我正在尝试解决以下问题。我有一个使用预先创建的数据库的应用程序。我将此数据库放在资产文件夹中。然后我在内部复制数据库。
我希望能够检查我的数据库的版本，所以当有新版本时，我删除旧数据库并从资产文件夹中复制新数据库。我将数据库版本存储为共享偏好数据。
运行以下代码时出现错误，应用程序停止运行（意外停止）。你能帮我找出问题所在吗？
public class DatabaseConnector
{
// database name
private static final String DATABASE_NAME = "Recipes";
private SQLiteDatabase database; // database object
private DatabaseOpenHelper databaseOpenHelper; // database helper
String selection;
// public constructor for DatabaseConnector
public DatabaseConnector(Context context)
{
// create a new DatabaseOpenHelper
databaseOpenHelper =
new DatabaseOpenHelper(context, DATABASE_NAME, null, 1);
} // end DatabaseConnector constructor
// open the database connection
// attempts to establish a connection to the db
// and throws an exception if it fails
public void open() throws SQLException
{
// create or open a database for reading/writing
database = databaseOpenHelper.getWritableDatabase();
} // end method open
// close the database connection
public void close()
{
if (database != null)
database.close(); // close the database connection
} // end method close
// inserts a new esoda in the database
public void insertEsoda(String title, String category, String ingredients, String process,
String notes, String image, String time, String calories, String difficulty)
{
....
} // end method insertEsoda
// updates a esoda in the database
public void updateEsoda(long id, String title, String category, String ingredients,
String process, String notes, String image, String time, String calories, String difficulty)
{
....
} // end method updateEsoda
// updates favorites in the database
public Boolean updateFavorite(long id)
{
....
} // end method update favorites
// return a Cursor with all esoda information in the database
public Cursor getAllEsoda()
{
....
} // end method getAllEsoda
// return a Cursor with all esoda information in the database
public Cursor getRecipeCategory(int position)
{
....
} // end method getAllEsoda
// get a Cursor containing all information about the esoda specified
// by the given id
public Cursor getOneEsoda(long id)
{
....
} // end method getOneEsoda
// delete the contact specified by the given String name
public void deleteEsoda(long id)
{
....
} // end method deleteEsoda
private class DatabaseOpenHelper extends SQLiteOpenHelper {
public Context mContext;
private static final int DATABASE_VERSION = 1;
private static final String SP_KEY_DB_VER = "db_ver";
// public constructor
public DatabaseOpenHelper(Context context, String title,
CursorFactory factory, int version)
{
super(context, title, factory, version);
mContext=context;
initialize();
} // end DatabaseOpenHelper constructor
private void initialize() {
if (databaseExists()) {
SharedPreferences prefs = PreferenceManager
.getDefaultSharedPreferences(mContext);
int dbVersion = prefs.getInt(SP_KEY_DB_VER, 1);
if (DATABASE_VERSION != dbVersion) {
File dbFile = mContext.getDatabasePath(DATABASE_NAME);
if (!dbFile.delete()) {
Log.w("TAG", "Unable to update database");
}
}
}
if (!databaseExists()) {
createDatabase();
}
}
private boolean databaseExists() {
File dbFile = mContext.getDatabasePath(DATABASE_NAME);
return dbFile.exists();
}
private void createDatabase() {
String parentPath = mContext.getDatabasePath(DATABASE_NAME).getParent();
String path = mContext.getDatabasePath(DATABASE_NAME).getPath();
File file = new File(parentPath);
if (!file.exists()) {
if (!file.mkdir()) {
Log.w("TAG", "Unable to create database directory");
return;
}
}
InputStream is = null;
OutputStream os = null;
try {
is = mContext.getAssets().open(DATABASE_NAME);
os = new FileOutputStream(path);
byte[] buffer = new byte[1024];
int length;
while ((length = is.read(buffer)) > 0) {
os.write(buffer, 0, length);
}
os.flush();
SharedPreferences prefs = PreferenceManager
.getDefaultSharedPreferences(mContext);
SharedPreferences.Editor editor = prefs.edit();
editor.putInt(SP_KEY_DB_VER, DATABASE_VERSION);
editor.commit();
} catch (IOException e) {
e.printStackTrace();
} finally {
if (is != null) {
try {
is.close();
} catch (IOException e) {
e.printStackTrace();
}
}
if (os != null) {
try {
os.close();
} catch (IOException e) {
e.printStackTrace();
}
}
}
}
// creates the contacts table when the database is created
@Override
public void onCreate(SQLiteDatabase db)
{
// query to create a new table named contacts
String createQuery = "CREATE TABLE recipes" +
"(_id integer primary key autoincrement," +
"title TEXT, category TEXT, ingredients TEXT, process TEXT, notes TEXT, image TEXT, " +
"time TEXT, calories TEXT, difficulty TEXT);";
db.execSQL(createQuery); // execute the query
} // end method onCreate
@Override
public void onUpgrade(SQLiteDatabase db, int oldVersion,
int newVersion)
{
} // end method onUpgrade
} // end class DatabaseOpenHelper
}
这是我的 LogCat 文本：
11-27 23:19:41.643: D/AndroidRuntime(297): Shutting down VM
11-27 23:19:41.643: W/dalvikvm(297): threadid=1: thread exiting with uncaught exception (group=0x4001d800)
11-27 23:19:41.673: E/AndroidRuntime(297): FATAL EXCEPTION: main
11-27 23:19:41.673: E/AndroidRuntime(297): java.lang.RuntimeException: Unable to instantiate activity ComponentInfo{development.nk.cretanrecipes/development.nk.cretanrecipes.RecipesMainActivity}: java.lang.NullPointerException
11-27 23:19:41.673: E/AndroidRuntime(297): at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2585)
11-27 23:19:41.673: E/AndroidRuntime(297): at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2679)
11-27 23:19:41.673: E/AndroidRuntime(297): at android.app.ActivityThread.access$2300(ActivityThread.java:125)
11-27 23:19:41.673: E/AndroidRuntime(297): at android.app.ActivityThread$H.handleMessage(ActivityThread.java:2033)
11-27 23:19:41.673: E/AndroidRuntime(297): at android.os.Handler.dispatchMessage(Handler.java:99)
11-27 23:19:41.673: E/AndroidRuntime(297): at android.os.Looper.loop(Looper.java:123)
11-27 23:19:41.673: E/AndroidRuntime(297): at android.app.ActivityThread.main(ActivityThread.java:4627)
11-27 23:19:41.673: E/AndroidRuntime(297): at java.lang.reflect.Method.invokeNative(Native Method)
11-27 23:19:41.673: E/AndroidRuntime(297): at java.lang.reflect.Method.invoke(Method.java:521)
11-27 23:19:41.673: E/AndroidRuntime(297): at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:868)
11-27 23:19:41.673: E/AndroidRuntime(297): at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:626)
11-27 23:19:41.673: E/AndroidRuntime(297): at dalvik.system.NativeStart.main(Native Method)
11-27 23:19:41.673: E/AndroidRuntime(297): Caused by: java.lang.NullPointerException
11-27 23:19:41.673: E/AndroidRuntime(297): at android.content.ContextWrapper.getResources(ContextWrapper.java:80)
11-27 23:19:41.673: E/AndroidRuntime(297): at android.widget.Toast.<init>(Toast.java:89)
11-27 23:19:41.673: E/AndroidRuntime(297): at android.widget.Toast.makeText(Toast.java:231)
11-27 23:19:41.673: E/AndroidRuntime(297): at development.nk.cretanrecipes.DatabaseConnector$DatabaseOpenHelper.initialize(DatabaseConnector.java:191)
11-27 23:19:41.673: E/AndroidRuntime(297): at development.nk.cretanrecipes.DatabaseConnector$DatabaseOpenHelper.<init>(DatabaseConnector.java:186)
11-27 23:19:41.673: E/AndroidRuntime(297): at development.nk.cretanrecipes.DatabaseConnector.<init>(DatabaseConnector.java:35)
11-27 23:19:41.673: E/AndroidRuntime(297): at development.nk.cretanrecipes.RecipesMainActivity.<init>(RecipesMainActivity.java:32)
11-27 23:19:41.673: E/AndroidRuntime(297): at java.lang.Class.newInstanceImpl(Native Method)
11-27 23:19:41.673: E/AndroidRuntime(297): at java.lang.Class.newInstance(Class.java:1429)
11-27 23:19:41.673: E/AndroidRuntime(297): at android.app.Instrumentation.newActivity(Instrumentation.java:1021)
11-27 23:19:41.673: E/AndroidRuntime(297): at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2577)
11-27 23:19:41.673: E/AndroidRuntime(297): ... 11 more