有没有办法忽略 Gson 中的 JsonSyntaxException

Question

我有一个如下所示的 json：

[
{
_id: "54b8f62fa08c286b08449b8f",
loc: [
   36.860983,
   31.0567
]
},
{
_id: "54b8f6aea08c286b08449b93",
loc: {
coordinates: [ ]
}
}
]

如您所见，loc 对象有时是 json 对象，有时是双精度数组。在不编写自定义反序列化器的情况下，有没有办法避免JsonSyntaxException 并将 loc 对象设置为 null 当它是 json 对象而不是双数组时。

Answer 1

没有任何简单的方法（我的意思是 Gson 的属性/方法调用）在 json 值处对特定字段进行自定义序列化/反序列化。

可以查看com.google.gson.internal.bind.ReflectiveTypeAdapterFactory的源码，调试其内部类Adapter的read方法。 （这就是你的JsonSyntaxException 出现的地方）

您可以阅读Custom serialization for JUST specific fields 并跟踪其链接。它可能会在 Gson 的未来版本中实现。 （在最新版本 2.2.4 中不可用）

我会为此编写一些代码。也许这不是您想要的，但它可能对其他人有所帮助。）

方案一（与第二种方案相比，该方案的代码更少，但第二种方案的性能要好得多）：

public class SubClass extends BaseClass {
    private double[] loc;
}

public class BaseClass {
    @SerializedName("_id")
    private String id;
}

public class CustomTypeAdapter extends TypeAdapter<BaseClass> {
    private Gson gson;
    public CustomTypeAdapter() {
        this.gson = new Gson();
    }

    @Override
    public void write(JsonWriter out, BaseClass value)
            throws IOException {
        throw new RuntimeException("Not implemented for this question!");
    }

    @Override
    public BaseClass read(JsonReader in) throws IOException {
        BaseClass instance;
        try {
            instance = gson.fromJson(in, SubClass.class);
        } catch (Exception e) {
            e.printStackTrace();
            instance = gson.fromJson(in, BaseClass.class);
        }
        return instance;
    }
}

测试：

private void test() {
    String json = "[{_id:\"54b8f62fa08c286b08449b8f\",loc:[36.860983,31.0567]},{_id:\"54b8f6aea08c286b08449b93\",loc:{coordinates:[]}}]";

    Type collectionType = new TypeToken<List<BaseClass>>(){}.getType();
    Gson gson = new GsonBuilder().registerTypeAdapter(BaseClass.class, new CustomTypeAdapter()).create();
    List<BaseClass> list = gson.fromJson(json, collectionType);

    for(BaseClass item : list) {
        if(item instanceof SubClass) {
            System.out.println("item has loc value");
            SubClass subClassInstance = (SubClass)item;
        } else {
            System.out.println("item has no loc value");
            BaseClass baseClassInstance = item;
        }
    }
}

解决方案 2（这是 Gson 开发人员的建议之一。See original post.）：

将下面的类复制到您的项目中。它将成为您自定义 TypeAdapterFactorys 的基类。

public abstract class CustomizedTypeAdapterFactory<C>
        implements TypeAdapterFactory {
    private final Class<C> customizedClass;

    public CustomizedTypeAdapterFactory(Class<C> customizedClass) {
        this.customizedClass = customizedClass;
    }

    @SuppressWarnings("unchecked") // we use a runtime check to guarantee that 'C' and 'T' are equal
    public final <T> TypeAdapter<T> create(Gson gson, TypeToken<T> type) {
        return type.getRawType() == customizedClass
                ? (TypeAdapter<T>) customizeMyClassAdapter(gson, (TypeToken<C>) type)
                : null;
    }

    private TypeAdapter<C> customizeMyClassAdapter(Gson gson, TypeToken<C> type) {
        final TypeAdapter<C> delegate = gson.getDelegateAdapter(this, type);
        final TypeAdapter<JsonElement> elementAdapter = gson.getAdapter(JsonElement.class);
        return new TypeAdapter<C>() {
            @Override public void write(JsonWriter out, C value) throws IOException {
                JsonElement tree = delegate.toJsonTree(value);
                beforeWrite(value, tree);
                elementAdapter.write(out, tree);
            }
            @Override public C read(JsonReader in) throws IOException {
                JsonElement tree = elementAdapter.read(in);
                afterRead(tree);
                return delegate.fromJsonTree(tree);
            }
        };
    }

    /**
     * Override this to muck with {@code toSerialize} before it is written to
     * the outgoing JSON stream.
     */
    protected void beforeWrite(C source, JsonElement toSerialize) {
    }

    /**
     * Override this to muck with {@code deserialized} before it parsed into
     * the application type.
     */
    protected void afterRead(JsonElement deserialized) {
    }
}

编写您的 POJO 和您的自定义 CustomizedTypeAdapterFactory。覆盖afterRead 方法并按照您的问题处理双数组：

public class MyClass {
    @SerializedName("_id")
    private String id;
    private double[] loc;
    // getters/setters
}

private class MyClassTypeAdapterFactory extends CustomizedTypeAdapterFactory<MyClass> {
    private MyClassTypeAdapterFactory() {
        super(MyClass.class);
    }

    @Override protected void afterRead(JsonElement deserialized) {
        try {
            JsonArray jsonArray = deserialized.getAsJsonObject().get("loc").getAsJsonArray();

            System.out.println("loc is not a double array, its ignored!");
        } catch (Exception e) {
            deserialized.getAsJsonObject().remove("loc");
        }
    }
}

测试：

private void test() {
    String json = "[{_id:\"54b8f62fa08c286b08449b8f\",loc:[36.860983,31.0567]},{_id:\"54b8f6aea08c286b08449b93\",loc:{coordinates:[]}}]";

    Gson gson = new GsonBuilder()
            .registerTypeAdapterFactory(new MyClassTypeAdapterFactory())
            .create();

    Type collectionType = new TypeToken<List<MyClass>>(){}.getType();
    List<MyClass> list = gson.fromJson(json, collectionType);

    for(MyClass item : list) {
        if(item.getLoc() != null) {
            System.out.println("item has loc value");
        } else {
            System.out.println("item has no loc value");
        }
    }
}

Answer 2

我就是这样做的。它更短，但我认为 @DevrimTuncer 的答案是最好的。

//This is just Double array to use as location object
public class Location extends ArrayList<Double> {
    public Double getLatidute() {
        if (this.size() > 0) {
            return this.get(0);
        } else {
            return (double) 0;
        }
    }

    public Double getLongitude() {
        if (this.size() > 1) {
            return this.get(1);
        } else {
            return (double) 0;
        }
    }

    public static class LocationDeserializer implements JsonDeserializer<Location> {
        @Override
        public Location deserialize(JsonElement json, Type typeOfT, JsonDeserializationContext context) throws JsonParseException {
            try {
                JsonArray array = json.getAsJsonArray();
                Location location = new Location();
                for (int i = 0; i < array.size(); i++) {
                    location.add(array.get(i).getAsDouble());
                }

                return location;
            } catch (Exception e) {
                return null;
            }
        }
    }
}