Android Room - error: Cannot figure out how to save this field into database

Question

详细日志

error: Cannot figure out how to save this field into database. You can 
consider adding a type converter for it.
private final java.util.Date mTime = null;

我有一个字段为

的实体

var mStartTime : Date = Date() // java.util.Date

为什么 Room 不能持久化 Date 对象？什么是 Date 的最佳转换器？

Answer 1

Date正是https://developer.android.com/training/data-storage/room/referencing-data中给出的例子。

For example, if we want to persist instances of Date, we can write the following TypeConverter to store the equivalent Unix timestamp in the database:
public class Converters {
    @TypeConverter
    public static Date fromTimestamp(Long value) {
        return value == null ? null : new Date(value);
    }
    @TypeConverter
    public static Long dateToTimestamp(Date date) {
        return date == null ? null : date.getTime();
    }
}
The preceding example defines 2 functions, one that converts a Date object to a Long object and another that performs the inverse conversion, from Long to Date. Since Room already knows how to persist Long objects, it can use this converter to persist values of type Date.

Next, you add the @TypeConverters annotation to the AppDatabase class so that Room can use the converter that you've defined for each entity and DAO in that AppDatabase:

AppDatabase.java
@Database(entities = {User.class}, version = 1)
@TypeConverters({Converters.class})
public abstract class AppDatabase extends RoomDatabase {
    public abstract UserDao userDao();
}

旁注：java.util.Date 被认为设计不当（java.util.Calendar 更糟）。如果您有任何重要的日期时间逻辑并且可以摆脱 API 级别 26（桌面上的 Java 8），通常最好使用 java.time package. And if you can't, see https://github.com/JakeWharton/ThreeTenABP 作为后向端口。

Answer 2

    // Java code will not convert to Kotlin very 
    // well so here is the Kotlin: Converter 
    // class

    public class Converters {
        @TypeConverter
        fun fromTimestamp( value: Long?) : 
                       java.sql.Date {
            return java.sql.Date(value ?: 0)
        }
        @TypeConverter
        fun dateToTimestamp(date :java.sql.Date?) 
                                 :Long {
            return date?.getTime() ?: 0
       }

    // Here is the type converters example in 
    // Kotlin
    @Database(entities = [DbNasaPictures::class], 
              version = 2)
    @TypeConverters(Converters::class)
    abstract class PicturesDatabase: 
                     RoomDatabase() {

Answer 3

以上所有答案都是针对以下 strings.But 的列表，可帮助您找到对象列表的转换器。

就在“YourClassName”的位置，添加您的对象 class.

 @TypeConverter
        public String fromValuesToList(ArrayList<**YourClassName**> value) {
            if (value== null) {
                return (null);
            }
            Gson gson = new Gson();
            Type type = new TypeToken<ArrayList<**YourClassName**>>() {}.getType();
            return gson.toJson(value, type);
        }
    
        @TypeConverter
        public ArrayList<**YourClassName**> toOptionValuesList(String value) {
            if (value== null) {
                return (null);
            }
            Gson gson = new Gson();
            Type type = new TypeToken<List<**YourClassName**>>() {
            }.getType();
            return gson.fromJson(value, type);
        }

Answer 4

在Kotlin中，需要在两个文件中添加@TypeConverters

数据库 class 其中扩展了 RoomDatabase

@Database(entities = [ToDo::class], version = 1, exportSchema = false)
@TypeConverters(DateConverter::class)
abstract class AppDatabase : RoomDatabase() {

    abstract fun todoDao(): ToDoDao

    companion object {
        @Volatile private var instance: AppDatabase? = null

        fun getDatabase(context: Context): AppDatabase =
            instance ?: synchronized(this) { instance ?: buildDatabase(context).also { instance = it } }

        private fun buildDatabase(appContext: Context) =
            Room.databaseBuilder(appContext, AppDatabase::class.java, "todo")
                .fallbackToDestructiveMigration()
                .build()
    }

}

数据class其中@Entity声明需要添加DateConverter class for Date

@Entity(tableName = "todo")
data class ToDo(
    @PrimaryKey
    val id: Int,
    val title: String,
    val description: String,
    val time: String,
    val date: String,
    val types: Int,
    @TypeConverters(DateConverter::class)
    val date_time: Date,
    val created: String
)

日期转换器class

import androidx.room.TypeConverter
import java.util.*

class DateConverter {

    @TypeConverter
    fun toDate(timestamp: Long): Date {
        return Date(timestamp)
    }

    @TypeConverter
    fun toTimestamp(date: Date): Long {
        return date.time
    }
}

Android Room - error: Cannot figure out how to save this field into database

Android Room - error: Cannot figure out how to save this field into database

java

android

kotlin

android-room