Android SQLite数据库demo。架构组件Room

Android架构组件Room

Room主要包含三个部分：

Database : 持有DB和DAO
Entity : 定义POJO类，即数据表结构
DAO(Data Access Objects) : 定义访问数据（增删改查）的接口

@Entity(tableName = "table_name**") 注解POJO类，定义数据表名称;
@PrimaryKey 定义主键，如果一个Entity使用的是复合主键，可以通过@Entity注解的primaryKeys 属性定义复合主键：@Entity(primaryKeys = {"firstName", "lastName"})
@ColumnInfo(name = “column_name”) 定义数据表中的字段名
@Ignore 用于告诉Room需要忽略的字段或方法
建立索引：在@Entity注解的indices属性中添加索引字段。例如：indices = {@Index(value = {"first_name", "last_name"}, unique = true), ...}, unique = true可以确保表中不会出现{"first_name", "last_name"} 相同的数据。

1.2 Entitiy间的关系

不同于目前存在的大多数ORM库，Room不支持Entitiy对象间的直接引用。（具体原因可以参考： Understand why Room doesn't allow object references）
但Room允许通过外键(Foreign Key)来表示Entity之间的关系。

@Entity(foreignKeys = @ForeignKey(entity = User.class,parentColumns = "id",childColumns = "user_id"))
class Book {@PrimaryKeypublic int bookId;public String title;@ColumnInfo(name = "user_id")public int userId;
}

如上面代码所示，Book对象与User对象是属于的关系。Book中的user_id,对应User中的id。那么当一个User对象被删除时，对应的Book会发生什么呢？

@ForeignKey注解中有两个属性onDelete和onUpdate，这两个属性对应ForeignKey中的onDelete()和onUpdate(), 通过这两个属性的值来设置当User对象被删除／更新时，Book对象作出的响应。这两个属性的可选值如下：

CASCADE：User删除时对应Book一同删除；更新时，关联的字段一同更新
NO_ACTION：User删除时不做任何响应
RESTRICT：禁止User的删除／更新。当User删除或更新时，Sqlite会立马报错。
SET_NULL：当User删除时， Book中的userId会设为NULL
SET_DEFAULT：与SET_NULL类似，当User删除时，Book中的userId会设为默认值

2. 创建数据访问对象（DAO）

@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);@Insertvoid insertAll(List<User> users);@Insert(onConflict = OnConflictStrategy.REPLACE)public void insertUsers(User... users);@Deletevoid delete(User user);@Updatepublic void updateUsers(List<User> users);
}

DAO 可以是一个接口，也可以是一个抽象类， Room会在编译时创建DAO的实现。

2.2 查询参数传递

@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);

看代码应该比较好理解，方法中传递参数arg, 在sql语句中用:arg即可。编译时R

2.3 查询表中部分字段的信息

在实际某个业务场景中，我们可能仅关心一个表部分字段的值，这时我仅需要查询关心的列即可。

定义子集的POJO类:

public class NameTuple {@ColumnInfo(name="first_name")public String firstName;@ColumnInfo(name="last_name")public String lastName;
}

在DAO中添加查询方法：

@Query("SELECT first_name, last_name FROM user")
public List<NameTuple> loadFullName();

这里定义的POJO也支持使用@Embedded

2.4 联表查询

Room支持联表查询，接口定义上与其他查询差别不大，主要还是sql语句的差别。

@Dao
public interface MyDao {@Query("SELECT * FROM book "+ "INNER JOIN loan ON loan.book_id = book.id "+ "INNER JOIN user ON user.id = loan.user_id "+ "WHERE user.name LIKE :userName")public List<Book> findBooksBorrowedByNameSync(String userName);
}

2.4 联表查询

Room支持联表查询，接口定义上与其他查询差别不大，主要还是sql语句的差别。

@Dao
public interface MyDao {@Query("SELECT * FROM book "+ "INNER JOIN loan ON loan.book_id = book.id "+ "INNER JOIN user ON user.id = loan.user_id "+ "WHERE user.name LIKE :userName")public List<Book> findBooksBorrowedByNameSync(String userName);
}

Room提供了Migration类来实现数据库的升级:

Room.databaseBuilder(getApplicationContext(), MyDb.class, "database-name").addMigrations(MIGRATION_1_2, MIGRATION_2_3).build();static final Migration MIGRATION_1_2 = new Migration(1, 2) {@Overridepublic void migrate(SupportSQLiteDatabase database) {database.execSQL("CREATE TABLE `Fruit` (`id` INTEGER, "+ "`name` TEXT, PRIMARY KEY(`id`))");}
};static final Migration MIGRATION_2_3 = new Migration(2, 3) {@Overridepublic void migrate(SupportSQLiteDatabase database) {database.execSQL("ALTER TABLE Book "+ " ADD COLUMN pub_year INTEGER");}
};

在创建Migration类时需要指定startVersion和endVersion, 代码中MIGRATION_1_2和MIGRATION_2_3的startVersion和endVersion是递增的， Migration其实是支持从版本1直接升到版本3，只要其migrate()方法里执行的语句正常即可。那么Room是怎么实现数据库升级的呢？其实本质上还是调用SQLiteOpenHelper.onUpgrade，Room中自己实现了一个SQLiteOpenHelper，在onUpgrade()方法被调用时触发Migration，当第一次访问数据库时，Room做了以下几件事：

创建Room Database实例
SQLiteOpenHelper.onUpgrade被调用，并且触发Migration
打开数据库

架构组件Room、demo了解：https://download.csdn.net/download/meixi_android/19386289

原生sqlite、demo链接：https://download.csdn.net/download/meixi_android/10710400

创建数据库版本v.1

/*** 作者：created by meixi* 邮箱：13164716840@163.com* 日期：2018/10/9 11*/
public class DBHelper extends SQLiteOpenHelper {private static final String DATABASE_NAME = "mall.db";private static final int DATABASE_VERSION = 1000;private static DBHelper instance = null;public DBHelper(Context context) {super(context, DATABASE_NAME, null, DATABASE_VERSION);}public synchronized static DBHelper getInstance(Context context) {if (instance == null) {instance = new DBHelper(context);}return instance;}@Overridepublic void onCreate(SQLiteDatabase db) {db.execSQL(SQL.CREATE_TABLE_FAVORITE);// 若不是第一个版本安装，直接执行数据库升级// 请不要修改FIRST_DATABASE_VERSION的值，其为第一个数据库版本大小final int FIRST_DATABASE_VERSION = 1000;onUpgrade(db, FIRST_DATABASE_VERSION, DATABASE_VERSION);}@Overridepublic void onUpgrade(SQLiteDatabase db, int oldVersion, int newVersion) {// 使用for实现跨版本升级数据库for (int i = oldVersion; i < newVersion; i++) {switch (i) {default:break;}}}
}

创建数据库语句：

public class SQL {public static final String T_FAVORITE = "favorite";public static final String CREATE_TABLE_FAVORITE ="CREATE TABLE IF NOT EXISTS " + T_FAVORITE + "(" +"_id integer primary key autoincrement, " +"title VARCHAR, " +"url VARCHAR, " +"createDate VARCHAR " +")";
}

实现增删改查activity：

public class MainActivity extends AppCompatActivity {private static String CREATE_TABLE ="create table number(_id integer primary key autoincrement ,phone real)";int ab  = 0,cd;@Overrideprotected void onCreate(Bundle savedInstanceState) {super.onCreate(savedInstanceState);setContentView(R.layout.activity_main);Button button1=findViewById(R.id.baocunid);Button button2=findViewById(R.id.duquid);Button button3=findViewById(R.id.shanchuid);Button button4=findViewById(R.id.xiugaiid);button1.setOnClickListener(new View.OnClickListener() {@Overridepublic void onClick(View view) {ab++;boolean nn = baocun(MainActivity.this, new String[] { "114499title","rullll---htt;s=="+ab,"加入的xxx" });Log.i("lgq","sbbbb---"+nn);}});button2.setOnClickListener(new View.OnClickListener() {@Overridepublic void onClick(View view) {getBai(MainActivity.this);}});button3.setOnClickListener(new View.OnClickListener() {@Overridepublic void onClick(View view) {String ss= Delete(MainActivity.this,"1");Log.i("lgq","shanchu=g==="+ss);}});button4.setOnClickListener(new View.OnClickListener() {@Overridepublic void onClick(View view) {String ss = updata(MainActivity.this,"");Log.i("lgq","xiugg=g==="+ss);}});}public static boolean baocun(Context context, String[] values) {SQLiteDatabase db2=null;DBHelper2 dbHelper = null;try {dbHelper =new DBHelper2(context);db2 = dbHelper.getWritableDatabase();db2=dbHelper.getWritableDatabase();db2.beginTransaction();db2.execSQL("insert into favorite (title,url,deleted) values (?,?,?)",values);db2.setTransactionSuccessful();db2.endTransaction();return true;} catch (Exception e) {e.printStackTrace();return false;}finally{}}public  List<String> getBai(Context context){List<String> list = new ArrayList<String>();SQLiteDatabase db2=null;DBHelper2 dbHelper = null;try {dbHelper= new DBHelper2(context);db2=dbHelper.getWritableDatabase();Cursor cursor =db2.rawQuery("select _id,title,deleted from favorite", null);while(cursor.moveToNext()){cd++;
//                int dbid = cursor.getInt(cursor.getColumnIndex("_id"));int id=cursor.getInt(cursor.getColumnIndex("_id"));String haoma = cursor.getString(cursor.getColumnIndex("title"))+"";String names = cursor.getString(cursor.getColumnIndex("deleted"));list.add(haoma);System.out.println("lgqs======"+haoma+"......."+names+"     "+cd+"    id===  "+id);}db2.setTransactionSuccessful();db2.endTransaction();} catch (Exception e) {e.printStackTrace();}return list;}public static String Delete(Context context,String string){SQLiteDatabase db2=null;DBHelper2 dbHelper = null;try {dbHelper =new DBHelper2(context);db2 = dbHelper.getWritableDatabase();db2=dbHelper.getWritableDatabase();db2.beginTransaction();db2.execSQL("delete from favorite where _id=?",new String[] { string });db2.setTransactionSuccessful();db2.endTransaction();return "成功";} catch (Exception e) {e.printStackTrace();return "失败";}}public static String updata(Context context,String string){SQLiteDatabase db2=null;DBHelper2 dbHelper = null;try {dbHelper =new DBHelper2(context);db2 = dbHelper.getWritableDatabase();db2=dbHelper.getWritableDatabase();db2.beginTransaction();db2.execSQL("update favorite set title ="+"2222999"+" where url =?",new String[]{"rullll---htt;s==3"});db2.setTransactionSuccessful();db2.endTransaction();return "成功";} catch (Exception e) {e.printStackTrace();return "失败";}}
}