java - 使用现有数据库在 Android 应用中显示数据
问题描述
嗨，我是 android 开发的新手。我已经创建了 SQLite 数据库并将其保存在 Android Studio 的资产文件夹中。我的应用程序必须使用现有数据库而不是创建新数据库。我面临的问题是，当我想在屏幕上显示数据时，它会在执行 SQL 语句的光标处引发错误。请帮忙。
数据库名是test.db，表名是MASTER。这是我的 DataBaseHelper 类
public class DataBaseHelper extends SQLiteOpenHelper {
private static String TAG = "DataBaseHelper"; // Tag just for the LogCat window
//destination path (location) of our database on device
private static String DB_PATH = "";
private static String DB_NAME ="test,db";// Database name
private SQLiteDatabase mDataBase;
private final Context mContext;
public DataBaseHelper(Context context)
{
super(context, DB_NAME, null, 1);// 1? Its database Version
if(android.os.Build.VERSION.SDK_INT >= 17){
DB_PATH = context.getApplicationInfo().dataDir + "/databases/";
}
else
{
DB_PATH = context.getApplicationInfo().dataDir + "/databases/";
}
this.mContext = context;
}
public void createDataBase() throws IOException
{
//If the database does not exist, copy it from the assets.
boolean mDataBaseExist = checkDataBase();
if(!mDataBaseExist)
{
this.getReadableDatabase();
this.close();
try
{
//Copy the database from assests
copyDataBase();
Log.e(TAG, "createDatabase database created");
}
catch (IOException mIOException)
{
throw new Error("ErrorCopyingDataBase");
}
}
}
//Check that the database exists here: /data/data/your package/databases/Da Name
private boolean checkDataBase()
{
File dbFile = new File(DB_PATH + DB_NAME);
//Log.v("dbFile", dbFile + " "+ dbFile.exists());
return dbFile.exists();
}
//Copy the database from assets
private void copyDataBase() throws IOException
{
InputStream mInput = mContext.getAssets().open(DB_NAME);
String outFileName = DB_PATH + DB_NAME;
OutputStream mOutput = new FileOutputStream(outFileName);
byte[] mBuffer = new byte[1024];
int mLength;
while ((mLength = mInput.read(mBuffer))>0)
{
mOutput.write(mBuffer, 0, mLength);
}
mOutput.flush();
mOutput.close();
mInput.close();
}
//Open the database, so we can query it
public boolean openDataBase() throws SQLException
{
String mPath = DB_PATH + DB_NAME;
//Log.v("mPath", mPath);
mDataBase = SQLiteDatabase.openDatabase(mPath, null, SQLiteDatabase.CREATE_IF_NECESSARY);
//mDataBase = SQLiteDatabase.openDatabase(mPath, null, SQLiteDatabase.NO_LOCALIZED_COLLATORS);
return mDataBase != null;
}
@Override
public synchronized void close()
{
if(mDataBase != null)
mDataBase.close();
super.close();
}
@Override
public void onCreate(SQLiteDatabase db) {
}
@Override
public void onUpgrade(SQLiteDatabase db, int oldVersion, int newVersion) {
}
}
这是我的 TestAdapter 类
public class TestAdapter
{
protected static final String TAG = "DataAdapter";
private final Context mContext;
private SQLiteDatabase mDb;
private DataBaseHelper mDbHelper;
public TestAdapter(Context context)
{
this.mContext = context;
mDbHelper = new DataBaseHelper(mContext);
}
public TestAdapter createDatabase() throws SQLException
{
try
{
mDbHelper.createDataBase();
}
catch (IOException mIOException)
{
Log.e(TAG, mIOException.toString() + " UnableToCreateDatabase");
throw new Error("UnableToCreateDatabase");
}
return this;
}
public TestAdapter open() throws SQLException
{
try
{
mDbHelper.openDataBase();
mDbHelper.close();
mDb = mDbHelper.getReadableDatabase();
}
catch (SQLException mSQLException)
{
Log.e(TAG, "open >>"+ mSQLException.toString());
throw mSQLException;
}
return this;
}
public void close()
{
mDbHelper.close();
}
public Cursor getTestData() {
try
{
String sql ="SELECT * FROM MASTER;";
Cursor mCur = mDb.rawQuery(sql, null);
if (mCur!=null)
{
mCur.moveToNext();
}
return mCur;
}
catch (SQLException mSQLException)
{
Log.e(TAG, "getTestData >>"+ mSQLException.toString());
throw mSQLException;
}
}
}
这是我的 MainActivity 类
public class MainActivity extends AppCompatActivity {
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
Button button=findViewById(R.id.submit);
button.setOnClickListener(new View.OnClickListener() {
@Override
public void onClick(View v) {
TestAdapter mDbHelper = new TestAdapter(MainActivity.this);
mDbHelper.createDatabase();
mDbHelper.open();
Cursor testdata = mDbHelper.getTestData();
Toast.makeText(MainActivity.this,testdata.getString(0),Toast.LENGTH_SHORT).show();
mDbHelper.close();
}
});
}
}
这是logcat
2019-02-04 15:47:30.227 2594-2594/com.example.myapplication E/SQLiteLog: (1) no such table: MASTER
2019-02-04 15:47:30.228 2594-2594/com.example.myapplication E/DataAdapter: getTestData >>android.database.sqlite.SQLiteException: no such table: MASTER (code 1): , while compiling: SELECT * FROM MASTER;
2019-02-04 15:47:30.228 2594-2594/com.example.myapplication D/AndroidRuntime: Shutting down VM
2019-02-04 15:47:30.242 2594-2594/com.example.myapplication E/AndroidRuntime: FATAL EXCEPTION: main
Process: com.example.myapplication, PID: 2594
android.database.sqlite.SQLiteException: no such table: MASTER (code 1): , while compiling: SELECT * FROM MASTER;
at android.database.sqlite.SQLiteConnection.nativePrepareStatement(Native Method)
at android.database.sqlite.SQLiteConnection.acquirePreparedStatement(SQLiteConnection.java:890)
at android.database.sqlite.SQLiteConnection.prepare(SQLiteConnection.java:501)
at android.database.sqlite.SQLiteSession.prepare(SQLiteSession.java:588)
at android.database.sqlite.SQLiteProgram.<init>(SQLiteProgram.java:58)
at android.database.sqlite.SQLiteQuery.<init>(SQLiteQuery.java:37)
at android.database.sqlite.SQLiteDirectCursorDriver.query(SQLiteDirectCursorDriver.java:46)
at android.database.sqlite.SQLiteDatabase.rawQueryWithFactory(SQLiteDatabase.java:1392)
at android.database.sqlite.SQLiteDatabase.rawQuery(SQLiteDatabase.java:1331)
at com.example.myapplication.TestAdapter.getTestData(TestAdapter.java:63)
at com.example.myapplication.MainActivity$1.onClick(MainActivity.java:38)
at android.view.View.performClick(View.java:6297)
at android.view.View$PerformClick.run(View.java:24797)
at android.os.Handler.handleCallback(Handler.java:790)
at android.os.Handler.dispatchMessage(Handler.java:99)
at android.os.Looper.loop(Looper.java:164)
at android.app.ActivityThread.main(ActivityThread.java:6626)
at java.lang.reflect.Method.invoke(Native Method)
at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:438)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:811)
logcat 说没有MASTER表，但是当我通过 SQLiteDB 浏览器查看数据库时，它当时就在那里。
解决方案
我相信您的主要问题是您使用过private static String DB_NAME ="test,db";// Database name
代替private static String DB_NAME ="test.db";// Database name
也就是说，您编写了逗号,而不是句点.，因此将找不到资产文件夹中的数据库文件，因此不会被复制。
第一次运行文件test,db时，由于使用this.getReadableDatabase();，将创建数据库，这将导致创建数据库，该数据库将为空，因此对于后续运行将不会尝试从资产文件夹作为数据库存在，因此由于数据库为空，因此访问该表的尝试失败，因为该表不存在。
- 注意 getRedableDatabase 在大多数情况下实际上会得到一个可写的数据库
创建和/或打开数据库。这将与 getWritableDatabase() 返回的对象相同，除非某些问题（例如磁盘已满）需要以只读方式打开数据库。在这种情况下，将返回一个只读数据库对象。如果问题得到解决，未来对 getWritableDatabase() 的调用可能会成功，在这种情况下，只读数据库对象将被关闭，并且将来会返回读/写对象。 获取可读数据库
我相信 usinggetReadableDatabase仅用于规避最初数据库文件夹不存在的问题，因此尝试从资产复制文件失败，因为父文件夹不存在。更好的解决方案是不使用getReadableDatabase，而是检查目录是否存在，如果不存在则创建它。
当使用 Android 9+ 作为默认值时，这种使用getReabableDatabase引入了更大的问题，然后使用 WAL（预写日志记录），这会产生额外的文件（以 -shm 和 -wal 为后缀的数据库名称）。
因此使用：-
//Check that the database exists here: /data/data/your package/databases/Da Name
private boolean checkDataBase()
{
File dbFile = new File(DB_PATH + DB_NAME);
if (dbFile.exists()) return true;
if (!dbFile.getParentFile().exists()) dbFile.getParentFile().mkdirs();
return false;
}
getReabableDatabase即使由于资产文件不存在而导致随后的副本失败，也无需使用以及实际创建数据库文件所产生的复杂性。
为了更加小心并应对 -shm 和 -wal 文件可能无意中存在的可能性，那么上述内容甚至可以扩展到：-
private boolean checkDataBase()
{
File dbFile = new File(DB_PATH + DB_NAME);
if (dbFile.exists()) return true;
if (!dbFile.getParentFile().exists()) dbFile.getParentFile().mkdirs();
if (new File(DB_PATH + DB_NAME + "-shm").exists())
new File(DB_PATH + DB_NAME + "-shm").delete();
if ((new File(DB_PATH + DB_NAME + "-wal")).exists())
new File(DB_PATH + DB_NAME + "-wal").delete();
return false;
}
通常DB_PATH = context.getApplicationInfo().dataDir + "/databases/";不建议使用，而是建议使用更具体DB_PATH = mContext.getDatabasePath(DB_NAME).getPath();的，因为不需要硬编码文件分隔符和文件夹名称。
以下可能是更好的整体数据库助手：-
public class DataBaseHelper extends SQLiteOpenHelper {
private static String TAG = "DataBaseHelper"; // Tag just for the LogCat window
//destination path (location) of our database on device
private static String DB_PATH = "";
private static String DB_NAME ="test.db";// Database name //<<<<<<<<<< CHANGED TO FIX PRIMARY ISSUE
private SQLiteDatabase mDataBase;
private final Context mContext;
public DataBaseHelper(Context context)
{
super(context, DB_NAME, null, 1);// 1? Its database Version
this.mContext = context;
DB_PATH = mContext.getDatabasePath(DB_NAME).getPath();
}
public void createDataBase() throws IOException
{
//If the database does not exist, copy it from the assets.
boolean mDataBaseExist = checkDataBase();
if(!mDataBaseExist)
{
//this.getReadableDatabase(); //<<<<<<<<<< REMOVED (commented out)
//this.close(); //<<<<<<<<<< REMOVED ()commented out
try
{
//Copy the database from assests
copyDataBase();
Log.e(TAG, "createDatabase database created");
}
catch (IOException mIOException)
{
mIOException.printStackTrace(); //<<<<<<<<<< might as well include the actual cause in the log
throw new Error("ErrorCopyingDataBase");
}
}
}
//Check that the database exists here: /data/data/your package/databases/Da Name
private boolean checkDataBase()
{
File dbFile = new File(DB_PATH); //<<<<<<<<<< just the path used
if (dbFile.exists()) return true; //<<<<<<<<<< return true of the db exists (see NOTE001)
if (!dbFile.getParentFile().exists()) dbFile.getParentFile().mkdirs();
if (new File(DB_PATH + "-shm").exists())
new File(DB_PATH + "-shm").delete();
if ((new File(DB_PATH + "-wal")).exists())
new File(DB_PATH + "-wal").delete();
return false;
}
/** NOTE001
* Just checking the file does leave scope for a non sqlite file to be copied from the assets folder
* and be copied resulting in an exception. The above could be extended to apply additional checks
* if considered required e.g. checking the first sixteen bytes for The header string: "SQLite format 3\000"
*/
//Copy the database from assets
private void copyDataBase() throws IOException
{
InputStream mInput = mContext.getAssets().open(DB_NAME);
String outFileName = DB_PATH; //<<<<<<<<<< just the path used
OutputStream mOutput = new FileOutputStream(outFileName);
byte[] mBuffer = new byte[1024];
int mLength;
while ((mLength = mInput.read(mBuffer))>0)
{
mOutput.write(mBuffer, 0, mLength);
}
mOutput.flush();
mOutput.close();
mInput.close();
}
//Open the database, so we can query it
public boolean openDataBase() throws SQLException
{
String mPath = DB_PATH;
//Log.v("mPath", mPath);
mDataBase = SQLiteDatabase.openDatabase(mPath, null, SQLiteDatabase.CREATE_IF_NECESSARY);
//mDataBase = SQLiteDatabase.openDatabase(mPath, null, SQLiteDatabase.NO_LOCALIZED_COLLATORS);
return mDataBase != null;
}
/**
* Note this can be added and the line uncommented (see below) to disable WAL logging which
* from Anroid 9 (Pie) is the default
*/
@Override
public void onConfigure(SQLiteDatabase db) {
super.onConfigure(db);
// db.disableWriteAheadLogging(); //<<<<<<<<<< uncomment if you want to not use WAL but use the less efficient joutnal mode.
}
@Override
public synchronized void close()
{
if(mDataBase != null)
mDataBase.close();
super.close();
}
@Override
public void onCreate(SQLiteDatabase db) {
}
@Override
public void onUpgrade(SQLiteDatabase db, int oldVersion, int newVersion) {
}
}
- 注意查看代码中的注释
额外/测试
如果使用上述（删除应用程序的数据或卸载应用程序以删除空数据库后）但资产文件夹中没有合适的文件（test，db未更改以进行测试），那么上述将导致更多解释： -
02-05 10:17:03.513 5502-5502/mjt.so54513838 W/System.err: java.io.FileNotFoundException: test,db
02-05 10:17:03.513 5502-5502/mjt.so54513838 W/System.err: at android.content.res.AssetManager.openAsset(Native Method)
02-05 10:17:03.513 5502-5502/mjt.so54513838 W/System.err: at android.content.res.AssetManager.open(AssetManager.java:313)
02-05 10:17:03.513 5502-5502/mjt.so54513838 W/System.err: at android.content.res.AssetManager.open(AssetManager.java:287)
02-05 10:17:03.513 5502-5502/mjt.so54513838 W/System.err: at mjt.so54513838.DataBaseHelper.copyDataBase(DataBaseHelper.java:75)
02-05 10:17:03.513 5502-5502/mjt.so54513838 W/System.err: at mjt.so54513838.DataBaseHelper.createDataBase(DataBaseHelper.java:42)
02-05 10:17:03.513 5502-5502/mjt.so54513838 W/System.err: at mjt.so54513838.TestAdapter.createDatabase(TestAdapter.java:29)
02-05 10:17:03.513 5502-5502/mjt.so54513838 W/System.err: at mjt.so54513838.MainActivity$1.onClick(MainActivity.java:23)
02-05 10:17:03.513 5502-5502/mjt.so54513838 W/System.err: at android.view.View.performClick(View.java:4780)
02-05 10:17:03.513 5502-5502/mjt.so54513838 W/System.err: at android.view.View$PerformClick.run(View.java:19866)
02-05 10:17:03.514 5502-5502/mjt.so54513838 W/System.err: at android.os.Handler.handleCallback(Handler.java:739)
02-05 10:17:03.514 5502-5502/mjt.so54513838 W/System.err: at android.os.Handler.dispatchMessage(Handler.java:95)
02-05 10:17:03.514 5502-5502/mjt.so54513838 W/System.err: at android.os.Looper.loop(Looper.java:135)
02-05 10:17:03.514 5502-5502/mjt.so54513838 W/System.err: at android.app.ActivityThread.main(ActivityThread.java:5254)
02-05 10:17:03.514 5502-5502/mjt.so54513838 W/System.err: at java.lang.reflect.Method.invoke(Native Method)
02-05 10:17:03.514 5502-5502/mjt.so54513838 W/System.err: at java.lang.reflect.Method.invoke(Method.java:372)
02-05 10:17:03.514 5502-5502/mjt.so54513838 W/System.err: at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:903)
02-05 10:17:03.514 5502-5502/mjt.so54513838 W/System.err: at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:698)
02-05 10:17:03.514 5502-5502/mjt.so54513838 D/AndroidRuntime: Shutting down VM
02-05 10:17:03.514 5502-5502/mjt.so54513838 E/AndroidRuntime: FATAL EXCEPTION: main
Process: mjt.so54513838, PID: 5502
java.lang.Error: ErrorCopyingDataBase
at mjt.so54513838.DataBaseHelper.createDataBase(DataBaseHelper.java:48)
at mjt.so54513838.TestAdapter.createDatabase(TestAdapter.java:29)
at mjt.so54513838.MainActivity$1.onClick(MainActivity.java:23)
at android.view.View.performClick(View.java:4780)
at android.view.View$PerformClick.run(View.java:19866)
at android.os.Handler.handleCallback(Handler.java:739)
at android.os.Handler.dispatchMessage(Handler.java:95)
at android.os.Looper.loop(Looper.java:135)
at android.app.ActivityThread.main(ActivityThread.java:5254)
at java.lang.reflect.Method.invoke(Native Method)
at java.lang.reflect.Method.invoke(Method.java:372)
at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:903)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:698)
如果应用程序在没有任何更改的情况下再次运行，那么上述情况也会发生，而不是更令人困惑的table not found。
注意在运行上述代码之前，或者即使只是将 test,db 更改为 test.db，也必须删除数据库文件。这可以通过删除/清除应用程序的数据或卸载应用程序来轻松实现。
以上内容已在 Android 5.0 (lollipop) (API 22) 和 Andorid 9 (Pie)(API 28) 上进行了测试，生成的 Toast 显示表（尽管为方便起见，表已从 MASTER 更改为 sqlite_master（保存使用现有数据库文件创建数据库文件））。
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