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一个容错的Gson新世界

1. 闯入背景:

公司项目中使用Gson框架对服务器传过来的Json数据进行解析,而服务器后台数据很大程度上是通过运营后台人员配置。由于各种原因运营可能将某一字段类型配置错误,比如集合类型配置成字符串类型。虽然业务层会进行异常的捕获,但是仅因为一个字段的错误,导致整个Json数据失效,因小失大,甚至可能会造成重大损失,比如直播间礼物墙,因为一个礼物的某一个字段的错误,导致整个礼物墙展示为空,在线上环境这个算是重大事故了。于是,一个对基本类型容错的Gson改造库的需求油然而生,对于错误的数据以默认值填充。

干货地址:类型容错的Gson

2. Gson官方库地址:

Github地址

3. 前提说明

a. 当前分析的Gson版本号为2.8.1。
b. Gson的处理过程主要分为两个流向,一个是序列化,将javabean对象转化为json字符串;另一个是反序列化,将json字符串映射成javabean对象。
c. 这两个流向处理前都有一个共同的操作,从传入的java实例对象或者字节码对象中获取 TypeAdapter,对于序列化就通过Jsonwriter进行写,对于反序列化就通过JsonReader进行读,所以此篇只分析Gson读的过程,写处理操作流程一样。

4. Gson 关键列的梳理

  • Gson 开发者直接使用的类,只对输入和输出负责。
  • TypeToken 封装“操作类”(Gson.fromJson(json,People.class、Gson.toJson(new People)) 两处的People都是操作类)的类型。
  • TypeAdapter 直接操作序列化与反序列化的过程,所以该抽象类中存在read()和write方法。
  • TypeAdapterFactory 用于生产TypeAdapter的工厂类。
  • GsonReader和GsonWriter是Gson处理内容的包装流,核心的操作有:
    • peek() 流中下一个需要处理的内容
    • nextName() 读取json的key
    • nextString() 读取一个String类型的value
    • nextInt() 读取一个String类型的value
    • nextBoolean() 读取一个Boolean类型的value
    • ...

5. 源码分析。

从Gson.from(json, People.class) 突入

  fromJson(json,Peolple.class)的调用链
  
  public <T> T fromJson(String json, Class<T> classOfT) throws JsonSyntaxException {
    Object object = fromJson(json, (Type) classOfT);
    return Primitives.wrap(classOfT).cast(object);
  }
  
  public <T> 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> 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> 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;
    } ...
  }
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上面是从fromJson(String json, Class classOfT)切入,亦或者是从fromJson(JsonElement json, Class classOfT)也好,最终都是由 fromJson(JsonReader reader, Type typeOfT)处理。

整个Json的解析过程分三步过程:

  • TypeToken对象的获取
  • 根据TypeToken获取TypeAdapter对象
  • 由TypeAdapter对象解析json字符串

根据以上的三步,我们逐一突破


我们先从简单的入手,请记住我们的例子:

gson.fromJson("hello gson",String.class)

1. TypeToken的获取

 public static TypeToken<?> get(Type type) {
    return new TypeToken<Object>(type);
  }
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没什么好瞅的~ 看new吧!

 TypeToken(Type type) {
    this.type = $Gson$Types.canonicalize($Gson$Preconditions.checkNotNull(type));
    this.rawType = (Class<? super T>) $Gson$Types.getRawType(this.type);
    this.hashCode = this.type.hashCode();
  }
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采用契约式对传入的type判空处理,然后获取type的(type、rawType和hashcode),分别看看type和rawtype的获取流程

1. type的获取(type的华丽包装)
public static Type canonicalize(Type type) {
    if (type instanceof Class) {
      Class<?> c = (Class<?>) type;
      return c.isArray() ? new GenericArrayTypeImpl(canonicalize(c.getComponentType())) : c;

    } else if (type instanceof ParameterizedType) {
      ParameterizedType p = (ParameterizedType) type;
      return new ParameterizedTypeImpl(p.getOwnerType(),
          p.getRawType(), p.getActualTypeArguments());

    } else if (type instanceof GenericArrayType) {
      GenericArrayType g = (GenericArrayType) type;
      return new GenericArrayTypeImpl(g.getGenericComponentType());

    } else if (type instanceof WildcardType) {
      WildcardType w = (WildcardType) type;
      return new WildcardTypeImpl(w.getUpperBounds(), w.getLowerBounds());

    } else {
      // type is either serializable as-is or unsupported
      return type;
    }
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进入条件的筛选,第一个if还是好理解,后面的是什么鬼? 不用着急,待我给施主梳理,之前Gson.from(json, People.class)的调用链中有一个fromJson(Reader json, Type typeOfT) ,用户使用时的切入点如果是它就可能是筛选情况的其他条件,此返回的type相对于对传入的java类型进行的类型的重新包装。

2. rawType的获取(type的简单粗暴说明)
public static Class<?> getRawType(Type type) {
    if (type instanceof Class<?>) {
      // type is a normal class.
      return (Class<?>) type;

    } else if (type instanceof ParameterizedType) {
      ParameterizedType parameterizedType = (ParameterizedType) type;

      // I'm not exactly sure why getRawType() returns Type instead of Class.
      // Neal isn't either but suspects some pathological case related
      // to nested classes exists.
      Type rawType = parameterizedType.getRawType();
      checkArgument(rawType instanceof Class);
      return (Class<?>) rawType;

    } else if (type instanceof GenericArrayType) {
      Type componentType = ((GenericArrayType)type).getGenericComponentType();
      return Array.newInstance(getRawType(componentType), 0).getClass();

    } else if (type instanceof TypeVariable) {
      // we could use the variable's bounds, but that won't work if there are multiple.
      // having a raw type that's more general than necessary is okay
      return Object.class;

    } else if (type instanceof WildcardType) {
      return getRawType(((WildcardType) type).getUpperBounds()[0]);

    } else {
      String className = type == null ? "null" : type.getClass().getName();
      throw new IllegalArgumentException("Expected a Class, ParameterizedType, or "
          + "GenericArrayType, but <" + type + "> is of type " + className);
    }
  }
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两处对比的看,其实type和rawtype很相似,type通过类来包装说明,而rawtype脱去华丽的衣服。type为GenericArrayType的,把衣服一脱,赤身裸体的一看,擦,原来是个array数组,这就是rawtype。

2. TypeAdapter的获取。

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 cannot handle " + type);
    }
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如果缓存中没有该Type对应TypeAdapter,就创建TypeAdapter。前面提过TypeAdapter是由TypeAdapterFactory创建的,所以有代码:

 for (TypeAdapterFactory factory : factories) {
        TypeAdapter<T> candidate = factory.create(this, type);
        if (candidate != null) {
          call.setDelegate(candidate);
          typeTokenCache.put(type, candidate);
          return candidate;
        }
      }
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遍历所有的TypeAdapterFactory,如果该工厂能创建该Type的TypeAdapter就返回该TypeAdapter对象。

那么重点来了,factories这么多的TypeAdapterFactory是怎么来了的?

在我们new Gson的时候,就往factories中塞入了不同类型的TypeAdapterFactory,包括StringTypeAdapterFactory等等,代码如下:

    public Gson(xxx)
    {
        ...
        factories.add(TypeAdapters.STRING_FACTORY);
        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);
        ...
    }
   
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在遍历factories过程中通过create(this,type)方法来生成TypeAdapter。

我们就以第一个STRING_FACTORY为例先进行说明。
public static final TypeAdapterFactory STRING_FACTORY = newFactory(String.class, STRING);
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接着往下看

  public static <TT> TypeAdapterFactory newFactory(
      final Class<TT> type, final TypeAdapter<TT> typeAdapter) {
    return new TypeAdapterFactory() {
      @SuppressWarnings("unchecked") // we use a runtime check to make sure the 'T's equal
      @Override public <T> TypeAdapter<T> create(Gson gson, TypeToken<T> typeToken) {
        return typeToken.getRawType() == type ? (TypeAdapter<T>) typeAdapter : null;
      }
      @Override public String toString() {
        return "Factory[type=" + type.getName() + ",adapter=" + typeAdapter + "]";
      }
    };
  }
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STRING_FACTORY = newFactory(String.class, STRING)的时候,STRING就是处理String类型的TypeAdapter,STRING_FACTORY中的create方法就是判断需要处理的类型是不是String类型的,如果是就返回STRING,否则返回null,即该类型不用STRING来处理。

总的来说,在创建Gson的实例对象时,创建TypeAdapterFactory的集合。每种TypeAdapterFactory实例包含能处理的Type类型和Type类型的TypeAdapter,不能处理的Type类型返回的TypeAdapter为null,所以在遍历factories过程中有:

for (TypeAdapterFactory factory : factories) {
        TypeAdapter<T> candidate = factory.create(this, type);
        if (candidate != null) {
            ...
          return candidate;
        }
      }
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3. 由TypeAdapter对象解析json字符串

我们回到最初的代码:

    TypeToken<T> typeToken = (TypeToken<T>)TypeToken.get(typeOfT);
TypeAdapter<T> typeAdapter = getAdapter(typeToken);
T object = typeAdapter.read(reader);
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STRING就是处理String类型的TypeAdapter,然后我们看它的read()方法。

public static final TypeAdapter<String> STRING = new TypeAdapter<String>() {
    @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();
    }
   ...
  };
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到这里位置,我们就将gson.fromJson("hello gson",String.class)的String类型“hello gson”返回。


刚刚是只是牛刀小试,我们的主材料来了,看看有多丰盛...

Gson.from("{ "name": "zhangsan", "age": 15, "grade": [ 95, 98 ] }", Student.class)

我们重新走刚刚的流程,看看怎么处理的

Step one : 获取TypeToken

这一步没有什么与众不同

Step Two: TypeAdapter的获取。

factories中包含了很多基本类型的TypeAdapterFactory,同时也包含用户自定义的类型Factory,看源码:

        // 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));
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此处我们能匹配上的是ReflectiveTypeAdapterFactory,然后我们看它的create()方法,关键的地方到了!!!

  @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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a. constructorConstructor 获取Student类的构造器
b. getBoundFields()通过反射获取Student每一个字段的的TypeAdapter,并且包装到Map<String, BoundField>中,后面会讲解getBoundFields()的方法。

Step Three 通过TypeAdapter的read()输出对象

@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;
    }
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到了这一步就似乎海阔天空了,通过传入的构造器创建Student类的实例,在JsonReader进行处理,in.beginObject()相当于跳过“{”,in.endObject()相当于跳过“}”,其中通过in.hasNext()判断是否处理完成。 在in.nextName()读取json字符串中的key值,然后在boundFields根据key获取对应的BoundField ,最后调用BoundField.read(in,instance)去处理细节,即每个字段的映射,我们看一下内部的细节:

new ReflectiveTypeAdapterFactory.BoundField(name, serialize, deserialize) {
      ...
      @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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当Filed都处理完成后,instance实例的每一个需要处理的字段都赋值成功,最终将这个对象return出去。


细节说明:

a. getBoundFields()

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;
        }
        field.setAccessible(true);
        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;
  }
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遍历Student类的每一个字段,遍历过程中做了两件事情:

  • a. 该字段能否被序列化和反序列化,如果都不行就没有必要处理该字段,主要通过注解和排除器(Excluder)进行判断。
  • b. 对字段进行BoundField的包装。

b. JsonReader.doPeek()

int doPeek() throws IOException {
    int peekStack = stack[stackSize - 1];
    if (peekStack == JsonScope.EMPTY_ARRAY) {
      stack[stackSize - 1] = JsonScope.NONEMPTY_ARRAY;
    } else if (peekStack == JsonScope.NONEMPTY_ARRAY) {
      // Look for a comma before the next element.
      int c = nextNonWhitespace(true);
      switch (c) {
      case ']':
        return peeked = PEEKED_END_ARRAY;
      case ';':
        checkLenient(); // fall-through
      case ',':
        break;
      default:
        throw syntaxError("Unterminated array");
      }
    } else if (peekStack == JsonScope.EMPTY_OBJECT || peekStack == JsonScope.NONEMPTY_OBJECT) {
      stack[stackSize - 1] = JsonScope.DANGLING_NAME;
      // Look for a comma before the next element.
      if (peekStack == JsonScope.NONEMPTY_OBJECT) {
        int c = nextNonWhitespace(true);
        switch (c) {
        case '}':
          return peeked = PEEKED_END_OBJECT;
        case ';':
          checkLenient(); // fall-through
        case ',':
          break;
        default:
          throw syntaxError("Unterminated object");
        }
      }
      int c = nextNonWhitespace(true);
      switch (c) {
      case '"':
        return peeked = PEEKED_DOUBLE_QUOTED_NAME;
      case '\'':
        checkLenient();
        return peeked = PEEKED_SINGLE_QUOTED_NAME;
      case '}':
        if (peekStack != JsonScope.NONEMPTY_OBJECT) {
          return peeked = PEEKED_END_OBJECT;
        } else {
          throw syntaxError("Expected name");
        }
      default:
        checkLenient();
        pos--; // Don't consume the first character in an unquoted string.
        if (isLiteral((char) c)) {
          return peeked = PEEKED_UNQUOTED_NAME;
        } else {
          throw syntaxError("Expected name");
        }
      }
    } else if (peekStack == JsonScope.DANGLING_NAME) {
      stack[stackSize - 1] = JsonScope.NONEMPTY_OBJECT;
      // Look for a colon before the value.
      int c = nextNonWhitespace(true);
      switch (c) {
      case ':':
        break;
      case '=':
        checkLenient();
        if ((pos < limit || fillBuffer(1)) && buffer[pos] == '>') {
          pos++;
        }
        break;
      default:
        throw syntaxError("Expected ':'");
      }
    } else if (peekStack == JsonScope.EMPTY_DOCUMENT) {
      if (lenient) {
        consumeNonExecutePrefix();
      }
      stack[stackSize - 1] = JsonScope.NONEMPTY_DOCUMENT;
    } else if (peekStack == JsonScope.NONEMPTY_DOCUMENT) {
      int c = nextNonWhitespace(false);
      if (c == -1) {
        return peeked = PEEKED_EOF;
      } else {
        checkLenient();
        pos--;
      }
    } else if (peekStack == JsonScope.CLOSED) {
      throw new IllegalStateException("JsonReader is closed");
    }

    int c = nextNonWhitespace(true);
    switch (c) {
    case ']':
      if (peekStack == JsonScope.EMPTY_ARRAY) {
        return peeked = PEEKED_END_ARRAY;
      }
      // fall-through to handle ",]"
    case ';':
    case ',':
      // In lenient mode, a 0-length literal in an array means 'null'.
      if (peekStack == JsonScope.EMPTY_ARRAY || peekStack == JsonScope.NONEMPTY_ARRAY) {
        checkLenient();
        pos--;
        return peeked = PEEKED_NULL;
      } else {
        throw syntaxError("Unexpected value");
      }
    case '\'':
      checkLenient();
      return peeked = PEEKED_SINGLE_QUOTED;
    case '"':
      return peeked = PEEKED_DOUBLE_QUOTED;
    case '[':
      return peeked = PEEKED_BEGIN_ARRAY;
    case '{':
      return peeked = PEEKED_BEGIN_OBJECT;
    default:
      pos--; // Don't consume the first character in a literal value.
    }

    int result = peekKeyword();
    if (result != PEEKED_NONE) {
      return result;
    }

    result = peekNumber();
    if (result != PEEKED_NONE) {
      return result;
    }

    if (!isLiteral(buffer[pos])) {
      throw syntaxError("Expected value");
    }

    checkLenient();
    return peeked = PEEKED_UNQUOTED;
  }
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该操作逻辑处理较强,主要工作分为3点:

  • json的格式校验,格式不合法抛出异常
  • 根据当前的操作,决定下一步的操作方式
  • 流中下一部分的内容类型
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