尝试反序列化 json 时出现 NullPointerException答案

【问题标题】：Getting NullPointerException while trying to deserialize json尝试反序列化 json 时出现 NullPointerException
【发布时间】：2021-07-01 04:19:42
【问题描述】：

我在尝试反序列化 json 时收到 NullPointerException。我正在尝试仅反序列化来自 https://api.covid19india.org/data.json

的选择 JSON 对象

我写过代码

import java.util.List;
    import com.fasterxml.jackson.annotation.JsonIgnoreProperties;
    @JsonIgnoreProperties(ignoreUnknown = true)
    public class CovidData {
        List<Statewise> statewiseData;
        public List<Statewise> getStatewiseData() {
            return statewiseData;
        }
        public void setStatewiseData(List<Statewise> statewiseData) {
            this.statewiseData = statewiseData;
        }
        @Override
        public String toString() {
            return "CovidData [statewiseData=" + statewiseData + "]";
        }
    }

Statewise.java

public class Statewise {
    @JsonProperty("active")
    String active;
    
    public String getActive() {
        return active;
    }
    public void setActive(String active) {
        this.active = active;
    }
    
    @Override
    public String toString() {
        return "Statewise [active=" + active + ", confirmed=" + confirmed + ", deaths=" + deaths + ", deltaconfirmed="
                + deltaconfirmed + ", deltadeaths=" + deltadeaths + ", deltarecovered=" + deltarecovered
                + ", lastupdatedtime=" + lastupdatedtime + ", migratedother=" + migratedother + ", recovered="
                + recovered + ", state=" + state + ", statecode=" + statecode + ", statenotes=" + statenotes + "]";
    }
    
}

调用方法

@Test
    public void test1() {
        CovidData covidData=
        given()
        .when()
        .get("https://api.covid19india.org/data.json")
        .as(CovidData.class);
        
        List<Statewise> state=covidData.getStatewiseData();
        
        for (Statewise s : state)
            System.out.println(s.getActive());
    }

请帮忙

【问题讨论】：

能否提供异常的stacktrace。
java.lang.NullPointerException at RestAssured.com.restassured.DeserializationTest2.test1(DeserializationTest2.java:18) at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method) at sun.reflect.NativeMethodAccessorImpl.invoke (NativeMethodAccessorImpl.java:62)
我可以看到，该 JSON 中没有 statewiseData，您正在 for 循环中对其进行迭代。这可能是问题的关键。
它就在那里，它是长 JSON

标签： java json jackson

【解决方案1】：

返回的 JSON 包含“statewise”属性。因此，要正确反序列化它，您可以在 CovidData 类中将属性 List<Statewise> statewiseData 更改为 List<Statewise> statewise 或添加 JsonProperty 以将其映射到“statewise”

@JsonProperty("statewise")
List<Statewise> statewiseData;

【讨论】：

我忘记将 json 属性添加到 statewise 中。谢谢指出