分享一個(gè)Kotlin搭配Gson的坑

今天發(fā)現(xiàn)服務(wù)端返回的一個(gè)javaBean里面有三個(gè)List<T>其實(shí)是可以合并到一個(gè)List<T>的，所以打算合并一下，遂在javaBean中增加一個(gè)field；

改之前

    data class Example(
        @SerializedName("a_list") val aList : List<String>,
        @SerializedName("b_list") val bList : List<String>,
        @SerializedName("c_list") val cList : List<String>
    )

改之后

    data class Example(
        @SerializedName("a_list") val aList : List<String>,
        @SerializedName("b_list") val bList : List<String>,
        @SerializedName("c_list") val cList : List<String>,
        val mixList: MutableList<String> = arrayListOf()
    )

    fun mix(){
        //從服務(wù)端獲取
        val example = getExampleData()
        example.mixList.apply {
            addAll(example.aList)
            addAll(example.bList)
            addAll(example.cList)
        }
    }

    fun getExampleData() : Example{
        return Gson().fromJson(getData(),Example::class.java)
    }

然后拋出了mixList的NullPointException

解決方案很簡(jiǎn)單，在mix方法中給mixList賦值
通過(guò)閱讀Gson源碼分析一下
構(gòu)造一個(gè)Person類(lèi)試試

    data class Person(
        val name : String,
        val age : Int,
        val job : String = "coder"
    )

• 首先是fromJson

public <T> T fromJson(JsonReader reader, Type typeOfT) throws JsonIOException, JsonSyntaxException {
    ---
      TypeToken<T> typeToken = (TypeToken<T>)   TypeToken.get(typeOfT);
      TypeAdapter<T> typeAdapter = getAdapter(typeToken);
      T object = typeAdapter.read(reader);
      return object;
    ---

• 可以看到這里通過(guò)TypeAdapter來(lái)讀取字符串，
  通過(guò)斷點(diǎn)來(lái)到ReflectiveTypeAdapterFactory的getBoundFields方法，

private Map<String, BoundField> getBoundFields(Gson context, TypeToken<?> type, Class<?> raw) {
    Map<String, BoundField> result = new LinkedHashMap<String, BoundField>();
    if (raw.isInterface()) {
      return result;
    }

    Type declaredType = type.getType();
    while (raw != Object.class) {
      Field[] fields = raw.getDeclaredFields();
      for (Field field : fields) {
        boolean serialize = excludeField(field, true);
        boolean deserialize = excludeField(field, false);
        if (!serialize && !deserialize) {
          continue;
        }
        accessor.makeAccessible(field);
        Type fieldType = $Gson$Types.resolve(type.getType(), raw, field.getGenericType());
        List<String> fieldNames = getFieldNames(field);
        BoundField previous = null;
        for (int i = 0, size = fieldNames.size(); i < size; ++i) {
          String name = fieldNames.get(i);
          if (i != 0) serialize = false; // only serialize the default name
          BoundField boundField = createBoundField(context, field, name,
              TypeToken.get(fieldType), serialize, deserialize);
          BoundField replaced = result.put(name, boundField);
          if (previous == null) previous = replaced;
        }
        if (previous != null) {
          throw new IllegalArgumentException(declaredType
              + " declares multiple JSON fields named " + previous.name);
        }
      }
      type = TypeToken.get($Gson$Types.resolve(type.getType(), raw, raw.getGenericSuperclass()));
      raw = type.getRawType();
    }
    return result;
  }

• 看一下這個(gè)raw的來(lái)源

  @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));
  }

也就是我們傳進(jìn)來(lái)要轉(zhuǎn)換的結(jié)果class，gson通過(guò)默認(rèn)構(gòu)造函數(shù)構(gòu)造了一個(gè)Person對(duì)象，通過(guò)遍歷field，并進(jìn)行一一匹配填充，而沒(méi)有讀取到的初始值為空，所以我們?cè)?code>data class中賦的值沒(méi)有意義了～

構(gòu)造器.jpg

log.jpg

• 看一下Kotlin的構(gòu)造器問(wèn)題

默認(rèn)構(gòu)造.jpg

也就是說(shuō)，我們賦的默認(rèn)值失去了意義～
其實(shí)也沒(méi)啥，就是記錄一下