如何用jackson-databind反序列化toplyrics为列表？空指针问题求助

问题描述 #

使用jackson-databind解析JSON数据时，artist名称能正常解析，但解析toplyrics字段时出现NullPointerException，提示响应为null。

JSON结构 #

{"artist": {"id": "3ade68b5gd237eda3","desc": "Kiss","url": "/kiss/","pic_small": "/kiss/images/profile.jpg","pic_medium": "/kiss/images/kiss.jpg","rank": {"pos": "350","period": 202404,"views": "1372","uniques": "936","points": "5.0"},"genre": [{"name": "Hard Rock","url": "/browse/style/hard-rock.html"},{"name": "Heavy Metal","url": "/browse/style/heavy-metal.html"},{"name": "Rock","url": "/browse/style/rock.html"},{"name": "Classic Rock","url": "/browse/style/classic-rock.html"}],"related": [{"id": "3ade68b5gb2c7eda3","name": "Van Halen","url": "/van-halen/"},{"id": "3ade68b5g2cd6eda3","name": "Deep Purple","url": "/deep-purple/"},{"id": "3ade68b4gdda6eda3","name": "Black Sabbath","url": "/black-sabbath/"},{"id": "3ade68b5g0187eda3","name": "Ramones","url": "/the-ramones/"},{"id": "3ade68b3g0c86eda3","name": "Led Zeppelin","url": "/led-zeppelin/"},{"id": "3ade68b5gb1b8eda3","name": "Ozzy Osbourne","url": "/ozzy-osbourne/"},{"id": "3ade68b6g28c9eda3","name": "Rolling Stones","url": "/the-rolling-stones/"},{"id": "3ade68b5g1bf7eda3","name": "Queen","url": "/queen/"},{"id": "3ade68b5g2297eda3","name": "Scorpions","url": "/scorpions/"},{"id": "3ade68b5g8227eda3","name": "Iron Maiden","url": "/iron-maiden/"}],"toplyrics": {"item": [{"id": "3ade68b8gace4afa3","desc": "Danger Us","url": "/kiss/danger-us.html","turl": "/kiss/danger-us-traducao.html"},{"id": "3ade68b6gbfebeda3","desc": "Dark Light","url": "/kiss/dark-light.html","turl": "/kiss/dark-light-traducao.html"},{"id": "3ade68b8gb764efa3","desc": "Deadly Weapons (The Elder demo)","url": "/kiss/deadly-weapons-the-elder-demo.html"}]}}}}

原实体类代码 #

ApiResponse类 #

package com.pedruuv.sound.models;

import com.fasterxml.jackson.annotation.JsonAlias;
import com.fasterxml.jackson.annotation.JsonIgnoreProperties;

@JsonIgnoreProperties(ignoreUnknown = true)
public record ApiResponse(@JsonAlias("artist") ArtistData artist,
                        @JsonAlias("toplyrics") SongData lyrics,
                        @JsonAlias("albums") AlbumData album){

}

SongData类 #

package com.pedruuv.sound.models;

import java.util.List;

import com.fasterxml.jackson.annotation.JsonAlias;
import com.fasterxml.jackson.annotation.JsonIgnoreProperties;

@JsonIgnoreProperties(ignoreUnknown = true)
public record SongData(@JsonAlias("item") List<Music> songs) {

}

Music类 #

package com.pedruuv.sound.models;

import com.fasterxml.jackson.annotation.JsonAlias;
import com.fasterxml.jackson.annotation.JsonIgnoreProperties;

@JsonIgnoreProperties(ignoreUnknown = true)
public record Music(@JsonAlias("desc") String title) {

}

问题根源 #

观察JSON结构可知，toplyrics字段并非根节点下的直接字段，而是嵌套在artist对象内部的子字段。但原ApiResponse类中直接将toplyrics映射为根级属性，Jackson无法在根节点找到该字段，导致lyrics变量被赋值为null，后续访问其内部属性时就会触发NullPointerException。

解决方案 #

调整实体类结构，将toplyrics字段移到ArtistData类中，使其与JSON的层级结构匹配：

修改后的ArtistData类（补充原缺失字段） #

package com.pedruuv.sound.models;

import com.fasterxml.jackson.annotation.JsonAlias;
import com.fasterxml.jackson.annotation.JsonIgnoreProperties;
import java.util.List;

@JsonIgnoreProperties(ignoreUnknown = true)
public record ArtistData(
        @JsonAlias("id") String id,
        @JsonAlias("desc") String name,
        @JsonAlias("url") String url,
        @JsonAlias("pic_small") String picSmall,
        @JsonAlias("pic_medium") String picMedium,
        @JsonAlias("rank") RankData rank,
        @JsonAlias("genre") List<GenreData> genres,
        @JsonAlias("related") List<RelatedArtistData> relatedArtists,
        @JsonAlias("toplyrics") SongData topLyrics // 添加toplyrics字段
) {}

// 补充所需辅助实体类
@JsonIgnoreProperties(ignoreUnknown = true)
record RankData(
        @JsonAlias("pos") String position,
        @JsonAlias("period") int period,
        @JsonAlias("views") String views,
        @JsonAlias("uniques") String uniques,
        @JsonAlias("points") String points
) {}

@JsonIgnoreProperties(ignoreUnknown = true)
record GenreData(
        @JsonAlias("name") String name,
        @JsonAlias("url") String url
) {}

@JsonIgnoreProperties(ignoreUnknown = true)
record RelatedArtistData(
        @JsonAlias("id") String id,
        @JsonAlias("name") String name,
        @JsonAlias("url") String url
) {}

修改后的ApiResponse类 #

移除根级的toplyrics映射，因为该字段已归属到ArtistData内部：

package com.pedruuv.sound.models;

import com.fasterxml.jackson.annotation.JsonAlias;
import com.fasterxml.jackson.annotation.JsonIgnoreProperties;

@JsonIgnoreProperties(ignoreUnknown = true)
public record ApiResponse(
        @JsonAlias("artist") ArtistData artist,
        @JsonAlias("albums") AlbumData album
) {}

验证 #

修改后，Jackson会正确识别嵌套在artist下的toplyrics字段，将其反序列化为ArtistData中的topLyrics属性，避免null值出现，后续访问artist.topLyrics().songs()时就不会触发NullPointerException。

内容的提问来源于stack exchange，提问作者PedroVitorGl