java Jackson反序列化将空值映射为空字符串

Question

我试图为所有json Map<String, String>找到一个全局ObjectMapper配置（或任何其他定制），以便反序列化到没有null值的映射中（应该有一个空字符串）

例如

class MyClass {
  Map<String, String> dict;

  String str;

  public MyClass() {
  }

  public MyClass(Map<String, String> dict, String str) {
    this.dict = dict;
    this.str = str;
  }

  public Map<String, String> getDict() {
    return dict;
  }

  public String getStr() {
    return str;
  }
}
...

final Map<String, String> dict = new HashMap<>();
dict.put("k1", "v1");
dict.put("k2", null);
final MyClass myClass = new MyClass(dict);
final String valueAsString = objectMapper.writeValueAsString(myClass);
final MyClass deserialized = objectMapper.readValue(valueAsString, MyClass.class);
deserialized.getDict().get("k2"); // = "" (empty String)
deserialized.getStr(); // = null

---

我知道我可以通过用@JsonDeserialize(using = MyDeserializer.class)注释我的类来实现每个POJO 但我希望它对所有地图都有效

我还试图调整这个solution来实现反序列化，但它抛出NullPointerException

---

更新：“null到empty”转换只需要在映射上工作（使用字符串值）。（见上面更新的示例）

Answer 1

看，这对我有用：

import java.io.IOException;
import java.util.HashMap;
import java.util.Map;

import org.junit.Test;

import com.fasterxml.jackson.core.JsonGenerator;
import com.fasterxml.jackson.databind.JsonSerializer;
import com.fasterxml.jackson.databind.ObjectMapper;
import com.fasterxml.jackson.databind.SerializerProvider;
import com.fasterxml.jackson.databind.ser.DefaultSerializerProvider;

public class TestTest {

    @Test
    public void test_Map_null_EmptyString_() throws IOException {    
        ObjectMapper mapper = new ObjectMapper();
        DefaultSerializerProvider sp = new DefaultSerializerProvider.Impl();
        sp.setNullValueSerializer(new NullSerializer());
        mapper.setSerializerProvider(sp);

        final Map<String, String> dict = new HashMap<>();
        dict.put("k1", "v1");
        dict.put("k2", null);
        final MyClass myClass = new MyClass(dict);
        final String valueAsString = mapper.writeValueAsString(myClass);
        System.out.println(valueAsString);
        final MyClass deserialized = mapper.readValue(valueAsString, MyClass.class);
        System.out.println(deserialized.getDict().get("k2")); // = "" (empty String)
    }

    public class NullSerializer extends JsonSerializer<Object> {
        public void serialize(Object value, JsonGenerator jgen, SerializerProvider provider) throws IOException {
            jgen.writeString("");
        }
    }
}

class MyClass {
    Map<String, String> dict;

    public MyClass() {
    }

    public MyClass(Map<String, String> dict) {
        this.dict = dict;
    }

    public Map<String, String> getDict() {
        return dict;
    }
}

Answer 2

我最终使用了反序列化修改器技术，如下所示：

static class MapCustomDeserializerModifier extends BeanDeserializerModifier {

  @Override
  public JsonDeserializer<?> modifyMapDeserializer(DeserializationConfig config, MapType type, BeanDescription beanDesc, JsonDeserializer<?> deserializer {

    final JsonDeserializer<?> jsonDeserializer = super.modifyMapDeserializer(config, type, beanDesc, deserializer);
    if (type.getKeyType().isTypeOrSubTypeOf(String.class) && type.getContentType().isTypeOrSubTypeOf(String.class)) {
      return new MapCustomDeserializer();
    }

    return jsonDeserializer;
  }

}

static class MapCustomDeserializer extends JsonDeserializer<Map<String, String>> {

  @Override
  public Map<String, String> deserialize(JsonParser jsonParser, DeserializationContext ctxt) throws IOException, JsonProcessingException {

    final Map<String, String> map = jsonParser.readValueAs(Map.class);

    if (map != null) {
      map.replaceAll((key, value) -> Strings.nullToEmpty(value));
    }
    return map;
  }
}