我最近正在学习Room Database，但是这段代码似乎太奇怪了。
我试图理解这行代码，但也许我的Google搜索技能不是那么好。
你能解释一下吗？
public abstract NoDoDao noDoDao();
我不明白的是，这里的接口叫“ NoDoDa”，怎么能称呼它为“方法”或“功能”呢？这是我第一次看到这种语法。为什么还有“抽象关键字”？
package com.paige.room.data;
import android.content.Context;
import android.os.AsyncTask;
import androidx.annotation.NonNull;
import androidx.room.Database;
import androidx.room.Room;
import androidx.room.RoomDatabase;
import androidx.sqlite.db.SupportSQLiteDatabase;
import com.paige.room.model.NoDo;
@Database(entities = {NoDo.class}, version = 1)
public abstract class NoDoRoomDatabase extends RoomDatabase {
//Don't want to create a lot of instances, singleton
//volatile means "don't store in the cache"
private static volatile NoDoRoomDatabase INSTANCE;
public abstract NoDoDao noDoDao();
public static NoDoRoomDatabase getDatabase(final Context context){
if(INSTANCE == null){
//synchronized keyword, for UI Thread, force it to work as it is supposed to do
synchronized (NoDoRoomDatabase.class){
if(INSTANCE == null){
//create our db
INSTANCE = Room.databaseBuilder(
context,
NoDoRoomDatabase.class,
"nodo_database")
.addCallback(roomDatabaseCallBack)
.build();
}
}
}
return INSTANCE;
}
//callback
private static RoomDatabase.Callback roomDatabaseCallBack =
new RoomDatabase.Callback(){
@Override
public void onOpen(@NonNull SupportSQLiteDatabase db) {
super.onOpen(db);
new PopulateDBAsync(INSTANCE).execute();
}
};
private static class PopulateDBAsync extends AsyncTask<Void, Void, Void> {
private final NoDoDao noDoDao;
PopulateDBAsync(NoDoRoomDatabase db) {
this.noDoDao = db.noDoDao();
}
@Override
protected Void doInBackground(Void... voids) {
// noDoDao.deleteAll(); //removes all items from our table
// //for testing
// NoDo noDo = new NoDo("Buy a new Ferrari");
// noDoDao.insert(noDo);
//
// noDo = new NoDo("Buy a Big House");
// noDoDao.insert(noDo);
return null;
}
}
}
这是我的整个代码
最佳答案
NoDoDao是一个abstract类，无法实例化（构造），但可以使用该类。这是怎么回事。
您可能没有意识到，Room会生成代码，而正是这些抽象使底层代码可以代表您实现。
假设您的实体类别NoDO是：-
@Entity
public class NoDo {
@PrimaryKey
Long id;
}
而您的Dao类NoDoDao是：-
@Dao
public abstract class NoDoDao {
@Query("SELECT * FROM nodo")
abstract List<NoDo> getnothing();
}
然后编译（F9），然后得到：
查看NoDoDao_Impl，您会得到：-
public final class NoDoDao_Impl extends NoDoDao {
private final RoomDatabase __db;
public NoDoDao_Impl(RoomDatabase __db) {
this.__db = __db;
}
@Override
List<NoDo> getnothing() {
final String _sql = "SELECT * FROM nodo";
final RoomSQLiteQuery _statement = RoomSQLiteQuery.acquire(_sql, 0);
__db.assertNotSuspendingTransaction();
final Cursor _cursor = DBUtil.query(__db, _statement, false, null);
try {
final int _cursorIndexOfId = CursorUtil.getColumnIndexOrThrow(_cursor, "id");
final List<NoDo> _result = new ArrayList<NoDo>(_cursor.getCount());
while(_cursor.moveToNext()) {
final NoDo _item;
_item = new NoDo();
if (_cursor.isNull(_cursorIndexOfId)) {
_item.id = null;
} else {
_item.id = _cursor.getLong(_cursorIndexOfId);
}
_result.add(_item);
}
return _result;
} finally {
_cursor.close();
_statement.release();
}
}
}
注释处理器已经生成了真正的getNothing代码。
重要的是NoDoRoomDatabase_Impl为：-
public final class NoDoRoomDatabase_Impl extends NoDoRoomDatabase {
private volatile NoDoDao _noDoDao;
@Override
protected SupportSQLiteOpenHelper createOpenHelper(DatabaseConfiguration configuration) {
final SupportSQLiteOpenHelper.Callback _openCallback = new RoomOpenHelper(configuration, new RoomOpenHelper.Delegate(1) {
@Override
public void createAllTables(SupportSQLiteDatabase _db) {
_db.execSQL("CREATE TABLE IF NOT EXISTS `NoDo` (`id` INTEGER, PRIMARY KEY(`id`))");
_db.execSQL("CREATE TABLE IF NOT EXISTS room_master_table (id INTEGER PRIMARY KEY,identity_hash TEXT)");
_db.execSQL("INSERT OR REPLACE INTO room_master_table (id,identity_hash) VALUES(42, '1fdde93d9c60610ae88bf0c74771844a')");
}
@Override
public void dropAllTables(SupportSQLiteDatabase _db) {
_db.execSQL("DROP TABLE IF EXISTS `NoDo`");
if (mCallbacks != null) {
for (int _i = 0, _size = mCallbacks.size(); _i < _size; _i++) {
mCallbacks.get(_i).onDestructiveMigration(_db);
}
}
}
@Override
protected void onCreate(SupportSQLiteDatabase _db) {
if (mCallbacks != null) {
for (int _i = 0, _size = mCallbacks.size(); _i < _size; _i++) {
mCallbacks.get(_i).onCreate(_db);
}
}
}
@Override
public void onOpen(SupportSQLiteDatabase _db) {
mDatabase = _db;
internalInitInvalidationTracker(_db);
if (mCallbacks != null) {
for (int _i = 0, _size = mCallbacks.size(); _i < _size; _i++) {
mCallbacks.get(_i).onOpen(_db);
}
}
}
@Override
public void onPreMigrate(SupportSQLiteDatabase _db) {
DBUtil.dropFtsSyncTriggers(_db);
}
@Override
public void onPostMigrate(SupportSQLiteDatabase _db) {
}
@Override
protected RoomOpenHelper.ValidationResult onValidateSchema(SupportSQLiteDatabase _db) {
final HashMap<String, TableInfo.Column> _columnsNoDo = new HashMap<String, TableInfo.Column>(1);
_columnsNoDo.put("id", new TableInfo.Column("id", "INTEGER", false, 1, null, TableInfo.CREATED_FROM_ENTITY));
final HashSet<TableInfo.ForeignKey> _foreignKeysNoDo = new HashSet<TableInfo.ForeignKey>(0);
final HashSet<TableInfo.Index> _indicesNoDo = new HashSet<TableInfo.Index>(0);
final TableInfo _infoNoDo = new TableInfo("NoDo", _columnsNoDo, _foreignKeysNoDo, _indicesNoDo);
final TableInfo _existingNoDo = TableInfo.read(_db, "NoDo");
if (! _infoNoDo.equals(_existingNoDo)) {
return new RoomOpenHelper.ValidationResult(false, "NoDo(a.so59478461.NoDo).\n"
+ " Expected:\n" + _infoNoDo + "\n"
+ " Found:\n" + _existingNoDo);
}
return new RoomOpenHelper.ValidationResult(true, null);
}
}, "1fdde93d9c60610ae88bf0c74771844a", "1d145662ebde2538a8da3217996c0550");
final SupportSQLiteOpenHelper.Configuration _sqliteConfig = SupportSQLiteOpenHelper.Configuration.builder(configuration.context)
.name(configuration.name)
.callback(_openCallback)
.build();
final SupportSQLiteOpenHelper _helper = configuration.sqliteOpenHelperFactory.create(_sqliteConfig);
return _helper;
}
@Override
protected InvalidationTracker createInvalidationTracker() {
final HashMap<String, String> _shadowTablesMap = new HashMap<String, String>(0);
HashMap<String, Set<String>> _viewTables = new HashMap<String, Set<String>>(0);
return new InvalidationTracker(this, _shadowTablesMap, _viewTables, "NoDo");
}
@Override
public void clearAllTables() {
super.assertNotMainThread();
final SupportSQLiteDatabase _db = super.getOpenHelper().getWritableDatabase();
try {
super.beginTransaction();
_db.execSQL("DELETE FROM `NoDo`");
super.setTransactionSuccessful();
} finally {
super.endTransaction();
_db.query("PRAGMA wal_checkpoint(FULL)").close();
if (!_db.inTransaction()) {
_db.execSQL("VACUUM");
}
}
}
@Override
public NoDoDao noDoDao() {
if (_noDoDao != null) {
return _noDoDao;
} else {
synchronized(this) {
if(_noDoDao == null) {
_noDoDao = new NoDoDao_Impl(this);
}
return _noDoDao;
}
}
}
}
查看如何将NoDoDao基本上重写为NoDoDao_Impl