rest - InvalidDefinitionException : Cannot construct instance of `com.vehicle.datatransferobject.VehicleDTO`

Question

在我在Spring Boot中构建的REST端点中，我试图将vehicleDTO传递给 Controller ​​。但是在到达我的 Controller 之前，有一个错误。

InvalidDefinitionException: Cannot construct instance of com.vehicle.datatransferobject.VehicleDTO (no Creators, like default construct, exist): cannot deserialize from Object value (no delegate- or property-based Creator)

载具DTO

import com.fasterxml.jackson.annotation.JsonIgnore;
import com.fasterxml.jackson.annotation.JsonInclude;
import com.fasterxml.jackson.annotation.JsonProperty;
import com.myvehicle.EngineType;

@JsonInclude(JsonInclude.Include.NON_NULL)
public class VehicleDTO {

    @JsonIgnore
    private Long id;

    @NotNull(message = "vehiclenumber can not be null!")
    private String vehiclenumber;

    @Min(2)
    @NotNull(message = "Seat count can not be less than 2!")
    private Integer vehicleseatcount;

    @NotNull(message = "Engine Type can not be null!")
    private EngineType enginetype;

    @Max(5)
    private Integer vehiclerating;



    private VehicleDTO(Long id, String vehiclenumber, Integer vehicleseatcount, EngineType enginetype,Integer vehiclerating){
        this.vehiclenumber=vehiclenumber;
        this.vehicleseatcount=vehicleseatcount;
        this.enginetype=enginetype;
        this.vehiclerating=vehiclerating;
        this.id=id;
    }

    public static VehicleDTOBuilder newBuilder()
    {
        return new VehicleDTOBuilder();
    }

    @JsonProperty
    public Long getId() {
        return id;
    }


    public String getvehiclenumber() {
        return vehiclenumber;
    }


    public Integer getvehicleseatcount() {
        return vehicleseatcount;
    }


    public EngineType getEnginetype() {
        return enginetype;
    }



    public Integer getvehiclerating() {
        return vehiclerating;
    }




    public static class VehicleDTOBuilder{

        private Long id;
        private String vehiclenumber;
        private Integer vehicleseatcount;
        private EngineType enginetype;
        private Integer vehiclerating;

        public VehicleDTOBuilder setId(Long id) {
            this.id = id;
            return this;
        }
        public VehicleDTOBuilder setvehiclenumber(String vehiclenumber) {
            this.vehiclenumber = vehiclenumber;
            return this;
        }
        public VehicleDTOBuilder setvehicleseatcount(Integer vehicleseatcount) {
            this.vehicleseatcount = vehicleseatcount;
            return this;
        }
        public VehicleDTOBuilder setEnginetype(EngineType enginetype) {
            this.enginetype = enginetype;
            return this;
        }
        public VehicleDTOBuilder setvehiclerating(Integer vehiclerating) {
            this.vehiclerating = vehiclerating;
            return this;
        }


        public VehicleDTO createVehicleDTO()
        {
            return new VehicleDTO(id, vehiclenumber, vehicleseatcount, enginetype,vehiclerating);
        }

    }

}

我的DTO具有一个称为EngineType的枚举类型

public enum EngineType {
    ELECTRIC, DIESEL
}

我的 Controller 看起来像这样

@PostMapping
    @ResponseStatus(HttpStatus.CREATED)
    public VehicleDTO addvehicle(@Valid @RequestBody VehicleDTO vehicleDTO) 
    {
        VehicleDO vehicleDO = Mapper.VehicleDO(vehicleDTO);
        return Mapper.makeVehicleDTO(Service.addvehicle(vehicleDO));
    }

Answer 1

此异常:

InvalidDefinitionException: Cannot construct instance of com.vehicle.datatransferobject.VehicleDTO (no Creators, like default construct, exist): cannot deserialize from Object value (no delegate- or property-based Creator)

意味着Jackson找不到实例化 VehicleDTO的方法，该方法是默认构造函数(无arg构造函数)或 JsonCreator。
在使用构建器模式时，您将配置 VehicleDTO类，使Jackson可以使用 VehicleDTO实例化 VehicleDTOBuilder，例如:

@JsonDeserialize(builder = VehicleDTO.VehicleDTOBuilder.class)
public class VehicleDTO {
      ...         
}

并用 JsonPOJOBuilder注释您的构建器为:

@JsonPOJOBuilder(buildMethodName = "createVehicleDTO", withPrefix = "set")
public static class VehicleDTOBuilder{ 
   ...
}

根据javadoc， JsonPOJOBuilder是:

used to configure details of a Builder class: instances of which are used as Builders for deserialized POJO values, instead of POJOs being instantiated using constructors or factory methods. Note that this annotation is NOT used to define what is the Builder class for a POJO: rather, this is determined by JsonDeserialize.builder() property of JsonDeserialize.