如何将java.lang.string的空白JSON字符串值反序列化为null？

Question

{
  "eventId" : 1,
  "title" : "sample event",
  "location" : "" 
}

public class EventBean {

    public Long eventId;
    public String title;
    public String location;

}

ObjectMapper mapper = new ObjectMapper();
mapper.disable(DeserializationFeature.FAIL_ON_UNKNOWN_PROPERTIES);
mapper.enable(DeserializationFeature.ACCEPT_EMPTY_STRING_AS_NULL_OBJECT);

try {
    File file = new   File(JsonTest.class.getClassLoader().getResource("event.txt").getFile());

    JsonNode root = mapper.readTree(file);
    // find out the applicationId

    EventBean e = mapper.treeToValue(root, EventBean.class);
    System.out.println("It is " + e.location);
}

Answer 1

对于其他对象，Jackson会给出null，但是对于String，它会给出空字符串。

但是您可以使用自定义的JSONDeserializer来完成此操作：

class CustomDeserializer extends JsonDeserializer<String> {

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

}

在课堂上，您必须将其用于位置字段：

class EventBean {
    public Long eventId;
    public String title;

    @JsonDeserialize(using = CustomDeserializer.class)
    public String location;
}