InstantiationException：在 Kotlin 中使实体字段不可为空时，没有实体的默认构造函数

Posted 2023-02-27

【中文标题】InstantiationException：在 Kotlin 中使实体字段不可为空时，没有实体的默认构造函数

【英文标题】：InstantiationException: No default constructor for entity when making an Entity field NOT nullable in Kotlin

【发布时间】：2020-05-03 07:46:24

【问题描述】：

我有一个项目实体（如下所示）。当我尝试改变时

var fullName: String?= null

到：

var fullName: String,

我突然从 Hibernate 得到一个 InstantiationException：

org.hibernate.InstantiationException: No default constructor for entity:

在此之前没有任何问题。可能出了什么问题？这是实体类：

package no.inmeta.ris.project.model.entity

import org.hibernate.annotations.GenericGenerator
import java.util.*
import javax.persistence.*

@Entity
data class Project (

    @Id
    @GeneratedValue(generator = "uuid2")
    @GenericGenerator(name = "uuid2", strategy = "uuid2")
    @Column(columnDefinition = "BINARY(16)")
    var id : UUID? = null,

    var fullName: String,

    var shortName: String? = null,

    var prevRefId: String? = null,

    var startDate: Date? = null,

    var endDate: Date?  = null,

    @ManyToMany(fetch = FetchType.EAGER)
    @JoinTable(name = "Associated_Projects",
        joinColumns = [JoinColumn(name = "associated_project")],
        inverseJoinColumns = [JoinColumn(name = "second_associated_project") ])
    var associatedProjects: List<Project?> = mutableListOf(),

    var shortDescription: String? = null,

    var summary: String?  = null,

    var url: String? = null,

    @ManyToMany(fetch = FetchType.EAGER)
    @JoinTable(name = "Project_Types",
        joinColumns = [JoinColumn(name = "project_id")],
        inverseJoinColumns = [JoinColumn(name = "type_id") ])
    var type: List<Type?> = mutableListOf()
    )

【问题讨论】：

嘿@Nora，我认为缺少默认构造函数。你添加了吗？

【参考方案1】：

为 fullName 参数提供默认值或保持为空。

在您的代码中，它没有默认值，因此在生成的 Java 字节码 Project 类中不会有没有参数的构造函数 - 它至少需要 1 个参数来初始化 fullName 字段。

【讨论】：

我正在创建一个 API 并试图避免任何默认值。同时我不希望 fullName 为空，因为它是必填字段

Hibernate 需要一个默认构造函数。默认构造函数是没有参数的构造函数。如果希望 Kotlin 为数据类生成默认构造函数，则必须为其主构造函数中声明的每个属性设置默认参数。

看这里的答案***.com/questions/37873995/…

【参考方案2】：

您应该提供一个默认构造函数。如果您不想在此类之外公开它，您可以将其设为私有：

package no.inmeta.ris.project.model.entity

import org.hibernate.annotations.GenericGenerator
import java.util.*
import javax.persistence.*

@Entity
data class Project (

    @Id
    @GeneratedValue(generator = "uuid2")
    @GenericGenerator(name = "uuid2", strategy = "uuid2")
    @Column(columnDefinition = "BINARY(16)")
    var id : UUID? = null,

    var fullName: String,

    var shortName: String? = null,

    var prevRefId: String? = null,

    var startDate: Date? = null,

    var endDate: Date?  = null,

    private constructor()

    @ManyToMany(fetch = FetchType.EAGER)
    @JoinTable(name = "Associated_Projects",
        joinColumns = [JoinColumn(name = "associated_project")],
        inverseJoinColumns = [JoinColumn(name = "second_associated_project") ])
    var associatedProjects: List<Project?> = mutableListOf(),

    var shortDescription: String? = null,

    var summary: String?  = null,

    var url: String? = null,

    @ManyToMany(fetch = FetchType.EAGER)
    @JoinTable(name = "Project_Types",
        joinColumns = [JoinColumn(name = "project_id")],
        inverseJoinColumns = [JoinColumn(name = "type_id") ])
    var type: List<Type?> = mutableListOf()
    )