Android使用Room持久化SQLite

原文链接

前言

Android中提供了SQLite数据库进行数据的持久化 ，并提供了对应API访问数据库，而Room框架提供了SQLite数据访问抽象层，为高效的数据库访问层带来便捷

APP可以缓存用户数据，当APP离线时便从SQLite读取数据，当重新连线时即可完成和服务器数据的同步

谷歌官方强烈推荐使用Room框架操作SQLite数据库

Hello World

首先在build.gradle中添加必要依赖

dependencies { def room_version = "1.1.1" implementation "android.arch.persistence.room:runtime:$room_version" annotationProcessor "android.arch.persistence.room:compiler:$room_version" // use kapt for Kotlin // optional - RxJava support for Room implementation "android.arch.persistence.room:rxjava2:$room_version" // optional - Guava support for Room, including Optional and ListenableFuture implementation "android.arch.persistence.room:guava:$room_version" // Test helpers testImplementation "android.arch.persistence.room:testing:$room_version" } 

创建实体类User,@Entity表示该类对应数据库中的表，@ColumnInfo后面的name属性对应数据库中的字段名，并实现该实体类的Getter，Setter方法

@Entity public class User { @PrimaryKey private int uid; @ColumnInfo(name = "first_name") private String firstName; @ColumnInfo(name = "last_name") private String lastName; public int getUid() { return uid; } public void setUid(int uid) { this.uid = uid; } public String getFirstName() { return firstName; } public void setFirstName(String firstName) { this.firstName = firstName; } public String getLastName() { return lastName; } public void setLastName(String lastName) { this.lastName = lastName; } @Override public String toString() { return "User{" + "uid=" + uid + ", firstName='" + firstName + '\'' + ", lastName='" + lastName + '\'' + '}'; } } 

创建实体类对应的dao层UserDao，完成User的增删改查(CRUD)接口定义，@Dao注解定义一个dao层,参数赋值(传递)使用:clumn_name进行赋值

 @Dao public interface UserDao { @Query("SELECT * FROM user") List<User> getAll(); @Query("SELECT * FROM user WHERE uid IN (:userIds)") List<User> loadAllByIds(int[] userIds); @Query("SELECT * FROM user WHERE first_name LIKE :first AND " + "last_name LIKE :last LIMIT 1") User findByName(String first, String last); @Insert(onConflict = OnConflictStrategy.REPLACE) void insertAll(User... users); @Delete void delete(User user); } 

创建AppDatabase，@Database注解表示这是一个数据库操作类，entities对应Entity实体类，version用于数据库版本升级，并在该抽象类中定义一个返回dao层的抽象方法

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

初始化用于操作数据库的实例对象AppDatabase，需要注意的是不能在主线程中初始化，必须新开启一个线程进行初始化，否则会报错，或者无法创建数据库

new Thread(new Runnable() { @Override public void run() { AppDatabase db = Room.databaseBuilder(getApplicationContext(), AppDatabase.class, "database-name").build(); } }).start(); 

测试

增加，也可以传一个User数组

 for (int i = 0; i < 10; i++) { User user = new User(); user.setUid(i); user.setFirstName("Shell" + i); user.setLastName("Hub" + i); insertAll(db, user); } 

如果报以下错误，修改dao层的注解为@Insert(onConflict = OnConflictStrategy.REPLACE)

android.database.sqlite.SQLiteConstraintException: UNIQUE constraint failed: User.uid (code 1555 SQLITE_CONSTRAINT_PRIMARYKEY) 

查询所有数据

for (User user : db.userDao().getAll()) { System.out.println(user); } 

other...

Room单例模式

最好使用设计模式中的单例模式获取数据库实例，因为每次获取数据库实例都很耗时并且耗内存，我们可以自定义一个类继承Application并定义一个public static方法获取数据库实例

 public class App extends Application { private static Context context; private static final String DATABASE_NAME = "SHELLHUB"; private static AppDatabase DB_INSTANCE; @Override public void onCreate() { super.onCreate(); context = getApplicationContext(); new Thread(new Runnable() { @Override public void run() { DB_INSTANCE = Room.databaseBuilder(getApplicationContext(), AppDatabase.class, DATABASE_NAME).build(); } }).start(); } public static AppDatabase getDB() { return DB_INSTANCE; } }