?Android架構(gòu)組件Room

Room主要包含三個部分：

• Database : 持有DB和DAO
• Entity : 定義POJO類，即數(shù)據(jù)表結(jié)構(gòu)
• DAO(Data Access Objects) : 定義訪問數(shù)據(jù)（增刪改查）的接口

• @Entity(tableName = "table_name**") 注解POJO類，定義數(shù)據(jù)表名稱;
• @PrimaryKey 定義主鍵，如果一個Entity使用的是復(fù)合主鍵，可以通過@Entity注解的primaryKeys 屬性定義復(fù)合主鍵：@Entity(primaryKeys = {"firstName", "lastName"})
• @ColumnInfo(name = “column_name”) 定義數(shù)據(jù)表中的字段名
• @Ignore 用于告訴Room需要忽略的字段或方法
• 建立索引：在@Entity注解的indices屬性中添加索引字段。例如：indices = {@Index(value = {"first_name", "last_name"}, unique = true), ...}, unique = true可以確保表中不會出現(xiàn){"first_name", "last_name"} 相同的數(shù)據(jù)。

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1.2 Entitiy間的關(guān)系

不同于目前存在的大多數(shù)ORM庫，Room不支持Entitiy對象間的直接引用。（具體原因可以參考： Understand why Room doesn't allow object references）
但Room允許通過外鍵(Foreign Key)來表示Entity之間的關(guān)系。

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@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; }

如上面代碼所示，Book對象與User對象是屬于的關(guān)系。Book中的user_id,對應(yīng)User中的id。 那么當(dāng)一個User對象被刪除時， 對應(yīng)的Book會發(fā)生什么呢？

@ForeignKey注解中有兩個屬性onDelete和onUpdate， 這兩個屬性對應(yīng)ForeignKey中的onDelete()和onUpdate(), 通過這兩個屬性的值來設(shè)置當(dāng)User對象被刪除／更新時，Book對象作出的響應(yīng)。這兩個屬性的可選值如下：

• CASCADE：User刪除時對應(yīng)Book一同刪除； 更新時，關(guān)聯(lián)的字段一同更新
• NO_ACTION：User刪除時不做任何響應(yīng)
• RESTRICT：禁止User的刪除／更新。當(dāng)User刪除或更新時，Sqlite會立馬報錯。
• SET_NULL：當(dāng)User刪除時， Book中的userId會設(shè)為NULL
• SET_DEFAULT：與SET_NULL類似，當(dāng)User刪除時，Book中的userId會設(shè)為默認(rèn)值

2. 創(chuàng)建數(shù)據(jù)訪問對象（DAO）

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@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 可以是一個接口，也可以是一個抽象類， Room會在編譯時創(chuàng)建DAO的實現(xiàn)。

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2.2 查詢參數(shù)傳遞

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@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);

看代碼應(yīng)該比較好理解， 方法中傳遞參數(shù)arg, 在sql語句中用:arg即可。編譯時R

2.3 查詢表中部分字段的信息

在實際某個業(yè)務(wù)場景中， 我們可能僅關(guān)心一個表部分字段的值，這時我僅需要查詢關(guān)心的列即可。

定義子集的POJO類:

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

在DAO中添加查詢方法：

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

這里定義的POJO也支持使用@Embedded

2.4 聯(lián)表查詢

Room支持聯(lián)表查詢，接口定義上與其他查詢差別不大， 主要還是sql語句的差別。

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@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 聯(lián)表查詢

Room支持聯(lián)表查詢，接口定義上與其他查詢差別不大， 主要還是sql語句的差別。

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@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類來實現(xiàn)數(shù)據(jù)庫的升級:

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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");} };

在創(chuàng)建Migration類時需要指定startVersion和endVersion, 代碼中MIGRATION_1_2和MIGRATION_2_3的startVersion和endVersion是遞增的， Migration其實是支持從版本1直接升到版本3，只要其migrate()方法里執(zhí)行的語句正常即可。那么Room是怎么實現(xiàn)數(shù)據(jù)庫升級的呢？其實本質(zhì)上還是調(diào)用SQLiteOpenHelper.onUpgrade，Room中自己實現(xiàn)了一個SQLiteOpenHelper， 在onUpgrade()方法被調(diào)用時觸發(fā)Migration，當(dāng)?shù)谝淮卧L問數(shù)據(jù)庫時，Room做了以下幾件事：

• 創(chuàng)建Room Database實例
• SQLiteOpenHelper.onUpgrade被調(diào)用，并且觸發(fā)Migration
• 打開數(shù)據(jù)庫

架構(gòu)組件Room、demo了解：https://download.csdn.net/download/meixi_android/19386289

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

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創(chuàng)建數(shù)據(jù)庫版本v.1

/*** 作者：created by meixi* 郵箱：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);// 若不是第一個版本安裝，直接執(zhí)行數(shù)據(jù)庫升級// 請不要修改FIRST_DATABASE_VERSION的值，其為第一個數(shù)據(jù)庫版本大小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實現(xiàn)跨版本升級數(shù)據(jù)庫for (int i = oldVersion; i < newVersion; i++) {switch (i) {default:break;}}} }

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創(chuàng)建數(shù)據(jù)庫語句：

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 " +")"; }

實現(xiàn)增刪改查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 "失敗";}}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 "失敗";}} }

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