今天在做 Room 迁移的时候，想测试Room 是否按预期工作，报上面错误：

检查发现 我报 它放在主线执行的使用报错了， 对数据库的操作由于是耗时操作，所以必须放在子线程中完成， 不允许在主线程上访问数据库，

Caused by: java.lang.IllegalStateException: Cannot access database on the main thread since it may potentially lock the UI for a long period of time.

java.lang.RuntimeException: Unable to start activity ComponentInfo{space.dua/com.fox.app.ui.local.AppActivity}: java.lang.IllegalStateException: Cannot access database on the main thread since it may potentially lock the UI for a long period of time.

/*** entities: 实体类* version: 数据库初始版本号* exportSchema: 是否允许数据库架构将导出到给定的文件夹中【 默认true 】* @Database(entities = [User::class], version = 1, exportSchema = false)*/

@Database(entities = [RuleBean::class, UserDate::class], version = 2)
abstract class BoxDatabase : RoomDatabase() {// 在类的主体内，声明一个返回 RuleDao 的抽象值。您可能有多个 DAO。abstract fun ruleDao(): RuleDao// TODO 定义一个 companion 对象。伴生对象允许客户端访问用于创建或获取数据库，而无需该类进行实例化的方法companion object {//TODO 提供一个Migration实例，指明迁移的版本号private val MIGRATION_1_2: Migration = object : Migration(1, 2) {override fun migrate(database: SupportSQLiteDatabase) {// Migration code here, if neededdatabase.execSQL("alter table Book add column chuban text");}}//TODO Room.databaseBuilder（）中传入上下文、数据库类，以及数据库的名称@SuppressLint("StaticFieldLeak")val instance = Room.databaseBuilder(ContextHolder.get(), BoxDatabase::class.java, "boxConfig").addMigrations(MIGRATION_1_2) .build()}}