轻触开源(四)-Gson项目源码解析_叁

ReflectiveTypeAdapterFactory完成Field属性之后,将生成TypeAdapter返回。Gson通过Adapter来生成指定类型的对象。生成对象的过程,被ReflectiveTypeAdapterFactory.Adapter类记录在自己的read方法中:

@Override public T read(JsonReader in) throws IOException {
      if (in.peek() == JsonToken.NULL) {
        in.nextNull();
        return null;
      }

      T instance = constructor.construct();

      try {
        in.beginObject();
        while (in.hasNext()) {
          String name = in.nextName();
          BoundField field = boundFields.get(name);
          if (field == null || !field.deserialized) {
            in.skipValue();
          } else {
            field.read(in, instance);
          }
        }
      } catch (IllegalStateException e) {
        throw new JsonSyntaxException(e);
      } catch (IllegalAccessException e) {
        throw new AssertionError(e);
      }
      in.endObject();
      return instance;
    }

Gson采用注入的方式往已生成的对象中注入属性。在注入之前,会先调用ObjectConstructor来生成所需要的对象。这部分,非墨已经在之前的文章中表述过了。由于ReflectiveTypeAdapterFactory.Adapter针对的是Gson对象的适配,因此在适配之前,会先调用JsonReader.beginObject来声明处理入口。

//CODE JsonReader.java
public void beginObject() throws IOException {
    int p = peeked;
    if (p == PEEKED_NONE) {
      p = doPeek();
    }
    if (p == PEEKED_BEGIN_OBJECT) {
      push(JsonScope.EMPTY_OBJECT);
      peeked = PEEKED_NONE;
    } else {
      throw new IllegalStateException("Expected BEGIN_OBJECT but was " + peek()
          + " at line " + getLineNumber() + " column " + getColumnNumber() + " path " + getPath());
    }
}
private void push(int newTop) {
    if (stackSize == stack.length) {
      int[] newStack = new int[stackSize * 2];
      int[] newPathIndices = new int[stackSize * 2];
      String[] newPathNames = new String[stackSize * 2];
      System.arraycopy(stack, 0, newStack, 0, stackSize);
      System.arraycopy(pathIndices, 0, newPathIndices, 0, stackSize);
      System.arraycopy(pathNames, 0, newPathNames, 0, stackSize);
      stack = newStack;
      pathIndices = newPathIndices;
      pathNames = newPathNames;
    }
    stack[stackSize++] = newTop;
  }

JsonReader调用beginObject方法,会调用push方法用于记录一下当前操作的词汇元素。这里将记录一个JsonScope.EMPTY_OBJECT的变量。我们用以下Json串用于测试:

String strJson = "{\"name\":david,age:19,room:{roomName:small,number:1}}";

这个Json串将被记录在JsonReader的Buffer中。Gson在调用fromJson的时候,会调用一下JsonReader的peek操作,而peek操作,会调用内部的doPeek操作,doPeek操作会改变JsonReader中peeked变量的值和buffer偏移pos。peeked变量用于记录下一个字符的属性,pos变量用于记录buffer缓存的偏移量。

此时,在Gson.fromJson中调用完reader的peek方法之后,JsonReader中pos索引的位置将指向' " '符号。

我们回到ReflectiveTypeAdapterFactory的Adapter的read方法中去看:

@Override public T read(JsonReader in) throws IOException {
      ...
      try {
        in.beginObject();
        while (in.hasNext()) {
          String name = in.nextName();
          BoundField field = boundFields.get(name);
          if (field == null || !field.deserialized) {
            in.skipValue();
          } else {
            field.read(in, instance);
          }
        }
      } catch (IllegalStateException e) {
        throw new JsonSyntaxException(e);
      } catch (IllegalAccessException e) {
        throw new AssertionError(e);
      }
      in.endObject();
      return instance;
    }

Gson在调用完beginObject之后调用了JsonReader的hasNext方法,这个方法顾名思义,就是用来查看输入源中是否还有待处理的字符。之后,如果此项为真,那么将调用JsonReader.nextName方法来获取Json对象的属性名字。

public String nextName() throws IOException {
    int p = peeked;
    if (p == PEEKED_NONE) {
      p = doPeek();
    }
    String result;
    if (p == PEEKED_UNQUOTED_NAME) {
      result = nextUnquotedValue();
    } else if (p == PEEKED_SINGLE_QUOTED_NAME) {
      result = nextQuotedValue('\'');
    } else if (p == PEEKED_DOUBLE_QUOTED_NAME) {
      result = nextQuotedValue('"');
    } else {
      throw new IllegalStateException("Expected a name but was " + peek()
          + " at line " + getLineNumber() + " column " + getColumnNumber() + " path " + getPath());
    }
    peeked = PEEKED_NONE;
    pathNames[stackSize - 1] = result;
    return result;
  }

nextName方法会先调用一个peek,查看下Buffer流的第一个字符是什么属性,并且只给三个属性给出了不同的操作,分别是:

PEEKED_UNQUOTED_NAME  :对应 name:value

PEEKED_SINGLE_QUOTED_NAME : 对应 'name':value

PEEKED_DOUBLE_QUOTED_NAME: 对应 "name":value

如果你希望Gson严格按照规范执行,也就是说你对Gson语法的宽容度为false(调用JsonReader.setLenient(false))。那么你所传入的Json串,只要涉及到String类型的数据,都必须采用PEEKED_DOUBLE_QUOTED_NAME方式记录。

在调用完不同类型的getName方法之后,取得第一个name值:"name"。之后将通过这个name来取得在对象field映射表boundFields中的BoundField值:

 BoundField field = boundFields.get(name);
          if (field == null || !field.deserialized) {
            in.skipValue();
          } else {
            field.read(in, instance);
          }

如果你的Field已经标注为不被反序列化,或者Json串中的name在你的对象中根本没被标记,JsonReader将通过skipValue来过滤掉这些无用的信息。但是如果映射表中存在有这样的Field,Adapter将调用BoundField的read方法来讲读取出来的值,注入到之前生成的instance对象中去。BoundField的read方法在ReflectiveTypeAdapterFactory中,以匿名类的方式来实现,

private ReflectiveTypeAdapterFactory.BoundField createBoundField(
      final Gson context, final Field field, final String name,
      final TypeToken<?> fieldType, boolean serialize, boolean deserialize) {
    final boolean isPrimitive = Primitives.isPrimitive(fieldType.getRawType());
    // special casing primitives here saves ~5% on Android...
    return new ReflectiveTypeAdapterFactory.BoundField(name, serialize, deserialize) {
      final TypeAdapter<?> typeAdapter = getFieldAdapter(context, field, fieldType);
      ....
      @Override void read(JsonReader reader, Object value)
          throws IOException, IllegalAccessException {
        Object fieldValue = typeAdapter.read(reader);
        if (fieldValue != null || !isPrimitive) {
          field.set(value, fieldValue);
        }
      }
       ....
    };
  }

  private TypeAdapter<?> getFieldAdapter(Gson gson, Field field, TypeToken<?> fieldType) {
    JsonAdapter annotation = field.getAnnotation(JsonAdapter.class);
    if (annotation != null) {
      TypeAdapter<?> adapter = getTypeAdapter(constructorConstructor, gson, fieldType, annotation);
      if (adapter != null) return adapter;
    }
    return gson.getAdapter(fieldType);
  }

BoundField会通过自己的TypeAdapter来read一个Value,然后通过调用set方法,往生成对象中注入。而内部的TypeAdapter的生成是通过getFieldAdapter方法来生成的,具体的内容,请参照非墨的上篇文章:

<轻触开源(三)-Gson项目源码解析_贰 https://my.oschina.net/u/874727/blog/750473>

 

来源：oschina

链接：https://my.oschina.net/u/874727/blog/751417