Gson源码分析（一）

Json解析一直是网络通信中重要的数据解析框架。而Gson和FastJson是最为常用的两个Json解析框架。这一章节主要介绍Gson的原理。Gson的一个典型使用例子如下：

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

Gson的fromJson()方法源码如下：

  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获取TypeToken
      TypeAdapter typeAdapter = getAdapter(typeToken);//2获取TypeAdapter
      T object = typeAdapter.read(reader);//3解析
      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);
    }
  }

如上，fromJson()方法主要分三个步骤：1获取TypeToken，2获取TypeAdapter，3利用TypeAdapter解析。
下面就按照这三个步骤分析。
（1）TypeToken
TypeToken的get()方法源码如下：

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