Jackson 映射对象或对象列表取决于 json 输入

问题描述

我有这个 POJO :

I have this POJO :

public class JsonObj {

    private String id;
    private List<Location> location;


    public String getId() {
        return id;
    }

    public List<Location> getLocation() {
        return location;
    }

    @JsonSetter("location")
    public void setLocation(){
        List<Location> list = new ArrayList<Location>();
        if(location instanceof Location){
            list.add((Location) location);
            location = list;
        }
    }
}

来自 json 输入的位置"对象可以是位置的简单实例或位置数组.当它只是一个实例时，我收到此错误:

the "location" object from the json input can be either a simple instance of Location or an Array of Location. When it is just one instance, I get this error :

Could not read JSON: Can not deserialize instance of java.util.ArrayList out of   START_OBJECT token

我尝试实现自定义设置器，但没有奏效.如何根据 json 输入映射位置或列表?

I've tried to implement a custom setter but it didn't work. How could I do to map either a Location or a List depending on the json input?

推荐答案

更新:Mher Sarkissian's soulution 作品很好，它也可以按照此处的建议与注释一起使用，如下所示:

Update: Mher Sarkissian's soulution works fine, it can also be used with annotations as suggested here, like so:.

@JsonFormat(with = JsonFormat.Feature.ACCEPT_SINGLE_VALUE_AS_ARRAY)
private List<Item> item;

我对这个最烦人的问题深表同情，我遇到了同样的问题，并在此处找到了解决方案:https://stackoverflow.com/a/22956168/1020871

稍加修改，我想出了这个，首先是泛型类:

With a little modification I come up with this, first the generic class:

public abstract class OptionalArrayDeserializer<T> extends JsonDeserializer<List<T>> {

    private final Class<T> clazz;

    public OptionalArrayDeserializer(Class<T> clazz) {
        this.clazz = clazz;
    }

    @Override
    public List<T> deserialize(JsonParser jp, DeserializationContext ctxt)
            throws IOException {
        ObjectCodec oc = jp.getCodec();
        JsonNode node = oc.readTree(jp);
        ArrayList<T> list = new ArrayList<>();
        if (node.isArray()) {
            for (JsonNode elementNode : node) {
                list.add(oc.treeToValue(elementNode, clazz));
            }
        } else {
            list.add(oc.treeToValue(node, clazz));
        }
        return list;
    }
}

然后是属性和实际的反序列化器类(Java 泛型并不总是很漂亮):

And then the property and the actual deserializer class (Java generics is not always pretty):

@JsonDeserialize(using = ItemListDeserializer.class)
private List<Item> item;

public static class ItemListDeserializer extends OptionalArrayDeserializer<Item> {
    protected ItemListDeserializer() {
        super(Item.class);
    }
}

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