【源碼閱讀】Gson源碼閱讀

時間  2019-11-07
標籤 源碼閱讀 gson 源碼 閱讀 简体版
原文   原文鏈接

前言

以前也立過FLAG，要閱讀gson的源碼，拖了很久，如今也算是填了以前的坑吧。html

使用

  • 轉換爲json
Gson gson = new Gson();
User user = new User(18, "張三");
String userJson = gson.toJson(user);
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  • 轉換爲對象
String json = "{\"age\":18,\"name\":\"張三\",\"sex\":\"女\"}";
User u = gson.fromJson(json, User.class);
User us = gson.fromJson(json, new TypeToken<User>() {
        }.getType());
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構造方法

  • 無參的 Gson gson = new Gson();
  • Builder Gson gson = new GsonBuilder().create(); 經常使用的方法有

registerTypeAdapter 添加本身的解析器java

registerTypeAdapterFactory 添加本身的解析器工廠json

setDateFormat 設置日期格式數組

fromJson

fromJson有多個重載方法，最後調用的是fromJson(JsonReader reader, Type typeOfT)方法緩存

public <T> 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<T> typeToken = (TypeToken<T>) TypeToken.get(typeOfT);
      TypeAdapter<T> typeAdapter = getAdapter(typeToken);
      T object = typeAdapter.read(reader);
      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);
    } catch (AssertionError e) {
      throw new AssertionError("AssertionError (GSON " + GsonBuildConfig.VERSION + "): " + e.getMessage(), e);
    } finally {
      reader.setLenient(oldLenient);
    }
  }
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首先，咱們來看JsonReader 它是個解析json數據的輔助類，主要有如下方法架構

  • peek() 返回下一個元素的JsonToken
  • doPeek() 返回當前元素的類型
  • hasNext() 是否有下個元素
  • beginArray() 數組開始
  • endArray() 數組結束
  • beginObject() 對象開始
  • endObject() 對象結束

而後是TypeAdapter<T> typeAdapter = getAdapter(typeToken);app

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

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

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

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

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

      if (requiresThreadLocalCleanup) {
        calls.remove();
      }
    }
  }
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首先，從緩存中取，若是取不到，則經過遍歷factories對比Type，拿到對應的TypeAdapter。 可是，這個factories是何時添加的呢？答案是構造方法裏ide

Gson(final Excluder excluder, final FieldNamingStrategy fieldNamingStrategy,
      final Map<Type, InstanceCreator<?>> instanceCreators, boolean serializeNulls,
      boolean complexMapKeySerialization, boolean generateNonExecutableGson, boolean htmlSafe,
      boolean prettyPrinting, boolean lenient, boolean serializeSpecialFloatingPointValues,
      LongSerializationPolicy longSerializationPolicy, String datePattern, int dateStyle,
      int timeStyle, List<TypeAdapterFactory> builderFactories,
      List<TypeAdapterFactory> builderHierarchyFactories,
      List<TypeAdapterFactory> factoriesToBeAdded) {
    ...省略...
    List<TypeAdapterFactory> factories = new ArrayList<TypeAdapterFactory>();

    // 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);

    // users' type adapters
    factories.addAll(factoriesToBeAdded);

    // 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<Number> 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);
  }
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能夠看到，基本上每一個基本類型都有對應的TypeAdapter，那麼問題就來了，這裏咱們使用的是自定義的實體類，是如何解析的？ 請注意最後添加的ReflectiveTypeAdapterFactory，它就是解決自定義的實體類而存在的。學習

最後是T object = typeAdapter.read(reader); 根據上一步獲得的TypeAdapter，調用它的read方法，這裏咱們就以ReflectiveTypeAdapterFactory#Adapter的read方法爲例ui

@Override public T read(JsonReader in) throws IOException {
      if (in.peek() == JsonToken.NULL) {
        in.nextNull();
        return null;
      }
      // 反射建立實例
      T instance = constructor.construct();

      try {
        in.beginObject();
        while (in.hasNext()) {
          // 獲取變量名
          String name = in.nextName();
          BoundField field = boundFields.get(name);
          if (field == null || !field.deserialized) {
            in.skipValue();
          } else {
            //調用 field.read設置值
            field.read(in, instance);
          }
        }
      } catch (IllegalStateException e) {
        throw new JsonSyntaxException(e);
      } catch (IllegalAccessException e) {
        throw new AssertionError(e);
      }
      in.endObject();
      return instance;
    }
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先用反射建立對象，再循環取出每一個值，而後調用field.read爲每一個變量賦值。其中，boundFields存放的是全部的變量名，賦值是在getBoundFields方法。

@Override public <T> TypeAdapter<T> create(Gson gson, final TypeToken<T> type) {
    Class<? super T> raw = type.getRawType();

    if (!Object.class.isAssignableFrom(raw)) {
      return null; // it's a primitive!
    }

    ObjectConstructor<T> constructor = constructorConstructor.get(type);
    return new Adapter<T>(constructor, getBoundFields(gson, type, raw));
  }
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BoundField的read方法

@Override void read(JsonReader reader, Object value) throws IOException, IllegalAccessException {
        Object fieldValue = typeAdapter.read(reader);
        if (fieldValue != null || !isPrimitive) {
          field.set(value, fieldValue);
        }
      }
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由此，有如下結論

  • 1.json中有該字段(對象)、實體類中沒有 or 實體類中有、json中沒有，都不影響解析
  • 2.賦值是調用反射，所以與實體類中是否有get、set方法無關。

toJson

看完fromJson，再來看toJson就會簡單不少 toJson也有不少重載方法，最後調用的是toJson(Object src, Type typeOfSrc, JsonWriter writer)

public void toJson(Object src, Type typeOfSrc, JsonWriter writer) throws JsonIOException {
    TypeAdapter<?> adapter = getAdapter(TypeToken.get(typeOfSrc));
    boolean oldLenient = writer.isLenient();
    writer.setLenient(true);
    boolean oldHtmlSafe = writer.isHtmlSafe();
    writer.setHtmlSafe(htmlSafe);
    boolean oldSerializeNulls = writer.getSerializeNulls();
    writer.setSerializeNulls(serializeNulls);
    try {
      ((TypeAdapter<Object>) adapter).write(writer, src);
    } catch (IOException e) {
      throw new JsonIOException(e);
    } catch (AssertionError e) {
      throw new AssertionError("AssertionError (GSON " + GsonBuildConfig.VERSION + "): " + e.getMessage(), e);
    } finally {
      writer.setLenient(oldLenient);
      writer.setHtmlSafe(oldHtmlSafe);
      writer.setSerializeNulls(oldSerializeNulls);
    }
  }
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這裏的邏輯和fromJson相似，根據傳入的Type獲得對應的TypeAdapter，而後調用它的write方法，這裏，咱們仍是以ReflectiveTypeAdapterFactory#Adapter爲例

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

      out.beginObject();
      try {
        for (BoundField boundField : boundFields.values()) {
          if (boundField.writeField(value)) {
            out.name(boundField.name);
            boundField.write(out, value);
          }
        }
      } catch (IllegalAccessException e) {
        throw new AssertionError(e);
      }
      out.endObject();
    }
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首先是JsonWriter，它也是輔助類，主要有如下方法

  • beginArray() 寫入"["
  • endArray() 寫入"]"
  • beginObject() 寫入"{"
  • endObject() 寫入"}"
  • beforeName() 寫入key前檢查
  • beforeValue() 寫入值前檢查
  • value(...) 寫入值

而後遍歷boundFields，循環調用。 BoundField的write方法

@Override void write(JsonWriter writer, Object value) throws IOException, IllegalAccessException {
        Object fieldValue = field.get(value);
        TypeAdapter t = jsonAdapterPresent ? typeAdapter
            : new TypeAdapterRuntimeTypeWrapper(context, typeAdapter, fieldType.getType());
        t.write(writer, fieldValue);
      }
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BoundField的writeField方法

@Override public boolean writeField(Object value) throws IOException, IllegalAccessException {
        if (!serialized) return false;
        Object fieldValue = field.get(value);
        return fieldValue != value; // 避免遞歸
      }
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總結

整個項目的架構是創建在Adapter模式上，經過區分Type從而拆分邏輯到各個Adapter，其中也不乏Factory模式、緩存的使用，其對JsonReader、JsonWriter封裝也是值得學習的。

你的承認，是我堅持更新博客的動力，若是以爲有用，就請點個贊，謝謝

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