杰克逊不能反序列化空数组

【问题标题】：Jackson Can not deserialize empty array杰克逊不能反序列化空数组
【发布时间】：2014-09-29 17:42:23
【问题描述】：

我正在阅读 Facebook Insights 并试图让 Jackson 将 JSON 映射到 Object。如果所有数据都没有空，我让它工作。但是我在尝试反序列化键值的空数组时遇到问题。即使尝试了这个帖子：How to prevent null values inside a Map and null fields inside a bean from getting serialized through Jackson 也没有解决问题:(

这是 JSON：

{"data":[{"id":"492640667465465\/insights\/page_fans_country\/lifetime","name":"page_fans_country","period":"lifetime","values":[{"value":{"MY":26315,"ID":311,"SG":77,"NP":63,"MM":56,"PH":51,"GB":44,"US":44,"KR":36,"TH":36,"IN":34,"BD":24,"PK":22,"BN":22,"AU":15,"TW":14,"VN":12,"KH":11,"YE":11,"CA":10,"JP":10,"EG":8,"ZA":7,"SA":6,"ES":6,"HK":6,"FR":6,"IT":5,"IL":5,"IR":5,"NG":5,"LK":5,"BR":5,"IQ":4,"AF":4,"AE":4,"GT":4,"RO":4,"LR":4,"RU":4,"PS":4,"DE":4,"CN":4,"LY":3,"JO":3},"end_time":"2014-08-02T07:00:00+0000"},{"value":{"MY":26326,"ID":315,"SG":77,"NP":63,"MM":56,"PH":54,"GB":44,"US":43,"TH":38,"KR":36,"IN":33,"BD":23,"BN":22,"PK":21,"AU":16,"TW":14,"VN":12,"KH":11,"YE":11,"CA":10,"JP":10,"EG":8,"ZA":7,"SA":7,"ES":6,"HK":6,"FR":6,"IT":5,"IL":5,"IR":5,"NG":5,"LK":5,"BR":5,"IQ":4,"RU":4,"CN":4,"GT":4,"RO":4,"LR":4,"AF":4,"PS":4,"DE":4,"AE":4,"LY":3,"CH":3},"end_time":"2014-08-03T07:00:00+0000"},{"value":{"MY":26338,"ID":312,"SG":79,"NP":63,"MM":55,"PH":52,"US":45,"GB":44,"TH":39,"KR":34,"IN":32,"BD":24,"BN":22,"PK":21,"AU":16,"TW":14,"KH":12,"VN":12,"CA":11,"YE":11,"JP":10,"EG":8,"ZA":7,"SA":7,"ES":6,"HK":6,"FR":6,"IT":5,"CN":5,"IR":5,"NG":5,"LK":5,"BR":5,"IL":5,"IQ":4,"AF":4,"AE":4,"GT":4,"RO":4,"LR":4,"RU":4,"PS":4,"DE":4,"NZ":3,"TR":3},"end_time":"2014-08-04T07:00:00+0000"}],"title":"Lifetime Likes by Country","description":"Lifetime: Aggregated Facebook location data, sorted by country, about the people who like your Page. (Unique Users)"},{"id":"492640667465465\/insights\/page_storytellers_by_country\/day","name":"page_storytellers_by_country","period":"day","values":[{"value":[],"end_time":"2014-08-02T07:00:00+0000"},{"value":[],"end_time":"2014-08-03T07:00:00+0000"},{"value":[],"end_time":"2014-08-04T07:00:00+0000"}],"title":"Daily Country: People Talking About This","description":"Daily: The number of People Talking About the Page by user country (Unique Users)"},{"id":"492640667465465\/insights\/page_storytellers_by_country\/week","name":"page_storytellers_by_country","period":"week","values":[{"value":{"MY":136,"IN":3,"ID":2,"BD":1,"US":1,"TN":1,"AU":1},"end_time":"2014-08-02T07:00:00+0000"},{"value":{"MY":131,"IN":3,"US":1,"TN":1,"AU":1,"ID":1},"end_time":"2014-08-03T07:00:00+0000"},{"value":{"MY":118,"IN":2,"KH":1,"TR":1,"US":1,"TN":1,"AR":1,"AU":1},"end_time":"2014-08-04T07:00:00+0000"}],"title":"Weekly Country: People Talking About This","description":"Weekly: The number of People Talking About the Page by user country (Unique Users)"},{"id":"492640667465465\/insights\/page_storytellers_by_country\/days_28","name":"page_storytellers_by_country","period":"days_28","values":[{"value":{"MY":492,"IN":5,"ID":3,"AU":2,"SG":2,"ZA":2,"US":2,"GB":2,"RO":1,"PH":1,"NP":1,"BD":1,"JO":1,"PS":1,"TN":1,"IR":1,"CA":1,"CN":1,"KR":1},"end_time":"2014-08-02T07:00:00+0000"},{"value":{"MY":499,"IN":5,"ID":3,"GB":2,"SG":2,"ZA":2,"US":2,"RO":1,"PH":1,"NP":1,"BD":1,"AU":1,"CN":1,"KR":1,"TN":1,"IR":1,"CA":1,"JO":1},"end_time":"2014-08-03T07:00:00+0000"},{"value":{"MY":501,"IN":4,"ID":3,"SG":2,"ZA":2,"US":2,"GB":2,"AU":1,"RO":1,"PH":1,"NP":1,"JO":1,"AR":1,"KR":1,"BD":1,"TR":1,"IR":1,"CA":1,"CN":1,"KH":1,"TN":1},"end_time":"2014-08-04T07:00:00+0000"}],"title":"28 Days Country: People Talking About This","description":"28 Days: The number of People Talking About the Page by user country (Unique Users)"}],"paging":{"previous":"https:\/\/graph.facebook.com\/v2.0\/492640667465465\/insights?since=1406649169&until=1406908369","next":"https:\/\/graph.facebook.com\/v2.0\/492640667465465\/insights?since=1407167569&until=1407426769"}}

我当前的代码根本不喜欢这样 --> "value":[]

以下是我的对象：

import java.util.Date;
import java.util.List;
import java.util.Map;

import com.fasterxml.jackson.annotation.JsonInclude;
import com.fasterxml.jackson.annotation.JsonInclude.Include;

public class Insights {
private Data[] data;
private Paging paging;

public Data[] getData() {
    return data;
}
public void setData(Data[] data) {
    this.data = data;
}
public Paging getPaging() {
    return paging;
}
public void setPaging(Paging paging) {
    this.paging = paging;
}




/**
 * inner class for Data
 * @author pohsoon.yap
 *
 */
public static class Data {
    private String id;
    private String name;
    private String period;
    private Values[] values;
    private String title;
    private String description;

    public String getId() {
        return id;
    }
    public void setId(String id) {
        this.id = id;
    }
    public String getName() {
        return name;
    }
    public void setName(String name) {
        this.name = name;
    }
    public String getPeriod() {
        return period;
    }
    public void setPeriod(String period) {
        this.period = period;
    }       
    public Values[] getValues() {
        return values;
    }
    public void setValues(Values[] values) {
        this.values = values;
    }
    public String getTitle() {
        return title;
    }
    public void setTitle(String title) {
        this.title = title;
    }
    public String getDescription() {
        return description;
    }
    public void setDescription(String description) {
        this.description = description;
    }


    /**
     * inner class for Values
     * @author pohsoon.yap
     *
     */
    public static class Values {
        // if "value":[]  then this will break
        private Map<String, Integer> Value;
        private String end_time;
        public Map<String, Integer> getValue() {
            return Value;
        }
        public void setValue(Map<String, Integer> value) {
            Value = value;
        }
        public String getEnd_time() {
            return end_time;
        }
        public void setEnd_time(String end_time) {
            this.end_time = end_time;
        }


    }
}

public static class Paging {
    private String previous;
    private String next;

    public String getPrevious() {
        return previous;
    }
    public void setPrevious(String previous) {
        this.previous = previous;
    }
    public String getNext() {
        return next;
    }
    public void setNext(String next) {
        this.next = next;
    }
}
}

我的代码sn-p如下：

    ObjectMapper mapper = new ObjectMapper();
    mapper.setSerializationInclusion(JsonInclude.Include.NON_EMPTY);
    List<Insights> insightList = new ArrayList();
    String insightStr = "";
    try {

        for (Operation operation : mq.getOperationList()){

            String apiEndPoint = this.facebookGraphApiUrl + operation.getApi();
            apiEndPoint = apiEndPoint.replace("{pageid}", mq.getFacebookPage().getPageId());
            uri = new URI(apiEndPoint);
            insightStr = facebook.getApi().restOperations().getForObject(uri, String.class);

            Insights insights = mapper.readValue(insightStr, Insights.class);

完整的堆栈跟踪：

com.fasterxml.jackson.databind.JsonMappingException：无法从 START_ARRAY 令牌中反序列化 java.util.LinkedHashMap 的实例 在 [来源：java.io.StringReader@625a80df;行：1，列：1603]（通过参考链：com.social.facebook.model.Insights["data"]->com.social.facebook.model.Data["values"]->com.social.facebook .model.Values["value"]) 在 com.fasterxml.jackson.databind.JsonMappingException.from(JsonMappingException.java:164) 在 com.fasterxml.jackson.databind.DeserializationContext.mappingException（DeserializationContext.java:599） 在 com.fasterxml.jackson.databind.DeserializationContext.mappingException（DeserializationContext.java:593） 在 com.fasterxml.jackson.databind.deser.std.MapDeserializer.deserialize(MapDeserializer.java:306) 在 com.fasterxml.jackson.databind.deser.std.MapDeserializer.deserialize(MapDeserializer.java:26) 在 com.fasterxml.jackson.databind.deser.SettableBeanProperty.deserialize(SettableBeanProperty.java:375) 在 com.fasterxml.jackson.databind.deser.impl.MethodProperty.deserializeAndSet(MethodProperty.java:98) 在 com.fasterxml.jackson.databind.deser.BeanDeserializer.deserializeFromObject(BeanDeserializer.java:308) 在 com.fasterxml.jackson.databind.deser.BeanDeserializer.deserialize(BeanDeserializer.java:121) 在 com.fasterxml.jackson.databind.deser.std.ObjectArrayDeserializer.deserialize(ObjectArrayDeserializer.java:147) 在 com.fasterxml.jackson.databind.deser.std.ObjectArrayDeserializer.deserialize(ObjectArrayDeserializer.java:18) 在 com.fasterxml.jackson.databind.deser.SettableBeanProperty.deserialize(SettableBeanProperty.java:375) 在 com.fasterxml.jackson.databind.deser.impl.MethodProperty.deserializeAndSet(MethodProperty.java:98) 在 com.fasterxml.jackson.databind.deser.BeanDeserializer.deserializeFromObject(BeanDeserializer.java:308) 在 com.fasterxml.jackson.databind.deser.BeanDeserializer.deserialize(BeanDeserializer.java:121) 在 com.fasterxml.jackson.databind.deser.std.ObjectArrayDeserializer.deserialize(ObjectArrayDeserializer.java:147) 在 com.fasterxml.jackson.databind.deser.std.ObjectArrayDeserializer.deserialize(ObjectArrayDeserializer.java:18) 在 com.fasterxml.jackson.databind.deser.SettableBeanProperty.deserialize(SettableBeanProperty.java:375) 在 com.fasterxml.jackson.databind.deser.impl.MethodProperty.deserializeAndSet(MethodProperty.java:98) 在 com.fasterxml.jackson.databind.deser.BeanDeserializer.deserializeFromObject(BeanDeserializer.java:308) 在 com.fasterxml.jackson.databind.deser.BeanDeserializer.deserialize(BeanDeserializer.java:121) 在 com.fasterxml.jackson.databind.ObjectMapper._readMapAndClose(ObjectMapper.java:2796) 在 com.fasterxml.jackson.databind.ObjectMapper.readValue(ObjectMapper.java:1942)

【问题讨论】：

  • JSON value 属性似乎是一个数组类型，但您的模型类需要一个映射。我想知道 value 属性不为空时的样子？
  • @AlexeyGavrilov 在我上面附加的同一个 JSON 中，“数据”数组有 4 个项目，其中只有一个项目的值为空，其余的看起来像这样：“值”：{“ MY":26315,"ID":311} 我认为是一张地图；一个键和一个值？

标签： java json serialization jackson


【解决方案1】：

正如其他人所解释的，您正在尝试将 JSON Array 映射到 Java Map，这是默认情况下不允许的。

但可能允许空 JSON 数组映射到 java.util.Map。通过启用DeserializationFeature.ACCEPT_EMPTY_ARRAY_AS_NULL_OBJECT:

objectMapper.enable(DeserializationFeature.ACCEPT_EMPTY_ARRAY_AS_NULL_OBJECT);

这至少在 POJO 类型的情况下有效；我不记得这是否适用于通常采用 JSON 对象的其他 Java 类型。

【讨论】：

    【解决方案2】：

    模型中的value 字段声明为Map，而相应的JSON 属性可以是空数组或键值映射。 Jackson 无法将空数组分配给地图字段。

    假设您希望在客户端解决问题，您可以修改setValue 方法以接受泛型Object，然后验证它是映射还是数组（实际上是List，因为Jackson 反序列化数组作为 Java 集合）。这是一个例子：

    public class JacksonArrayAsMap {

    public static class Bean {
        private Map<String, Object> value;

        public void setValue(Object value) {
            if (value instanceof Map) {
                this.value = (Map<String, Object>) value;
            } else if (value instanceof List && ((List) value).size() == 0){
                this.value = Collections.EMPTY_MAP;
            } else {
                throw new IllegalArgumentException("Invalid value: " + value);
            }
        }

        @Override
        public String toString() {
            return "Bean{" +
                    "value=" + value +
                    '}';
        }
    }

    public static void main(String[] args) throws IOException {
        final String json1 = "{\"value\":{}}";
        final String json2 = "{\"value\":[]}";
        final String json3 = "{\"value\":{\"a\":\"b\"}}";
        ObjectMapper mapper = new ObjectMapper();
        System.out.println(mapper.readValue(json1, Bean.class));
        System.out.println(mapper.readValue(json2, Bean.class));
        System.out.println(mapper.readValue(json3, Bean.class));
    }
}

    输出：

    Bean{value={}}
Bean{value={}}
Bean{value={a=b}}

    【讨论】：

    • 感谢您的回答，我戴上我的爱好帽后会试试这个:)
    • 耶！确认它有效 :) 我将继续处理来自 Graph API 的其他可用 JSON 提要。
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