Jackson基于推导的反序列化遇字段缺失时的类型歧义问题

我在用Jackson处理下面这个接口和实现类的反序列化：

import com.fasterxml.jackson.annotation.JsonSubTypes;
import com.fasterxml.jackson.annotation.JsonTypeInfo;

@JsonTypeInfo(use = JsonTypeInfo.Id.DEDUCTION)
@JsonSubTypes({
        @JsonSubTypes.Type(value = DatiSocieta.class, name = "datiSocieta"),
        @JsonSubTypes.Type(value = DatiPersonaFisica.class, name = "datiPersonaFisica") ,
        @JsonSubTypes.Type(value = DatiDittaIndividuale.class, name = "datiDittaIndividuale") }
)
public interface Dati {
}

问题出在DatiPersonaFisica和DatiDittaIndividuale这两个类上——前者的所有字段都被后者包含了，导致Jackson反序列化时没法区分：

@Data
public class DatiPersonaFisica implements Dati {
    private String codiceFiscale;
    private String sesso;
    private String nome;
    private String cognome;
    @JsonFormat(pattern = "yyyy-MM-dd")
    private Date dataNascita;
    private String luogoNascita;
    private String provinciaNascita;
    private String nazioneNascita;
}

@Data
public class DatiDittaIndividuale implements Dati {
    private String denominazione;
    private String codiceFiscale;
    private String sesso;
    private String nome;
    private String cognome;
    @JsonFormat(pattern = "yyyy-MM-dd")
    private Date dataNascita;
    private String luogoNascita;
    private String provinciaNascita;
    private String nazioneNascita;
    private String numeroRea;
    private String provinciaCCIA;
    private String partitaIVA;
}

运行时直接抛出错误：

JSON parse error: Could not resolve subtype of [simple type, class
com.XXXX.cmp.cgw.webclient.bo.Dati]: Cannot deduce unique subtype of
com.xxxx.cmp.cgw.webclient.bo.Dati (2 candidates match); nested
exception is
com.fasterxml.jackson.databind.exc.InvalidTypeIdException: Could not
resolve subtype of [simple type, class
com.xxxx.cmp.cgw.webclient.bo.Dati]: Cannot deduce unique subtype of
com.xxxx.cmp.cgw.webclient.bo.Dati (2 candidates match)\n at
[Source: (org.springframework.util.StreamUtils$NonClosingInputStream);
line: 12, column: 5] (through reference chain:
com.xxxx.cmp.cgw.webclient.dto.ClienteDTO["soggetto"]->com.xxxx.cmp.cgw.webclient.bo.Soggetto["anagrafica"])

我想要的效果是：如果JSON里没有DatiDittaIndividuale独有的字段，就自动反序列化成DatiPersonaFisica，试了半天没找到现成方法，求帮忙。

核心问题是DatiPersonaFisica是DatiDittaIndividuale的字段子集，Jackson默认的类型推导机制没法判断该选哪个，得手动添加判断规则：

1. 自定义反序列化器（最靠谱） #

写一个JsonDeserializer，直接根据字段存在情况判断目标类型：

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

import java.io.IOException;

public class DatiDeserializer extends JsonDeserializer<Dati> {
    @Override
    public Dati deserialize(JsonParser p, DeserializationContext ctxt) throws IOException {
        JsonNode node = p.getCodec().readTree(p);
        
        // 先判断是否为DatiSocieta（假设它有独有的denominazione字段，可根据实际情况调整判断条件）
        if (node.has("denominazione") && !node.has("nome")) {
            return ctxt.readValue(p, DatiSocieta.class);
        }
        // 检查是否存在DatiDittaIndividuale的专属字段，只要有一个就归为该类型
        boolean isDitta = node.has("numeroRea") || node.has("partitaIVA") || node.has("provinciaCCIA");
        if (isDitta) {
            return ctxt.readValue(p, DatiDittaIndividuale.class);
        }
        // 剩余情况全部归为DatiPersonaFisica
        return ctxt.readValue(p, DatiPersonaFisica.class);
    }
}

然后在Dati接口上添加该反序列化器的注解：

@JsonDeserialize(using = DatiDeserializer.class)
@JsonTypeInfo(use = JsonTypeInfo.Id.DEDUCTION)
@JsonSubTypes({
        @JsonSubTypes.Type(value = DatiSocieta.class, name = "datiSocieta"),
        @JsonSubTypes.Type(value = DatiPersonaFisica.class, name = "datiPersonaFisica"),
        @JsonSubTypes.Type(value = DatiDittaIndividuale.class, name = "datiDittaIndividuale")
})
public interface Dati {
}

2. 调整类继承关系 #

把DatiPersonaFisica改成DatiDittaIndividuale的父类，让Jackson在类型推导时，优先匹配带专属字段的子类，没有子类字段时自动使用父类：

@Data
public class DatiPersonaFisica implements Dati {
    // 原有字段保持不变
}

@Data
public class DatiDittaIndividuale extends DatiPersonaFisica {
    // 只保留DatiDittaIndividuale的专属字段
    private String denominazione;
    private String numeroRea;
    private String provinciaCCIA;
    private String partitaIVA;
}

然后修改Dati接口的注解：

@JsonTypeInfo(use = JsonTypeInfo.Id.DEDUCTION)
@JsonSubTypes({
        @JsonSubTypes.Type(value = DatiSocieta.class),
        @JsonSubTypes.Type(value = DatiPersonaFisica.class),
        @JsonSubTypes.Type(value = DatiDittaIndividuale.class)
})
public interface Dati {
}

这种方式适合允许调整类结构的场景，不过如果JSON刚好只包含父类字段，Jackson偶尔还是会有匹配犹豫，但实际测试中大多会优先匹配父类。

内容的提问来源于stack exchange，提问作者Lore