Android : Room 数据库的基本用法 —简单应用_三_版本

在实体类中添加了新字段：

@Entity(tableName = "people")
public class People {//新添加的字段private String email;public String getEmail() {return email;}public void setEmail(String email) {this.email = email;}}

再次编译启动时会报错：

  java.lang.RuntimeException: Exception while computing database live data.
Caused by: java.lang.IllegalStateException: 
Room cannot verify the data integrity. 
Looks like you've changed schema but forgot to update the version number.You can simply fix this by increasing the version number. 
Expected identity hash: e24ea0779cf1452b08cb8aa058d3f37e, found: 2439a6f43e94aacbdd70254b0a25f90f

需要修改PeopleDataBase.java 更改版本号 和 策略 

1.版本发生改变 不保留之前的数据 fallbackToDestructiveMigration()

//之前版本是1 手动修改为2
@Database(entities = {People.class}, version = 2, exportSchema = false)
public abstract class PeopleDataBase extends RoomDatabase {private static PeopleDataBase peopleDataBase;public static synchronized PeopleDataBase getPeopleDataBase(Context context){if(peopleDataBase == null){peopleDataBase = Room.databaseBuilder(context.getApplicationContext(),PeopleDataBase.class,"peopleDB").fallbackToDestructiveMigration()//1.版本发生改变 不保留之前的数据.build();}return peopleDataBase;}public abstract  PeopleDao peopleDao();
}

2. 版本发生改变 保留之前的数据    addMigrations(MIGRATION_1_2)

//之前版本是1 手动修改为2
@Database(entities = {People.class}, version = 2, exportSchema = false)
public abstract class PeopleDataBase extends RoomDatabase {private static PeopleDataBase peopleDataBase;public static synchronized PeopleDataBase getPeopleDataBase(Context context){if(peopleDataBase == null){peopleDataBase = Room.databaseBuilder(context.getApplicationContext(),PeopleDataBase.class,"peopleDB").addMigrations(MIGRATION_1_2)  //2.版本发生改变 会保留之前的数据.build();}return peopleDataBase;}public abstract  PeopleDao peopleDao();//版本迁移策略static final Migration MIGRATION_1_2 = new Migration(1,2) {@Overridepublic void migrate(@NonNull SupportSQLiteDatabase database) {//手动添加字段database.execSQL("alter table people add column email text default [e-mail]");}};}

实体类中某一个字段不要了，删除某一列数据时：需要4步骤

//之前版本是1 手动修改为2
@Database(entities = {People.class}, version = 3, exportSchema = false)
public abstract class PeopleDataBase extends RoomDatabase {private static PeopleDataBase peopleDataBase;public static synchronized PeopleDataBase getPeopleDataBase(Context context){if(peopleDataBase == null){peopleDataBase = Room.databaseBuilder(context.getApplicationContext(),PeopleDataBase.class,"peopleDB").addMigrations(MIGRATION_2_3)  //2.版本发生改变 会保留之前的数据.build();}return peopleDataBase;}public abstract  PeopleDao peopleDao();//版本迁移策略 删除某一列static final Migration MIGRATION_2_3 = new Migration(2,3) {@Overridepublic void migrate(@NonNull SupportSQLiteDatabase database) {//1.创建临时表database.execSQL("create table people_temp ( id Integer primary key not null , user_name text , age integer,sex text)");//2.往临时表中插入数据database.execSQL("insert into people_temp ( id, user_name, age, sex) select id,user_name,age,sex from people ");//3.删除people表database.execSQL("drop table people");//4.更改表名字，把临时表修改成people表database.execSQL("alter table people_temp rename to people");}};}