如何将 SQLite 数据库与片段结合起来？
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【中文标题】如何将 SQLite 数据库与片段结合起来？【英文标题】：How to combine a SQLite database with a fragment? 【发布时间】：2019-08-15 23:47:00 【问题描述】：我想创建一个摩尔质量计算器。为此，我需要访问我存储群众的数据库。 我为此创建了一个片段。如果我单击按钮，应用程序应显示数据库中的质量。
如果我把它放在一个活动中，它就可以工作。
这是我的 DatabaseOpenHelper：
public class DatabaseOpenHelper extends SQLiteAssetHelper
private static final String DATABASE_NAME = "molmassen.db";
private static final int DATABASE_VERSION = 1;
public DatabaseOpenHelper(Context context)
super(context, DATABASE_NAME, null, DATABASE_VERSION);
对数据库的访问：
public class DatabaseAccess
private SQLiteOpenHelper openHelper;
private SQLiteDatabase db;
private static DatabaseAccess instance;
Cursor c = null;
public DatabaseAccess(Context context)
this.openHelper = new DatabaseOpenHelper(context);
public static DatabaseAccess getInstance (Context context)
if (instance == null)
instance = new DatabaseAccess(context);
return instance;
public void open()
this.db=openHelper.getWritableDatabase();
public void close()
if(db!=null)
this.db.close();
public String getMolmasse(String kürzel)
c=db.rawQuery("select Molmasse from Molmassen where Kürzel = '"+kürzel+"'",new String[]);
StringBuffer buffer = new StringBuffer();
while (c.moveToNext())
String masse = c.getString(0);
buffer.append(""+masse);
return buffer.toString();
和片段（我想问题就在那里......）
public class MolmasseFragment extends Fragment implements View.OnClickListener
public DatabaseAccess databaseAccess;
public View onCreateView(@NonNull LayoutInflater inflater, @Nullable ViewGroup container, @Nullable Bundle savedInstanceState)
View view = inflater.inflate(R.layout.fragment_molmasse, container, false);
name = view.findViewById(R.id.name);
Button query_button = (Button) view.findViewById(R.id.query_button);
result = view.findViewById(R.id.result);
databaseAccess = new DatabaseAccess(getActivity());
button.setOnClickListener(this);
query_button.setOnClickListener(this);
return view;
public void onClick(View v)
switch (v.getId())
case R.id.query_button:
DatabaseAccess databaseAccess = DatabaseAccess.getInstance(getActivity());
databaseAccess.open();
String kürzel = name.getText().toString();
String masse = databaseAccess.getMolmasse(kürzel);
result.setText(masse);
databaseAccess.close();
break;
default:
break;
我收到这个错误
com.readystatesoftware.sqliteasset.SQLiteAssetHelper$SQLiteAssetException: Missing databases/molmassen.db file (or .zip, .gz archive) in assets, or target folder not writable
我真的希望有人可以帮助我:)
【问题讨论】：
【参考方案1】：我认为这是 SQLiteAssetHelper 无法应对使用 WAL（预写日志记录）作为默认值引入的 SQLite 数据库的问题（在 API 28 之前，日志模式一直是默认值）。
如果不是，则基本上是 molmassen.file（确切命名）不在 assets/databases 文件夹中。在任何一种情况下，如果数据库文件名为 molmassen.db 并且它位于 assets 文件夹的数据库文件夹中，则以下修复将起作用。
因此，我建议更改为使用非 SQLiteAssetHelper 方法来复制数据库。
例如，考虑以下替代数据库助手，即 DatabaseOpenHelperAlt
public class DatabaseOpenHelperAlt extends SQLiteOpenHelper
private static final String DATABASE_NAME = "molmassen.db";
private static final int DATABASE_VERSION = 1;
Context myContext;
int buffer_size = 1024 * 4, blocks_copied = 0, bytes_copied = 0;
public DatabaseOpenHelperAlt(Context context)
super(context, DATABASE_NAME, null, DATABASE_VERSION);
myContext = context;
if (!checkDataBase())
try
copyDataBase();
catch (IOException e)
e.printStackTrace();
throw new RuntimeException("Error copy database file " + DATABASE_NAME + " - see stack-trace above" );
@Override
public void onCreate(SQLiteDatabase db)
@Override
public void onUpgrade(SQLiteDatabase db, int oldVersion, int newVersion)
private boolean checkDataBase()
/**
* Does not open the database instead checks to see if the file exists
* also creates the databases directory if it does not exists
* (the real reason why the database is opened, which appears to result in issues)
*/
File db = new File(myContext.getDatabasePath(DATABASE_NAME).getPath()); //Get the file name of the database
Log.d("DBPATH","DB Path is " + db.getPath()); //TODO remove for Live App
if (db.exists()) return true; // If it exists then return doing nothing
// Get the parent (directory in which the database file would be)
File dbdir = db.getParentFile();
// If the directory does not exits then make the directory (and higher level directories)
if (!dbdir.exists())
db.getParentFile().mkdirs();
dbdir.mkdirs();
return false;
private void copyDataBase() throws IOException
final String TAG = "COPYDATABASE";
//Open your local db as the input stream
Log.d(TAG,"Initiated Copy of the database file " + DATABASE_NAME + " from the assets folder."); //TODO remove for Live App
//Note how the databases folder is hard coded (to suit the same location as used by SQLiteAssetHelper)
InputStream myInput = myContext.getAssets().open("databases" + File.separator + DATABASE_NAME); // Open the Asset file
String dbpath = myContext.getDatabasePath(DATABASE_NAME).getPath();
Log.d(TAG,"Asset file " + DATABASE_NAME + " found so attmepting to copy to " + dbpath); //TODO remove for Live App
// Path to the just created empty db
//String outFileName = DB_PATH + DB_NAME;
//Open the empty db as the output stream
File outfile = new File(myContext.getDatabasePath(DATABASE_NAME).toString());
Log.d("DBPATH","path is " + outfile.getPath()); //TODO remove for Live App
//outfile.setWritable(true); // NOT NEEDED as permission already applies
//OutputStream myoutputx2 = new FileOutputStream(outfile);
/* Note done in checkDatabase method
if (!outfile.getParentFile().exists())
outfile.getParentFile().mkdirs();
*/
OutputStream myOutput = new FileOutputStream(outfile);
//transfer bytes from the inputfile to the outputfile
byte[] buffer = new byte[buffer_size];
int length;
while ((length = myInput.read(buffer))>0)
blocks_copied++;
Log.d(TAG,"Ateempting copy of block " + String.valueOf(blocks_copied) + " which has " + String.valueOf(length) + " bytes."); //TODO remove for Live App
myOutput.write(buffer, 0, length);
bytes_copied += length;
Log.d(TAG,
"Finished copying Database " + DATABASE_NAME +
" from the assets folder, to " + dbpath +
String.valueOf(bytes_copied) + "were copied, in " +
String.valueOf(blocks_copied) + " blocks of size " +
String.valueOf(buffer_size) + "."
); //TODO remove for Live App
//Close the streams
myOutput.flush();
myOutput.close();
myInput.close();
Log.d(TAG,"All Streams have been flushed and closed."); //TODO remove for Live App
将上述内容与修改后的 DatabaseAccess 类一起使用（调用备用 DatabaseHelper 以及获取底层 SQLiteDatabase 进行测试的方法，而无需与您的数据库完全相同）检查 cmets： -
public class DatabaseAccess
private SQLiteOpenHelper openHelper;
private SQLiteDatabase db;
private static DatabaseAccess instance;
Cursor c = null;
public DatabaseAccess(Context context)
//this.openHelper = new DatabaseOpenHelper(context); //<<<<<<<<<< OLD REPLACED
this.openHelper = new DatabaseOpenHelperAlt(context); //<<<<<<<<<< NEW
db = this.openHelper.getWritableDatabase(); //<<<<<<<<<< force open
//!!!!!!!!! ADDED for simple testing
public SQLiteDatabase getDB()
return db;
public static DatabaseAccess getInstance (Context context)
if (instance == null)
instance = new DatabaseAccess(context);
return instance;
public void open()
this.db=openHelper.getWritableDatabase();
public void close()
if(db!=null)
this.db.close();
public String getMolmasse(String kürzel)
c=db.rawQuery("select Molmasse from Molmassen where Kürzel = '"+kürzel+"'",new String[]);
StringBuffer buffer = new StringBuffer();
while (c.moveToNext())
String masse = c.getString(0);
buffer.append(""+masse);
return buffer.toString();
片段中包含以下内容的正在运行的应用程序（为方便起见而减少）：-
public class MolassesFragment extends Fragment
public DatabaseAccess databaseAccess;
private MolassesViewModel mViewModel;
public static MolassesFragment newInstance()
return new MolassesFragment();
@Nullable
@Override
public View onCreateView(@NonNull LayoutInflater inflater, @Nullable ViewGroup container,
@Nullable Bundle savedInstanceState)
View view = inflater.inflate(R.layout.molasses_fragment, container, false);
databaseAccess = new DatabaseAccess(getContext());
SQLiteDatabase db = databaseAccess.getDB();
Cursor csr = db.query("sqlite_master",null,null,null,null,null,null);
DatabaseUtils.dumpCursor(csr);
csr.close();
return view;
@Override
public void onActivityCreated(@Nullable Bundle savedInstanceState)
super.onActivityCreated(savedInstanceState);
mViewModel = ViewModelProviders.of(this).get(MolassesViewModel.class);
// TODO: Use the ViewModel
第一次运行（复制数据库的运行），它可以工作并且日志显示：-
2019-03-26 12:14:15.419 7929-7929/aaa.so55342826preexistdbassethelperexample D/DBPATH: DB Path is /data/user/0/aaa.so55342826preexistdbassethelperexample/databases/molmassen.db
2019-03-26 12:14:15.419 7929-7929/aaa.so55342826preexistdbassethelperexample D/COPYDATABASE: Initiated Copy of the database file molmassen.db from the assets folder.
2019-03-26 12:14:15.419 7929-7929/aaa.so55342826preexistdbassethelperexample D/COPYDATABASE: Asset file molmassen.db found so attmepting to copy to /data/user/0/aaa.so55342826preexistdbassethelperexample/databases/molmassen.db
2019-03-26 12:14:15.419 7929-7929/aaa.so55342826preexistdbassethelperexample D/DBPATH: path is /data/user/0/aaa.so55342826preexistdbassethelperexample/databases/molmassen.db
2019-03-26 12:14:15.419 7929-7929/aaa.so55342826preexistdbassethelperexample D/COPYDATABASE: Ateempting copy of block 1 which has 4096 bytes.
2019-03-26 12:14:15.420 7929-7929/aaa.so55342826preexistdbassethelperexample D/COPYDATABASE: Ateempting copy of block 2 which has 4096 bytes.
.....
2019-03-26 12:14:15.422 7929-7929/aaa.so55342826preexistdbassethelperexample D/COPYDATABASE: Ateempting copy of block 27 which has 4096 bytes.
2019-03-26 12:14:15.422 7929-7929/aaa.so55342826preexistdbassethelperexample D/COPYDATABASE: Finished copying Database molmassen.db from the assets folder, to /data/user/0/aaa.so55342826preexistdbassethelperexample/databases/molmassen.db110592were copied, in 27 blocks of size 4096.
2019-03-26 12:14:15.423 7929-7929/aaa.so55342826preexistdbassethelperexample D/COPYDATABASE: All Streams have been flushed and closed.
2019-03-26 12:14:15.452 7929-7929/aaa.so55342826preexistdbassethelperexample
........日志的其余部分因太大而被省略，但足以说明预期的表格已显示。
【讨论】：
非常感谢:)，不过我自己修好了。我不知道是什么问题。我在另一台计算机上运行该应用程序 - 突然它工作了......在我自己的计算机上再次尝试后，它仍然可以工作。以上是关于如何将 SQLite 数据库与片段结合起来？的主要内容，如果未能解决你的问题，请参考以下文章
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