如何解决Room报错：Entities and POJOs must have a usable public constructor

问题代码 #

定义Kotlin data class作为Room实体代码如下：

@Entity(tableName = "training_session")
data class SessionEntity(
    @PrimaryKey(autoGenerate = false) val id: Long,
    @ColumnInfo(name = "current_state_marker") val currentState: Short,
    @Embedded val states: List<Int>
)

编译报错信息 #

> Task :training-infrastructure:kaptDebugKotlin FAILED
error: Entities and POJOs must have a usable public constructor. You can have an empty constructor or a constructor whose parameters match the fields (by name and type). - java.util.List
error: Entities and POJOs must have a usable public constructor. You can have an empty constructor or a constructor whose parameters match the fields (by name and type). - java.util.List

同项目内另一个写法类似、同样包含List字段的Entity未触发该报错。

解决方案 #

调整TypeConverter定义即可解决问题，调整后的代码如下：

data class SessionStateList (val stateList : List<Int>) 

class SessionStateListConverter {

    @TypeConverter
    fun fromArraySessionStateList(sh: List<Int>?): String? {
        return Gson().toJson(sh)
    }
    @TypeConverter
    fun toArraySessionStateList(sh: String?): List<Int>? {
        val listType: Type = object : TypeToken<ArrayList<Int?>?>() {}.type
        return Gson().fromJson(sh,listType)
    }
}

操作提示：需要将上述Converter注册到你的Room Database类上，同时删除List字段上的@Embedded注解，@Embedded仅支持自定义POJO类型，无法作用于集合类型。

补充说明 #

之前提到的未报错的包含Embedded val something: List<Int>的Entity，实际是因为没有被添加到@Database注解的实体列表中，所以才没有触发编译错误。

内容的提问来源于stack exchange，提问作者hba