GSON反序列化空值不起作用

Question

我正在尝试反序列化 json 字符串。我尝试了不同的 API，但没有找到解决方案。在这里，我试图反序列化下面的 json 并想要读取每个字段/元素的值。下面的例子 -

 String inputJson = "{"phone":null, "address":"underworld"}";
 LinkedTreeMap map = new Gson().fromJson(inputJson , LinkedTreeMap.class);

当我说 map.containsKey("phone) 时，它给出的是 false，这意味着 json 字符串中不存在“phone”元素。但是，这是不正确的，因为我们可以看到这个元素存在于输入的 json。

任何人都可以帮助我开发任何可以为密钥提供价值的 API。

使用 Spring Boot 可以接受空值的正确杰克逊反序列化配置是什么？目前我正在使用如下 -

pubic ObjectMapper objectMapper(Jckson3OjectMapperBuilder builder) {
      ObjectMapper  mapper = builder.build();
      mapper.configure(DeserializationFeature.FAIL_ON_UNKNOWN_PROPERTIES, false);
      return mapper;
   }

Answer 1

我已经编写了一些反序列化和序列化您的案例的测试，也许这会对您有所帮助

GSON 总是反序列化 null 对象，如果你想改变，写你的适配器

我使用 JDK1.8 和 com.google.code.gson:gson:2.8.6

package pl.jac.mija.gson;

import com.google.gson.Gson;
import com.google.gson.GsonBuilder;
import com.google.gson.TypeAdapter;
import com.google.gson.internal.LinkedTreeMap;
import com.google.gson.internal.bind.ObjectTypeAdapter;
import com.google.gson.reflect.TypeToken;
import com.google.gson.stream.JsonReader;
import com.google.gson.stream.JsonToken;
import com.google.gson.stream.JsonWriter;
import org.junit.Test;
import sun.reflect.generics.reflectiveObjects.NotImplementedException;

import java.io.IOException;
import java.util.ArrayList;
import java.util.HashMap;
import java.util.List;
import java.util.Map;
import java.util.stream.Collectors;

import static org.junit.Assert.assertEquals;
import static org.junit.Assert.assertTrue;

public class GsonWithNullTest {

  @Test
  public void deserializeWithNull() {
    //given
    String inputJson = "{\"phone\":null, \"address\":\"underworld\"}";
    //when
    LinkedTreeMap<String, Object> map = new Gson().fromJson(inputJson, LinkedTreeMap.class);
    boolean phone = map.containsKey("phone");
    //then
    assertEquals(true, phone);
  }

  @Test
  public void deserializeWithoutNull_V1_use_adapter() {
    //given
    String inputJson = "{\"phone\":null, \"address\":\"underworld\"}";
    //when

    Gson gson = new GsonBuilder().registerTypeAdapter(LinkedTreeMap.class, new MyAdapterSkipNull()).create();
    LinkedTreeMap<String, Object> map = gson.fromJson(inputJson, LinkedTreeMap.class);
    //then
    boolean isPhone = map.containsKey("phone");
    boolean isAddress = map.containsKey("address");
    assertEquals(false, isPhone);
    assertEquals(true, isAddress);
  }

  @Test
  public void deserializeWithoutNull_V2_use_post_filter_null() {
    //given
    String inputJson = "{\"phone\":null, \"address\":\"underworld\"}";
    //when

    Gson gson = new GsonBuilder().registerTypeAdapter(LinkedTreeMap.class, new MyAdapterSkipNull()).create();
    LinkedTreeMap<String, Object> map = new Gson().fromJson(inputJson, LinkedTreeMap.class);
    Map<String, Object> collect = map.entrySet().stream().filter(x -> x.getValue() != null).collect(Collectors.toMap(Map.Entry::getKey, Map.Entry::getValue));
    //then
    boolean isPhone = collect.containsKey("phone");
    boolean isAddress = collect.containsKey("address");
    assertEquals(false, isPhone);
    assertEquals(true, isAddress);
  }

  @Test
  public void serializeWithoutNull() {
    //given
    Map<String, Object> map = new HashMap<>();
    map.put("phone", null);
    map.put("address", "underworld");
    //when
    Gson gson = new GsonBuilder().serializeNulls().create();
    String json = gson.toJson(map);
    //then
    List<String> answert = new ArrayList<>();
    answert.add("{\"address\":\"underworld\",\"phone\":null}");
    answert.add("{\"phone\":null,\"address\":\"underworld\"}");
    assertTrue(answert.contains(json));
  }

  @Test
  public void serializeWithNull() {
    //given
    Map<String, Object> map = new HashMap<>();
    map.put("phone", null);
    map.put("address", "underworld");
    //when
    Gson gson = new Gson();
    String json = gson.toJson(map);
    //then
    assertEquals("{\"address\":\"underworld\"}", json);
  }

}

class MyAdapterSkipNull extends TypeAdapter<LinkedTreeMap<String, Object>> {


  @Override
  public void write(JsonWriter out, LinkedTreeMap<String, Object> value) throws IOException {
    throw new NotImplementedException();
  }

  @Override
  public LinkedTreeMap<String, Object> read(JsonReader in) throws IOException {
    JsonToken peek = in.peek();
    if (peek == JsonToken.NULL) {
      in.nextNull();
      return null;
    }
    TypeAdapter<Object> objectTypeAdapter = ObjectTypeAdapter.FACTORY.create(new Gson(), TypeToken.get(Object.class));
    LinkedTreeMap<String, Object> map = new LinkedTreeMap<>();
    in.beginObject();
    while (in.hasNext()) {
      String key = in.nextName();
      JsonToken peek1 = in.peek();
      if (JsonToken.NULL.equals(peek1)) {
        in.skipValue(); //skip NULL
      } else {
        Object read = objectTypeAdapter.read(in);
        map.put(key, read);
      }
    }
    in.endObject();
    return map;
  }
}

Answer 2

我通过将 Spring Boot 端点方法参数签名从 Object 更改为 String 解决了这个问题。早些时候它是对象类型，因为它只是忽略了字符串中具有空值的键。在控制器中，我正在检查密钥是否存在，如下所示 -

public ResponseEntity<Object> validate(@RequestBody String requestBody) {
Object requestObject = new ObjectMapper().readValue(requestBody, Object.class);
LinkedTreeMap requestObjectMap = new Gson().fromJson(requestObject.toString(), LinkedTreeMap.class);
List<FieldError> fieldErrors = new ArrayList<>();
final boolean isKeyExists = requestObjectMap.containsKey("keyname");
final Object fieldValue = requestObjectMap.get(optionalField);
if (isKeyExists && (Objects.isNull(fieldValue)) {
  System.out.println("Key exists but its value is null in the input Json request");
}

// other logic

}