Gsonソース分析(一)

Json解析はネットワーク通信において重要なデータ解析フレームワークである.GsonとFastJsonは最もよく使われる2つのJson解析フレームワークである.この章では主にGsonの原理を紹介する.Gsonの典型的な使用例は以下の通りである.

        String jsonString = "{\"name\":\"renyiguang\"}";
        Gson gson = new Gson();
        TestModel testModel = gson.fromJson(jsonString, TestModel.class);

GsonのfromJson()メソッドのソースコードは次のとおりです.

  public  T fromJson(String json, Class classOfT) throws JsonSyntaxException {
    Object object = fromJson(json, (Type) classOfT);
    return Primitives.wrap(classOfT).cast(object);
  }

  public  T fromJson(String json, Type typeOfT) throws JsonSyntaxException {
    if (json == null) {
      return null;
    }
    StringReader reader = new StringReader(json);
    T target = (T) fromJson(reader, typeOfT);
    return target;
  }

  public  T fromJson(Reader json, Type typeOfT) throws JsonIOException, JsonSyntaxException {
    JsonReader jsonReader = newJsonReader(json);
    T object = (T) fromJson(jsonReader, typeOfT);
    assertFullConsumption(object, jsonReader);
    return object;
  }

  public  T fromJson(JsonReader reader, Type typeOfT) throws JsonIOException, JsonSyntaxException {
    boolean isEmpty = true;
    boolean oldLenient = reader.isLenient();
    reader.setLenient(true);
    try {
      reader.peek();
      isEmpty = false;
      TypeToken typeToken = (TypeToken) TypeToken.get(typeOfT);//1  TypeToken
      TypeAdapter typeAdapter = getAdapter(typeToken);//2  TypeAdapter
      T object = typeAdapter.read(reader);//3  
      return object;
    } catch (EOFException e) {
      /*
       * For compatibility with JSON 1.5 and earlier, we return null for empty
       * documents instead of throwing.
       */
      if (isEmpty) {
        return null;
      }
      throw new JsonSyntaxException(e);
    } catch (IllegalStateException e) {
      throw new JsonSyntaxException(e);
    } catch (IOException e) {
      // TODO(inder): Figure out whether it is indeed right to rethrow this as JsonSyntaxException
      throw new JsonSyntaxException(e);
    } finally {
      reader.setLenient(oldLenient);
    }
  }

以上のようにfromJson()メソッドは主に3つのステップに分けられる:1 Type Tokenを取得し,2 Type Adapterを取得し,3 Type Adapterを用いて解析する.以下、この3つのステップに従って分析します.(1)TypeToken TypeTokenのget()メソッドのソースコードは以下の通りである.

  public static TypeToken> get(Type type) {
    return new TypeToken

TypeTokenは解析対象データのTypeタイプを保存している.(2)TypeAdapter TypeAdapterはgetAdapter()メソッドで取得する.getAdapter()メソッドのソースコードは次のとおりです.

  public  TypeAdapter getAdapter(TypeToken type) {
    TypeAdapter> cached = typeTokenCache.get(type == null ? NULL_KEY_SURROGATE : type);//    TypeAdapter
    if (cached != null) {
      return (TypeAdapter) cached;
    }

    Map, FutureTypeAdapter>> threadCalls = calls.get();
    boolean requiresThreadLocalCleanup = false;
    if (threadCalls == null) {
      threadCalls = new HashMap, FutureTypeAdapter>>();
      calls.set(threadCalls);
      requiresThreadLocalCleanup = true;
    }

    // the key and value type parameters always agree
    FutureTypeAdapter ongoingCall = (FutureTypeAdapter) threadCalls.get(type);
    if (ongoingCall != null) {
      return ongoingCall;
    }

    try {
      FutureTypeAdapter call = new FutureTypeAdapter();
      threadCalls.put(type, call);

      for (TypeAdapterFactory factory : factories) {// factories   
        TypeAdapter candidate = factory.create(this, type);
        if (candidate != null) {
          call.setDelegate(candidate);
          typeTokenCache.put(type, candidate);
          return candidate;
        }
      }
      throw new IllegalArgumentException("GSON cannot handle " + type);
    } finally {
      threadCalls.remove(type);

      if (requiresThreadLocalCleanup) {
        calls.remove();
      }
    }
  }

TypeTokenCacheにTypeToken対応のTypeAdapterがない場合、factoriesから対応するTypeAdapterを検索します.factoriesがType Adapterを検索する基準は、Type AdapterFactoryのcreate()メソッドが空でないことを返します.factoriesの初期化はGsonの構造方法において以下の通りである.

  Gson(final Excluder excluder, final FieldNamingStrategy fieldNamingStrategy,
      final Map> instanceCreators, boolean serializeNulls,
      boolean complexMapKeySerialization, boolean generateNonExecutableGson, boolean htmlSafe,
      boolean prettyPrinting, boolean lenient, boolean serializeSpecialFloatingPointValues,
      LongSerializationPolicy longSerializationPolicy,
      List typeAdapterFactories) {
    this.constructorConstructor = new ConstructorConstructor(instanceCreators);
    this.excluder = excluder;
    this.fieldNamingStrategy = fieldNamingStrategy;
    this.serializeNulls = serializeNulls;
    this.generateNonExecutableJson = generateNonExecutableGson;
    this.htmlSafe = htmlSafe;
    this.prettyPrinting = prettyPrinting;
    this.lenient = lenient;

    List factories = new ArrayList();

    // built-in type adapters that cannot be overridden
    factories.add(TypeAdapters.JSON_ELEMENT_FACTORY);
    factories.add(ObjectTypeAdapter.FACTORY);

    // the excluder must precede all adapters that handle user-defined types
    factories.add(excluder);

    // user's type adapters
    factories.addAll(typeAdapterFactories);//      TypeAdapterFactory

    // type adapters for basic platform types
    factories.add(TypeAdapters.STRING_FACTORY);
    factories.add(TypeAdapters.INTEGER_FACTORY);
    factories.add(TypeAdapters.BOOLEAN_FACTORY);
    factories.add(TypeAdapters.BYTE_FACTORY);
    factories.add(TypeAdapters.SHORT_FACTORY);
    TypeAdapter longAdapter = longAdapter(longSerializationPolicy);
    factories.add(TypeAdapters.newFactory(long.class, Long.class, longAdapter));
    factories.add(TypeAdapters.newFactory(double.class, Double.class,
            doubleAdapter(serializeSpecialFloatingPointValues)));
    factories.add(TypeAdapters.newFactory(float.class, Float.class,
            floatAdapter(serializeSpecialFloatingPointValues)));
    factories.add(TypeAdapters.NUMBER_FACTORY);
    factories.add(TypeAdapters.ATOMIC_INTEGER_FACTORY);
    factories.add(TypeAdapters.ATOMIC_BOOLEAN_FACTORY);
    factories.add(TypeAdapters.newFactory(AtomicLong.class, atomicLongAdapter(longAdapter)));
    factories.add(TypeAdapters.newFactory(AtomicLongArray.class, atomicLongArrayAdapter(longAdapter)));
    factories.add(TypeAdapters.ATOMIC_INTEGER_ARRAY_FACTORY);
    factories.add(TypeAdapters.CHARACTER_FACTORY);
    factories.add(TypeAdapters.STRING_BUILDER_FACTORY);
    factories.add(TypeAdapters.STRING_BUFFER_FACTORY);
    factories.add(TypeAdapters.newFactory(BigDecimal.class, TypeAdapters.BIG_DECIMAL));
    factories.add(TypeAdapters.newFactory(BigInteger.class, TypeAdapters.BIG_INTEGER));
    factories.add(TypeAdapters.URL_FACTORY);
    factories.add(TypeAdapters.URI_FACTORY);
    factories.add(TypeAdapters.UUID_FACTORY);
    factories.add(TypeAdapters.CURRENCY_FACTORY);
    factories.add(TypeAdapters.LOCALE_FACTORY);
    factories.add(TypeAdapters.INET_ADDRESS_FACTORY);
    factories.add(TypeAdapters.BIT_SET_FACTORY);
    factories.add(DateTypeAdapter.FACTORY);
    factories.add(TypeAdapters.CALENDAR_FACTORY);
    factories.add(TimeTypeAdapter.FACTORY);
    factories.add(SqlDateTypeAdapter.FACTORY);
    factories.add(TypeAdapters.TIMESTAMP_FACTORY);
    factories.add(ArrayTypeAdapter.FACTORY);
    factories.add(TypeAdapters.CLASS_FACTORY);

    // type adapters for composite and user-defined types
    factories.add(new CollectionTypeAdapterFactory(constructorConstructor));
    factories.add(new MapTypeAdapterFactory(constructorConstructor, complexMapKeySerialization));
    this.jsonAdapterFactory = new JsonAdapterAnnotationTypeAdapterFactory(constructorConstructor);
    factories.add(jsonAdapterFactory);
    factories.add(TypeAdapters.ENUM_FACTORY);
    factories.add(new ReflectiveTypeAdapterFactory(
        constructorConstructor, fieldNamingStrategy, excluder, jsonAdapterFactory));

    this.factories = Collections.unmodifiableList(factories);
  }

上記factoriesにはGsonが解析できるすべてのデータフォーマットが含まれています.以下STRING_FACTORYを例に説明します.

  public static final TypeAdapterFactory STRING_FACTORY = newFactory(String.class, STRING);

  public static  TypeAdapterFactory newFactory(
      final Class type, final TypeAdapter typeAdapter) {
    return new TypeAdapterFactory() {
      @SuppressWarnings("unchecked") // we use a runtime check to make sure the 'T's equal
      @Override public  TypeAdapter create(Gson gson, TypeToken typeToken) {
        return typeToken.getRawType() == type ? (TypeAdapter) typeAdapter : null;
      }
      @Override public String toString() {
        return "Factory[type=" + type.getName() + ",adapter=" + typeAdapter + "]";
      }
    };
  }

newFactory()メソッドが返すTypeAdapterFactoryのcreate()メソッドは、typeの一貫性を検証するために1つのことをしました.つまり、処理するデータ型がStringの場合はType Adapter、そうでなければnullを返します.Stringタイプに対応するType Adapterのソースコードは以下の通りです.

  public static final TypeAdapter STRING = new TypeAdapter() {
    @Override
    public String read(JsonReader in) throws IOException {
      JsonToken peek = in.peek();
      if (peek == JsonToken.NULL) {
        in.nextNull();
        return null;
      }
      /* coerce booleans to strings for backwards compatibility */
      if (peek == JsonToken.BOOLEAN) {
        return Boolean.toString(in.nextBoolean());
      }
      return in.nextString();
    }
    @Override
    public void write(JsonWriter out, String value) throws IOException {
      out.value(value);
    }
  };

Gson解析については,簡単な流れが紹介されている.解析の観点から分析を重視します.