java - jackson 忽略了特定的属性(property)，但能够检查它是否可用

Question

有没有办法在反序列化时跳过某些属性，但同时知道它们是否存在？

{
    "id": 123,
    "name": "My Name",
    "picture": {
        // a lot of properties that's not important for me
    }
}

@JsonIgnoreProperties(ignoreUnknown=true)
@JsonInclude(JsonInclude.Include.NON_NULL)
public class User {
    private int id;
}

因此，我 ignoreUnknown 是我想要的默认行为，因为我不想要 name 字段以及所有其他可能存在的字段。 picture 字段的值也不重要。我只是想知道 picture 属性是否可用。我怎样才能做到这一点？

Answer 1

您可以添加一个 boolean 属性和自定义反序列化器，它只读取给定值并返回 true。仅当有效负载中存在属性时，Jackson 才会调用自定义反序列化器。

import com.fasterxml.jackson.annotation.JsonIgnoreProperties;
import com.fasterxml.jackson.annotation.JsonInclude;
import com.fasterxml.jackson.annotation.JsonProperty;
import com.fasterxml.jackson.core.JsonParser;
import com.fasterxml.jackson.databind.DeserializationContext;
import com.fasterxml.jackson.databind.JsonDeserializer;
import com.fasterxml.jackson.databind.ObjectMapper;
import com.fasterxml.jackson.databind.annotation.JsonDeserialize;

import java.io.File;
import java.io.IOException;

public class JsonApp {

    public static void main(String[] args) throws Exception {
        File jsonFile = new File("./src/main/resources/test.json");
        ObjectMapper mapper = new ObjectMapper();
        System.out.println(mapper.readValue(jsonFile, User.class));
    }
}

class PropertyExistsJsonDeserializer extends JsonDeserializer<Boolean> {
    @Override
    public Boolean deserialize(JsonParser p, DeserializationContext ctxt) throws IOException {
        p.readValueAsTree(); //consume value
        return Boolean.TRUE;
    }
}

@JsonIgnoreProperties(ignoreUnknown = true)
@JsonInclude(JsonInclude.Include.NON_NULL)
class User {
    private int id;

    @JsonDeserialize(using = PropertyExistsJsonDeserializer.class)
    @JsonProperty("picture")
    private boolean pictureAvailable;

    //getters, setters, toString
}

上面的代码打印:

User{id=123, pictureAvailable=true}