一、基礎(chǔ)使用

1.1 Room三個(gè)主要操作類

Entity 實(shí)體類代表一個(gè)表中的字段
Dao數(shù)據(jù)庫操作接口提供增刪改查
管理創(chuàng)建數(shù)據(jù)庫

1.2 引入依賴

https://developer.android.com/jetpack/androidx/releases/room
選擇性引入

dependencies {
  def room_version = "2.2.5"

  implementation "androidx.room:room-runtime:$room_version"
  annotationProcessor "androidx.room:room-compiler:$room_version" // For Kotlin use kapt instead of annotationProcessor

  // optional - Kotlin Extensions and Coroutines support for Room
  // implementation "androidx.room:room-ktx:$room_version"

  // optional - RxJava support for Room
  // implementation "androidx.room:room-rxjava2:$room_version"

  // optional - Guava support for Room, including Optional and ListenableFuture
  // implementation "androidx.room:room-guava:$room_version"

  // Test helpers
  testImplementation "androidx.room:room-testing:$room_version"
}

1.3 創(chuàng)建數(shù)據(jù)庫實(shí)體 Entity

@Entity(tableName = "User")  //數(shù)據(jù)庫中表名
public class User {
    //主鍵 自增
    @PrimaryKey(autoGenerate = true)
    private int id;
    @ColumnInfo(name = "user_name", typeAffinity = ColumnInfo.TEXT) //實(shí)際數(shù)據(jù)庫中的字段user_name 數(shù)據(jù)類型
    private String name;
    @ColumnInfo(name = "user_gender")
    private String gender;
    private int age;
    @Ignore //忽略字段
    private int flag;

    public User(int id, String name, String gender, int age) {
        this.id = id;
        this.name = name;
        this.gender = gender;
        this.age = age;
    }

    public User(String name, String gender, int age) {
        this.name = name;
        this.gender = gender;
        this.age = age;
    }

    @Ignore
    public User(String gender, int age) {
        this.gender = gender;
        this.age = age;
    }
  // 省略 get set ......
}

構(gòu)造方法中的形參代表查詢時(shí)需要查詢的字段儒老，如果有多個(gè)構(gòu)造方法可以使用注解@Ignore忽略, 同樣

1.4 創(chuàng)建數(shù)據(jù)庫操作接口 Dao

@Dao //Database access object  數(shù)據(jù)庫訪問接口 所有增刪改查等操作都在此聲明
public interface UserDao {

    // long Not sure how to handle insert method's return type.long
    @Insert
    void insertUser(User... users);

    // int 影響的行數(shù)
    @Update
    int updateUser(User... users);

    @Delete
    void deleteUser(User... users);

    @Query("DELETE FROM USER")
    void deleteUser();

    @Query("SELECT * FROM USER ORDER BY ID DESC")
    List<User> getAllUser();
}

1.4.1 唯一約束+REPLACE實(shí)現(xiàn)有則更新無則插入

實(shí)體類中通過indices 將name字段設(shè)置成唯一約束

@Entity(tableName = "chatrow", indices = {@Index(value = {"name"}, unique = true)} )  //數(shù)據(jù)庫實(shí)體類
public class User {...}

Dao中插入語句設(shè)置onConflict插入模式

public interface UserDao {
    @Insert(onConflict = OnConflictStrategy.REPLACE)
    void insertUser(User... users);
}

插入時(shí)如果表中有名字相同則更新本條數(shù)據(jù) 否則插入
更多語法參考官網(wǎng) https://developer.android.com/training/data-storage/room/accessing-data?hl=zh-cn

1.5 創(chuàng)建數(shù)據(jù)庫管理 Database

// entities ={多個(gè)表逗號隔開}
@Database(entities = {User.class}, version = 1, exportSchema = false)
public abstract class MyDatabase extends RoomDatabase {
    public abstract UserDao getUserDao();
    // 若entities有多個(gè)實(shí)例，則應(yīng)該寫多個(gè)Dao
}

1.6 使用

操作數(shù)據(jù)庫必須在子線程中挣棕，為了簡單此處直接使用allowMainThreadQueries()強(qiáng)制在主線程運(yùn)行，正常開發(fā)不允許

MyDatabase myDatabase = Room.databaseBuilder(this, MyDatabase.class, "user")
        .allowMainThreadQueries()//強(qiáng)制在主線程運(yùn)行 不建議
        .build();
UserDao userDao = myDatabase.getUserDao();
//增加
userDao.insertUser(new User("nikola", "男", 22));
//修改ID為3的
int row = userDao.updateUser(new User(3, "kolia", "女", 22));
//查詢所有
List<User> allUser = userDao.getAllUser();

二亲桥、使用單例注意事項(xiàng)

Google官方稱創(chuàng)建數(shù)據(jù)庫實(shí)例較耗時(shí)洛心，所以使用單例模式，但要注意: 無參構(gòu)造方法不能使用private修飾 否則Room自動生成實(shí)現(xiàn)類時(shí)會報(bào)錯(cuò)

// entities ={多個(gè)集合逗號隔開}
@Database(entities = {User.class}, version = 1, exportSchema = false)
public abstract class MyDatabase extends RoomDatabase {

    private static final String DATABASE_NAME="my_db.db";
    private static MyDatabase myDatabase;

    public MyDatabase() {
    }
    
    synchronized public static MyDatabase getInstance(Context context) {
        if (null == myDatabase) {
            myDatabase = Room.databaseBuilder(context.getApplicationContext(), MyDatabase.class, DATABASE_NAME)
                    .allowMainThreadQueries() //強(qiáng)制在主線程運(yùn)行 不建議
                    .build();
        }
        return myDatabase;
    }

    public abstract UserDao getUserDao();
    // 若entities有多個(gè)實(shí)例题篷，則應(yīng)該寫多個(gè)Dao
}

指定數(shù)據(jù)庫db文件保存路徑

synchronized public static MyDatabase getInstance(Context context) {
        if (null == myDatabase) {
            File priExternalDir = FileUtil.getPriExternalDir(context, Environment.DIRECTORY_DOWNLOADS);
            final String dbPath = new File( priExternalDir.getAbsolutePath()+File.separator+DATABASE_NAME).getPath();
            myDatabase = Room.databaseBuilder(context.getApplicationContext(), MyDatabase.class, dbPath)
                    .allowMainThreadQueries() //強(qiáng)制在主線程運(yùn)行 不建議
                    .build();
        }
        return myDatabase;
    }

三词身、 LiveData 監(jiān)聽數(shù)據(jù)庫變化

3.1 返回值使用LiveData包裝

數(shù)據(jù)變化只在查詢時(shí)提現(xiàn)，所以修改UserDao中查詢所有的返回值類型, 且LiveData內(nèi)部已經(jīng)實(shí)現(xiàn)再子線程中執(zhí)行悼凑，可以直接調(diào)用

@Query("SELECT * FROM USER ORDER BY ID DESC")
    // List<User> getAllUser();
    LiveData<List<User>> getAllUserLive();

3.2 設(shè)置監(jiān)聽

LiveData<List<User>> listLiveData  = userDao.getAllUserLive();
listLiveData.observe(this, new Observer<List<User>>() {
    @Override
    public void onChanged(List<User> users) {
     ......
    }
});

當(dāng)對數(shù)據(jù)庫進(jìn)行增刪改時(shí)會回調(diào)onChanged

四偿枕、數(shù)據(jù)庫版本遷移

變更Entity中的字段要增加 Database中的version，
遷移數(shù)據(jù)庫有保留原有數(shù)據(jù)與破壞式清空原有數(shù)據(jù)

4.1 破壞式

allowMainThreadQueries 不保留數(shù)據(jù)直接刪除原有數(shù)據(jù)庫

MyDatabase myDatabase = Room.databaseBuilder(this, MyDatabase.class, "user")
    .allowMainThreadQueries()//強(qiáng)制在主線程運(yùn)行 不建議
    .fallbackToDestructiveMigration() //破壞式: 當(dāng)數(shù)據(jù)庫版本變化時(shí)不做數(shù)據(jù)遷移
    .build();

4.2 保留原有數(shù)據(jù)遷移

當(dāng)Entity字段改變時(shí)調(diào)用.addMigrations(Migration... migrations)户辫，指定從某個(gè)版本遷移到某個(gè)版本需要做的某項(xiàng)操作渐夸，具體操作定義在Migration中

MyDatabase myDatabase = Room.databaseBuilder(this, MyDatabase.class, "user")
    .allowMainThreadQueries()//強(qiáng)制在主線程運(yùn)行 不建議
    .addMigrations(migration) //保留原有數(shù)據(jù)
    .build();

表中增加type字段時(shí)的addMigrations 實(shí)參 migration如下，需要自行寫增加字段語句

static final Migration migration = new Migration(2,3) {
    @Override
    public void migrate(@NonNull SupportSQLiteDatabase database) {
        database.execSQL("ALTER TABLE user ADD COLUMN type INTEGER NOT NULL DEFAULT 1");
    }
};

刪除某個(gè)字段渔欢，sqlLet沒有刪除字段語句墓塌，只能創(chuàng)建新的數(shù)據(jù)庫定義需要的字段，將原有數(shù)據(jù)庫數(shù)據(jù)復(fù)制過去奥额，刪除舊數(shù)據(jù)庫后再將新數(shù)據(jù)庫重命名
new Migration(3,4) 表示從版本3升級到版本4

static final Migration migration = new Migration(3,4) {
    @Override
    public void migrate(@NonNull SupportSQLiteDatabase database) {
        database.execSQL("CREATE TABLE user_temp (id INTEGER PRIMARY KEY NOT NULL , user_name TEXT," +
                "user_gender TEXT)");
        database.execSQL("INSERT INTO user_temp (id, user_name, user_gender) " +
                "SELECT id, user_name, user_gender FROM user");
        database.execSQL("DROP TABLE user");
        database.execSQL("ALTER TABLE user_temp RENAME to user");
    }
};

Jetpack Room數(shù)據(jù)庫