解析Json到包含对象的对象

Question

我从accuweather获得了以下带有json的代码

{  
 "Headline":{  
      "EffectiveDate":"2019-07-29T07:00:00+06:00",
      "EffectiveEpochDate":1564362000,
      "Severity":3,
      "Text":"The heat wave will continue through Friday",
      "Category":"heat",
      "EndDate":"2019-08-02T19:00:00+06:00",
      "EndEpochDate":1564750800
   },
   "DailyForecasts":[  
      {  
         "Date":"2019-07-29T07:00:00+06:00",
         "EpochDate":1564362000,
         "Temperature":{  
            "Minimum":{  
               "Value":19.1,
               "Unit":"C",
               "UnitType":17
            },
            "Maximum":{  
               "Value":36.7,
               "Unit":"C",
               "UnitType":17
            }
         },
         "Day":{  
            "Icon":30,
            "IconPhrase":"Hot",
            "HasPrecipitation":false
         },
         "Night":{  
            "Icon":35,
            "IconPhrase":"Partly cloudy",
            "HasPrecipitation":false
         },
         "Sources":[  
            "AccuWeather"
         ]
      }
   ]
}

我尝试通过Jackson将此对象解析为POJO

public static void main( String[] args )
{
    String x = "{\"Headline\":{\"EffectiveDate\":\"2019-07-29T07:00:00+06:00\",\"EffectiveEpochDate\":1564362000,\"Severity\":3,\"Text\":\"The heat wave will continue through Friday\",\"Category\":\"heat\",\"EndDate\":\"2019-08-02T19:00:00+06:00\",\"EndEpochDate\":1564750800},\"DailyForecasts\":[{\"Date\":\"2019-07-29T07:00:00+06:00\",\"EpochDate\":1564362000,\"Temperature\":{\"Minimum\":{\"Value\":19.1,\"Unit\":\"C\",\"UnitType\":17},\"Maximum\":{\"Value\":36.7,\"Unit\":\"C\",\"UnitType\":17}},\"Day\":{\"Icon\":30,\"IconPhrase\":\"Hot\",\"HasPrecipitation\":false},\"Night\":{\"Icon\":35,\"IconPhrase\":\"Partly cloudy\",\"HasPrecipitation\":false},\"Sources\":[\"AccuWeather\"]}]}";
    ObjectMapper objectMapper = new ObjectMapper();

    try {
        Weather weather = objectMapper.readValue(x, Weather.class);

        System.out.println(weather);
    } catch (IOException e) {
        e.printStackTrace();
    }
}

我有json中指定的所有模型，如标题、每日预测数组、温度，由温度项组成（在json中命名为最小值和最大值）等，它们都有私有字段和公共构造函数、getter和setter。但是我没有一些字段，因为我想省略它们（Day、night、EpochDate、Source）。

当我运行程序时，我得到了错误

com.fasterxml.jackson.databind.exc.无法构造test.models.weather.Weather的实例（不存在Creator，如默认构造）：无法从Object值反序列化（没有基于委托或属性的Creator）

我也尝试过Gson但它返回空值的对象

我做错什么了吗？还有别的方法吗？

编辑：这些是模型，@LazerBass是对的，因为我首先没有包括默认构造函数，现在错误已经改变了：

com.fasterxml.jackson.databind.exc.无法识别的属性：无法识别的字段标题（类test.models.weather.天气），未标记为可忽略（2个已知属性：标题，每日预测）

public class TemperatureItem {
    public double value;
    public String unit;
    public String unitType;

    public TemperatureItem() {

    }

    //Getters and setters
}


public class Temperature {
    private TemperatureItem maximum;
    private TemperatureItem minimum;

    public Temperature(TemperatureItem maximum, TemperatureItem minimum) {
        this.maximum = maximum;
        this.minimum = minimum;
    }

    public Temperature() {

    }
    //Getters and setters
}

public class DailyForecasts {
    private LocalDateTime date;
    private Temperature temperature;

    public DailyForecasts(LocalDateTime date, Temperature temperature) {
        this.date = date;
        this.temperature = temperature;
    }

    public DailyForecasts() {

    }
    //Getters and setters
}

public class Headline {
    private LocalDateTime effectiveDate;
    private int severity;
    private String text;
    private String category;
    private LocalDateTime endDate;

    public Headline() {

    }

    public Headline(LocalDateTime effectiveDate, Integer severity, String text, String category, LocalDateTime endDate) {
        this.effectiveDate = effectiveDate;
        this.severity = severity;
        this.text = text;
        this.category = category;
        this.endDate = endDate;
    }
    //Getters and setters
}

public class Weather {
        private Headline headline;
        private DailyForecasts[] dailyForecasts;

        public Weather() {

        }
        public Weather(Headline headline, DailyForecasts[] dailyForecasts) {
            this.headline = headline;
            this.dailyForecasts = dailyForecasts;
        }
        //Getters and setters
}

我发现，如果我将json字符串转换为小写，我可以得到一些值，尽管没有解析数组和LocalDateTime

Answer 1

从异常消息判断，我猜您的Weather类使用的是无参数构造函数。尝试添加一个。例如。

public class Weather {
    public Weather() {
        // no arg constructor needed for jackson
    }
}

Answer 2

当Jackson失败并带有诸如“no Creator，如默认构造，存在”之类的消息时，它需要默认的public无参数构造函数为每个POJO您在模型中拥有的类。

当它失败时，如“无法识别的字段...未标记为可忽略...”您需要禁用FAIL_ON_UNKNOWN_PROPERTIES功能。

另请参见：

1. Jackson解组具有未知属性的JSON
2. 杰克逊延迟反序列化字段

Answer 3

要生成Weather类及其相应的类，请使用以下链接并选择源类型为json。它将根据json字符串生成所需的类。

http://www.jsonschema2pojo.org/

生成类后，可以使用@JsonIgnore注释不需要的字段。