使用 Jackson 和 @JsonCreator 反序列化 JSON

Question

我从正在开发的第三方工具收到 JSON 文档，即 JSON 经常扩展。我想坚持使用最简单的反序列化机制，以避免增加自定义反序列化器带来的维护。

这是JSON的相关部分：

{
"key1": "value1",
"key2": "value2",
"variants": {
    "columns": ["chr", "pos", "id", "ref", "alt", "qual", "filter", "type", "genotype", "alignpos", "basepos", "signalpos"],
    "rows": [
        ["17", 19561093, ".", "G", "C", 51, "PASS", "SNV", "het.", 97, 147, 1761],
        ["17", 19561123, ".", "T", "G", 51, "PASS", "SNV", "het.", 127, 177, 2120],
        ["17", 19561149, ".", "G", "A", 51, "PASS", "SNV", "het.", 153, 203, 2432]],
    "xranges": [
        [829, 1129],
        [1480, 1780],
        [1492, 1792]],
}}

问题出在变量标签上，特别是 "rows"。看起来好像 "rows" 是用 @JsonValue 注释序列化的，以便摆脱字段名称，否则这些字段名称将成为每个 "row" 的一部分。相反，一个额外的 "columns" 字段被序列化以至少提及一次列名。

没有 variants 标签，我已经使用 ObjectMapper 成功地将 JSON 反序列化为 POJO:

public class Pojo {
    private String key1;
    private String key2;
    public String getKey1() {
        return key1;
    }
    public void setKey1(String key1) {
        this.key1 = key1;
    }
    public String getKey2() {
        return key2;
    }
    public void setKey2(String key2) {
        this.key2 = key2;
    }
}

和

public static void main(String[] args) {
    final ObjectMapper objectMapper = new ObjectMapper();
    objectMapper.configure(DeserializationConfig.Feature.FAIL_ON_UNKNOWN_PROPERTIES, false);
    Pojo pojo = objectMapper.readValue(jsonFile, Pojo.class);
}

现在，为了反序列化变体，我将其添加到我的原始 POJO

@JsonProperty("variants")
private PojoVariants pojoVariants;

然后，实现 PojoVariants

public class PojoVariants {

    @JsonProperty("columns")
    private String[] columnNames;

    @JsonProperty("rows")
    private PojoVariant[] pojoVariantArr;

    public String[] getColumnNames() {
        return columnNames;
    }

    public void setColumnNames(String[] columnNames) {
        this.columnNames = columnNames;
    }

    public PojoVariant[] getPojoVariantArr() {
        return pojoVariantArr;
    }

    public void setPojoVariantArr(PojoVariant[] pojoVariantArr) {
        this.pojoVariantArr = pojoVariantArr;
    }

}

最后是 PojoVariant

@JsonPropertyOrder({
        "chr", "pos",
        "id", "ref", "alt",
        "quality", "filter", "type", "genotype",
        "alignPos", "basePos", "signalPos" })
public class PojoVariant {

    private String chr;
    private int pos;

    private String id;
    private String ref;
    private String alt;

    private int quality;
    private String filter;
    private String type;
    private String genotype;

    private int alignPos;
    private int basePos;
    private int signalPos;

    @JsonCreator
    public PojoVariant(
            String chr, int pos,
            String id, String ref, String alt,
            int quality, String filter, String type, String genotype,
            int alignPos, int basePos, int signalPos) {

    }

}

设置@JsonPropertyOrder，我希望Jackson 能够弄清楚我想将每个"row" 读入一个PojoVariant 实例。但是，使用原始的 main 方法，我得到了这个堆栈跟踪：

org.codehaus.jackson.map.JsonMappingException: Argument #0 of constructor [constructor for my.json.stuff.PojoVariant, annotations: {interface org.codehaus.jackson.annotate.JsonCreator=@org.codehaus.jackson.annotate.JsonCreator()}] has no property name annotation; must have name when multiple-paramater constructor annotated as Creator

我知道 Jackson 希望我用属性-names 注释构造函数参数。但是 JSON 首先包含 none 。我错过了什么？或者这种方法根本不受支持 - 我是否必须实现自定义解串器？

Answer 1

来自jackson annotations javadoc，@JsonPropertyOrder用于序列化，不是反序列化：

Annotation that can be used to define ordering (possibly partial) to use when serializing object properties.

此外，当构造函数被注释为 @JsonCreator 时，构造函数必须是：

Single-argument constructor/factory method without JsonProperty annotation for the argument: if so, this is so-called "delegate creator", in which case Jackson first binds JSON into type of the argument, and then calls creator.

Constructor/factory method where every argument is annotated with either JsonProperty or JacksonInject, to indicate name of property to bind to.

问题是您尝试从 json 列表 ["17", 19561093, ".", ..] 反序列化为 PojoVariant。杰克逊很聪明，但没那么聪明。他在这里需要一些帮助，您可以帮助他实现自定义反序列化器。我们不想那样做，所以希望我们能找到别的东西。

提示来自 @JsonCreator javadoc（第一个项目符号，粗体文本）。而且，因为 Jackson 知道如何将列表绑定到对象数组中，我们可以像这样重写 PojoVariant 构造函数：

@JsonCreator
public PojoVariant(Object[] params) {
    this.chr = (String) params[0];
    this.pos = (int) params[1];
    this.id = (String) params[2];
    this.ref = (String) params[3];
    this.alt = (String) params[4];
    this.quality = (int) params[5];
    this.filter = (String) params[6];
    this.type = (String) params[7];
    this.genotype = (String) params[8];
    this.alignPos = (int) params[9]; 
    this.basePos = (int) params[10];
    this.signalPos = (int) params[11];
}

使用 Jackson 和 @JsonCreator 反序列化 JSON

Deserialize JSON with Jackson and @JsonCreator

java

json

jackson

json-deserialization

objectmapper