHttpResponseParser.Builder<ResponseT> newBuilder() {
    return new ApiMessageHttpResponseParser.Builder<>();
  }

  @Override
  public ResponseT parse(InputStream httpResponseBody) {
    return parse(httpResponseBody, null);
  }

  @Override
  public ResponseT parse(InputStream httpResponseBody, TypeRegistry registry) {
    return parse(new InputStreamReader(httpResponseBody, StandardCharsets.UTF_8), registry);
  }

  @Override
  public ResponseT parse(Reader httpResponseBody, TypeRegistry registry) {
    if (getResponseInstance() == null) {
      return null;
    } else {
      Type responseType = getResponseInstance().getClass();
      try {
        return getResponseMarshaller().fromJson(httpResponseBody, responseType);
      } catch (JsonIOException | JsonSyntaxException e) {
        throw new RestSerializationException(e);
      }
    }
  }

  @Override
  public String serialize(ResponseT response) {
    return getResponseMarshaller().toJson(response);
  }

  public static class Builder<ResponseT extends ApiMessage> {
    private ResponseT responseInstance;

    private Builder() {}

    public Builder<ResponseT> setResponseInstance(ResponseT responseInstance) {
      this.responseInstance = responseInstance;
      return this;
    }

    public ApiMessageHttpResponseParser<ResponseT> build() {
      return ApiMessageHttpResponseParser.create(responseInstance);
    }
  }
}