json反序列化把空字符串转为null

一直使用jackson处理json,最近ui传来的数据{"funcCode":"Read_Coils","action":"pass","startaddr":"","endaddr":""},处理时需要把""转为null,
看了许久才明白其中的道理.
先写一个自定义反序列化类

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;

import java.io.IOException;

/**
 * Created by li on 2/17/17.
 */
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();
    }

}

然后在POJO类需要转换的字段加注解

private String action;
    @JsonDeserialize(using = CustomDeserializer.class)
    private String startaddr;
    @JsonDeserialize(using = CustomDeserializer.class)
    private String endaddr;

主要是引用
import com.fasterxml.jackson.databind.annotation.JsonDeserialize;

转换的代码

ObjectMapper mapper = new ObjectMapper();
        mapper.configure(DeserializationFeature.FAIL_ON_UNKNOWN_PROPERTIES, false);

ModbusTcp modbusTcp = mapper.readValue(json, ModbusTcp.class);

导入库
import com.fasterxml.jackson.databind.DeserializationFeature;
import com.fasterxml.jackson.databind.ObjectMapper;

注意是fastxml不是codehua,否则不起作用