我想创建一个摩尔质量计算器。为此，我需要访问存储群众的数据库。 我为此创建了一个片段。如果单击按钮，则该应用程序应显示数据库中的质量。
如果我将其放在活动中，则它可以正常工作。
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;
}
}
}
我收到此错误
com.readystatesoftware.sqliteasset.SQLiteAssetHelper$SQLiteAssetException: Missing databases/molmassen.db file (or .zip, .gz archive) in assets, or target folder not writable
我真的希望有人能帮助我：）
答案 0 :(得分：0)
我认为这是SQLiteAssetHelper的一个问题，无法解决使用WAL（预写日志记录）作为默认设置（日志模式一直是API 28之前的默认设置）的SQLite数据库的引入。
否则，基本上是molmassen.file（确切命名为）不在 assets / databases 文件夹中。在这两种情况下，如果数据库文件名为molmassen.db并且位于资产文件夹的databases文件夹中，则下面的修复程序将起作用。
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帮助器，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
}
}
结合使用上述内容和经过修改的DatabaseAccess类（调用替代的DatabaseHelper以及用于获取基础SQLiteDatabase进行测试的方法，而不必拥有与您的数据库完全相同的数据库）检查注释： -
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();
}
}
正在运行的应用程序，该应用程序的片段中包含以下内容（为方便起见已减少）：-
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），它起作用，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
........由于剩余的日志太大，因此省略了其余日志，但足以表明已显示期望的表。