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[Android] Room - Alternative to SQLite

2022-07-31 02:45:00 【Bin Youwei】

SQLitehow comeRoom给替代了？

SQLiteWhy is it replaced？

Sqlite从2000年问世至今,已有20多年历史,在这二十多年里,Sqlite已由原先的1.0version upgrade current3.0版本,but,even nowAndroid最新支持的2.3.0版本,它在AndroidThere are still a series of shortcomings,如下：

没有原始 SQL 查询的编译时验证.
requires a lot of boilerplate code in SQL 查询和数据对象之间进行转换.
虽然这些 API 功能强大,but they are quite low level,Requires a lot of time and effort to use.

针对Sqlite一系列的问题,realm（适用于ios、Android）、ObjectBox（跨端,Claims to be the fastest）、greenDaoetc. local database target,all to replaceSQLite,甚至到后面,Even the official out of a new database —— Room,It's going to go completelySqlite化？

答案是否定的,以上的数据库中,greenDAO 、Room都是在Sqlite进行修改、优化.ObjectBox是由greenDAO products developed by the development team,他们在greenDAO的Github首页,向大家推荐ObjectBox数据库,而greenDAOFor more than a year did not update,也许,是greenDaoThe peak has passed,It is difficult to stand on the top again, so they are slowly losing hope for the future（个人猜想）.

那为啥,取代SQLite的是Room而不是greenDao呢？个人认为,It's all because of the official“官宣”导致的.在Android Developers官网找到SQLite's introductory page and user guide,you will find words like this：

we highly recommended using the Room Persistence Library as an abstraction layer for accessing information in your app’s SQLite databases.
我们强烈建议使用 Room Persistence Library as an abstraction layer to access the application's SQLite 数据库中的信息.

在这里插入图片描述
放弃SQLite,persuade us to useRoom的原因就是SQLite的缺点与Room的优点.

Room优点：

简化的数据库迁移路径.
SQL 查询的编译时验证.
Handy Notes,Minimize duplication and error-prone boilerplate code.

集成Room

Room 包含三个主要组件,分别是：

Database：The database class that holds the database and serves as the main access point for the underlying connection to the application's persistent data.
Entity：A data entity representing a table in the application database.
Dao：数据访问对象,Provide your application can be used to query the database、更新、Methods of inserting and deleting data.

导入依赖

implementation("androidx.room:room-runtime:2.4.2")
annotationProcessor("androidx.room:room-compiler:2.4.2")

// 使用 Kotlin 注释处理工具 (kapt)
kapt("androidx.room:room-compiler:2.4.2")

注意：Kotlin Plugin support for annotation processors,如 Dagger 或 DBFlow . in order for them to be Kotlin 类一起工作,使用kapt需要应用kotlin-kapt插件.

apply plugin: 'kotlin-kapt'

Click to view the latest version dependencies

RoomThere are three steps to using,分别是Database、Entity、Dao,The data can only be written after the creation is completed、修改等操作.

Entity（数据实体）

在 Room 中,You can define entities to represent the objects you want to store.Each entity corresponds to the associated Room 数据库中的一个表,Each instance of an entity represents a row of data in the corresponding table.

Kotlin 版本的 Entity 与 Java 版本的 Entity 写法不一,Java 版本的 Entity The normal way of writing is to create a new entity class and implement variablesget、set方法,Kotlin 版本的 Entity 使用了data class关键字,在 Kotlin 中,使用data classThe marked class is a data class,This class will automatically rewrite Kotlin 的超类Any的equals(other: Any?)、hashCode()、toString()方法,For more details, please go to→Kotlin 数据类(Data)

将data class声明为 Room 的数据类,需使用@Entity注解声明,默认情况下,Room 会使用类名作为数据库表名,If you do not want the class name to be the same as the database table name,可在@Entity注解设置tableName参数,如：@Entity(tableName = "自定义的数据库表名"),添加参数后,At this time, the database table name generated by this class will no longer be the class name..

@Entity
data class Classes(
    @PrimaryKey val classesId: String,
    @ColumnInfo val classesName: String
)

其它注解

Annotation	Describe
@PrimaryKey	用于唯一标识相应数据库表中的每一行
@ColumnInfo	Define the column names of the data table
@Ignore	Do not add fields to datatables
@NonNull	The return value of a field or method cannot benull

@Entity详细使用方式见：使用 Room 实体定义数据

Dao（数据访问对象）

Dao：Provides methods for the rest of the application to interact with data in the data table.

Dao是一个接口类,Do not need to write the method body,Annotation can be used directly instead,Additions, deletions, and changes correspond to the following four annotations respectively：

Annotation	Describe
@Insert	增
@Delete	删
@Update	改
@Query	查

注解使用方式如下：

@Dao
interface ClassesDao {
    
    /** * Add multiple classes */
    @Insert
    fun addMultipleClasses(vararg classes: Classes)
	/** * 删除某条数据 */
    @Delete
    fun deleteClasses(classes: Classes)
    /** * 修改某条数据 */
    @Update
    fun updateClasses(classes: Classes)
    /** * 查询classes表的数据 */
    @Query("select * from classes")
    fun getAllClasses() : List<Classes>
}

在 Room 中,Queries with values are represented by colons,如下代码,means looking for className Equal to invoke the interface with the value.

@Query("select * from classes where className = :classesName")
fun getAllClasses(classesName: String) : List<Classes>

Dao详细使用方式见：使用 Room DAO 访问数据

Database（数据库）

Entity、Dao都已经定义了,The last step is to create the database class to hold the data,The created class must meet the three conditions of the database：

The class must be an extensionRoomDatabase类的抽象类 .
对于与数据库关联的每个 Dao 类,The database class must define a class with zero parameters and return Dao abstract method of class instance.
该类必须使用@Database包含一个entitiesAnnotation of the array to annotate,This array lists all data entities associated with the database（包括数据库版本）.

@Database(entities = [Classes::class/*Entities contained in the database*/], version = 1, exportSchema = false)
abstract class SchoolDatabase : RoomDatabase() {
    
    /** * 调用该方法获取Dao,来调用Classestable interface methods */
    abstract fun classDao(): ClassesDao
}

使用

Room.databaseBuilder(applicationContext, SchoolDatabase::class.java, "数据库名称")
     .build()
     .classDao()
     .addClasses(classes)

效果

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数据迁移

Why do databases need to migrate data？

Everyone may encounter such a problem in the process of developing a local database,After modifying the fields in the database,Restart will crash.

When some students encounter this problem, they may use such keywords to search：The solution to the flashback of database field modification.The result of the search is that you will be asked to uninstallapp重新安装就好,problem can be solved.但是,This seemingly correct method is actually fatal.

Modify the local database once,打包发布,用户更新appThen if you use a local database,会报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.的异常错误.

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一个appinterface if in8秒内没有加载完成,就会流失70%用户,8second law,How many users will be lost in a flashback?？解决上述问题,Only the database can be migrated,And data migration,Divided into automatic migration and manual migration.

自动迁移

Automatic migration provides four annotations for automatic migration,分别是：

使用方式如下：

// Database class before version update.
@Database(
  version = 1,
  entities = [User::class]
)
abstract class AppDatabase : RoomDatabase() {
    
  ...
}

// Database class after version update.
@Database(
  version = 2,
  entities = [User::class],
  exportSchema = true,
  autoMigrations = [
    AutoMigration (from = 1, to = 2)
  ]
)
abstract class AppDatabase : RoomDatabase() {
    
  ...
}

Since currently only the above four annotations are officially provided for automatic migration,How to add fields becomes a mystery,If you learned about the newly added data table fields of automatic migration when watching this article,Hope you can share it in the comments.

注意：
1、使用自动迁移exportSchema必须设置为true,Avoid migration failures.
2、Automatic migration is only available for 2.4.0-alpha01 及更高版本,低版本、Scenarios involving complex schema changes must use manual migrations.

参考文章：Room 数据库迁移

手动迁移

The first step of manual database migration,Modify the data table structure first.

@Entity
data class Classes(
    @PrimaryKey(autoGenerate = true) val classesId: Int = 0,
    @ColumnInfo(name = "className") var classesName: String,
    // 新增的列
    @ColumnInfo(name = "classesNum") var classesNum: String
)

Manual migration using toRoomDatabase.Builder的addMigrations(@NonNull Migration... migrations)方法,将新增的MigrationClasses are put in to complete the migration.

Room 的底层使用的是 SQLite ,The data migration usesSQLite 的 execSQL(String sql)方法执行 sql 语句对数据库进行修改,to complete the migration,代码如下：

var db: SchoolDatabase ?= null
// 单例模式的双重检查锁
fun getDataBase(context: Context) : SchoolDatabase{
    
    if (db == null){
    
        synchronized(SchoolDatabase::class.java){
    
            if (db == null){
    
                db = Room.databaseBuilder(context.applicationContext, SchoolDatabase::class.java, "school")
                    // .fallbackToDestructiveMigration()
                    // 添加数据迁移
                    .addMigrations(MIGRATION_1_TO_2)
                    .build()
            }
        }
    }
    return db as SchoolDatabase;
}

// MigrationThe two parameters of the instance refer to what version the current version is upgraded to
val MIGRATION_1_TO_2 : Migration = object : Migration(1, 2) {
    
    override fun migrate(database: SupportSQLiteDatabase) {
    
        // Add a new column using 底层SQLite + sql 语法,The same is true for other additions, deletions and modifications
        database.execSQL("alter table classesNum add column 新增的列名 列名的数据类型 default 'null'")
    }
}

Finally, don't forget to put on the database class name@Database注解的versionparameter to the latest version,再次 run 即可完成迁移.

参考：Android开发基础教程

结合RxJava使用

Room 执行数据库操作,By default, child threads are used to execute,Callable if it needs to be executed on the main threadRoom.databaseBuilder的allowMainThreadQueries()来禁用 Room The main thread query checks,但不建议这样操作,This will increase the trigger ANR 的概率.

如果每次执行 Room Create a new operationThread并实现Runnable接口,It's not easier to trigger ANR 吗？

为此,officially launched a Rxjava Libraries used in combination,Easy to switch between threads.

// optional - RxJava3 support for Room
implementation("androidx.room:room-rxjava3:2.4.2")
// RxJava3
implementation ("io.reactivex.rxjava3:rxjava:3.1.3")
implementation ("io.reactivex.rxjava3:rxandroid:3.0.0")