java - JsonDeserializer不适用于该类，而仅适用于该类的单个元素

Question

我创建了一个新的Deserializer，以便能够将空字符串写为null

public class CustomDeserializer extends JsonDeserializer<String> {
    @Override
    public String deserialize(JsonParser jsonParser, DeserializationContext context) throws IOException  {
        JsonNode node = jsonParser.readValueAsTree();
        if (node.asText().isEmpty()) {
            return null;
        }
        return node.toString();
    }
}

尝试在每个User字段上创建单个注释，“自定义”起作用，但是通过在整个类上插入注释，我不再能够打印Json消息

@JsonDeserialize(using = CustomDeserializer.class)
public class User {
    private String firstName;
    private String lastName;
    private String age;
    private String address; }

CustomExceptionHandler引发此错误: 类MethodArgumentNotValidException
这是我的Kafka Consumer，是我唯一输入验证注释的人，但即使删除它也会给我同样的错误

public class KafkaConsumer {

    @Autowired
    private UserRepository userRepository;

    @KafkaListener(topics = "${spring.kafka.topic.name}")
    public void listen(@Validated User user) {

        User  user = new User(user);
        UserRepository.save(user.getName(), user);
    }
}

对象映射器

public ObjectMapper getObjectMapper() {
    ObjectMapper mapper = new ObjectMapper();
    mapper.registerModule(new JavaTimeModule());
    mapper.configure(SerializationFeature.WRITE_DATES_AS_TIMESTAMPS, false);
    mapper.configure(DeserializationFeature.FAIL_ON_UNKNOWN_PROPERTIES, false);
    mapper.setSerializationInclusion(Include.NON_NULL);
    return mapper;
}

是否有可能使它适用于整个类(class)？

Answer 1

如果您希望将表示整个整个对象的空String视为null，则可以启用默认情况下禁用的ACCEPT_EMPTY_STRING_AS_NULL_OBJECT Jackson deserialization feature。
您可以在配置ObjectMapper时包括它:

public ObjectMapper getObjectMapper() {
    ObjectMapper mapper = new ObjectMapper();
    mapper.registerModule(new JavaTimeModule());
    mapper.configure(SerializationFeature.WRITE_DATES_AS_TIMESTAMPS, false);
    mapper.configure(DeserializationFeature.FAIL_ON_UNKNOWN_PROPERTIES, false);
    mapper.setSerializationInclusion(Include.NON_NULL);
    // Enable ACCEPT_EMPTY_STRING_AS_NULL_OBJECT deserialization feature
    mapper.enable(DeserializationFeature.ACCEPT_EMPTY_STRING_AS_NULL_OBJECT);
    return mapper;
}

如上所述，当您要将代表整个对象的整个 的空String视为null时，这很有用；但是，它不适用于String类型的单个属性:在以后的情况下，您可以安全地使用自定义反序列化器，因此，解决方案实际上是两种方法的组合，使用ACCEPT_EMPTY_STRING_AS_NULL_OBJECT反序列化功能来处理整个对象，和用于处理各个String属性的自定义解串器。
请参阅this和this other相关的SO问题。
您也可以改进自定义的User反序列化器。请考虑一下示例(为了清楚起见，我将名称重构为UserDeserializer):

import java.io.IOException;
import java.lang.annotation.Annotation;
import java.lang.reflect.Field;
import java.time.LocalDate;
import java.time.format.DateTimeFormatter;
import java.util.HashMap;
import java.util.Iterator;
import java.util.Map;

import com.fasterxml.jackson.annotation.JsonAlias;
import com.fasterxml.jackson.core.JsonParser;
import com.fasterxml.jackson.core.JsonProcessingException;
import com.fasterxml.jackson.databind.DeserializationContext;
import com.fasterxml.jackson.databind.JsonDeserializer;
import com.fasterxml.jackson.databind.JsonNode;

public class UserDeserializer extends JsonDeserializer<User> {

  @Override
  public User deserialize(JsonParser jsonParser, DeserializationContext deserializationContext) throws IOException, JsonProcessingException {
    JsonNode node = jsonParser.readValueAsTree();
    Iterator<String> fieldNames = node.fieldNames();
    // Process Jackson annotations looking for aliases
    Map<String, String> fieldAliases = this.getAliases();
    User user = new User();
    boolean anyNonNull = false;
    // Iterate over every field. The deserialization process assume simple properties
    while(fieldNames.hasNext()) {
      String fieldName = fieldNames.next();
      JsonNode fieldValue = node.get(fieldName);
      String fieldValueTextRepresentation = fieldValue.asText();
      if (fieldValueTextRepresentation != null && !fieldValueTextRepresentation.trim().isEmpty()) {
        // Check if the field is aliased
        String actualFieldName = fieldAliases.get(fieldName);
        if (actualFieldName == null) {
          actualFieldName = fieldName;
        }

        this.setFieldValue(user, actualFieldName, fieldValueTextRepresentation);
        anyNonNull = true;
      }
    }

    return anyNonNull ? user : null;
  }

  // Set field value via Reflection
  private void setFieldValue(User user, String fieldName, String fieldValueTextRepresentation) {
    try {
      Field field = User.class.getDeclaredField(fieldName);
      Object fieldValue = null;
      Class clazz = field.getType();
      // Handle each class type: probably this code can be improved, but it is extensible and adaptable,
      // you can include as many cases as you need.
      if (clazz.isAssignableFrom(String.class)) {
        fieldValue = fieldValueTextRepresentation;
      } else if (clazz.isAssignableFrom(LocalDate.class)) {
        // Adjust the date pattern as required
        // For example, if you are receiving the information
        // like this: year-month-day, as in the provided example,
        // you can use the following pattern
        fieldValue = LocalDate.parse(fieldValueTextRepresentation, DateTimeFormatter.ofPattern("yyyy-MM-dd"));
      } else if (clazz.isAssignableFrom(Integer.class)) {
        fieldValue = Integer.parseInt(fieldValueTextRepresentation);
      }
      field.setAccessible(true);
      field.set(user, fieldValue);
    } catch (Exception e) {
      // Handle the problem as appropriate
      e.printStackTrace();
    }
  }
  
  /* Look for Jackson aliases */
  private Map<String, String> getAliases() {
    Map<String, String> fieldAliases = new HashMap<>();

    Field[] fields = User.class.getDeclaredFields();
    for (Field field: fields) {
      Annotation annotation = field.getAnnotation(JsonAlias.class);
      if (annotation != null) {
        String fieldName = field.getName();
        JsonAlias jsonAliasAnnotation = (JsonAlias) annotation;
        String[] aliases = jsonAliasAnnotation.value();
        for (String alias: aliases) {
          fieldAliases.put(alias, fieldName);
        }
      }
    }

    return fieldAliases;
  }
}

有了此序列化程序后，给出了一个类似于User的类:

import java.time.LocalDate;

import com.fasterxml.jackson.annotation.JsonAlias;
import com.fasterxml.jackson.databind.annotation.JsonDeserialize;

@JsonDeserialize(using = UserDeserializer.class)
public class User {
  private String firstName;
  private String lastName;
  private Integer age;
  private String address;
  @JsonAlias("dateofbirth")
  private LocalDate dateOfBirth;

  // Setters and getters omitted for brevity

  @Override
  public boolean equals(Object o) {
    if (this == o) return true;
    if (o == null || getClass() != o.getClass()) return false;

    User user = (User) o;

    if (firstName != null ? !firstName.equals(user.firstName) : user.firstName != null) return false;
    if (lastName != null ? !lastName.equals(user.lastName) : user.lastName != null) return false;
    if (age != null ? !age.equals(user.age) : user.age != null) return false;
    if (address != null ? !address.equals(user.address) : user.address != null) return false;
    return dateOfBirth != null ? dateOfBirth.equals(user.dateOfBirth) : user.dateOfBirth == null;
  }

  @Override
  public int hashCode() {
    int result = firstName != null ? firstName.hashCode() : 0;
    result = 31 * result + (lastName != null ? lastName.hashCode() : 0);
    result = 31 * result + (age != null ? age.hashCode() : 0);
    result = 31 * result + (address != null ? address.hashCode() : 0);
    result = 31 * result + (dateOfBirth != null ? dateOfBirth.hashCode() : 0);
    return result;
  }

以及以下JSON(我仅为了测试别名而更改为dateofbirth字段的名称):

{"firstName":"John","age":40,"dateofbirth":"1978-03-16"}

您应该获得适当的结果，请考虑以下测试:

  public static void main(String... args) throws JsonProcessingException {
    User user = new User();
    user.setFirstName("John");
    user.setAge(40);
    user.setDateOfBirth(LocalDate.of(1978, Month.MARCH, 16));

    ObjectMapper mapper = new ObjectMapper();
    mapper.setSerializationInclusion(JsonInclude.Include.NON_NULL);

    String json = "{\"firstName\":\"John\",\"age\":40,\"dateofbirth\":\"1978-03-16\"}";

    User reconstructed = mapper.readValue(json, User.class);

    System.out.println(user.equals(reconstructed));
  }

最后，请注意，为了允许您的@KafkaListener处理null值，您必须将@Payload批注与required = false结合使用，例如:

public class KafkaConsumer {

    @Autowired
    private UserRepository userRepository;

    @KafkaListener(topics = "${spring.kafka.topic.name}")
    public void listen(@Payload(required = false) User user) {
        // Handle null value
        if (user == null) {
          // Consider logging the event
          // logger.debug("Null message received");
          System.out.println("Null message received");
          return;
        }

        // Continue as usual
        User  user = new User(user);
        UserRepository.save(user.getName(), user);
    }
}

请参阅相关的Spring Kafka documentation和此Github issue以及相关的commit。 This SO question也可能是相关的。